InfoMagic Standards 1993 July/Disc.iso/ccitt/1988/troff/3_1_07.tro

Text File | 1991-12-12 | 82.0 KB | 2,888 lines

.rs
.\" Troff code generated by TPS Convert from ITU Original Files
.\"                 Not Copyright ( c) 1991
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.\" Assumes tbl, eqn, MS macros, and lots of luck.
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.EN
.nr LL 40.5P
.nr ll 40.5P
.nr HM 3P
.nr FM 6P
.nr PO 4P
.nr PD 9p
.po 4P

.rs
\v | 5i'
.LP
\fBMONTAGE : FIN DE LA RECOMMANDATION G.164 EN T\* | TE DE CETTE PAGE\fR
.sp 2P
.LP
\v'12P'
\fBRecommendation\ G.165\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fBECHO\ CANCELLERS\fR
.EF '%    Fascicle\ III.1\ \(em\ Rec.\ G.165''
.OF '''Fascicle\ III.1\ \(em\ Rec.\ G.165    %'
.ce 0
.sp 1P
.ce 1000
\fI(Geneva, 1980; amended at Malaga\(hyTorremolinos, 1984\fR
.sp 9p
.RT
.ce 0
.sp 1P
.ce 1000
\fIand at Melbourne, 1988)\fR
.ce 0
.sp 1P
.LP
\fB1\fR     \fBGeneral\fR
.sp 1P
.RT
.PP
1.1
Echo cancellers are voice operated devices placed in the 4\(hywire portion
of a circuit (which may be an individual circuit path or a path
carrying a multiplexed signal) and are used for reducing the echo by
subtracting an estimated echo from the circuit echo. They may be characterized
by whether the transmission path or the subtraction of the echo is by
analogue or digital means (see Figures\ 1/G.165, 2/G.165 and 3/G.165).
.sp 9p
.RT
.PP
1.2
This Recommendation is applicable to the design of echo cancellers
using digital or analogue techniques, and intended for use in an international
circuit. Echo cancellers designed to this Recommendation will be compatible
with each other and with echo suppressors designed in accordance with
Recommendations\ G.161\ [1] and\ G.164. Compatibility is defined in
Recommendation\ G.164, \(sc\ 1.4. Freedom is permitted in design details not
covered by the requirements.
.PP
Echo cancellers may be used for purposes other than network echo control
on international circuits, e.g.\ in active 2\(hywire/4\(hywire hybrids
or
2\(hywire repeaters, but this Recommendation does not apply to such echo
cancellors.
.LP
.rs
.sp 16P
.ad r
\fBFigure\ 1/G.165 p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 17P
.ad r
\fBFigure\ 2/G.165 p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 14P
.ad r
\fBFigure\ 3/G.165 p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 2P
.LP
\fB2\fR     \fBDefinitions relating to echo cancellers\fR
.FS
These definitions
assume that nonlinear processing, e.g.\ centre clipping, is not present
in the send or receive paths unless otherwise specified and that the signal
at
\fIS\fR\di\\dn\uis purely echo.
.FE
.sp 1P
.RT
.PP
In the definition and text, \fIL\fR will refer to the relative power level
of a signal, expressed in\ dBm0 and
\fIA\fR \ will refer to the attenuation or loss of a signal path expressed
in\ dB.
.RT
.sp 1P
.LP
2.1
    \fBecho canceller\fR (see Figure\ 4/G.165)
.sp 9p
.RT
.LP
    \fIF:\fR     \fIannuleur d'\*'echo\fR
.LP
    \fIS:\fR     \fIcompensador de eco; cancelador de eco\fR
.PP
A voice operated device placed in the 4\(hywire portion of a circuit and
used for reducing near\(hyend echo present on the send path by subtracting
an estimation of that echo from the near\(hyend echo.
.RT
.LP
.sp 1
.bp
.LP
.rs
.sp 16P
.ad r
\fBFigure 4/G.165 p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
2.2
    \fBecho loss (A\fR \s6\fBECHO\fR .PS 10
\fB)\fR
.sp 9p
.RT
.LP
    \fIF:\fR     \fIaffaiblissement d'\*'echo (A\fR \s6\fIECHO\fR .PS 10
\fI)\fR
.RT
.LP
    \fIS:\fR     \fI atenuaci\*'on del eco (A\fR \s6\fIECO\fR .PS 10
\fI)\fR
.RT
.PP
The attenuation of a signal from the receive\(hyout port
(R\do\\du\\dt\u) to the send\(hyin port (S\di\\dn\u) of an echo
canceller,
due to transmission and hybrid loss, i.e.\ the loss in the echo path.
.PP
\fINote\fR \ \(em\ This definition does not strictly adhere to the echo loss
definition given in Recommendation\ G.122, \(sc\ 2.2 which applies to loss
of the
\fIa\fR \(hy\fIt\fR \(hy\fIb\fR path viewed from the virtual switching
point of the international circuit. The echo canceller may be located closer
to the echo reflection
point.
.RT
.sp 1P
.LP
2.3
    \fBcancellation (A\fR \s6\fBCANC\fR .PS 10
\fB)\fR
.sp 9p
.RT
.LP
    \fIF:\fR     \fIannulation (A\fR \s6\fINL\fR .PS 10
\fI)\fR
.RT
.LP
    \fIS:\fR     \fIcompensaci\*'on; cancelaci\*'on (A\fR \s6\fICOMP\fR .PS 10
\fI)\fR
.RT
.PP
The attenuation of the echo signal as it passes through the send path of
an echo canceller. This definition specifically excludes any nonlinear
processing on the output of the canceller to provide for further
attenuation.
.RT
.sp 1P
.LP
2.4
    \fBresidual echo level (L\fR \s6\fBRES\fR .PS 10
\fB)\fR
.sp 9p
.RT
.LP
    \fIF:\fR     \fIniveau d'\*'echo r\*'esiduel (N\fR \s6\fIRES\fR .PS 10
\fI)\fR
.RT
.LP
    \fIS:\fR     \fInivel de eco residual (N\fR \s6\fIRES\fR .PS 10
\fI)\fR
.RT
.PP
The level of the echo signal which remains at the send\(hyout port of an
operating echo canceller after imperfect cancellation of the circuit echo.
It is related to the receive\(hyin signal\ \fIL\fR \s6Rin
.PS 10
by
\v'6p'
.RT
.sp 1P
.ce 1000
\fIL\fR \s6RES
.PS 10
\ =\ \fIL\fR \s6Rin
.PS 10
\ \(em
\fIA\fR \s6ECHO
.PS 10
\ \(em\ \fIA\fR \s6CANC
.PS 10
.RT
.ce 0
.sp 1P
.PP
.sp 1
Any nonlinear processing is not included.
.bp
.sp 1P
.LP
2.5
    \fBnonlinear processor (NLP)\fR
.sp 9p
.RT
.LP
    \fIF:\fR     \fIprocesseur non lin\*'eaire (PNL)\fR
.LP
    \fIS:\fR     \fIprocesador no lineal (PNL)\fR
.PP
A device having a defined suppression threshold level and in
which:
.RT
.LP
    a)
     signals having a level detected as being below the threshold are suppressed,
and
.LP
    b)
     signals having a level detected as being above the threshold are passed
although the signal may be distorted.
.PP
\fINote 1\fR \ \(em\ The precise operation of a nonlinear processor depends
upon the detection and control algorithm used.
.PP
\fINote 2\fR \ \(em\ An example of a nonlinear processor is an analogue
centre clipper in which all signal levels below a defined threshold are
forced to some minimum value.
.RT
.sp 1P
.LP
2.6
    \fBnonlinear processing loss (A\fR \s6\fBNLP\fR .PS 10
\fB)\fR
.sp 9p
.RT
.LP
    \fIF:\fR     \fIaffaiblissement par traitement non lin\*'eaire\fR
\fI(A\fR \s6TNL
.PS 10
\fI)\fR
.RT
.LP
    \fIS:\fR     \fIatenuaci\*'on por procesamiento (o tratamiento)\fR
\fIno lineal\fR \fI(A\fR \s6PNL
.PS 10
\fI)\fR
.RT
.PP
Additional attenuation of residual echo level by a nonlinear
processor placed in the send path of an echo canceller.
.PP
\fINote\fR \ \(em\ Strictly, the attenuation of a nonlinear process cannot be
characterized by a loss in\ dB. However, for purposes of illustration and
discussion of echo canceller operation, the careful use of
\fIA\fR \s6NLP
.PS 10
is helpful.
.RT
.sp 1P
.LP
2.7
    \fBreturned echo level (L\fR \s6\fBRET\fR .PS 10
\fB)\fR
.sp 9p
.RT
.LP
    \fIF:\fR     \fIniveau de retour d'\*'echo (N\fR \s6RET
.PS 10
\fI)\fR
.RT
.LP
    \fIS:\fR     \fInivel del eco devuelto (N\fR \s6DEV
.PS 10
\fI)\fR
.RT
.PP
The level of the signal at the send\(hyout port of an operating echo canceller
which will be returned to the talker. The attenuation of a nonlinear processor
is included, if one is normally present. \fIL\fR \s6RET
.PS 10
is related to \fIL\fR \s6Rin
.PS 10
by
\v'6p'
.RT
.sp 1P
.ce 1000
\fIL\fR \s6RET
.PS 10
\ =\ \fIL\fR \s6Rin
.PS 10
\ \(em
(\fIA\fR \s6ECHO
.PS 10
\ +\ \fIA\fR \s6CANC
.PS 10
\ +
\fIA\fR \s6NLP
.PS 10
).
.RT
.ce 0
.sp 1P
.PP
.sp 1
If nonlinear processing is not present, note that
\fIL\fR \s6RES
.PS 10
\ =\ \fIL\fR \s6RET
.PS 10
.
.RT
.sp 1P
.LP
2.8
    \fBcombined loss (A\fR \s6\fBCOM\fR .PS 10
\fB)\fR
.sp 9p
.RT
.LP
    \fIF:\fR     \fIaffaiblissement combin\*'e (\fIA\fR \s6COM
.PS 10
\fI)\fR
.RT
.LP
    \fIS:\fR     \fIatenuaci\*'on combinada (A\fR \s6COMB
.PS 10
\fI)\fR
.RT
.PP
The sum of echo loss, cancellation loss and nonlinear processing loss (if
present). This loss relates \fIL\fR \s6Rin
.PS 10
to \fIL\fR \s6RET
.PS 10
by:
\v'6p'
.RT
.sp 1P
.ce 1000
\fIL\fR \s6RET
.PS 10
\ =\ \fIL\fR \s6Rin
.PS 10
\ \(em
\fIA\fR \s6COM
.PS 10
, where
\fIA\fR \s6COM
.PS 10
\ =\ \fIA\fR \s6ECHO
.PS 10
\ +
\fIA\fR \s6CANC
.PS 10
\ +\ \fIA\fR \s6NLP
.PS 10
.
.RT
.ce 0
.sp 1P
.LP
.sp 1
.sp 1P
.LP
2.9
    \fBconvergence\fR
.sp 9p
.RT
.LP
    \fIF:\fR     \fIconvergence\fR
.LP
    \fIS:\fR     \fIconvergencia\fR
.PP
The process of developing a model of the echo path which will be used in
the echo estimator to produce the estimate of the circuit echo.
.RT
.sp 1P
.LP
2.10
    \fBconvergence time\fR
.sp 9p
.RT
.LP
    \fIF:\fR     \fItemps de convergence\fR
.LP
    \fIS:\fR     \fItiempo de convergencia\fR
.PP
For a defined echo path, the interval between the instant a
defined test signal is applied to the receive\(hyin port of an echo canceller
with the estimated echo path impulse response initially set to zero, and
the instant the returned echo level at the send\(hyout port reaches a defined
level.
.bp
.RT
.sp 1P
.LP
2.11
    \fBleak time\fR
.sp 9p
.RT
.LP
    \fIF:\fR     \fItemps de fuite\fR
.LP
    \fIS:\fR     \fItiempo de fuga\fR
.PP
The interval between the instant a test signal is removed from the receive\(hyin
port of a fully\(hyconverged echo canceller and the instant the echo
path model in the echo canceller changes such that, when a test signal is
reapplied to \fIR\fR\di\\dn\uwith the convergence circuitry
inhibited, the returned echo is at a defined level.
.PP
This definition refers to echo cancellers employing, for example,
leaky integrators in the convergence circuitry.
.RT
.sp 2P
.LP
\fB3\fR     \fBCharacteristics of echo cancellers\fR
.sp 1P
.RT
.sp 1P
.LP
3.1
    \fIGeneral\fR
.sp 9p
.RT
.PP
This Recommendation is applicable to the design of echo cancellers. The
echo cancellers are assumed to be \*Qhalf\*U echo cancellers, i.e.\ those
in
which cancellation takes place only in the send path due to signals present
in the receive path.
.RT
.sp 1P
.LP
3.2
    \fIPurpose, operation and environment\fR
.sp 9p
.RT
.PP
Echo, in any 2\(hywire or combination 2\(hy and 4\(hywire telephone
circuit, is caused by impedance mismatches. An echo canceller can be used to
reduce this echo to tolerable levels.
.PP
The echo present at the send\(hyin port of an echo canceller is a
distorted and delayed replica of the incoming speech from the far end,
i.e.\ the echo is the incoming speech as modified by the echo path. The
echo path is
commonly described by its impulse response (see Figure\ 5/G.165). This
response of a typical echo path shows a pure delay \fIt\fR\d\fIr\fR\u,
due to the delays
inherent
in the echo path transmission facilities, and a dispersed signal due to band
limiting and multiple reflections. The sum of these is the echo path delay,
\fIt\fR\d\fId\fR\u. The values of delay and dispersion will vary depending
on the
properties of the echo paths, e.g.\ they may vary for different national
networks. It is assumed that the echo paths are basically linear and not
continuously varying
.FS
Echo cancellers designed specifically for echo paths which are nonlinear
and/or time variant are likely to be much more complex than those not so
designed. It is felt that insufficient information exists to
include such echo cancellers in this Recommendation. Echo cancellers conforming
to this Recommendation are adaptive and will cope with slowly varying echo
paths when only receive speech is present.
.FE
, e.g.\ have no phase roll (see
.PP
Recommendation\ G.164). In addition, the loss of the echo path in\ dB (see
\(sc\ 2.2 above) is likely to be such that the minimum loss from
\fIR\fR\do\\du\\dt\uto \fIS\fR\di\\dn\uof the echo canceller will be equal to
the difference between relative levels at these two ports plus 6\ dB.
Echo cancellers designed to this Recommendation will perform properly for
echo loss (\fIA\fR \s6ECHO
.PS 10
) of 6\ dB or greater. For
(\fIA\fR \s6ECHO
.PS 10
) less than 6\ dB they may also work
but with degraded performance. It is not possible to quantify this degraded
performance.
.RT
.LP
.rs
.sp 12P
.ad r
\fBFigure 5/G.165 p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.PP
An echo canceller must be able to synthesize a replica of the
echo path impulse response
. Many echo cancellers model the echo path
using a sampled data representation, the sampling being at the
Nyquist
rate
(8000\ Hz). Such an echo canceller, to function properly, must have
sufficient storage capacity for the required number of samples
.FS
Echo
cancellers having storage capacities of 16\ ms to 40\ ms have been successfully
demonstrated. Maximum echo path
delay\ \fIt\fR\d\fId\fR\u, in the network in which the canceller will be
used will determine the required storage capacity.
.FE
.  Typically, too few storage locations will prevent adequate synthesis of all
echo paths: too many storage locations will create undesirable additional
noise due to the unused locations which, because of estimation noise, are
generally not zero. It should be recognized that an echo canceller introduces
an
additional parallel echo path. If the impulse response of the echo path
model is sufficiently different from the echo path impulse response, the
total
returned echo may be larger than that due to the echo path only.
.PP
The echo paths change as the echo canceller is used in successive
connections. When speech first arrives at \fIR\fR\di\\dn\u, the echo
canceller must adapt or converge to the new echo path, and it is desirable
that this be fairly rapid, e.g.\ about one\(hyhalf second. Also the residual
echo should be small regardless of the level of the receive speech and
the characteristics of the echo path. Some Administrations feel that a
slightly higher residual
echo level may be permitted provided it is further reduced using a small
amount of nonlinear processing (see \(sc\ 5).
.PP
When there is receive speech and the near party begins to double
talk, an echo canceller may interpret the transmit signal as a new echo
signal and attempt to adapt to it. This can seriously degrade the subjective
quality of the connection. Not only is the echo cancellation reduced but
distortion of the double talking speech may occur as the echo canceller
dynamically attempts to adapt. Two common approaches are taken as a solution.
The first is to use
algorithm which causes slow adaptation during periods of double talk. The
second is to employ a
double talk detector
, similar to that used in
echo suppressors. The echo canceller double talk detector, however, generally
should favour break\(hyin at the expense of false operation on echo. This
differs from the double talk detector in an echo suppressor.
.PP
Thus, echo cancellers have the following fundamental
requirements:
.RT
.LP
    1)
    rapid convergence;
.LP
    2)
    subjective low returned echo level during
single
talk;
.LP
    3)
    low divergence during
double talk.
.PP
When echo cancellers are located on the subscriber side of the
international signalling equipment, signalling tones do not pass through the
cancellers so no special action is necessary. When cancellers are on the
international side of the signalling equipment they are normally disabled by
the switch during the active signalling exchange intervals in order to
prevent distortion of the signalling tones by the echo canceller. When
signalling tones simultaneously appear at the canceller receive and send
ports (double talk) the receive signal will be processed through the echo
path model contained in the canceller. The signal estimate produced by
the canceller may sufficiently
distort the send side signal so that it will not be properly recognized
by the signalling receive unit (Note\ 1). An echo canceller must de disabled
during the transmission of the CCITT No.\ 6 and No.\ 7 continuity check
signal (Note\ 2).
.PP
If an echo canceller conforming to Recommendation\ G.165 is located on the
international side of CCITT No.\ 5 signalling units an enabled canceller, it
will interfere with the continuously compelled signalling exchange CCITT
No.\ 5 unless additional special precautions are taken. See Recommendation\
Q.115 for details.
.PP
\fINote\ 1\fR \ \(em\ For some echo cancellers this problem may not occur when
the send and receive frequencies are different.
.PP
\fINote\ 2\fR \ \(em\ CCITT Recommendation Q.271 on CCITT No. 6 and
Recommendation\ Q.724 on CCITT No.\ 7 both include the following statement:
\*QAs the presence of active echo suppressors in the circuit would interfere
with
the continuity check, it is necessary to disable the suppressors during the
check and to re\(hyenable them, if required, after the check has been
completed.\*U
.RT
.LP
.sp 1
.bp
.sp 1P
.LP
3.3
    \fIExternal enabling/disabling\fR
.sp 9p
.RT
.PP
An option should be included in the echo canceller to provide for enabling
or disabling by an externally derived ground (earth) from the trunk
circuit. The enabler should function to permit or prevent normal echo canceller
operation. Certain type\ C echo cancellers may be disabled directly by
a digital signal. Some digital data signals may require Type\ C echo cancellers
to provide 64\ kbit/s bit sequence integrity in the externally disabled
state.
.RT
.sp 2P
.LP
3.4
     \fITests and requirements for performance with inputs signals applied\fR
\fIto the send and receive paths\fR
.sp 1P
.RT
.sp 1P
.LP
3.4.1
    \fITransmission performance\fR
.sp 9p
.RT
.PP
The appropriate transmission performance requirements of
Recommendation\ G.164 also apply to echo cancellers except as noted below.
.RT
.sp 1P
.LP
3.4.1.1
    \fIDelay distortion\ \(em\ Type A\fR
.sp 9p
.RT
.PP
The delay distortion relative to the minimum delay shall not exceed the
values given in Table\ 1/G.165.
.RT
.ce
\fBH.T. [T1.165]\fR
.ce
TABLE\ 1/G.165
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(60p) | cw(60p) .
Frequency band  (Hz)    Delay distortion  (\(*ms)
_
.T&
cw(60p) | cw(60p) .
\ 500\(hy\ 600    300
.T&
cw(60p) | cw(60p) .
\ 600\(hy1000    150
.T&
cw(60p) | cw(60p) .
1000\(hy2600    \ 50
.T&
cw(60p) | cw(60p) .
2600\(hy3000    250
_
.TE
.nr PS 9
.RT
.ad r
\fBTable [1/G.165] [T1.165], p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
.sp 1
3.4.1.2
    \fIAttenuation distortion\ \(em\ Type A\fR
.sp 9p
.RT
.PP
The attenuation distortion shall be such that if \fIQ\fR \ dB is the
attenuation at 800\ Hz (or 1000\ Hz) the attenuation shall be within the range
(\fIQ\fR \ +\ 0.5)\ dB to (\fIQ\fR \ \(em\ 0.2)\ dB at any frequency in
the band 300\(hy3400\ Hz and at 200\ Hz, within the range of (\fIQ\fR \
+\ 1.0)\ dB to (\fIQ\fR \ \(em\ 0.2)\ dB.
.RT
.sp 1P
.LP
3.4.1.3
    \fIGroup delay\ \(em\ Type C\fR
.sp 9p
.RT
.PP
The group delay in the send path should be kept to a minimum and
should not exceed 1\ ms. No significant delay should occur in the receive path.
.PP
\fINote\fR \ \(em\ The creation of frame slips in the echo path can lead
to an occasional degradation of the echo cancellation. If a delay is necessary
to
synchronize the digital send and receive paths, the global admissible delay
on the send path, including the group delay mentioned above, must not exceed
1\ ms and on the receive path 250\ \(*ms.
.RT
.sp 1P
.LP
3.4.1.4
    \fIGroup delay \(em Type D\fR
.sp 9p
.RT
.PP
The group delay in the send and receive paths shall meet the
requirements of \(sc\ 3.4.1.3 for Type\ C echo cancellers with the addition
of the delay allowed for codecs as given in Recommendation\ G.712.
.bp
.RT
.sp 1P
.LP
3.4.2
    \fIEcho canceller performance\fR
.sp 9p
.RT
.PP
The performance requirements which follow are for echo cancellers which
include nonlinear processors (see Annex\ A for echo cancellers which do
not include a nonlinear processor).
.PP
In the tests, it is assumed that the nonlinear processor can be
disabled, that the
echo path impulse response store
(H\ register) can be cleared (set to zero) and that adaptation can be inhibited.
.PP
The requirements are described in terms of tests made by applying
signals to R\di\\dn\uand S\di\\dn\uof an echo canceller, and measuring the
S\do\\du\\dt\usignals. The test set\(hyup is as shown in Figure\ 6/G.165.
The ports are assumed to be at equal relative level points. Band\(hylimited
noise is used as the receive input test signal. The echo loss is independent
of frequency.
.RT
.LP
.rs
.sp 29P
.ad r
\fBFIGURE 6/G.165 p.\fR
.sp 1P
.RT
.ad b
.RT
.PP
The primary purpose of an echo canceller is to control the echo of a speech
stimulus signal. This is done by synthesizing a replica of the echo
path impulse response and using it to generate an estimate of the echo
which is subtracted from the actual circuit echo. The synthesis must be
accomplished
using a speech input signal. Because of the difficulty of defining a speech
test signal, the following tests are type tests and rely upon the use of a
band\(hylimited noise test signal primarily for measurement convenience and
.LP
repeatability. These tests should be performed on an echo canceller only
after the design has been shown to properly synthesize a replica of the
echo path
impulse response from a speech input signal and its corresponding echo.
Speech signals are not used in the tests in this section. Additionally,
the nonlinear processor in the echo canceller should be designed to minimize
and potentially avoid the perceptible effects of double\(hytalk clipping
and noise contrast [see Recommendation\ G.164, Table\ 1, Note\ a)]. Tests
to ensure proper operation are under study.
.bp
.sp 1P
.LP
3.4.2.1
    \fITest No.\ 1\ \(em\ Steady state residual and returned echo level\fR
\fItest\fR
.sp 9p
.RT
.PP
This test is meant to ensure that the steady state cancellation
(\fIA\fR \s6CANC
.PS 10
) is sufficient to produce a residual echo
level which is sufficiently low to permit the use of nonlinear processing
without undue reliance on it.
.RT
.PP
The H register is initially cleared and a receive signal is applied
for a sufficient time for the canceller to converge producing a steady state
residual echo level.
.RT
.sp 1P
.LP
    \fIRequirement (provisional)\fR
.sp 9p
.RT
.PP
With the H register initially set to zero, the nonlinear processor disabled
for all values of receive input signal level such that
\fIL\fR \s6Rin
.PS 10
\ \(>="\ \(em30\ dBm0 and \(=\ \(em10\ dBm0 and for all values of echo
loss \ 6\ dB and echo path delay, \fIt\fR\d\fId\fR\u\ \(=\ \(*D\ ms
.FS
Different echo
cancellers may be designed to work satisfactorily for different echo path
delays depending on their application in various networks. Thus \(*D, whenever
it appears in this Recommendation, represents the echo path delay, \fIt\fR\d\fId\fR\u,
for which the echo canceller is designed.
.FE
, the residual echo level should be less than or equal to that shown in
Figure\ 7/G.165. When the nonlinear
processor is enabled, the returned echo level must be less than \(em65\ dBm0.
.RT
.PP
\fINote\fR \ \(em\ Recommendation G.113 allows for up to 5\ PCM codecs in the
echo path. Meeting the requirement of Figure\ 7/G.165 under those conditions
has not been verified. This is under study.
.RT
.LP
.rs
.sp 19P
.ad r
\fBFigure 7/G.165, p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
3.4.2.2
    \fITest No.\ 2\ \(em\ Convergence test\fR
.sp 9p
.RT
.PP
This test is meant to ensure that the echo canceller converges
rapidly for all combinations of input signal levels and echo paths and
that the returned echo level is sufficiently low. The H\ register is initially
cleared
and adaption is inhibited. The double talk detector, if present, is put
in the double talk mode by applying signals to S\di\\dn\uand R\di\\dn\u.
The
signal at\ S\di\\dn\uis removed and simultaneously adaption is enabled. The
degree of adaption, as measured by the returned echo level, will depend
on the convergence characteristics of the echo canceller and the double
talk detection hangover time.
.bp
.PP
The test procedure is to clear the H\ register and inhibit adaption.
Signal\ N is applied at a level \(em10\ dBm0 and a signal is applied
at\ R\di\\dn\u. Then\ N is removed and simultaneously adaption is enabled
(see Figure\ 8/G.165). After 500\ ms inhibit adaption and measure the returned
echo level. The nonlinear processor should be enabled.
.RT
.LP
.rs
.sp 13P
.ad r
\fBFigure 8/G.165 p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
    \fIRequirement\fR
.sp 9p
.RT
.PP
With the H\ register initially set to zero, for all values
\fIL\fR \s6Rin
.PS 10
\ \(>="\ \(em30\ dBm0 and \(=\ \(em10\ dBm0 and present for 500\ ms and
for all values of echo loss\ \ 6\ dB and echo path delay, \fIt\fR\d\fId\fR\u\
\(=\ \(*D\ ms, the
combined loss (\fIA\fR \s6COM
.PS 10
\ =\ \fIA\fR \s6ECHO
.PS 10
\ +
\fIA\fR \s6CANC
.PS 10
\ +\ \fIA\fR \s6NLP
.PS 10
) should
be\ \(>="\ 27\ dB.
.RT
.sp 1P
.LP
3.4.2.3
    \fITest No.\ 3\ \(em\ Performance under conditions of double talk\fR
.sp 9p
.RT
.PP
The two parts of this test are meant to test the performance of the canceller
under various conditions of double talk. The tests make the
assumption that, upon detection of double talk, measures are taken to prevent
or slow adaption in order to avoid excessive reduction in cancellation.
.RT
.PP
3.4.2.3.1
Test No. 3 |  is meant to ensure that the double talk
detection is not so sensitive that echo and low level near\(hyend speech
falsely cause operation of the double talk detector to the extent that
adaption does
not occur. The test procedure is to clear the H\ register; then for some
value of echo delay and echo loss, a signal is applied to\ R\di\\dn\u.
Simultaneously (see Figure\ 9/G.165) an interfering signal which is sufficiently
low in level to not seriously hamper the ability of the echo canceller
to
converge, is applied at\ S\di\\dn\u. This signal should not cause the
double talk detector to be activated, and adaption and cancellation should
occur. After 1\ s the adaption is inhibited and the residual echo measured.
The nonlinear process should be \fIdisabled\fR .
.sp 9p
.RT
.LP
.rs
.sp 10P
.ad r
\fBfigure 9/G.165 p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
    \fIRequirement\fR
.sp 9p
.RT
.PP
With the H\ register initially set to zero for all values of
\fIL\fR \s6Rin
.PS 10
\ \(>="\ \(em25\ dBm0 and\ \(=\ \(em10\ dBm0,
\fIN\fR \ =\ \fIL\fR \s6Rin
.PS 10
\ \(em15\ dB, \fIA\fR \s6ECHO
.PS 10
\ \(>="\ 6\ dB and echo path delay, \fIt\fR\d\fId\fR\u\ \(=\ \(*D\ ms, convergence
should occur
within 1.0\ s and \fIL\fR \s6RES
.PS 10
should be\ \(=\ \fIN\fR .
.RT
.PP
3.4.2.3.2
Test No.\ 3 |  is meant to ensure that the double talk detector is sufficiently
sensitive and operates fast enough to prevent large divergence during double
talking.
.sp 9p
.RT
.PP
The test procedure is to fully converge the echo canceller for a given
echo path. A signal is then applied to R\di\\dn\u. Simultaneously
(see Figure\ 10/G.165) a signal\ \fIN\fR is applied to\ S\di\\dn\uwhich
has a level at least that of\ R\di\\dn\u. This should cause the double
talk detector to
operate. After any arbitrary time, \(*dt\ >\ 0, the adaption is inhibited
and the
residual echo measured. The nonlinear processor should be disabled.
.LP
.rs
.sp 10P
.ad r
\fBfigure 10/G.165 p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
    \fIRequirement\fR
.sp 9p
.RT
.PP
With the echo canceller initially in the fully converged state for all
values of\ \fIL\fR \s6Rin
.PS 10
\ \(>="\ \(em30\ dBm0 and\ \(=\ \(em10\ dBm0, and for all values of\ \fIN\fR
\ \(>="\ \fIL\fR \s6Rin
.PS 10
and for all values of echo
loss\ \(>="\ 6\ dB and echo path delay \fIt\fR\d\fId\fR\u\ \(=\ \(*D\ ms,
the residual echo level after the simultaneous application of\ \fIL\fR
\s6Rin
.PS 10
and\ \fIN\fR for any time period should not increase more than 10\ dB over
the steady state requirements of Test No.\ 1.
.RT
.sp 1P
.LP
3.4.2.4
    \fITest No.\ 4\ \(em\ Leak rate test\fR
.sp 9p
.RT
.PP
This test is meant to ensure that the leak time is not too fast,
i.e. that the contents of the H\ register do not go to zero too rapidly.
.PP
The test procedure is to fully converge the echo canceller for a given
echo path and then to remove all signals from the echo canceller. After
two
minutes the contents of the H\ register are frozen, a signal applied
to\ \fIR\fR\di\\dn\uand the residual echo measured (see Figure\ 11/G.165). The
nonlinear process is used in normal operation, it should be \fIdisabled\fR .
.RT
.LP
.rs
.sp 10P
.ad r
\fBfigure 11/G.165 p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
    \fIRequirement\fR
.sp 9p
.RT
.PP
With the echo canceller initially in the fully converged state for all
values of\ \fIL\fR \s6Rin
.PS 10
\ \(>="\ \(em30\ dBm0 and\ \(=\ \(em10\ dBm0, two minutes after the removal
of the \fIR\fR \s6in
.PS 10
\ signal, the residual echo
level should not increase more than 10\ dB over the steady state requirement
of Test No.\ 1.
.RT
.sp 1P
.LP
3.4.2.5
    \fITest No.\ 5\ \(em\ Infinite return loss convergence test\fR
.sp 9p
.RT
.PP
This test is meant to ensure that the echo canceller has some means to
prevent the unwanted generation of echo. This may occur when the H\ register
contains an echo path model, either from a previous connection or the current
connection, and the echo path is opened (circuit echo vanishes) while a
signal is present at \fIR\fR\di\\dn\u.
.PP
The test procedure is to fully converge the echo canceller for a given
echo path. The echo path is then interrupted while a signal is applied
to\ \fIR\fR\di\\dn\u. 500\ ms after interrupting the echo path the
returned echo signal at\ \fIS\fR\do\\du\\dt\ushould be measured (see
Figure\ 12/G.165). The nonlinear processor should be \fIdisabled\fR .
.RT
.LP
.rs
.sp 17P
.ad r
\fBFIGURE 12/G.165, p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
    \fIRequirement (provisional)\fR
.sp 9p
.RT
.PP
With the echo canceller initially in the fully converged state for all
values of echo loss\ \(>="\ 6\ dB, and for all values
of\ \fIL\fR \s6Rin
.PS 10
\ \(>="\ \(em30\ dBm0 and \(= | (em10\ dBm0, the returned echo level at\
\fIS\fR \s6out
.PS 10
, 500\ ms after the echo path is interrupted,  should be \(= | (em37\ dBm0.
.RT
.sp 1P
.LP
3.4.2.6
    \fITest No.\ 6\ \(em\ Stability test\fR
.sp 9p
.RT
.PP
Under study.
.RT
.sp 2P
.LP
\fB4\fR     \fBCharacteristics of an echo canceller tone disabler\fR
.sp 1P
.RT
.sp 1P
.LP
4.1
    \fIGeneral\fR
.sp 9p
.RT
.PP
To ensure proper operation of all currently specified V\(hyseries
modems, the echo cancellers covered by this Recommendation should be equipped
with a tone detector that conforms to this section. This tone detector
responds to a disabling signal which is different from that used to disable
the echo
suppressor as described in Recommendation\ G.164, \(sc\ 5 and consists
of a 2100\ Hz tone with periodic phase reversals inserted in that tone.
The tone disabler
should respond only to the specified in\(hyband signal. It should not respond
to other in\(hyband signals, e.g.\ speech, or a 2100\ Hz tone without a
phase reversal. The tone disabler should detect and respond to a disabling
signal which may be present in either the send or the receive path.
.bp
.PP
The requirements for echo canceller disabling to ensure proper
operation with ATME No.\ 2 equipment that transmits the 2100\ Hz tone with
phase reversals could be met by using either the tone disabler specified
in this
section, or the
echo suppressor tone disabler
specified in
Recommendation\ G.164, \(sc\ 5. However, use of the Recommendation\ G.164,
\(sc\ 5
disabler does not assure proper operation with all currently specified
V\(hyseries modems.
.PP
The term disabled in this section refers to a condition in which the echo
canceller is configured in such a way as to no longer modify the signals
which pass through it in either direction. Under this condition, no echo
estimate is subtracted from the send path, the non\(hylinear processor is made
transparent, and the delay through the echo canceller still meets the
conditions
specified in \(sc\ 3.4.1. However, no relationship between the circuit
conditions before and after disabling should be assumed. For one thing,
the operation of echo cancellers with tonal inputs (such as the disabling
tone) is unspecified. Additionally, the impulse response stored in the
echo canceller prior to
convergence (and prior to the disabling tone being sent) is arbitrary.
This can lead to apparent additional echo paths which, in some echo canceller
implementations, remain unchanged until the disabling tone is recognized.
Also note that echo suppressors could be on the same circuit and there
is no
specified relationship between their delay in the enabled and disabled
states. In spite of the above, it is possible, for example, to measure
the round\(hytrip delay of a circuit with the disabling tone but the trailing
edge of the tone
burst should be used and sufficient time for all devices to be disabled
should be allotted before terminating the disabling tone and starting the
timing.
.PP
It should be noted that this condition does not necessarily fulfil the
requirements for 64\ kbit/s bit sequence integrity, for which case other
means of disabling in line with Recommendation\ G.165, \(sc\ 3.4 will apply.
.PP
A reference tone disabler is described in Annex\ B.
.RT
.sp 1P
.LP
4.2
    \fIDisabler characteristics\fR
.sp 9p
.RT
.PP
The echo canceller tone disabler requires the detection of a
2100\ Hz tone with phase reversals of that tone. The characteristics of the
transmitted signal are defined in Recommendation\ V.25. Phase variations
in the range of 180\(de \(+- | 5\(de must be detected while those in the
range of 0\(de \(+- | 10\(de must not be detected.
.PP
The frequency characteristics of the tone detector are the same as the
characteristics of the echo suppressor tone detector given in
Recommendation\ G.164, \(sc\ 5.2.
.PP
The dynamic range of this detector should be consistent with the input
levels as specified in Recommendation\ V.2 and H.51 with allowances for
variation introduced by the public switched telephone network.
.RT
.sp 1P
.LP
4.3
    \fIGuardband characteristics\fR
.sp 9p
.RT
.PP
Similar to that defined in Recommendation G.164, \(sc\ 5.3, consistent
with the dynamic range given in \(sc\ 4.2 above with the following exception.
The detector should operate perfectly with white noise less than or equal
to 11\ dB below the level of the 2100\ Hz signal. No definitive guidelines
can be given
for the range between 5\ and 11\ dB because of the variations in the test
equipment used. In particular, performance may vary with the peak\(hyto\(hyaverage
ratio of the noise generator used. As a general guideline, however, the
percentage of correct operation (detection of phase variations of 180\(de
\(+- | 5\(de
and non\(hydetection of phase variations of 0\(de \(+- | 10\(de) should
fall by no more than 1% for each dB reduction in signal\(hyto\(hynoise
below 11\ dB. The Administration
of the Federal Republic of Germany mentions the possibility of designing a
detector capable of operating perfectly at 5\ dB signal\(hyto\(hynoise ratio.
.RT
.sp 1P
.LP
4.4
    \fIHolding\(hyband characteristics\fR
.sp 9p
.RT
.PP
Same as defined in Recommendation\ G.164, \(sc\ 5.4.
.RT
.sp 1P
.LP
4.5
    \fIOperate time\fR
.sp 9p
.RT
.PP
The operate time must be sufficiently long to provide immunity from false
operation due to voice signals, but not so long as to needlessly extend
the time to disable. The tone disabler is required to operate within one
second of the receipt of the disabling signal.
.RT
.sp 1P
.LP
4.6
    \fIFalse operation due to speech currents\fR
.sp 9p
.RT
.PP
Same as in Recommendation G.164, \(sc\ 5.6.
.bp
.RT
.sp 1P
.LP
4.7
    \fIFalse operation due to data signals\fR
.sp 9p
.RT
.PP
It is desirable that the tone disabler should rarely operate
falsely on data signals from data sets that would be adversely affected by
disabling of the echo canceller. To this end, a reasonable objective is
that, for an echo canceller installed on a working circuit, usual data
signals from such data sets should not, on the average, cause more than
10\ false operations during 100\ hours of data transmissions.
.RT
.sp 1P
.LP
4.8
    \fIRelease time\fR
.sp 9p
.RT
.PP
Same as in Recommendation G.164, \(sc\ 5.7.
.RT
.sp 1P
.LP
4.9
    \fIOther considerations\fR
.sp 9p
.RT
.PP
Both the echo of the disabling tone and the echo of the calling
tone may disturb the detection of the echo canceller disabling tone. As
such, it is not recommended to add the receive and transmit signal inputs
together to form an input to a single detector.
.PP
Careful attention should be given to the number of phase reversals
required for detection of the disabling tone. Some Administrations favour
relying on\ 1 to improve the probability of detection even in the presence of
slips, impulse noise, and low signal\(hyto\(hynoise ratio. Other Administrations
favour relying on\ 2 to improve the probability of correctly distinguishing
between non\(hyphase\(hyreversed and phase\(hyreversed 2100\ Hz tones.
.RT
.sp 2P
.LP
\fB5\fR     \fBNonlinear processors for use in echo cancellers\fR
.sp 1P
.RT
.sp 1P
.LP
5.1
    \fIScope\fR
.sp 9p
.RT
.PP
For the purpose of this Recommendation the term \*Qnonlinear
processor\*U is intended to mean only those devices which fall within the
definition given in \(sc\ 2.5 and which have been proven to be effective
in echo
cancellers. It is possible to implement such nonlinear processors in a
number of ways (centre clippers being just one example), with fixed or
adaptive
.PP
operating features, but no recommendation is made for any particular
implementation. General principles and guidelines are given in \(sc\ 5.2. More
detailed and concrete information requires reference to specific
implementations. This is done in Annex\ C for the particular case of a
\*Qreference nonlinear processor\*U. The use of this term denotes an implementation
given for guidance and illustration only. It does not exclude other
implementations nor does it imply that the reference nonlinear processor is
necessarily the most appropriate realization on any technical, operational
or economic grounds.
.RT
.LP
5.2
    \fIGeneral principles and guidelines\fR
.sp 1P
.RT
.sp 2P
.LP
5.2.1
    \fIFunction\fR
.sp 1P
.RT
.sp 1P
.LP
5.2.1.1
    \fIGeneral\fR
.sp 9p
.RT
.PP
The
nonlinear processor
is located in the send path between the output of the subtractor and the
send\(hyout port of the echo canceller.
Conceptually, it is a device which blocks low level signals and passes high
level signals. Its function is to further reduce the residual echo level
(\fIL\fR \s6RES
.PS 10
as defined in \(sc\ 2.4) which remains after imperfect cancellation of
the circuit echo so that the necessary low returned echo level (\fIL\fR
\s6RET
.PS 10
as defined in \(sc\ 2.7) can be achieved.
.RT
.sp 1P
.LP
5.2.1.2
    \fINetwork performance\fR
.sp 9p
.RT
.PP
Imperfect cancellation can occur because echo cancellers which
conform to this Recommendation may not be capable of adequately modelling
echo paths which generate significant levels of nonlinear distortion (see
\(sc\ 3.2).
Such distortion can occur, for example, in networks conforming to
Recommendation\ G.113 in which up to five pairs of PCM codecs (conforming to
Recommendation\ G.712) are permitted in an echo path. The accumulated
quantization distortion from these codecs may prevent an echo canceller from
achieving the necessary \fIL\fR \s6RET
.PS 10
by using linear cancellation techniques alone. It is therefore recommended
that all echo cancellers capable only of modelling the linear components
of echo paths but intended for general network use should incorporate suitable
nonlinear processors.
.bp
.RT
.sp 1P
.LP
5.2.1.3
    \fILimitations\fR
.sp 9p
.RT
.PP
This use of nonlinear processors represents a compromise in the
circuit transparency which would be possible by an echo canceller which
could achieve the necessary \fIL\fR \s6RET
.PS 10
by using only modelling and
cancellation techniques. Ideally, the non\(hylinear processor should not cause
distortion of near\(hyend speech. In practical devices it may not be possible
to sufficiently approach this ideal in this case it is recommended that
nonlinear
.RT
.PP
processors should not be active under double talk or near\(hyend single\(hytalk
conditions. From this it follows that excessive dependence must not be
placed on the nonlinear processor and that \fIL\fR \s6RES
.PS 10
must be low
enough to prevent objectionable echo under double\(hytalk
conditions.
.RT
.sp 1P
.LP
5.2.1.4
    \fIData transmission\fR
.sp 9p
.RT
.PP
Nonlinear processors may affect the transmission of data through an enabled
echo canceller. This is under study.
.RT
.sp 2P
.LP
5.2.2
    \fISuppression threshold\fR
.sp 1P
.RT
.sp 1P
.LP
5.2.2.1
    \fIGeneral\fR
.sp 9p
.RT
.PP
The suppression threshold level (\fIT\fR \s6SUP
.PS 10
)   of a nonlinear processor is expressed in dBm0 and is equal to the highest
level of a sine\(hywave signal at a given moment that is just suppressed.
Either fixed or adaptive suppression threshold levels may be used.
.RT
.sp 1P
.LP
5.2.2.2
    \fIFixed suppression threshold\fR
.sp 9p
.RT
.PP
With a fixed suppression threshold level the appropriate level to use will
depend upon the cancellation achieved and the statistics of speech
levels and line conditions found in the particular network in which the echo
canceller is to be used. It is therefore recommended that the actual level
should be field selectable to permit the user to adjust it for the actual
network environment. Values of fixed suppression threshold levels to be used
are under study \(em see Notes\ 1 and\ 2.
.PP
\fINote 1\fR \ \(em\ As an interim guide, it is suggested that the suppression
threshold level should be set a few decibels above the level that would
result in the \fIpeaks\fR of \fIL\fR \s6RES
.PS 10
for a \*Q2\(*s\(hytalker\*U and a \*Q2\(*s\(hyecho
return loss\*U being suppressed.
.RT
.PP
\fINote 2\fR \ \(em\ Results of a field trial reported by one Administration
indicated that a fixed suppression threshold level of \(em36\ dBm0 gave a
satisfactory performance. A theoretical study, by another Administration,
of an echo path contianing five pairs of PCM codecs showed that for an
\fIL\fR \s6R
.PS 10
of \(em10\ dBm0, the quantization noise could result in an  \fIL\fR \s6RES
.PS 10
of \(em38\ dBm0.
.RT
.sp 1P
.LP
5.2.2.3
    \fIAdaptive suppression threshold\fR
.sp 9p
.RT
.PP
A good compromise can be made between using a high
\fIT\fR \s6SUP
.PS 10
to prevent it being exceeded by loud talker residual echo and using a low
\fIT\fR \s6SUP
.PS 10
to reduce speech distortion on   break\(hyin by making \fIT\fR \s6SUP
.PS 10
adaptive to the actual circuit
conditions and speech levels. This may be achieved in a number of ways
and no recommendation is made for any particular implementation. General
guidelines
applicable to the control algorithm and suppression threshold levels are
under study.
.RT
.sp 2P
.LP
5.2.3
    \fIControl of nonlinear processor activation\fR
.sp 1P
.RT
.sp 1P
.LP
5.2.3.1
    \fIGeneral\fR
.sp 9p
.RT
.PP
To conform to the recommendation made in \(sc\ 5.2.1.3, it is necessary
to control the activation of the nonlinear processor so that it is not
active when near\(hyend speech is likely to be present. When \*Qactive\*U,
the nonlinear
processor should function as intended to reduce
\fIL\fR \s6RES
.PS 10
.
When \*Qinactive\*U, it should not perform any nonlinear processing on
any signal passing through the echo canceller.
.bp
.RT
.sp 1P
.LP
5.2.3.2
    \fIControl guidelines\fR
.sp 9p
.RT
.PP
It is recommended that the following two guidelines should govern control
of the activation of a nonlinear processor. First, because they are
intended to further reduce \fIL
\s6RES
.PS 10
, they should be
active
when \fIL\fR \s6RES
.PS 10
is at a significant level. Second, because they should not distort near\(hyend
speech, they should be inactive when near\(hyend
speech is present. Where these two guidelines conflict the control function
should favour the second.
.RT
.sp 1P
.LP
5.2.3.3
    \fIStatic characteristics\fR
.sp 9p
.RT
.PP
A conceptual diagram showing the two operational states of a
nonlinear processor is shown in
Figure\ 13/G.165. The \fIL\fR \s6S
.PS 10
\fIL\fR \s6R
.PS 10
plane is divided into two regions, W and Z by the
threshold WZ. In the W region the nonlinear processor is inactive while
in the Z region it is active. Proper control of the nonlinear processor
to ensure
operation in the appropriate region requires recognition of the double\(hytalk
condition or the presence of near\(hyend speech. Imperfect detection of
double\(hytalk combined with a high suppression threshold level will result in
distortion of near\(hyend speech. The echo canceller then exhibits some of the
characteristics of an echo suppressor. A low suppression level will permit
easy double\(hytalking, even if a detection error is made because the near\(hyend
speech will suffer only a low level of non\(hylinear distortion. If the
suppression
threshold level is too low then peaks of residual echo may be heard.
.RT
.LP
.rs
.sp 21P
.ad r
\fBFigure 13/G.165, p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
5.2.3.4
    \fIDynamic characteristics\fR
.sp 9p
.RT
.PP
The dynamic characteristics can be specified by stating the time
that elapses when the signal conditions pass from a point in one area to a
point in the other area before the state appropriate to the second area is
established. Four such transitions are shown by arrows in Figure\ 13/G.165.
.RT
.sp 1P
.LP
    \fITransition No. 1 \(em W to Z, L\fI
.sp 9p
.RT
.EF '%    \fIS\ constant,\''
.OF '''\fIS\ constant,\    %'
.EF '%    \fIR\ increasing''
.OF '''\fIR\ increasing    %'
.PP
In this case the \fIL\fR\dS\usignal occurred first and the \fIL\fR\dR\uis
increasing to a sufficiently high level to override the \fIL\fR\dS\usignal
in the control path and cause the nonlinear processor to change from the
inactive to the active state. Since this will cause distortion of the \fIL\fR\dS\usignal
(near talker speech in this case) the action should not be initiated too
quickly.
.bp
.RT
.sp 1P
.LP
    \fITransition No. 2 \(em Z to W, L\fI
.sp 9p
.RT
.EF '%    \fIS\ constant,\''
.OF '''\fIS\ constant,\    %'
.EF '%    \fIR\ decreasing''
.OF '''\fIR\ decreasing    %'
.PP
In this case the \fIL\fR\dR\usignal has overriden the \fIL\fR\dS\usignal
in the control path and the nonlinear processor is in the active state.
The
\fIL\fR\dR\usignal is now decreasing. The nonlinear processor should remain
in the active state sufficiently long to prevent echo, which is stored
in the echo
path, from being hear by the far talker.
.RT
.sp 1P
.LP
    \fITransition No. 3 \(em Z to W, L\fI
.sp 9p
.RT
.EF '%    \fIR\ constant,\''
.OF '''\fIR\ constant,\    %'
.EF '%    \fIS\ increasing''
.OF '''\fIS\ increasing    %'
.PP
This transition is replicating the onset of double talk. As soon as possible
after the \fIL\fR\dS\usignal is detected the nonlinear processor should
be switched to the inactive state in order to minimise any distortion of
the near talker speech.
.RT
.sp 1P
.LP
    \fITransition No. 4 \(em W to Z, L\fI
.sp 9p
.RT
.EF '%    \fIR\ constant,\''
.OF '''\fIR\ constant,\    %'
.EF '%    \fIS\ decreasing''
.OF '''\fIS\ decreasing    %'
.PP
In this case \fIL\fR\dS\uhas been recognised but is decreasing. Any
action which is taken should favour continuing to permit the \fIL\fR\dS\usignal
to pass. This implies there should be some delay in switching the nonlinear
processor back to the active state.
.RT
.sp 1P
.LP
5.2.4
    \fIFrequency limits of control paths\fR
.sp 9p
.RT
.PP
Under study.
.PP
\fINote\fR \ \(em\ Depending on the particular implementation of the
nonlinear processor, the considerations and frequency response limits given
in Recommendation\ G.164, \(sc\ 3.2.4.2 for the suppression and break\(hyin
control paths of echo suppressors may also be applicable to similar control
paths used in
nonlinear processors. These control paths may include the activation control
and adaptive suppression threshold level control.
.RT
.sp 1P
.LP
5.2.5
    \fISignal attenuation below threshold level\fR
.sp 9p
.RT
.PP
The attenuation of signals having a level below that of the
suppression threshold level of a nonlinear processor in the active state
must be such that the requirements of \(sc\ 3.4.2.1 are met.
.RT
.sp 1P
.LP
5.2.6
    \fITesting of nonlinear processors\fR
.sp 9p
.RT
.PP
The nonlinear processor may be considered as a special case of an echo
suppressor which is limited to suppressing only low level signals. The
types of test required to determine the nonlinear processor performance
characteristics are very similar to the echo suppressor tests given in
Recommendation\ G.164. However, depend
ing on the specific implementation of a nonlinear processor, the transitions
between areas W and Z of
Figure\ 13/G.165
may not be as sharply defined as is the case for echo suppressors. Signals
observed at the send\(hyout port of the echo canceller may be distorted
for short periods when transitions between the W and Z operating regions
occur. Although Recommendation\ G.164 may be used as a guide to the testing
of nonlinear
processors it may be necessary to introduce unique test circuit modifications
in order to make measurements on some specific nonlinear processor
implementations. No recommendation can be given for a universal test circuit
appropriate for all nonlinear processor implementations.
\v'6p'
.RT
.ce 1000
ANNEX\ A
.ce 0
.ce 1000
(to Recommendation G.165)
.sp 9p
.RT
.ce 0
.ce 1000
\fBEcho cancellers without nonlinear processing\fR
.sp 1P
.RT
.ce 0
.PP
It may be possible to implement echo cancellers without the
inclusion of nonlinear processing. For these echo cancellers the total echo
loss is provided by echo cancellation. The achievable echo cancellation is
limited by the characteristics of the echo path and by the method of
implementing the echo canceller. In particular, if one pair of codecs
conforming to Recommendation\ G.712 is used in the echo path or in the echo
canceller, the maximum echo cancellation (considering quantizing errors
in the echo canceller and other impairments) is that shown by the solid
line in
Figure\ A\(hy1/G.165.
.bp
.sp 1P
.RT
.PP
Echo cancellers conforming to the solid line in Figure\ A\(hy1/G.165 have
been tested and found to provide acceptable performance in Japan. Other
tests, however, suggest that the echo cancellation required in echo cancellers
for general application is at least that shown by the broken line in
Figure\ A\(hy1/G.165. Further study is needed. Pending the results of that
study, echo cancellers which do not include nonlinear processors are not
yet
recommended for general application.
.PP
All the provisions and tests in the body of Recommendation\ G.165 apply
to these echo cancellers except as follows:
.RT
.LP
    a)
     \(sc\ 3.4.2.1: the residual echo level requirement is that shown by the
solid line of Figure\ A\(hy1/G.165.
.LP
    b)
     For all other tests, any reference to non\(hylinear processing should
be ignored.
.LP
.rs
.sp 26P
.ad r
\fBFigure A\(hy1/G.165, p.\fR
.sp 1P
.RT
.ad b
.RT
.ce 1000
ANNEX\ B
.ce 0
.ce 1000
(to Recommendation G.165)
.sp 9p
.RT
.ce 0
.ce 1000
\fBDescription of an\fR
\fBecho canceller reference tone disabler\fR
.sp 1P
.RT
.ce 0
.LP
B.1
    \fIGeneral\fR
.sp 1P
.RT
.PP
This annex describes the characteristics of an echo canceller
reference tone disabler. The use of the term \fIreference\fR denotes a
disabling
implementation given for guidance only. It does not exclude alternative
implementations of a tone disabler which responds to the signal as defined
in Recommendation\ V.25, and which also meets all of the criteria for reliability
of operation and protection from false operation by speech signals.
.bp
.RT
.sp 1P
.LP
B.2
    \fIDisabler characteristics\fR
.sp 9p
.RT
.PP
The echo canceller reference tone disabler described in this annex detects
a 2100\ Hz tone with periodic phase reversals which occur every 450
\(+-\ 25\ ms. The characteristics of the transmitted signal are defined in
Recommendation\ V.25.
.RT
.sp 1P
.LP
B.2.1
    \fITone detection\fR
.sp 9p
.RT
.PP
The frequency characteristics of the tone detector used in this
reference tone disabler are the same as the characteristics of the echo
suppressor tone detector given in Recommendation\ G.164, \(sc\ 5.2, except
that the upper limit of the dynamic range is \(em6\ dBm0.
.RT
.sp 1P
.LP
B.2.2
    \fIPhase reversal detection\fR
.sp 9p
.RT
.PP
The reference tone disabler responds to a signal which contains
phase reversals of 108\(de\ \(+-\ 10\(de at its source (as specified in
Recommendation\ V.25) when this signal has been modified by allowable
degradations caused by the network, e.g.\ noise, phase jitter, etc. This
disabler is insensitive to phase jitter of \(+-\ 15\(de peak\(hyto\(hypeak
in the frequency range of 0\(hy120\ Hz. This accommodates to the phase
jitter permitted by
Recommendations\ H.12 and G.229. In order to minimize the probability of
false disabling of the echo canceller due to speech currents and network\(hyinduced
phase changes, this reference tone disabler does not respond to single phase
changes of the 2100\ Hz tone in the range 0\(de \(+- 110\(de occurring
in a one second
period. This number has been chosen since it represents the approximate
phase shift caused by a single frame slips in a PCM system.
.RT
.sp 1P
.LP
B.3
    \fIGuardband characteristics\fR
.sp 9p
.RT
.PP
Meet requirements in Recommendation G.164, \(sc\ 5.3.
.PP
\fINote\fR \ \(em\ The possibility of interference during the phase reversal
detection period has been taken into account. One potential source of
interference is the presence of calling tone as specified in
Recommendation\ V.25. If the calling tone interferes with the detection
of the phase reversal, the entire disabling detection sequence is restarted,
but only one time. Recommendation\ V.25 ensures at least one second of
quiet time between calling tone burst.
.RT
.sp 1P
.LP
B.4
    \fIHolding\(hyband characteristics\fR
.sp 9p
.RT
.PP
Meet requirements in Recommendation\ G.164, \(sc\ 5.4.
.RT
.sp 1P
.LP
B.5
    \fIOperate time\fR
.sp 9p
.RT
.PP
The reference tone disabler operates within one second of the
receipt, without interference, of the sustained 2100\ Hz tone with periodic
phase reversals, having the level in the range \(em6 to \(em31\ dBm0. The
one second operate time permits the detection of the 2100\ Hz tone and
ensures that two
phase reversals will occur (unless a slip or impulse noise masks one of the
phase reversals).
.RT
.sp 1P
.LP
B.6
    \fIFalse operation due to speech currents\fR
.sp 9p
.RT
.PP
Meets requirements in Recommendation G.164, \(sc\ 5.6.
.RT
.sp 1P
.LP
B.7
    \fIFalse operation due to data signals\fR
.sp 9p
.RT
.PP
Meets the requirement in Recommendation\ G.165, \(sc\ 4.7. To this end,
the tone disabler circuitry becomes inoperative if one second of clear
(i.e.\ no phase reversals or other interference) 2100\ Hz tone is detected.
The detected circuit remains inoperative during the data transmission and
only becomes
operative again 250 \(+- 150\ ms after a signal in the holding band falls
at least 3\ dB below the maximum holding sensitivity. Thus the possibility
of inadvertent disabling of the echo canceller during data transmission
is minimized.
.RT
.sp 1P
.LP
B.8
    \fIRelease time\fR
.sp 9p
.RT
.PP
Meets the requirements in Recommendation G.164, \(sc\ 5.7.
.bp
.RT
.ce 1000
ANNEX\ C
.ce 0
.ce 1000
(to Recommendation G.165)
.sp 9p
.RT
.ce 0
.ce 1000
\fBDescription of a\fR
\fBreference nonlinear processor\fR
.sp 1P
.RT
.ce 0
.LP
C.1
    \fIGeneral\fR
.sp 1P
.RT
.PP
This annex, which is for the purposes of illustration only and
not intended as a detailed design (see \(sc\ 5.1), describes a reference
nonlinear processor based upon concepts that are as simple as possible
but having
included in it a sufficient number of features to give guidance for a wide
range of possible implementations. To this end two variants of the reference
nonlinear processor are included. Both are based on a centre clipper having
either of the idealized transfer functions illustrated in Figure\ C\(hy1/G.165.
The suppression threshold level (determined, in this case by the clipping
level) in the first variant is adaptive, adaptation being by reference
to \fIL\fR\dR\u. Activation control is by reference to the difference between
\fIL\fR\dR\uand
\fIL\fR\dS\u. In the second variant the suppression threshold is fixed.
.PP
It is assumed that the reference nonlinear processor is used in an echo
canceller which can achieve a cancellation of the linear components of any
returned echo of at least \fIN\fR \ dB. The value of \fIN\fR is under study.
.RT
.LP
.rs
.sp 13P
.ad r
\fBFigure C\(hy1/G.165, p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
C.2
    \fISuppression threshold\fR  | \fIT\fR\dS\\dU\\dP\u)
.sp 9p
.RT
.PP
Adaptive
\fIT\fR\dS\\dU\\dP\u= (\fIL\fR\dR\u\(em \fIx\fR \(+- 3) dBm0 for \(em30
\(= \fIL\fR\dR\u\(= \(em10 dBm0
.PP
Fixed \fIT\fR\dS\\dU\\dP\u= \fIx\fR `\ dBm0
.PP
\fINote\fR \ \(em\ Values of \fIx\fR and \fIx\fR ` are under study. Values
of 18 for
\fIx\fR and \(em36 for \fIx\fR ` have been suggested by confimation is
required that
these values are appropriate for use in all networks.
.RT
.sp 1P
.LP
C.3
    \fIStatic characteristics of activation control\fR
.sp 9p
.RT
.PP
\fIT\fR\dW\\dZ\u= (\fIL\fR\dR\u\(em \fIy\fR \(+- 3) dBm0 for \(em30 \(=
\fIL\fR\dR\u\(=
\(em10 dBm0
.PP
\fINote 1\fR \ \(em\ \fIT\fR\dW\\dZ\uis as defined in \(sc\ 5.2.3.3.
.PP
\fINote 2\fR \ \(em\ The value of \fIy\fR  | ay be different for each variant,
and
this is under study. Values of \fIx\fR \ dB in the case of the adaptive
\fIT\fR\dS\\dU\\dP\uand \(>="\ 6\ dB for \fIy\fR in the case of the fixed
\fIT\fR\dS\\dU\\dP\useem reasonable.
.RT
.sp 1P
.LP
C.4
    \fIDynamic characteristics of activation control\fR
.sp 9p
.RT
.PP
Dynamic characteristics of the activation control are given in
Table\ C\(hy1/G.165 and\ C\(hy2/G.165. Also see Figure\ 13/G.165.
.RT
.sp 1P
.LP
C.5
    \fIFrequency limits of control paths\fR
.sp 9p
.RT
.PP
See Recommendation G.165, \(sc\ 5.2.4.
.RT
.sp 1P
.LP
C.6
    \fITesting\fR
.sp 9p
.RT
.PP
Tables C\(hy1/G.165 and C\(hy2/G.165 indicate, by reference to
Recommendation\ G.164 how the dynamic performance of nonlinear processor
activation control may be checked using sine wave signals. Figures\ C\(hy2/G.165
and C\(hy3/G.165 show the traces obtained on an oscilloscope for these tests.
.bp
.RT
.ce
\fBH.T. [T2.165]\fR
.T&
lw(48p) | lw(24p) sw(30p) | lw(24p) sw(30p) | lw(30p) | lw(18p) | lw(42p)
| lw(42p) | lw(42p) , ^ | l | l | l | l | ^ | ^ | ^ | ^ | ^ .


.T&
cw(48p) | cw(24p) sw(30p) | cw(24p) sw(30p) | lw(30p) | lw(18p) | lw(42p)
| lw(42p) | lw(42p) , ^ | c | c | c | l | ^ | ^ | ^ | ^ | ^ .
Initial signal    Send \fIL\fR S (dBm0)    Send \fIL\fR S (dBm0)
    Receive \fIL\fR R (dBm0)    Final signal     {
Receive \fIL\fR
R (dBm0)
Recommended value (ms)
Test No. (Rec. G.164)
Excursion
(see Figure
13/G.165)
Test circuit,
Figure:
Oscilloscope
trace
 }
_
.T&
cw(18p) | cw(30p) | cw(24p) | cw(30p) | cw(24p) | cw(30p) | cw(30p) | lw(18p)
| lw(42p) | lw(42p) | lw(42p) , ^ | c | c | c | c | c | l | ^ | ^ | ^ |
^ .
Fixed    \(em25      \(em10      \(em25      \(em30      15\(hy64    Adaptive
     \(em55 \(em40 \(em30 \(em20 \(em15 \(em\ 5 \(em55 \(em40 \(em30 \(em40
\(em40 \(em30  {
\(*D\ua\d\u)\d
5
Transition
2
14/G.164
Trace 1 and trace 2
of Figure
C\(hy3/G.165
(\(*b)
W/Z
 }
_
.T&
cw(18p) | cw(30p) | cw(24p) | cw(30p) | cw(24p) | cw(30p) | cw(30p) | lw(18p)
| lw(42p) | lw(42p) | lw(42p) , ^ | c | c | c | c | c | l | ^ | ^ | ^ |
^ .
Fixed    \(em15      \(em25      \(em40      \(em25      16\(hy120    Adaptive
     \(em40 \(em40 \(em25 \(em50 \(em30 \(em15 \(em55 \(em55 \(em40 \(em50
\(em30 \(em15  {
30\(hy50
6
Transition
4
17/G.164
Trace 1 and trace 2
of Figure
C\(hy2/G.165
(\(*b)
\ua\d\u)\d\ \(*D is defined in \(sc\ 3.4.2.1 [footnote \u4\d\u)\d].
 }
_
.TE
.nr PS 9
.RT
.ad r
\fBTABLEAU C\(hy1/G.165 [T2.165] \*`a l'italienne, p.17\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.ce
\fBH.T. [T3.165]\fR
.T&
lw(48p) | lw(24p) sw(30p) | lw(24p) sw(30p) | lw(30p) | lw(18p) | lw(42p)
| lw(42p) | lw(42p) , ^ | l | l | l | l | ^ | ^ | ^ | ^ | ^ .


.T&
cw(48p) | cw(24p) sw(30p) | cw(24p) sw(30p) | lw(30p) | lw(18p) | lw(42p)
| lw(42p) | lw(42p) , ^ | c | c | c | l | ^ | ^ | ^ | ^ | ^ .
Initial signal    Send \fIL\fR S (dBm0)    Send \fIL\fR S (dBm0)
    Receive \fIL\fR R (dBm0)    Final signal     {
Receive \fIL\fR
R (dBm0)
Recommended value (ms)
Test No. (Rec. G.164)
Excursion
(see Figure
13/G.165)
Test circuit,
Figure:
Oscilloscope
trace
 }
_
.T&
cw(18p) | cw(30p) | cw(24p) | cw(30p) | cw(24p) | cw(30p) | cw(30p) | lw(18p)
| lw(42p) | lw(42p) | lw(42p) , ^ | c | c | c | c | c | l | ^ | ^ | ^ |
^ .
Fixed    \(em25      \(em30      \(em25      \(em10      16\(hy120    Adaptive
     \(em55 \(em40 \(em30 \(em40 \(em40 \(em30 \(em55 \(em40 \(em30 \(em20
\(em15 \(em\ 5  {
15\(hy75
4
Transition
1
14/G.164
Trace 2
of Figure
C\(hy3/G.165
(\(*a)
Z/W
 }
_
.T&
cw(18p) | cw(30p) | cw(24p) | cw(30p) | cw(24p) | cw(30p) | cw(30p) | lw(18p)
| lw(42p) | lw(42p) | lw(42p) , ^ | c | c | c | c | c | l | ^ | ^ | ^ |
^ .
Fixed    \(em40      \(em25      \(em15      \(em25      \(= |     Adaptive
     \(em55 \(em55 \(em40 \(em50 \(em30 \(em15 \(em40 \(em40 \(em25 \(em50
\(em30 \(em15  {
\(= |
6
Transition
3
17/G.164
Trace 2
of Figure
C\(hy2/G.165
(\(*a)
 }
_
.TE
.nr PS 9
.RT
.ad r
\fBTABLEAU C\(hy2/G.165 [T3.165] \*`a l'italienne, p.18\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 22P
.ad r
\fBFIGURE C\(hy2/G.165, p.19\fR
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 22P
.ad r
\fBFIGURE C\(hy3/G.165, p.20\fR
.sp 1P
.RT
.ad b
.RT
.sp 2P
.LP
    \fBReference\fR
.sp 1P
.RT
.LP
[1]
     CCITT Recommendation \(em \fIEcho suppressors suitable for circuits having\fR
\fIeither short or long propagation time\fR , Orange Book, Volume\ III.1,
Recommendation\ G.161, ITU, Geneva, 1977.
.bp
.sp 2P
.LP
\fBRecommendation\ G.166\fR
.RT
.sp 2P
.ce 1000
\fBCHARACTERISTICS\ OF\ SYLLABIC\ COMPANDORS\fR
.EF '%    Fascicle\ III.1\ \(em\ Rec.\ G.166''
.OF '''Fascicle\ III.1\ \(em\ Rec.\ G.166    %'
.ce 0
.sp 1P
.ce 1000
\fBFOR\ TELEPHONY\ ON\ HIGH\ CAPACITY\ LONG\ DISTANCE\ SYSTEMS\fR
.ce 0
.sp 1P
.ce 1000
\fI(Malaga\(hyTorremolinos, 1986; amended at Melbourne, 1988)\fR
.sp 9p
.RT
.ce 0
.sp 1P
.PP
Compandors adhering to Recommendation G.162, \fIYellow Book\fR ,
were intended for use in small capacity network systems and their use in
large capacity network long\(hydistance systems is not recommended. Compandors
adhering to this Recommendation are intended for use in large capacity
long\(hydistance
systems. Their use on small capacity network systems is optional. They are
not intended for use in subscriber applications such as mobile communication
systems.
.sp 1P
.RT
.sp 2P
.LP
\fB1\fR     \fBGeneral\fR
.sp 1P
.RT
.PP
1.1
Syllabic compandors are devices in which gain variations
occur at a rate comparable to the syllabic rate of speech. A compandor
consists of a combination of a compressor at one point in a communication
path, for
reducing the amplitude range of signals followed by an expander at another
point for a complementary increase in the amplitude range. The compandor
enhances the subjective speech performance primarily due to two actions. The
compressor increases the average speech level of weaker signals prior to
entering a communication path where increased noise is expected to be
encountered. The expander, in returning the speech signal to its original
dynamic range provides a subjective enhancement to the communication path by
attenuating the noise perceived by the listening party during silences.
For a further description of compandor operation see Annex\ A.
.sp 9p
.RT
.PP
1.2
This Recommendation does not specify the detector
characteristics, e.g.,\ peak, r.m.s. or average.
.PP
The performance recommended may not be sufficient to ensure
compatibility between compandors conforming to this Recommendation but
which are of different design. Before using compressors and expanders of
different design origins at opposite ends of the same circuit, Administrations
should test them for compatibility. The tests should take account of the
sensitivity of compandor performance to the characteristics of the test
signal.
.PP
1.3
The use of a number of syllabic compandors on circuits carried on the same
FDM carrier may result in a changed load being presented to the FDM system.
The FDM system operating parameters could, therefore, require
appropriate adjustment as a function of the load.
.sp 9p
.RT
.PP
1.4
It should be noted that the subjective enhancement which occurs
on speech, when syllabic compandors are used, does not apply to transmission
of non\(hyspeech signals which may experience a signal\(hyto\(hynoise degradation
on
syllabic compandored circuits.
.LP
.PP
1.5
Some of the clauses given below specify the joint
characteristics of a compressor and an expander in the same direction of
transmission of a 4\(hywire circuit. The characteristics specified in this
way can be obtained more easily if the compressors and expanders are of
similar design; in certain cases close cooperation between Administrations
may be necessary.
Application rules for syllabic compandors address this issue.
.sp 2P
.LP
\fB2\fR     \fBDefinitions\fR
.sp 1P
.RT
.sp 1P
.LP
2.1
    \fBunaffected level\fR
.sp 9p
.RT
.PP
\fI\fR The unaffected level is the absolute level, at a point of zero
relative level on the line between the compressor and the expander of a
signal at 800\ Hz, which remains unchanged whether the circuit is operated
with the
compressor or not. The unaffected level is defined in this way in order
not to impose any particular values of relative level at the input to the
compressor or the output of the expander.
.bp
.PP
To make allowances for the increase in mean power introduced by the
compressor, and to avoid the risk of increasing the intermodulation noise
and the overload which might result, the unaffected level must be adjusted
taking into account the capacity of the system. (See Reference\ [1], Chapter\
II,
Annex\ 4, for detailed discussion of this adjustment.)
.RT
.sp 1P
.LP
2.2
    \fBratio of compression\fR
.sp 9p
.RT
.PP
The ratio of compression of a compressor is defined by the
formula:
\v'6p'
.RT
.sp 1P
.ce 1000
\(*a =
@ { fIL\fR~\d1\~CIN~\u~\(em~\fIL\fR~\d2\~CIN~\u } over { fIL\fR~\d1\~COUT~\u~\(em~\fIL\fR~\d2\~COUT~\u } @
.ce 0
.sp 1P
.LP
.sp 1
where
.LP
    \fIL\fR\d1\u\dC\\dI\\dN\uand \fIL\fR\d2\u\dC\\dI\\dN\uare any two
different compressor input levels within the compressor operating range.
.LP
     \fIL\fR\d1\u\dC\\dO\\dU\\dT\uand \fIL\fR\d2\u\dC\\dO\\dU\\dT\uare the
compressor output
levels corresponding to input levels\ \fIL\fR\d1\u\ \dC\\dI\\dN\uand
\fIL\fR\d2\u\ \dC\\dI\\dN\urespectively.
.sp 1P
.LP
2.3
    \fBratio of expansion\fR
.sp 9p
.RT
.PP
The ratio of expansion of an expander is defined by the
formula:
\v'6p'
.RT
.sp 1P
.ce 1000
\(*b =
@ { fIL\fR~\d1\~EOUT~\u~\(em~\fIL\fR~\d2\~EOUT~\u } over { fIL\fR~\d1\~EIN~\u~\(em~\fIL\fR~\d2\~EIN~\u } @
.ce 0
.sp 1P
.LP
.sp 1
.LP
where
.LP
    \fIL\fR\d1\u\dE\\dI\\dN\uand \fIL\fR\d2\u\ \dE\\dI\\dN\uare any two
different expander input levels within the expander operating range.
.LP
     \fIL\fR\d1\u\ \dE\\dO\\dU\\dT\uand \fIL\fR\d2\u\ \dE\\dO\\dU\\dT\uare
the expander output levels
corresponding to input levels\ \fIL\fR\d1\u\ \dE\\dI\\dN\uand
\fIL\fR\d2\u\ \dE\\dI\\dN\urespectively.
.sp 2P
.LP
\fB3\fR     \fBCharacteristics of syllabic compandors\fR
.sp 1P
.RT
.sp 1P
.LP
3.1
    \fIUnaffected level\fR
.sp 9p
.RT
.PP
A nominal value of \(em10 dBm0 for the unaffected level is recommended
for high capacity systems. However, Administrations are free to mutually
negotiate a different unaffected level to allow optimal loading of their
transmission systems. Such variation is expected to be in the range\ \(em10
to\ \(em24\ dBm0. The loading effects of pilot tones should be considered.
.RT
.sp 1P
.LP
3.2
    \fIRatio of compression\fR \(*a
.sp 9p
.RT
.PP
The compandor compression ration \(*a should be 2 over the range of level
specified in \(sc\ 3.4 and over the temperatura range +10 | (deC to +40 | (deC.
The difference between the measured level and the calculated level at the
output of the compressor assuming a value of exactly\ 2 should not exceed
\(+- | .25\ dB.
.RT
.sp 1P
.LP
3.3
    \fIRatio of expansion\fR \(*b
.sp 9p
.RT
.PP
The compandor expansion ratio \(*b should be 2 over the range of level
specified in \(sc\ 3.4 and over the temperature range +10 | (deC to +40 | (deC.
The
difference between the measured level and the calculated level at the output
of the expander assuming a value of exactly\ 2 should not exceed \(+- | .4\
dB.
.RT
.sp 1P
.LP
3.4
    \fIRange of level\fR
.sp 9p
.RT
.PP
Under study
.PP
The range of level over which the recommended value of \(*a and \(*b
should apply, should extend at least:
.RT
.LP
    from +5 to \(em60 dBm0 at the input of the compressor, and
.LP
    from +5 to \(em65 dBm0 at the nominal output of the expander.
.bp
.sp 1P
.LP
3.5
    \fIVariation of compressor gain\fR
.sp 9p
.RT
.PP
The level at the output of the compressor, measured at 800 Hz, for an input
level equal to the unaffected level, should not vary from its nominal value
by more than \(+- | .25\ dB for a temperature range of\ +10 | (deC to\
+40 | (deC and a deviation of the supply voltage of \(+- | % from its nominal
value.
.RT
.sp 1P
.LP
3.6
    \fIVariation of expander gain\fR
.sp 9p
.RT
.PP
The level at the output of the expander, measured at 800 Hz for an input
level equal to the unaffected level, should not vary from its nominal
value by more than \(+- | .5\ dB for a temperature range of +10 | (deC
to +40 | (deC and a deviation of the supply voltage of \(+- | % from its
nominal value.
.RT
.sp 1P
.LP
3.7
     \fITolerances on the output levels of the combination of compressor\fR
\fIand expander in the same direction of transmission of a 4\(hywire circuit\fR
.sp 9p
.RT
.PP
The compressor and expander are connected in tandem. A loss (or
gain) is inserted between the compressor output and expander input equal
to the nominal loss (or gain) between these points in the actual circuit
in which they will be used. Figure\ 1/G.166 shows, as a function of level
of 800\ Hz input
signal to the compressor, the permissible limits of difference between
expander output level and compressor input level. (Positive values indicate
that the
expander output level exceeds the compressor input level.)
.PP
The limits shall be observed at all combinations of temperature of
compressor and temperature of expander in the range\ +10 | (deC to\ +40 | (deC.
They
shall also be observed when the test is repeated with the loss (or gain)
between the compressor and expander increased or decreased by 2\ dB and the
measurement corrected by \(+- | .0\ dB, assuming a\ \(*b of\ 2.00.
.RT
.LP
.rs
.sp 19P
.ad r
\fBFIGURE\ 1/G.166, p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
3.8
    \fIConditions for stability\fR
.sp 9p
.RT
.PP
See descriptions given in \(sc 2.6 of Recommendation G.162, Volume\ III
of the \fIYellow Book\fR , ITU, Geneva, 1981, \(sc\ 2 of Recommendation\
G.143,
\fIRed Book\fR , and Reference\ [1].
.PP
The limits shall be observed at all combinations of temperature of
compressor and temperature of expander in the range\ +10 | (deC to\ +40 | (deC.
They
shall also be observed when the test is repeated with the loss (or gain)
between the compressor and expander increased or decreased by 2\ dB.
.PP
\fINote\fR \ \(em\ The change of gain (or loss) of 2 dB mentioned in \(sc\
3.7 above is equal to twice the standard deviation of transmission loss
recommended as an objective for international circuits routed on single
group links in
Recommendation\ G.151, \(sc\ 3.
.bp
.RT
.sp 2P
.LP
\fB4\fR     \fBImpedances and return loss\fR
.sp 1P
.RT
.PP
The nominal value of the input and output impedances of both
compressor and expander should be 600\ ohms (nonreactive).
.PP
The return loss with respect to the nominal impedance of the input and
the output of both the compressor and the expander should be no less than
20\ dB over the frequency range 300\ to 3400\ Hz and for any measurement
level between +5 and \(em60\ dBm0 at the compressor input or the expander
output.
.RT
.sp 2P
.LP
\fB5\fR     \fBOperating characteristics at various frequencies\fR
.sp 1P
.RT
.sp 1P
.LP
5.1
    \fIFrequency characteristic with control circuit clamped\fR
.sp 9p
.RT
.PP
The control circuit is said to be clamped when the control
current (or voltage) derived by rectification of the signal is replaced by a
constant direct current (or voltage) supplied from an external source. For
purposes here, the value of this current (or voltage) should be equal to the
value of the control current (or voltage) obtained when the input signal is
set to the unaffected level.
.PP
For the compressor and the expander taken separately, the variations of
loss or gain with frequency should be contained within the limits of a
diagram that can be deduced from Figure\ 1/G.132 by dividing the tolerance
shown by\ 8, the measurement being made with a constant input level corresponding
to the unaffected level.
.RT
.sp 1P
.LP
5.2
    \fIFrequency characteristic with control circuit operating normally\fR
.sp 9p
.RT
.PP
The limits given in \(sc\ 5.1 should be observed for the compressor
when the control circuit is operating normally, the measurement being made
with a constant input level corresponding to the unaffected level.
.PP
For the expander, under the same conditions of measurement, the limits
can be deduced from
Figure\ 1/G.132 by dividing the tolerances
shown by\ 4.
.PP
These limits should be observed over the temperature range +10 | (deC to
+40 | (deC.
.RT
.sp 2P
.LP
\fB6\fR     \fBNonlinear distortion\fR
.sp 1P
.RT
.sp 1P
.LP
6.1
    \fIHarmonic distortion\fR
.sp 9p
.RT
.PP
The total harmonic distortion, measured with an 800\ Hz sine wave at the
unaffected level, should not exceed\ 0.5% for the compressor and the
expander taken separately.
.PP
\fINote\fR \ \(em\ Even in an ideal compressor, high output peaks will occur
when the signal level is suddenly raised. The most severe case seems to
be that of voice\(hyfrequency signalling, although the effect can also
occur during
speech. It may be desirable, in exceptional cases, to fit the compressor
with an amplitude limiter to avoid disturbance due to transients during
voice\(hyfrequency signalling.
.RT
.sp 1P
.LP
6.2
    \fIIntermodulation tests\fR
.sp 9p
.RT
.PP
It is necessary to add a measurement of intermodulation to the
measurements of harmonic distortion whenever compandors are intended for
international circuits (regardless of the signalling system used), as well
as in all cases where they are provided for national circuits over which
multi\(hyfrequency signalling, or data transmission using similar types of
signals, is envisaged.
.PP
The intermodulation products of concern to the operation of
multi\(hyfrequency telephone signalling receivers are those of the third
order, of type (2\fIf\fR\d1\u\ \(em\ \fIf\fR\d2\u) and (2\fIf\fR\d2\u\
\(em\ \fIf\fR\d1\u), where
\fIf\fR\d1\u\ and\ \fIf\fR\d2\uare two signalling frequencies.
.PP
Two signals at frequencies 900\ Hz and 1020\ Hz are recommended for
these tests.
.PP
Two test conditions should be considered: the first, where each of the
signals at \fIf\fR\d1\uand \fIf\fR\d2\uis at a level of \(em5\ dBm0 and
the second, where they are each at a level of \(em15\ dBm0. These levels
are to be understood to be at the input to the compressor or at the output
of the expander (uncompressed levels).
.bp
.PP
The limits for the intermodulation products are defined as the
difference between the level of either of the signals at
frequencies\ \fIf\fR\d1\uor \fIf\fR\d2\uand the level of either of the
intermodulation products at frequencies (2\fIf\fR\d1\u\ \(em\ \fIf\fR\d2\u)
or
(2\fIf\fR\d2\u\ \(em\ \fIf\fR\d1\u).
.PP
A value for this difference which seems adequate for the requirements of
multi\(hyfrequency telephone signalling (including end\(hyto\(hyend signalling
over three circuits in tandem, each equipped with a compandor) is 32\ dB
for the
compressor and the expander separately.
.PP
\fINote\ 1\fR \ \(em\ These values seem suitable for Signalling System No.\ 5,
which will be used on some long international circuits.
.PP
\fINote\ 2\fR \ \(em\ It is inadvisable to make measurements on a compressor
plus expander in tandem, because the individual intermodulation levels
of the compressor and of the expander might be quite high, although much
less
intermodulation is given in tandem measurements since the characteristics of
compressor and expander may be closely complementary. The compensation
encountered in tandem measurements on compressor and expander may not be
encountered in practice, either because there may be phase distortion in the
line or because the compressor and expander at the two ends of the line
may be less closely complementary than the compressor and expander measured
in
tandem.
.PP
Hence the measurements have to be performed separately for the
compressor and the expander. The two signals at frequencies\ \fIf\fR\d1\uand\
\fIf\fR\d2\umust be applied simultaneously, and the levels at the output
of the compressor or expander measured selectively.
.RT
.sp 2P
.LP
\fB7\fR     \fBNoise
.sp 1P
.RT
.PP
The effective value of the sum of all noise referred to a zero
.PP
relative level point, the input and the output being terminated with
resistances of 600\ ohms, shall be less than or equal to the following
values:
.RT
.LP
    \(em
    at\ the\ output\ of\ the\ compressor:
    \(em45 dBm0p
.LP
    \(em
    at\ the\ output\ of\ the\ expander:
    \(em80 dBm0p.
.sp 2P
.LP
\fB8\fR     \fBTransient response\fR
.sp 1P
.RT
.PP
The overall transient response of the combination of a compressor and expander
which are to be used in the same direction of transmission of a
4\(hywire circuit fitted with compandors shall be checked as follows:
.PP
The compressor and expander are connected in tandem, the appropriate
loss (or gain) being inserted between them as in\ \(sc\ 3.7.
.PP
A 12\(hydB step signal at a frequency of 2000\ Hz is applied to the input
of the compressor, the actual values being a change from \(em16\ to \(em4\
dBm0 for
attack, and from \(em4 to \(em16\ dBm0 for recovery. The envelope of the
expander
output is observed. The overshoot (positive or negative), after an upward
12\(hydB step expressed as a percentage of the final steady\(hystate voltage,
is a
.PP
measure of the overall transient distortion of the compressor\(hyexpander
combination for attack. The overshoot (positive or negative) after a downward
12\(hydB step, expressed as a percentage of the final steady\(hystate voltage
is a
measure of the overall transient distortion of the compressor\(hyexpander
combination for recovery. For both these quantities the permissible limits
shall be \(+- | 0%. These limits shall be observed for the same conditions of
temperature and of variation of loss (or gain) between compressor and expander
as for the test in \(sc\ 3.7.
.PP
In addition, the attack and recovery times of the compressor alone
shall be measured as follows:
.PP
Using the same 12\(hydB steps as above for attack and recovery
respectively, the attack time is defined as the time between the instant
when the sudden change is applied and the instant when the output voltage
envelope reaches a value equal to 1.5\ times its steady\(hystate value.
The recovery time is defined as the time between the instant when the sudden
change is applied and the instant when the output voltage envelope reaches
a value equal to
0.75\ times its steady\(hystate value.
.PP
The permissible limits shall be:
.RT
.LP
    \(em
    3\ ms minimum, 5 ms maximum for the attack time, and
.LP
    \(em
    13.5 ms minimum, 22.5 maximum for the recovery time.
.bp
.ce 1000
ANNEX\ A
.ce 0
.ce 1000
(to Recommendation G.166)
.sp 9p
.RT
.ce 0
.ce 1000
\fBCompandor enhancement characteristics\fR
.sp 1P
.RT
.ce 0
.PP
The improvement which the compandor makes available is based on the fact
that interference is most objectionable during quiet speech or
pauses, but is masked by relatively loud speech. While it will not be
necessary, therefore, to alter the performance of the system for speech
signals at a high level, an improvement has to be provided when the signal
level is
.sp 1P
.RT
.LP
low. This noise reduction can be arranged by introducing loss at the receiving
end of the circuit during periods when the signal is faint or absent. The
loss so introduced will affect the noise or crosstalk which has crept in
along the route, so that the interference is reduced by the amount of this
loss. However, the desired signals are also affected, and in order that
the speech level
finally received shall be unchanged by the insertion of the compandor,
an equal amount of gain has to be introduced at the sending end. The overall
equivalent of the circuit is thereby kept constant, and also the low level
signals are
raised above the background of interference on the line.
.PP
The above\(hymentioned condition must not, however, be allowed to
persist when high\(hylevel signals have to be transmitted, or overloading could
occur in the line amplifiers along the route. The function of the compandors
is to introduce the required amounts of gain and loss automatically in
just such a way that the overall circuit equivalent remains unchanged irrespective
of the speech level, while the signal\(hyto\(hynoise ratio is increased
for low\(hylevel
signals. This is shown schematically in the level diagram of Figure\ A\(hy1/G.166.
For one particular level, called the \fIunaffected\fR level\ \fIX\fR ,
the use of the
compandor at no point introduces gain or loss, and the signal passes at an
unchanged level throughout the system, as shown by\ (1), (2), (3).
.PP
Any given level of speech (4) would also normally (i.e. without
compandors) pass at an unchanged level through the system as shown at\
(4), (5), (6). If we suppose that the level of interference on the system
(noise,
crosstalk,\ etc.) is that shown by\ (7), the signal/interference ratio is then
given by\ \fIa\fR , and the interference level appearing at the output
is that shown by\ (8), during both speech and pauses.
.PP
By the introduction of the compandor, however, the incoming speech
level\ (4) is raised to\ (9), thereby giving a signal/interference ratio
within the system of\ \fIb\fR . The level of the speech is restored to\
(6) at the receiving end, and the corresponding interference level \fIduring
speech\fR is shown at\ (10). However, as stated earlier, of even greater
significance is the interference
level during pauses, which is that shown at\ (11). Thus the effective ratio
between speech signals and interference heard \fIduring pauses\fR has the value
shown by\ \fIc\fR .
.RT
.PP
The part of the compandor at the sending end is called the
compressor, because the range of levels of the incoming speech signals is
compressed. The unaffected level recommended by the CCITT for high capacity
systems is \(em10\ dBm0. However, Administrations may mutually negotiate a
different unaffected level to permit optimal loading of their transmission
systems. The unaffected level is expected to range from\ \(em10 to \(em24\
dBm0. The
selected unaffected level will affect the mean power per channel.
.PP
The part of the compandor at the receiving end is called the expander,
and the same level remains unchanged.
.PP
It will be seen from the foregoing that, when compandors are required,
one compandor has to be inserted at each end of the telephone circuit in
the
voice\(hyfrequency 4\(hywire path, with the compressor in the sending channel
and the expander in the receiving channel.
.RT
.LP
.rs
.sp 7P
.ad r
Blanc
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 35P
.ad r
\fBFigure A\(hy1/G.166, p.22\fR
.sp 1P
.RT
.ad b
.RT
.sp 2P
.LP
    \fBReference\fR
.sp 1P
.RT
.LP
[1]
    CCITT Manual \fITransmission planning of switched telephone networks\fR ,
ITU, Geneva, 1976.
.LP
.rs
.sp 10P
.ad r
Blanc
.ad b
.RT
.LP
.bp