If it is assumed (\(sc\ D.1.1 above) that \fISLR\fR = \fICSRE\fR \(em\
5, \fI\fR
\fIRLR\fR \ =\ \fICRRE\fR \(em\ 5, and \fID\fR `\d0\u\ =\ \(em4 (since the
Recommendations were originally applied to old\(hytype subscriber's stations),
then\ \fIZ\fR \ =\ \fIY\fR \ \(em\ 6\ dB is obtained.
.PP
In fact, the recommended values were derived from \fIZ\fR \ =\ \fIY\fR
\(em\ 5\ dB,
which is not a significant difference, but the recommendations for the
national system are a little more stringent, because the ADE of national
long distance circuits was included in the national system.
.RT
.sp 2P
.LP
D.2
\fILRs recommended in 1988\fR
.sp 1P
.RT
.PP
D.2.1
The maximum values and the minimum for sending have been retained; other
values differ from those recommended in\ 1984, as explained below.
.sp 9p
.RT
.sp 1P
.LP
D.2.2
\fIOptimum value\fR
.sp 9p
.RT
.PP
Values directly determined in terms of overall LR (Recommendations P.78
or P.79) during conversation tests are available as follows:
.PP
British Telecom [1], in the presence of room noise, found a maximum
mean opinion score (MOS) for OLR\ =\ 3\ dB and a minimum difficulty percentage
for OLR\ =\ 7.2\ dB. It was proposed to adopt 5\ dB as the optimum value
and an
almost equally good performance was found in a range from 1\ to 10\ dB.
.bp
.PP
NTT [2] found values between 4 and 6 dB according to noise conditions;
an optimum OLR\ =\ 5.34\ dB is used in the OPINE model.
.PP
According to the TRANSRAT model [3], maximum MOS is obtained
for\ \fIL\fR\d\fIe\fR\u\ =\ 7.5 (corresponding to \fIL\fR `\fI
\fI\d\fIe\fR\u\ =\ 8.5 in Supplement No.\ 3, \(sc\ 1, of Volume\ V, where\
\fIL\fR\d\fIe\fR\u\ =\ OLR(EARS). There are reasons to think that\ \fIL\fR\d\fIe\fR\uis
higher than OLR (See Recommendation\ P.79) by a few dB, so that this should
not differ significantly from the above values; this point is being studied
under Question\ 7/X.II.
.PP
In any case, such maxima are very flat and there is evidence that
higher values would apply in the presence of echoes. It may be provisionally
concluded that to obtain the best performance, OLR (See Recommendation\ P.79)
should not exceed about 10\ dB, but should not be much smaller.
.RT
.sp 1P
.LP
D.2.3
\fITraffic weighted mean values\fR
.sp 9p
.RT
.PP
An optimum OLR of 10 dB was adopted and it was subdivided between sending
and receiving in the same manner as for the LRs of digital
subscriber's sets (the latter being referred to a 0\ dBr point). This gives
the long\(hyterm objectives.
.PP
The values of \fIA\fR (see \(sc\ D.1.3) used previously, which took into
account both effects of attenuation distortion on loudness and naturalness
of speech, were replaced by a fixed allowance of 2\ dB (1\ dB in each national
system, see \(sc\ A.3) when analogue subscriber's stations are used.
.PP
This, combined with a small margin in the previous Recommendation version
(see \(sc\ D.1.3 of this Annex), made it possible to increase the traffic\(hyweighted
means for sending by about 4\ dB and to keep the same overall values.
.RT
.sp 1P
.LP
D.3
\fIConclusion\fR
.sp 9p
.RT
.PP
Table D\(hy1/G.111 recapitulates the values of LR recommended in\ 1984
and those which are recommended now.
.RT
.ce
\fBH.T. [T3.111]\fR
.ce
TABLE\ D\(hy1/G.111
.ce
\fBValues (dB) of sending, receiving, circuit and overall loudness\fR
.ce
\fBrating\fR
.ce
\fBcited in Recommendations G.111 and G.121\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
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