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*********************************************************************
* NOTE: The following ASCII text file (without graphics) *
* is contained in a printed technical paper available *
* from Broadcast Electronics Inc. Unfortunately, it *
* was not possible to reproduce the graphics portions *
* of this paper within this text file. If you find the *
* information in this file of interest, you may request *
* a complimentary, printed, copy including figures and *
* graphics from: BROADCAST ELECTRONICS INC. *
* P.O. BOX 3606 *
* 4100 N. 24TH STREET *
* QUINCY, IL. 62305-3606 *
* ATTN: SALES DEPARTMENT *
* PH 217-224-9600 *
* FAX 217-224-9607 *
* *
* The contents of this technical paper are *
* Copyrighted (c) 1986, by Broadcast Electronics Inc. *
* All rights reserved. *
*********************************************************************
TESTING TELEVISION TRANSMISSION SYSTEMS
FOR MULTICHANNEL SOUND COMPATIBILITY
BY:
Geoffrey N. Mendenhall, P.E.
Vice President of Engineering
Broadcast Electronics Inc.
Quincy, Illinois
BACKGROUND INFORMATION
Although stereo programming is not yet widely available to most television
stations, there is great interest in testing, evaluating, and preparing each
station's transmission system for multichannel sound transmission. One
approach to testing, is to measure the various audio parameters including;
frequency response, separation, crosstalk, and distortion using a BTSC encoder
with the aural exciter and a consumer type BTSC receiver decoder for the
"off-air" measurements. This approach may lead to an incorrect evaluation of
the transmission system due to errors in the encoding/decoding process since
precision encoders and precision modulation monitor/decoders are not yet
available with guaranteed minimum performance specifications that are
applicable to the entire system.
This engineering application note describes a procedure for evaluating
television transmission systems using readily available standard test
instruments and without relying on BTSC encoding and decoding equipment.
The primary objective is to transmit the BTSC composite waveform to the
stereo decoder in the receiver at the correct level (deviation of the aural
exciter) and without altering the amplitude and phase relationships of the
various components within this waveform. The composite signal path from the
output of the BTSC encoder to the input of the decoder in the receiver is
subject to many interacting and cumulative errors so it is necessary to devise
a test procedure that can identify the magnitude and type of error within each
functional block in the system.
PROBLEM AREAS FOR THE COMPOSITE SIGNAL
There are three areas for signal degradation to occur:
1. The composite link to the TV aural FM modulator.
(BTSC stereo generator, SAP generator, PRO
generator, composite processor, and STL equipment)
2. The aural FM modulator.
3. The RF path to the demodulator.
(aural exciter, IPA, PA, diplexer, and antenna system.)
Each of these three areas has its own special effect on the baseband signal
and each subsystem must be individually optimized before the complete
transmission system can give the best possible performance.
THE COMPOSITE LINK
The composite path from the stereo, SAP, and PRO generators to the aural FM
modulator should be linear in both amplitude -vs- frequency and in phase -vs-
frequency response. Simply stated, this means that no frequency component
within the baseband should be attenuated more than any other frequency
component. Furthermore, all frequency components should propagate thru the
system at the same speed (constant group delay) and thus arrive at the
modulator at the same time. Equation-1A and Equation-1B mathematically relate
stereo separation to amplitude response. Equation-2B mathematically relates
stereo separation to phase response.
STEREO SEPARATION AS A FUNCTION OF BOTH AMPLITUDE ERROR AND PHASE ERROR
EQUATION-1A
_ _
| 2 2 | 1/2
| (cos b + A) + (sin b) |
SEPARATION (A,a) = |--------------------------| General Form
| 2 2 |
|_ (cos b - A) + (sin b)
STEREO SEPARATION AS A FUNCTION OF AMPLITUDE ERROR ONLY
EQUATION-1B
_ _
| 2 | 1/2
| (1 + A) |
SEPARATION (A) = |------------| IF b=0 (perfect phase)
| 2 |
|_ (1 - A) _|
STEREO SEPARATION AS A FUNCTION OF PHASE ERROR ONLY
EQUATION-2B
_ _
| 2 2 | 1/2
| (cos b + 1) + (sin b) |
SEPARATION (b) = |--------------------------| IF A=1 (no amplitude error)
| 2 2 |
|_ (cos b - 1) + (sin b) _|
SUB L - R
WHERE: (A) = ------ = ------- = AMPLITUDE RATIO and (b) = PHASE ERROR
MAIN L + R (in degrees)
Figure-1 graphically shows the combined effect of amplitude and phase
response on stereo separation between the right and left channels.
BTSC SEPARATION -vs- COMBINED AMPLITUDE AND PHASE ERRORS
IN THE COMPOSITE BASEBAND
FIGURE-1
Correct phasing and equal group delay of the (Fh) pilot tone is also
essential to achieving stereo separation.
The final stereo performance of the complete system will be determined by
the algebraic summation of the individual composite amplitude response and
composite phase response of each device within the composite signal path.
The aural exciter, STL link, and any other composite device should specify
these composite performance parameters so that total system performance can be
easily predicted. In order to maintain a system separation capability of 40dB
as suggested by Zenith, the composite amplitude response must be within +/-
0.17dB (50Hz to 47KHz) and the composite phase response must be less than
+/-1.15 degrees from linear phase (50Hz to 47KHz).
COMPOSITE PROCESSING
In an effort to achieve maximum modulation density (loudness), some FM
broadcasters use composite processing to remove the low energy overshoots in
the amplitude of the composite waveform caused by complex audio input
filtering. Overshoots will also occur in the peak to peak amplitude of the BTSC
composite waveform, but are not considered significant to the lower modulation
density (wider dynamics) desired in television broadcasting. Since overshoots
have no effect on compandor tracking or any other audio performance parameter
other than achieving the last dB in loudness, composite processing is not
recommended for use with the BTSC system. The use of any non-linear devices,
such as clippers or limiters in the composite line will alter not only the peak
amplitude of the baseband, but also the frequency spectrum of the baseband.
This generates several types of distortion at the receiver.
Figure 2A and Figure 2B show the waveform and spectrum of unprocessed
baseband while Figure 2C and Figure 2D show the same waveform and spectrum
after 1.0dB of composite clipping.
SUMMARY OF TYPES OF DISTORTION
CAUSED BY COMPOSITE PROCESSING
1. Intermodulation of all baseband frequency components
causing extraneous spectral components.
2. Harmonic distortion of baseband causing degradation
of crosstalk and separation.
3. Modulation of pilot injection level causing loss
of lock at the synchronous detector.
BTSC BASEBAND WITHOUT CLIPPING
Figure 2-A Figure 2-B
(OUTPUT FROM BEI TZ-30 STEREO GENERATOR WITH
ONLY ONE CHANNEL MODULATED @ 50KHz PILOT OFF)
BTSC BASEBAND AFTER 1.0dB COMPOSITE CLIPPING
Figure 2-C Figure 2-D
(OUTPUT FROM BEI TZ-30 STEREO GENERATOR FOLLOWED
BY 1.0dB OF COMPOSITE CLIPPING WITH ONLY ONE CHANNEL
MODULATED @ 5KHZ, PILOT OFF)
The received audio is high in intermodulation distortion and non-correlated
information due to aliasing of the extraneous spectral components added by
composite processing. If minimum system distortion is the goal, composite
processing should not be used. Audio processing should be performed before the
audio is multiplexed into baseband.
Distortion of the composite baseband signal can also be caused by transient
intermodulation distortion (TIM) within the amplifier stages. Transient
intermodulation distortion of the baseband signal is caused by the same
mechanisms that produce TIM in audio signals. The composite amplifiers must
have sufficient feedback bandwidth to accept baseband frequencies to 100kHz and
should slew symmetrically to minimize slew-induced distortion. The TIM
performance becomes largely a matter of operational amplifier selection and
circuit configuration.
AURAL MODULATOR LINEARITY
The composite baseband signal is translated to a frequency modulated
carrier frequency by the modulated oscillator. Frequency modulation is produced
by applying the composite baseband signal to a voltage tunable RF oscillator.
The modulated oscillator usually operates at the carrier frequency and is
voltage tuned by varactor diodes, operating in a parallel LC circuit.
To have perfect modulation linearity, the RF output frequency must change
in direct proportion to the composite modulating voltage applied to the
varactor diodes. This requirement implies that the capacitance of the varactor
diodes must change as nearly the square of the modulating voltage.
Unfortunately, the voltage versus capacitance characteristic of practical
varactor diodes is not the desired square law relationship. All varactor-tuned
oscillators have an inherently non-linear modulating characteristic. This
non-linearity is very predictable and repeatable for a given circuit
configuration, making correction by complementary predistortion of the
modulating signal feasible. Suitable predistortion can be applied by using a
piece-wise linear approximation to the desired complementary transfer
function.
Any distortion of the baseband signal caused by the modulated oscillator
will have secondary effects on stereo, SAP, and PRO crosstalk, which are quite
noticeable at the receiver in spite of the rather small amounts of distortion
to the baseband. For example, if the harmonic distortion to the baseband is
increased from .05% to 1.0%, as much as 26dB additional crosstalk into the SAP
can be expected.
THE RF PATH
THE AURAL TRANSMITTER SIDEBAND STRUCTURE
The frequency modulated RF output spectrum contains many sideband frequency
components, theoretically an infinite number. They consist of pairs of
sideband components spaced from the carrier frequency by multiples of the
modulating frequency. The total transmitter RF output power remains constant
with modulation, but the distribution of that power into the sidebands varies
with the modulation index so that power at the carrier frequency is reduced by
the amount of power added to the sidebands.
OCCUPIED BANDWIDTH
After examining the resulting spectra, it becomes clear that the occupied
bandwidth of an FM signal is far greater than the amount of deviation from the
carrier that one might incorrectly assume as the bandwidth. In fact, the
occupied bandwidth is infinite if all the sidebands are taken into account, so
that a frequency modulation system requires the transmission of all of these
sidebands for perfect demodulation of information. In practice, a signal of
acceptable quality can be transmitted in the limited bandwidth assigned to the
TV aural channel.
EFFECTS OF BANDWIDTH LIMITATION
Practical considerations in the transmitter RF circuitry make it necessary
to restrict the RF bandwidth. As a result, the higher order sidebands will be
altered in amplitude and phase. Bandwidth limitation will cause distortion in
any FM system. The amount of distortion in any practical FM system will depend
on the amount of bandwidth available versus the modulation index being
transmitted.
LIMITING FACTORS WITHIN THE AURAL TRANSMITTER
Relating the specific quantitative effect of the bandwidth limitations
imposed by a particular transmitter to the actual distortion of the demodulated
composite baseband is a complicated problem indeed. Some of the factors
involved are:
1. Total number of tuned circuits involved.
2. Amplitude and phase response of the total combination of tuned
circuits in the RF path.
3. Amount of drive (saturation effects) to each amplifier stage.
4. Non-linear transfer function within each amplifier stage.
IMPROVEMENT OF THE AURAL RF PATH
The following design techniques can help improve the transmitter's
bandwidth:
1. Maximize bandwidth by using a broadband exciter and a broad-
band IPA stage.
2. Use a single-tube design or a broadband, completely solid-
state, design where feasible.
3. Optimize both the input circuit and output circuit of the
tuned stage for the best possible bandwidth.
4. Minimize the number of interactive tuned networks.
5. Use a delay equalized multiple cavity diplexer.
6. Use a broadband antenna system with a low standing wave
ratio at the aural carrier frequency.
SYSTEM TEST PROCEDURE
The composite amplitude and phase characteristics must be measured to a
high degree of accuracy. (tenths of a decibel and tenths of a degree from phase
linear).
A high accuracy audio network analyzer could be used to directly measure
the composite characteristics, but most stations do not have access to this
equipment.
Another simple yet effective way to evaluate the system performance is to
send a multi-tone test signal consisting of a low (L+R) audio frequency and
ultrasonic (L-R) frequency components of equal values through the system and
display the resulting waveform on an oscilloscope whose sweep is synchronized
to the low frequency audio component. The resulting waveform is shown in
Figure-3.
1:1 RATIO TEST WAVEFORM WITHOUT PILOT
FIGURE-3
The amplitude of the (L+R) and (L-R) components should be exactly equal at
each point throughout the composite system to the demodulator. The propagation
time through the system should also be equal for (L+R) and (L-R) components.
The key property of this test signal is that the (L+R) and (L-R) components are
equal (1:1 ratio) so that any change in this ratio due to system problems can
easily be observed on an oscilloscope. The composite signal output from the
BTSC stereo generator does not have a fixed and equal ratio between (L+R) and
(L-R) so it cannot be used for this test. Figure-4 shows what the BTSC
composite baseband looks like if viewed on an oscilloscope with the
peak-to-peak amplitude shown as a function of time. It is difficult to
accurately measure the amplitude ratio and phase relationship of (L+R) to
(L-R) since the ratios vary with the level of companding and are never equal.
The required (1:1) test signal can be obtained from a standard FM broadcast
stereo generator by turning the pilot off and modulating only one channel since
the (L+R) and (L-R) information is output in equal amounts under these
conditions.
The TZ-30 TV stereo generator has a special test mode to provide the
required 1:1 ratio test signal with or without the (Fh) pilot tone.
BTSC COMPOSITE WAVEFORM
FIGURE-4
INTERPRETING THE COMPOSITE WAVEFORM
During all of the tests the external trigger input to the oscilloscope is
connected to the audio generator which feeds only one input of the stereo
generator. The other audio input is shorted and the pilot is turned off. The
composite output from a wideband RF demodulator such as the Boonton model 82AD
or the Hewlett-Packard model 8901A modulation analyzer is fed to the wideband
vertical input of the oscilloscope. The composite waveform can also be
checked at other points within the system to determine the error contribution
from each subsystem.
If both the amplitude and phase response are correct, the base line of the
waveform will be perfectly flat even when closely examined by expanding the
vertical scale on the oscilloscope as shown in Figure-5.
EXPANDED SCALE TO CHECK BASELINE FLATNESS
FIGURE-5
An amplitude and delay equalizer for the composite baseband is available as
part of the TZ-30 BTSC stereo generator. Equalization for amplitude and phase
deficiencies in the STL or Aural exciter will improve the overall system
performance. The adjustments of the equalizer are made while observing the
demodulated composite baseband to minimize deviation from a flat baseline.
If the baseline deviates from flat in a (curved or bowed) symmetrical
manner as shown in Figure-6A and Figure-6B there is an amplitude error only.
FIGURE-6A
FIGURE-6B
If the baseline deviates from flat in a (straight line) tilted manner as
shown in Figure-7A and Figure-7B, there is a phase (time delay) error only.
FIGURE-7A
FIGURE-7B
MEASURING STEREO SEPARATION DIRECTLY FROM THE COMPOSITE WAVEFORM
Figure-8 illustrates a composite waveform with a mixture of amplitude and
phase errors as indicated by the asymmetrical deviation of the base line from
flat. The separation can be calculated by taking twenty times the log to the
base ten of the ratio of the total peak to peak value of the waveform to the
peak to peak deviation from flat base line. The sample calculation in
Figure-8 shows a separation of approximately 28dB.
HOW TO ADJUST THE AURAL TRANSMITTER FOR BEST
BTSC-MTS PERFORMANCE
All optimization should be done with the transmitter connected to the
normal diplexer and antenna system. The transmitter is first tuned for normal
output power and proper efficiency according to the manufacturer's instruction
manual. The meter readings should closely agree with those listed on the manu-
facturer's final test data sheet.
A simple method for centering the transmitter passband on the carrier
frequency involves adjustment for minimum synchronous AM. Synchronous AM is a
measure of the amount of incidental amplitude modulation introduced onto the
carrier by the presence of FM modulation. This measurement is very useful for
determining the proper tuning of the aural transmitter. Since all transmitters
have limited bandwidth, there will be a slight drop-off in power output as the
carrier frequency is swept to either side of the center frequency.
This slight change in RF output level follows the waveform of the signal
being applied to the FM modulator causing AM modulation in synchronization with
the FM modulation. Minimizing synchronous AM will assure that the transmitter
passband is centered on the aural channel.
Care must be taken when making these measurements that the test set-up does
not introduce synchronous AM and give erroneous readings which would cause the
operator to mistune the transmitter to compensate for errors in the measuring
equipment.
The input impedance of the envelope detector must provide a nearly perfect
match so that there is a very low VSWR on the sampling line. Any significant
VSWR on the sampling line will produce synchronous AM at the detector because
the position of the voltage peak caused by the standing wave moves along this
line with FM modulation. Unfortunately, the AM detectors supplied with some
modulation monitors do not provide a good enough match to be useful for this
measurement. Precision envelope detectors are available that present a good
match (30dB return loss) to the sampling line.
A typical adjustment procedure is to FM modulate 100% at 400Hz and
fine-adjust the transmitter's input tuning and output tuning controls for
minimum 400Hz AM modulation as detected by a wideband envelope detector (diode
and line probe). It is helpful to display the demodulated output from the AM
detector on an oscilloscope while making this adjustment.
Note that as the minimum point of synchronous AM is reached, the
demodulated output from the AM detector will double in frequency to 800Hz,
because the fall-off in output power is symmetrical about the center frequency
causing the amplitude variations to go through two complete cycles for every
one FM sweep cycle. This effect is illustrated in Figure-9. It should be
possible to minimize synchronous AM while maintaining output power and
efficiency in a properly designed power amplifier.
SYNCHRONOUS AM WAVEFORMS
FIGURE-9
Another more sensitive test is to tune for minimum intermodulation
distortion in left only or right only stereo transmissions. Stereo separation
will also vary with tuning.
For stations employing a SAP, transmitter tuning becomes very critical to
minimizing crosstalk into the SAP. Modulate one channel only on the stereo
generator to 100% with a 7867Hz tone. This will place the upper second
harmonic (L-R) stereo sideband on top of 78.67KHz SAP. Activate the SAP at
normal injection level without modulation on the SAP. Tune the transmitter for
minimum output from the SAP demodulator. This adjustment can also be made by
listening to the residual SAP audio while normal stereo programming is being
broadcast.
FIELD ADJUSTMENT TECHNIQUES
1. Tune for minimum synchronous AM noise.
2. Tune for minimum IMD in left or right only
channel.
3. Tune for minimum crosstalk into unmodulated
SAP subcarrier.
In any of these tests, the input tuning is frequently more critical than
the plate tuning. This is because the impedance match into the input
capacitance becomes the bandwidth limiting factor. Even though the amplitude
response appears flattened when the input is heavily driven into saturation,
the phase response still has a serious effect on the higher order FM sidebands.
TEST EQUIPMENT SET-UP
Figure-10 shows a block diagram of the required test equipment set-up for
making composite waveform measurements. Note that the composite baseband is
checked at various points along the transmission path in order to verify the
performance of each subsystem.
Observing the composite waveform while using a low modulating frequency of
50Hz will usually indicate any low frequency problems due to coupling
capacitors in the system that are of insufficient size. Composite tests using
a high modulating frequency of 15kHz will reveal rolloff in the high frequency
response of the system which attenuates the (L-R) components more than the
15kHz component.
A precision envelope detector is also included in the test set-up so that
the synchronous AM waveforms can be observed while tuning the aural
transmitter.
ACKNOWLEDGMENTS
The author wishes to thank Rick Carpenter and Bill Resch for their
assistance in conducting the tests and editing this paper. Special thanks to
Kathy Klingler for typing and word processing, Jeff Houghton for the
illustrations, and Kim Dopheide for word processing.
THE AUTHOR
Geoffrey N. Mendenhall earned his BEE degree from the Georgia Institute of
Technology in Atlanta, Georgia.
Mr. Mendenhall has designed communications and telemetry equipment for E.F.
Johnson and Harris Corporation. He led the design efforts for both the Harris
MS-15 product line and the Broadcast Electronics FX-30 FM exciter.
His practical field experience has involved engineering and operations work
for several radio and television stations.
Mr. Mendenhall is presently serving as Vice President of Engineering for
Broadcast Electronics Inc. in Quincy, Illinois. He is an active amateur radio
operator and is experimenting with home satellite reception.
The author holds three U.S. Patents for electronic designs and is a
registered professional engineer in the State of Illinois. He has authored
numerous technical papers and is a senior member of the Institute of Electrical
and Electronics Engineers.
