LEAMING INDUSTRIES MTS-4A BTSC STEREO GENERATOR

LEAMING INDUSTRIES MTS-4A
BTSC STEREO GENERATOR
INSTRUCTION BOOK
IB 090941-01 C
ALL ENGINEERING DESIGNS, DRAWINGS AND DATA CONTAINED HEREIN ARE
PROPRIETARY. NO PART OF THIS BOOK MAY BE COPIED OR OTHERWISE USED
WITHOUT WRITTEN AUTHORIZATION
Copyright 24 May, 2002
by
LEAMING INDUSTRIES
3972 BARRANCA PKWY. J608
IRVINE, CA 92606
(949) 743-5233
MTS-4A TABLE OF CONTENTS
SECTION
1.0
2.0
3.0
3.1
3.1.1
3.2
3.3
3.3.1
3.3.2
3.3.3
3.4
3.5
4.0
4.1
4.1.1
4.1.2
4.2
4.3
4.3.1
4.4
4.4
4.5
4.6
5.0
6.0
7.0
7.1
7.2
7.2.1
7.3
7.3.1
7.3.2
PAGE
QUICKSTART
INTRODUCTION TO THE MTS-4A
INSTALLATION
AUDIO INPUT
SECOND (B) AUDIO INPUT
VIDEO SYNC IN CONNECTIONS
COMPOSITE BASEBAND OR 4.5 MHz FEED TO TV MODULATOR
BASEBAND STEREO MULTIPLEX TO TV MODULATOR
4.5 MHz AUDIO CARRIER WITH VIDEO
4.5 MHz AUDIO CARRIER SEPARATE FROM VIDEO
VIDEO SCRAMBLING
LOCAL & REMOTE-CONTROLLED AUDIO INPUT SELECTION
OPERATION
SETTING THE AUDIO CARRIER DEVIATION
SPECTRUM ANALYZER AND TEST TONE METHODS
USING THE METER OR PEAK FLASHER ON THE TV MODULATOR
AUDIO CARRIER LEVEL ADJUSTMENT
"A"/"B" AUDIO INPUTS
LOCAL & REMOTE AUDIO INPUT SELECTION
STEREO AUDIO LEVELS and AGC CONSIDERATIONS
UNLOCK LIGHT
AUDIO AGC ON INDICATOR
STEREO / VIDEO SYNC LED
SPECIFICATIONS
AUDIO PERFORMANCE EVALUATION
TROUBLESHOOTING
COMMON SETUP ERRORS
TROUBLESHOOTING WITH A STEREO DECODER
USE YOUR EARS
TROUBLESHOOTING WITHOUT A STEREO DECODER
TROUBLESHOOTING WITH A SPECTRUM ANALYZER
TROUBLESHOOTING WITH AN OSCILLOSCOPE
APPENDIX
24 May, 2002
PARAGRAPH
3
3
4
4
4
4
5
5
6
6
6
6
7
7
7
8
8
9
9
9
9
10
10
11
13
13
13
14
15
15
15
15
CONNECTION DRAWINGS
MTS-4A PAGE 2 of 19
END
1.0
QUICKSTART
This section is provided for those who want to skip directly to connecting the MTS-4A
into their system. At the minimum, please see the Connection Diagrams in the
Appendix at the rear of this manual, and read the remainder of this section (1.X).
1.1
Configure Video Modulator to accept BTSC Stereo in the format you wish to use.
Refer to Section 3.3.
1.2
Loop baseband video through the MTS-4A on its way to the video modulator.
Refer to Section 3.2.
1.3
Connect the output of the MTS-4A to your TV channel modulator (use either
baseband multiplex or 4.5 MHz). Refer to Section 3.3.
1.4
Connect Program Audio to the MTS-4A. Refer to Section 3.1.
1.5
Connect AC Line Power.
1.6.1 If using Baseband Multiplex Stereo feed to audio modulator in video modulator,
adjust Aural Carrier Deviation at video modulator. Refer to Section 4.1.
1.6.2 If using 4.5 MHz feed to video modulator, adjust CARRIER LEVEL output of
MTS-4A. Refer to Section 4.2.
1.7
Adjust AUDIO LEVELS at MTS-4A. Refer to Section 4.4
2.0
INTRODUCTION TO THE MTS-4A
The Leaming MTS-4A provides a space-efficient means of generating highquality TV audio in BTSC stereo format.
The MTS-4A generates the U.S. broadcast-standard BTSC format, including
dbx companding. Left and Right baseband audio inputs are processed into a
composite multiplex stereo signal. This signal is available from the MTS-4A both
at baseband and on a 4.5 MHz carrier. Separate video sync input and 4.5 MHz
audio carrier output loops are standard.
The MTS-4A has dual stereo inputs. The second input may be used for local ad
insertion, or as an audio backup.
The MTS-4A includes automatic audio gain controls (AGC), which can
significantly reduce the program level variations that often occur when the
program source changes.
The MTS-4A displays stereo audio program levels with dual peak-reading
5-segment LED bargraphs.
24 May, 2002
MTS-4A PAGE 3 of 19
In addition, for those who need a SAP generator, a SAP carrier input is provided
on the MTS-4A. The Leaming SAP-1 or SAP-2 can be powered by the MTS-4A.
A Bessel-null calibration test tone is built-in to ensure a simple and accurate
setup when the composite baseband interface is used.
A thorough complement of front-panel switches, level controls, and LED
indicators facilitates use of the MTS-4A's features.
3.0
INSTALLATION
Refer to the CONNECTIONS drawings at the end of this book for illustrations of
how the MTS-4A hooks to the other components in the system. Note that it is
always necessary for the video to loop through the MTS-4A in order to phaselock the stereo pilot to the horizontal sync of the video.
3.1
AUDIO INPUT
For signals from a balanced source, connect a shielded audio cable pair from the
left channel of your stereo audio program source to the left channel of Input "A"
("A Audio In","LEFT") on the MTS-4A. Run an additional shielded cable pair from
the right channel of your program source to the right "A" input ("A AUDIO
IN","RIGHT"). Be sure that the + goes to the +, and the - goes to the -.
For signals from an unbalanced source, connect the high to the "+" and the shield
to the "-". Also, tie the "-" to the "G" (Ground) terminal if the two chassis are not
otherwise electrically grounded to each other.
NOTE:
The screw-terminal strips located on the back panel of the MTS-4A may be
removed (unplugged) for ease in wiring, and detachment also facilitates
reading the pin identification on the rear label.
3.1.1 SECOND (B) AUDIO INPUT
"Input B" can be used for local ad insertion or backup audio. The B input is wired
in the same procedure as that listed in Section 3.1, with the exception that it is
wired into Input B (B Audio In) on the back of the MTS-4A, rather than Input A.
See Section 4.3 for information regarding the selection of A/B inputs.
3.2
VIDEO SYNC IN CONNECTIONS
The MTS-4A's "VIDEO LOOP IN" must be connected to baseband video
(unscrambled), to permit the stereo pilot within the MTS-4A to frequency-lock to
it. The "VIDEO LOOP OUT" is to be connected as noted in 3.3.1, 3.3.2, or 3.3.3,
depending on the format selected.
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MTS-4A PAGE 4 of 19
3.3
COMPOSITE BASEBAND OR 4.5 MHz FEED TO TV MODULATOR
TV channel modulators generally accept BTSC stereo in either of two common
formats:
1) As a Composite (Multiplexed) Baseband "audio" or
2) On a 4.5 MHz subcarrier.
The MTS-4A provides both formats simultaneously. Either format produces
satisfactory results. The selection of which format to use is dependent on what
your TV modulator will accept, and which has been adopted as the preferred
method in your system.
Refer to connection diagrams D2 through D4 (located at the rear of this manual)
3.3.1 BASEBAND STEREO MULTIPLEX TO TV MODULATOR
This method uses the audio modulator built into the video modulator; the
4.5 MHz modulator in the MTS-4A is not used. First, verify that your TV
modulator is capable of accepting a baseband multiplex BTSC audio signal.
NOTE:
All video modulators require configuration to accept a BTSC stereo multiplex
signal, instead of mono audio. In most cases, this may be accomplished by
re-positioning a few switches or jump-jacks in the modulator. Specifically, the
75 µS pre-emphasis, and any audio limiting, if present, must be removed. The
modulator must have a wide "audio" bandwidth (flat through approx. 110 kHz).
Contact your TV modulator manufacturer for configuration instructions.
Once your TV modulator has been properly configured to accept BTSC stereo as
a baseband multiplex signal, refer to diagram D2 (at the rear of this manual) for
an illustration of the external hookup:
Connect the "VIDEO SYNC IN" loop "OUT" on the MTS-4A to the "Video In" on
your TV modulator.
Then connect a shielded audio cable from the "BTSC MULTIPLEX OUT" on the
MTS-4A to the "Audio In" on the TV modulator.
NOTE:
The BTSC MULTIPLEX OUTPUT of the MTS-4A is not balanced, When
driving a balanced load, the + (Red) goes to the +, any one of the MTS-4A's G
(Chassis Ground) terminals goes to - (Black), and another G (chassis ground)
terminal goes to the shield (drain) wire.
For signals to an unbalanced load, connect the + (red, high) to the +, and the
shield (and Black, if present) to "G" (Ground) terminals at both the MTS-4A and
the video modulator.
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MTS-4A PAGE 5 of 19
NOTE:
If you are using the 4.5 MHz (Audio Carrier) output of the MTS-4A, instead of
the baseband multiplex output of the MTS-4A, there is no need to connect
anything to the "Audio In" of the video modulator or the "BTSC MULTIPLEX
OUT" on the MTS-4A.
3.3.2 4.5 MHz AUDIO CARRIER WITH VIDEO
If the TV modulator you are using accepts the 4.5 MHz audio carrier on the same
cable as video, rather than on a separate cable from video, refer to diagram D3:
Connect a jumper cable from the "VIDEO SYNC OUT" of the MTS-4A to the
"AUDIO CARRIER IN" of the MTS-4A.
Connect another cable from the "AUDIO CARRIER OUT" on the MTS-4A to the
"Video plus 4.5 MHz Input" on your TV modulator.
NOTE:
If you are using the BTSC MULTIPLEX OUT (baseband) of the MTS-4A,
instead of the AUDIO CARRIER OUT (4.5 MHz) of the MTS-4A, there is no
need to connect anything to the AUDIO CARRIER OUT of the MTS-4A.
3.3.3 4.5 MHz AUDIO CARRIER SEPARATE FROM VIDEO
If the TV modulator you are using requires separate video and 4.5 MHz inputs,
refer to diagram D4:
Connect one cable from the "VIDEO SYNC OUT" connector on the MTS-4A to
the "Video In" on the TV modulator.
Connect another cable from the "AUDIO CARRIER OUT" on the MTS-4A to the
"Audio Carrier In" on the TV channel modulator. Then install a 75 ohm
terminating resistor on the "AUDIO CARRIER IN" connector on the MTS-4A.
3.4
VIDEO SCRAMBLING
Because most video scramblers work at TV I.F. (45.75 & 41.25 MHz), the stereo
connections are generally unaffected. However, if there is any possibility that the
video is scrambled at baseband, loop the unscrambled video through the MTS4A before the scrambler; the MTS-4A will not function properly if it is receiving
sync-suppressed baseband video (i.e. the stereo pilot cannot sync to basebandscrambled video).
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MTS-4A PAGE 6 of 19
4.0
OPERATION
4.1
SETTING THE AUDIO CARRIER DEVIATION
If you are using the 4.5 MHz Audio Carrier Output of the MTS-4A, its deviation
has been calibrated at the factory; it is not field-adjustable. Skip to Section 4.2.
If you are using the composite baseband multiplex output of the MTS-4A, the
aural carrier deviation must be accurately set at the TV channel modulator.
(Otherwise, stereo channel separation will be less than optimum.)
The TEST position of the "TEST MONO STEREO" switch (located on the
MTS-4A's front panel) activates a test tone, and shuts off the audio inputs, to
facilitate setting the deviation of an external audio modulator.
The most convenient and accurate way of verifying correct deviation is to use the
test tone and a spectrum analyzer to observe the first Bessel-null of the carrier.
As alternate methods, you may use either the meter or the peak modulation
(±25 kHz) light on your TV modulator.
NOTE:
If a video scrambler is being used in conjunction with the BTSC stereo
generator, to avoid possible interaction with the calibration tone, the scrambler
should be off or in the bypass mode while setting the audio deviation.
4.1.1 SPECTRUM ANALYZER & TEST TONE METHODS:
Move the "TEST MONO STEREO" switch to the "TEST" (left) position. Both the
"A" & "B" input lights will extinguish. Using a spectrum analyzer, look at the
audio carrier from your TV modulator. (For a clear picture, 20 kHz to 50 kHz
resolution per division is suggested.) Using the front-panel deviation control on
the TV modulator, null the carrier (to the first Bessel-null; see diagram 4-1).
To verify that the deviation is adjusted correctly, check the amplitude of the
15.734 kHz stereo pilot sidebands relative to the audio carrier. To do so, set the
test switch to the "STEREO" (right) position and, at reduced audio input levels,
look at the audio carrier and both sidebands using a spectrum analyzer (one
sideband should be 15.734 kHz above the audio carrier and another sideband
should be 15.734 kHz lower than the audio carrier). If the deviation is properly
adjusted, the two sidebands should be approximately 16 dB down from the
carrier (Refer to diagram 4-2). If they are not, after repeating the Bessel Null
procedure, repair and calibration of the equipment is needed.
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MTS-4A PAGE 7 of 19
Figure 4-1A
Correct Bessel-null
Figure 4-1B
Incorrect Bessel-null
Horizontal:
20 kHz/div
Figure 4-2
Main Audio Carrier with
Pilot Carrier Sidebands
Vertical:
10 dB/div
(All Views)
4.1.2 SING THE METER OR PEAK FLASHER ON THE TV MODULATOR:
If you are using the meter on your TV modulator to set the deviation, move the
"TEST MONO STEREO" switch to "TEST" (left position). Increase the Aural
Carrier Deviation (not Level) until the "±25 kHz" or "100%" or "0 VU" light on the
TV modulator comes on. Then, back it off until the light "just" goes off.
NOTE:
If SAP or scrambling is being used, be sure that it is off or in the bypass mode
while setting the deviation.
OPERATIONAL NOTE:
The audio peak flasher on the TV modulator may flash during
normal stereo operation, and more so if SAP is also present.
4.2
AUDIO CARRIER LEVEL ADJUSTMENT
When using the 4.5 MHz audio carrier output from the MTS-4A, the carrier level
must be adjusted, generally both at the MTS-4A and at the TV modulator. In a
typical case, such as when an MTS-4A feeds a Scientific Atlanta 6350, the audio
carrier level is adjusted first at the MTS-4A while monitoring the TV channel
output of the SA 6350 with a spectrum analyzer: Increase the carrier level at the
MTS-4A until the output of the 6350 ceases to increase, indicating that the 6350
has achieved limiting. This is the optimum drive level to the 6350; less drive
results in noisier audio, and more drive may splatter the audio into the video.
Then adjust the Aural Carrier Level on the 6350 to set it at the desired output
level, which is usually 15 dB below the (unmodulated) video carrier.
4.3
"A"/"B" AUDIO INPUTS
LEDS indicate which input is active, as selected remotely, or by the front-panel
switch (which can enable or override the remote control). If power is applied, and
24 May, 2002
MTS-4A PAGE 8 of 19
neither "A" nor "B" is illuminated, the MTS-4A's TEST/MONO/STEREO switch is
in Test mode.
4.3.1 LOCAL & REMOTE AUDIO INPUT SELECTION:
Selection of "A" or "B" inputs may be accomplished either locally, or remotely.
The "A" and "B" positions of the front panel switch will override remote A/B
control.
By grounding a line connected to the A/B terminal on the rear panel of the
MTS-4A, if the front-panel switch is in the center "REM" position, the "B" inputs
will be selected.
The "REM" terminal is normally pulled to +5 volts by a resistor; any switch, relay,
or appropriate logic device capable of sinking 0.5 mA and pulling the line to within
a volt of ground may be used.
4.4
STEREO AUDIO LEVELS and AGC CONSIDERATIONS
The MTS-4A is furnished with two sets of level controls on the front panel: "A
LEVEL" "LEFT" & "RIGHT" and "B LEVEL" "LEFT" & "RIGHT". Stereo audio
program levels are monitored with dual peak-reading LED bargraph meters.
While listening to the audio program, with the audio AGC OFF, use a small flatblade screwdriver thru the front-panel access holes to adjust the corresponding
level controls so that both the left and the right meters read approximately 0 VU
(regular flashes of the yellow LEDs on program peaks). The red +3 VU LEDs
may flash briefly with very loud program peaks.
NOTE:
It is recommended that the program be listened to while adjusting the audio
level controls, to ascertain that the peak indicators are flashing only on material
that is intended to be loud.
If desired, the audio AGC may be switched on after setting the audio levels.
There should be no significant level change during normal program, but any overmodulation will be quickly reduced (within milliseconds); conversely, prolonged
under-modulation will be gradually increased. The under-modulation correction is
1:2 dB; that is, if the 15-second time-averaged peak input level drops 10 dB, the
peak output level will drop only 5 dB. The maximum under-modulation gain
increase is limited to approximately 12 dB, in order to avoid increasing
background noise excessively.
24 May, 2002
MTS-4A PAGE 9 of 19
4.5
AUDIO AGC ON INDICATOR:
When the audio AGC is on, the AGC ON LED is illuminated whenever the
incoming audio program level is within the normal range of the AGC, where the
AGC can "silently" maintain good program dynamics.
The AGC ON LED may dim if the program level remains very low for approx. 15
seconds or more. If the AGC ON LED dims regularly, listen to the audio program
to determine whether or not it is intended to be very quiet. If not, re-adjust the
appropriate front-panel potentiometers so that the LED is usually illuminated.
If the incoming program level increases substantially from the initial setting, the
yellow 0 VU LED may remain steadily illuminated, indicating that the audio AGC is
maintaining a relatively constant peak output level. If the yellow 0 VU LED is on
steadily, or for a significant time, the input level should be manually reduced
somewhat with the appropriate front-panel potentiometers, in order to maintain
optimum program dynamics.
In general, when the AGC is on, the ON LED should always be fully illuminated,
unless there has been at least 15 seconds of near-silence (signal peak levels over
20 dB below the recent peak levels), in which case the ON LED will dim. At that
time, the AGC will return to its standby gain setting, awaiting the arrival of
program material, at which time it will automatically re-activate.
Whenever setting the audio levels, observe the VU indicators, and listen to and
watch the program, to ensure that the adjustments you are making are
appropriate. Unless you are compensating for a known problem elsewhere,
mechanically position the Left and Right potentiometers identically to maintain
normal channel balance.
4.6
STEREO / VIDEO SYNC LED
The STEREO LED, on the front panel beside the "TEST MONO STEREO" switch,
will dim if the MTS-4A has lost video sync. The MTS-4A will continue to produce
the BTSC signal, but the stereo image may shift slightly. If the Stereo LED dims,
the cabling between the video source and the MTS-4A should be checked.
24 May, 2002
MTS-4A PAGE 10 of 19
5.0
SPECIFICATIONS
Frequency Response
Stereo:
(overall)
20 Hz to 14 kHz, ±1 dB
Distortion
Stereo:
0.5% max. THD
Separation
>30 dB, 20 Hz to 10 kHz typ.;
>26 dB, 20 Hz to 13.5 kHz
Signal-to-Noise Ratio
>65 dB, overall through
receiver & expander
Pilot Protection
>50 dB at 15,734 Hz
Test Tone
2.5 Vp-p @ 10.4 kHz
Compressor, L-R
dbx
Nominal Input Level (APL)
0 dBm, adjustable ±10 dB
Peak Input Level
10 dB above nominal
Audio Input Impedance
100 k-ohms, balanced
Composite Stereo Baseband
Output Level
0.1 Vp-p/kHz dev. into hi-z load
(Source impedance = 75 ohms;
min. load 600 ohms)
4.5 MHz Output Level
0.4 V p-p into 75-ohm loop,
adjustable
4.5 MHz Freq. Tolerance
Locked to video at 286 FH;
if no video, may drop 1 kHz
to 4.499 MHz.
4.5 MHz Harmonics
Deviation, Peak
licensed (BTSC
Standard)
-50 dB re: .250 V p-p
±25 kHz, mono
±50 kHz, stereo
±5 kHz, pilot
>±100 kHz maximum
Controls (Front Panel)
24 May, 2002
AGC On/Off
Input Levels, A & B, Left & Right
A/B Input sw. w/ Remote position
Test/Mono/Stereo switch
4.5 MHz Carrier Level
SPECIFICATIONS SUBJECT TO CHAN GE WITH OUT NOTICE
MTS-4A PAGE 11 of 19
5.0
SPECIFICATIONS, continued
LED Indicators
Dual (L&R) Peak-reading
5-segment bargraphs,
A/B Input
AGC ON
Stereo On
Connectors
(Rear Panel)
Video (sync) Input Loop;
4.5 MHz Output Loop
(Four F-type std.; BNC opt.)
Audio Input A;
Audio Input B;
Composite Stereo Out,
SAP carrier Input
Remote A/B Select & Sync Output
(Four detachable screwterminal plugs)
Audio Input Connectors
(Two)
PIN 1
PIN 2
PIN 3
PIN 4
PIN 5
BTSC Multiplex Output & SAP Carrier Input
PIN 1
PIN 2
PIN 3
PIN 4
R+
RG
L+
LSAP CXR
GroundBTSC MPX
Ground
Remote Control & Sync Output
PIN 1
PIN 2
PIN 3
PIN 4
PIN 5
A/B Input Select
Sync Output
Ground
18 V Output
Reserved
Size
1.6 inches H x 5.5 inches W
x 18 inches D. Mounts on 1/3 of
a PMU413 19-inch panel mount
Weight
5 lb.
Power
105-125 V, 50-60 Hz, 10 VA
210-250 VAC (factory config.)
24 May, 2002
SPECIFICATIONS SUBJECT TO CHAN GE WITH OUT NOTICE
MTS-4A PAGE 12 of 19
6.0
AUDIO PERFORMANCE EVALUATION
When verifying performance specifications of the MTS-4A, be sure that
none of its features interfere with the item being tested. Specifically,
frequency response tests should be run with the AGC off, and at a level
approximately 20 dB below 100% modulation. The reason is that the
standard pre-emphasis will cause apparent frequency response errors,
when running either with the AGC on, or at 100% modulation at high
frequencies, due to AGC action and/or over-modulation limiting.
7.0
TROUBLESHOOTING
7.1
COMMON SETUP ERRORS
If the problem is low program level when listening in mono, but the stereo
level seems OK, the phase of either the Left channel or the Right channel
(but not both) needs to be reversed. Refer to sections 3.1, 4.3, and 4.4.
If the problem is somewhat excessive noise, and you are using the
4.5 MHz connection to the video modulator (not the baseband multiplex
connection), the front-panel Audio Carrier Level control of the MTS-4A
probably needs to be set higher. Refer to section 4.2.
If the program is overly sibilant ("hissy"; too much treble), and you are
using the baseband multiplex connection to the video modulator (not the
4.5 MHz aural carrier), it is very likely that the TV modulator has not been
configured to accept Baseband Multiplex Stereo. In addition, poor
separation will be caused by this same error. Once the video modulator is
properly configured, follow the setup instructions in section 4.1.
EXPLANATORY NOTES: The likely reason that the TV Channel Modulator may
not have been configured to accept a BTSC Multiplex Stereo signal is
that, when shipped from the factory, Channel Modulators are typically
configured to accept Mono Program Audio, not Multiplex Stereo.
Usually, it is simple to re-configure the modulator, for BTSC stereo,
using its internal jumper-jacks or switches. (Refer to the Channel
Modulator's instruction manual for instructions.)
Mono uses 75 microsecond pre-emphasis, and may have a 15 kHz
audio low-pass filter. However, BTSC stereo requires its pre-emphasis
to be in the stereo generator, not at the modulator. In addition, the
modulator must have flat frequency response up to at least 50 kHz, and
possibly up to 110 kHz (depending on whether SAP and PRO channels
are in use).
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7.2
TROUBLESHOOTING WITH A STEREO DECODER
First, determine that the MTS-4A alone is functioning normally. If you
have access to a Leaming TSD (Television Stereo Decoder), or any other
suitable stereo decoder, connect it directly to the output of the MTS-4A
stereo generator. Follow the instructions that came with the stereo
decoder.
7.2.1 USE YOUR EARS
Even if you are connecting audio-analyzer instrumentation to the stereo
decoder, also listen to the stereo program from the audio outputs of the
stereo decoder. Human ears can usually detect significant problems
much more quickly than meters, and audible traits of the problem will
generally reveal its cause.
Once the MTS-4A has been determined to be functioning normally,
connect the MTS-4A back to the video modulator, and connect the stereo
decoder to a monitor output of the cable system, through a set-top
converter box if necessary. Repeat the performance tests. Re-read
section 6.0 through 7.1.
If no professional stereo decoder is available, connect the channel to a
known-good stereo TV.
Use appropriate attenuators in the line to keep from overloading the
monitor's front-end.
7.3
TROUBLESHOOTING WITHOUT A STEREO DECODER
If no stereo decoder is available, but a spectrum analyzer or an
oscilloscope is available, and an audio signal generator is also available,
some troubleshooting is still possible.
7.3.1 TROUBLESHOOTING WITH A SPECTRUM ANALYZER
To help determine if the MTS-4A is at fault, check the 4.5 MHz output of
the MTS-4A (even if you are not using it), directly at the back panel the
MTS-4A, with a spectrum analyzer. Use the procedure in section 4 of this
manual. If the sidebands appear as described (and illustrated if Figs. 4-1A
and 4-2), the test tone and the stereo pilot of the MTS-4A are functioning
correctly.
Next, while still observing the stereo pilot on the spectrum analyzer, apply
audio to both Left and Right channels at 0 VU (100% modulation). The
aural carrier should appear frequency-modulated, deviating ±25 kHz from
the center frequency. Then turn down the audio on one channel (either
24 May, 2002
MTS-4A PAGE 14 of 19
Left or Right) while leaving the other at 0 VU. Modulated Sidebands
should appear, centered at approx. 32 kHz above and below the aural
carrier.
Reconnect the MTS-4A into the video modulator and connect the
spectrum analyzer to the channel output of the video modulator (or the
cable system) and tune the spectrum analyzer to the aural carrier
frequency of that TV channel. Repeat the tests from the preceding
paragraph; the results should look identical, if the interface with the video
modulator is correct, and if any video scrambling has been bypassed.
If these appear normal, the MTS-4A is generally functioning correctly.
However, this simple test only indicates that the MTS-4A has an output
which is modulated by the audio, and that the hookup to the video
modulator is possibly correct. It cannot show that the internal calibration
of the stereo generator is correct. For that, and to verify correct interface
with the video modulator, a stereo decoder is essential.
7.3.2 TROUBLESHOOTING WITH AN OSCILLOSCOPE
If just an oscilloscope is available, it is still possible to check the basic
normality of the composite baseband multiplex output of the MTS-4A.
Place the "TEST MONO STEREO" switch in the "TEST" position. The
output should be 10.4 kHz at 2.5 Vp-p. Next, with no audio input to the
MTS-4A, move the "TEST MONO STEREO" switch to the "STEREO"
position. The output should be the 15,734 Hz pilot at 0.5 Vp-p (ignore the
noise peaks; read the "average" peak). The absolute values are not
critical, but the 5:1 ratio is critical. If this ratio is correct, the test tone and
the stereo pilot of the MTS-4A are functioning correctly.
NOTE:
When in the stereo mode, the 15,734 Hz sine-wave-shape on the
oscilloscope will appear slightly noisy. This is normal, caused by the
difference-channel compander operating at very high gain in the
absence of an audio signal.
Next, while still observing the stereo pilot on the oscilloscope, connect an
audio signal generator to the Left and Right audio inputs of the MTS-4A.
Set the signal generator to produce a 1000 Hz tone at approx. 1 volt RMS.
Connect the oscilloscope to sync on the audio signal generator. Set the
MTS-4A input level controls to cause an indication of 0 VU (100%
modulation) on the MTS-4A's meters.
The baseband output should appear on the oscilloscope as a 2.5 Vp-p
1000 Hz sine wave with a 0.5 Vp-p 15,734 Hz sine wave riding on it. Then
turn down the audio on one channel (either Left or Right) while leaving the
24 May, 2002
MTS-4A PAGE 15 of 19
other at 0 VU. An envelope should appear on the oscilloscope similar to
that above, but with an amplitude-modulated 31,468 Hz tone added (the
Difference Channel Carrier). The phase and amplitude of the modulated
envelope are difficult to describe, because of the effects of the dbx
difference-channel companding, but if what you see resembles this, the
MTS-4A does have a modulated output. For more meaningful evaluation,
a stereo decoder is necessary.
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