Instruction manual

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8. Reset the COMPOSITE PROCESSING control to the full-
counterclockwise OUT position. NOTE: when composite clipping is
turned off, front-panel MPX metering will read –1dB when a steady-
state, 100%-modulation input signal is applied.
Stereo Pilot
Distortion Null
NOTE: This portion of 708 calibration must follow the preceding
Output Level Calibration procedure directly. At this point the
Modulation Monitor should indicate 90% Total Modulation of the
400Hz Audio Generator test signal, and 10% pilot injection.
1. Disconnect the Audio Generator from the PROGRAM LINE INPUT of
the 708 Stereo Generator.
2. Switch the Mod-Monitor to show 38kHz residual subcarrier.
3. Adjust R85 (to the right of the clock trimmer cap) to null the 38kHz
second-harmonic-distortion component of the stereo pilot. The Mod-
Monitor should indicate a 38kHz “residual” reading at the bottom
of its most sensitive scale (–70dB or better).
Subcarrier
Calibration
NOTE: This portion of 708 calibration must follow the preceding
Stereo Pilot Distortion Null procedure directly.
1. Downrange the Mod-Monitor display to verify that the 19kHz
stereo pilot reads 10% in the Total Modulation measurement mode.
2. Switch the PILOT switch OFF.
3. Connect the output of the Audio Generator to SUBCARRIER INPUT
1 of the 708. Reset the Audio Generator frequency to 80kHz,
output level to 0dBu.
4. Adjust the 708 front-panel SUB 1 INJ. control for a Mod-Monitor
indication of 10% Total Modulation.
5. Switch the 708 metering to SUB 1 and adjust R49 for a 0dB reading.
6. Disconnect the Audio Generator from SUBCARRIER INPUT 1 and
connect it to SUBCARRIER INPUT 2.
7. Adjust the SUB 2 INJ. control for a Mod-Monitor indication of 10%
Total Modulation.
8. Switch the 708 metering to SUB 2 and adjust R50 for a 0dB reading.
Separation Trim
The traditional method of adjusting and verifying stereo separation is
to visually monitor an oscilloscope display of the composite multiplex
signal (one channel driven and stereo pilot off), and adjust for the
straightest baseline of the DSB waveform. Although this technique
does permit a rough separation calculation, even today’s best ‘scopes
will show baseline distortion when overdriven to the extent necessary
to resolve stereo separation of 40dB or more. For the high figures of
separation which modern stereo generators can achieve, a precision
stereo demodulator will give much greater measurement and,
consequently, adjustment accuracy.
It is entirely feasible to trim the equalization and phase adjustments in
the 708 output circuitry to correct for shortcomings elsewhere in the
transmission system; for instance, in a “composite” STL, or even in a