Radica Games DAVID-II 716, Инструкция по эксплуатации и техническому обслуживанию

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PILOT Switch
In its OFF position, the front-panel PILOT switch, S3, is arranged to
interrupt the pilot sampling sequence and preset IC6 to a zero count.
This stops the stereo pilot at its zero crossing.
Subcarrier
Generation
The FM-multiplex “composite” waveform consists of a “main” channel,
conveying the L+R stereo sum in its normal audible frequency range,
and a “sub” channel with L–R stereo difference information in the
form of a 38kHz double-sideband, suppressed-carrier subcarrier. The
19kHz pilot is added to enable stereo receivers to reconstruct and re-
introduce the 38kHz carrier for difference signal demodulation.
Digital synthesis of the composite waveform is similar to the
generation of the Stereo Pilot previously discussed, but with sinusoidal
commutation between the left and the right stereo program channels at
a 38kHz rate.
Up/down counter IC4 is clocked at the 608kHz sampling rate. The
output is decoded by IC9 and, with gating provided by IC10D and CR7,
IC4 counts continuously from zero to 8, back to zero, etc. A Pulse from
IC12A presets the counter to its center position (count of 4) each time
the pilot reaches the proper phase relationship. This ensures proper
and constant synchronization between pilot and subcarrier.
IC13 and IC16D also decode the count, sequentially sampling each tap
of the resistor divider string which bridges the left and right program
audio signals. Sampled program audio is buffered by IC15B, center-
sampled by IC16B, and held between samples by C20 and buffer
IC17B. The stereo multiplex signal consists of 16 discrete,
sinusoidally-weighted steps.
When the front-panel
MODE switch, S2, is set to MONO , the counting
sequence is halted with the counter preset at the center position (count
of 4). This stops the subcarrier generation, and L+R audio appears at
the DAVID-II output. In the MONO position, S2 also stops the stereo
pilot, regardless of how S3 is switched.
Composite
Processor
Diodes CR8 and CR9 are biased to the same 100%-modulation level as
the clipper diodes in the overshoot compensation circuit. The buffered
composite signal, less the stereo pilot, is presented to this clipper
through front-panel COMPOSITE PROCESSING control R64. When
R64 is set to its fully counterclockwise position (
OUT
), the two diodes
catch only the occasional program peak which somehow has eluded the
earlier processing and clipping stages. As R64 is cautiously turned
clockwise, program peaks at the 100%-modulation value may be
subjected to as much as 3dB of clipping.
Even though the composite waveform is clipped prior to pilot injection,
good broadcasting practice demands judicious use of this feature. (See
Page 14.)