Instruction manual

— —
29
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 IC26 is clocked at the 608kHz sampling rate. The
output is decoded by IC25. With gating provided by IC20C and CR19,
IC26 counts continuously from zero to 8, back to zero, etc. A Pulse
from IC16C 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.
IC35 and IC28C 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 IC29B, center-
sampled by IC28D, and held between samples by C38 and buffer
IC29A. The stereo multiplex signal consists of 16 discrete,
sinusoidally-weighted steps.
When the front-panel MODE switch, S3, 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 708 output. In the MONO position, S3 also stops the stereo pilot,
regardless of how S4 is switched.
Composite
Processor
The emitters of transistors Q5 and Q6 are biased to the same 100%-
modulation level as the clipper diodes in the overshoot compensation
circuit. The transistor base/emitter junctions comprise the composite
clipping network. The buffered composite signal, less the stereo pilot,
is presented to this clipper through front-panel COMPOSITE
PROCESSING control R57. When R57 is set to its fully-
counterclockwise OUT position, the clipper is essentially a “safety”
clipper, catching only those occasional program peaks which somehow
have evaded the earlier overshoot protection circuitry. As R57 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 17.)
Q5 and Q6 also function in the common-collector mode to flash the
indicator adjacent to the COMPOSITE PROCESSING control.
Additional gain is provided by IC14 to light the LED, which gives a
visual indication of circuit action.
Combining
Amplifier
IC22B is a combining stage for the main/sub portion of the composite
signal, the stereo pilot, and any optional RDS or SCA subcarriers
applied to the rear-panel SUBCARRIER INPUT connectors. Adjustment
over the composite level delivered to the exciter is afforded by R58, the
front-panel OUTPUT LEVEL control in the feedback path of IC22A.