Instruction manual

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30
Output Filter Since all components of the composite output are digitally synthesized
at a 608kHz rate, only a simple filter is required to eliminate the
sampling frequency from the Stereo Generator output. L2, L1, C25
and C26 form a 4-pole filter with quasi-Butterworth response. This
filter is exceptionally flat to about 100kHz, but effectively prevents any
higher-order digital noise components from appearing at the 708
output.
Output
Phase and
Equalization
Trimmers
The output amplifier consists of IC21A, IC10B, and transistors Q3 and
Q4. R71 is the front-panel MPX EQUAL. control, affording variable
high frequency tip-up and an opportunity to optimize stereo separation
of the overall transmission system. R56 is an internal trim control to
calibrate multiplex signal phase response.
BARGRAPH METER
Display Selector Front-panel “up/down” selector switch S2 cycles the display among its
various internal monitoring functions. Three sections of IC13 de-
bounce the switch contacts; the fourth section, IC13A, presets the
counter to the default MPX display when power is first applied. The
BCD output of up/down counter IC12 is decoded by two, 8-channel
CMOS multiplexers. IC1 lights the appropriate identifier LED below
the bargraph display, and IC11 routes the selected signal to the
metering circuit.
Left- and right-channel program signals, taken just after pre-emphasis
amplifier IC45, are presented, independently, to two inputs of
multiplexer IC11. Left and right audio is also matrixed: IC6A derives
the L+R stereo sum, IC6A the L–R stereo difference. Sum and
difference signals are scaled to give a reading of 0dB when both
channels are driven to the 0dB point, in- and out-of-phase, respectively.
The combined composite multiplex signal, the stereo pilot, and the two
subcarrier input signals are also routed to multiplexer IC11. All
monitored signals may be trimmed by appropriate meter calibration
controls, except for the stereo sum and difference which are based on
trimmed left and right values.
Meter Rectifier
Voltage-gain stage IC10A feeds the full-wave, peak-responding rectifier
circuit. The value of positive-going waveform peaks is buffered by
IC4B and rectified by diodes CR8 and CR9. Negative-going values are
inverted by IC4A and rectified by CR6 and CR7. The absolute value is
held by C9, IC5A buffers the feedback around the rectifiers.
Buffer stage IC5B drives the three cascaded LED driver ICs. A small
amount of mains-frequency “dither’ is added to the DC level to provide
a smooth fade from one LED display segment to the next.
Each of the LED drivers contains ten voltage comparators and a
resistive voltage-divider reference. When the three are “stacked,” this
creates a 30-step voltage-linear readout, though only 29 segments
comprise the Model 708 display. The front-panel meter is scaled both