Instruction manual

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28
Components in the right channel filter have fixed values. The resonant
frequency of each of the left channel legs is variable over a small range
so that the two channels may be precisely matched in amplitude and
phase response.
A great “cookbook,” with this and other filter circuits which might prove
valuable to the broadcaster, is the Electronic Filter Design Handbook by
Arthur B. Williams, published by McGraw-Hill. The reader is kindly
directed to this source for a more informed discussion of how the FDNR
circuit works than we could possibly muddle-through here.
IC34B buffers the output of the low-pass filter and includes gain, both to
compensate for the 6dB filter loss and to present the proper amplitude
to the digital synthesis circuitry.
PILOT AND SUBCARRIER GENERATION
Clock IC18D is a crystal-controlled oscillator at 1.216Mhz. IC18A buffers the
clock, and binary divider IC15A furnishes the 608kHz digital synthesis
sampling frequency.
Pilot Generation IC19 is an up/down BCD counter clocked at the 608kHz sampling rate.
1-of-10 decoder IC24, OR gate IC20D, and binary divider IC17B work
together to keep IC19 continually counting from zero to 8, back down
to zero, etc. Counting logic is decoded by a 1-of-9 de-multiplexer (IC23
and IC28B) which samples a resistor string with sine-weighted values.
This generates one-half a sinewave for each counting cycle. IC15B
reverses the DC polarity applied to the top of the resistor string for
every-other up/down count, forming the complete 19kHz pilot sinewave
from 32 discrete steps. R85 introduces an offset to compensate for any
difference between the power supply rails, and is adjusted to null the
38kHz second harmonic component. The segmented Stereo Pilot is
buffered by IC27A
Center-
Sampling
Analog switch IC28A is controlled directly by the 1.216MHz clock,
turning on for one-half of one clock period precisely at the center of
each stepped pilot waveform sample. This charges C36 to the sample
voltage value, which is held by buffer stage IC27B until the next center
sample is taken. Center-sampling eliminates integration of switching
noise which is concurrent with leading and trailing edges of the
waveform steps.
PILOT Switch In its OFF position, the front-panel PILOT switch, S4, presets IC6 to a
zero count. This holds 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.