Processed, pre-emphasized left and right audio is passed through third-order lowpass filters
comprised of U202A (203A) and associated circuitry. The filters decrease the level of audio
products below 30 Hz. This low-frequency roll off is necessary to prevent disturbance to the
phase-lock loop in the RF frequency synthesizer by extremely low-frequency audio compo-
nents. (See caution at section 2.8.)
U204 is a precision, four-quadrant, analog multiplier. The output of U204 is the product of 38
kHz applied to the Y input and the difference of left and right audio (L-R signal) applied to the X
input. The resulting output is a double sideband, suppressed carrier/the L-R subcarrier.
Spectral purity of the stereo subcarrier is dependant on a pure 38-kHz sine wave at the multi-
plier input.
U207A and Y201 comprise a 7.6-MHz crystal oscillator from which the 19 and 38-kHz subcarri-
ers are digitally synthesized. U207F is a buffer. The 7.6 MHz is divided by 5 in U208A to pro-
vide 1.52 Mhz, used by switched-capacitor filters U5 and U11 in the audio section. 3.8 MHz,
1.9 MHz, and 304 kHz are also derived from dividers in U208. Exclusive-OR gates, U210C
and U210D, provide a stepped approximation of a 38-kHz sine wave. With the resistor ratios
used, the synthesized sine wave has very little harmonic energy below the 7th harmonic.
U210A and B generate the 19-kHz pilot subcarrier. U211 is a dual switched-capacitor filter,
configured as second-order, low-pass filters, each one with a Q of 5. The 38 and 19-kHz out-
puts of pins 1 and 20, respectively, are fairly pure sine waves. Harmonic distortion products
are better than 66 dB down-with a THD of less than 0.05%. SEPARATION control R244 sets
the 38-kHz level at the Y input of U204.
Resistor matrix R219, R220, R221, and R223 sum the L+R audio with the L-R subcarrier to
produce a current at the junction of R221 and R223 that will be converted to composite stereo
(less pilot) at the output of summing amplifier U206A. SCA signal is also injected at the input of
U206A. 19-kHz pilot is combined with composite stereo in summing amplifier U206B.
Analog switch U205, at the input of U206A, provides switching of left and right audio for stereo
and mono modes. In the mono mode, right channel audio is disabled, and the left channel au-
dio is increased from 45% modulation to 100%.
MON L and MON R outputs go to the AF Monitor jacks on the rear panel. R209+R210
(R214+R215) and C207 (C210) is a de-emphasis network. Processed, de-emphasized sam-
ples of the left and right audio are used for a studio monitor and for audio testing. Jumpers at
HD201 (HD202) allow selection of 50-μsec or 75-μsec de-emphasis.
VR212A and B supply +7 volts and -7 volts, respectively. A 5-volt reference from the audio
processor section supplies the subcarrier generators.
For an explanation of on-board adjustments see section 5.2.
4-5 Principles of Operation
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