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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 sub-
carrier.
Spectral purity of the stereo subcarrier is dependant on a pure 38-kHz sinewave at the mul-
tiplier input.
U207A and Y201 comprise a 7.6-MHz crystal oscillator from which the 19 and 38-kHz sub-
carriers are digitally synthesized. U207F is a buffer. The 7.6 MHz is divided by 5 in U208A
to provide 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 outputs 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 ste-
reo (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 ste-
reo and mono modes. In the mono mode, right channel audio is disabled, and the left chan-
nel audio 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
samples 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.
Principles of Operation
4-5
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