The Programmable Sound Generators, H4 and K4, receive data from N4 (output port) and three control lines from G3 (octal D type latch). Both Programmable Sound Generators use a 2 MHz clock on pin 14 from SI, (synchronous 4-bit counter).
The audio outputs of H4 on pins 15, 17 and 19 are pulled down by R4, R7 and R8 and are summed by R42 through R45 and an op amp HI, pins 1, 2 and 3. The output from HI pin 1, is capacitively coupled by C23 to the main summer. The audio outputs of K4 on pins 15, 17 and 19, are pulled down by R22, R23 and R24 and summed by R29, R30, R32, R36 and HI (op amp pins 12, 13 and 14). The output from HI pin 14 is filtered by two 1.6 KHz low pass filters. The first filter consists of C5, R37, R38 and op amp HI, pins 8, 9 and 10. The second filter consists of C6, R40, R41 and HI (op amp pins 5, 6 and 7). The output from the second filter is capacitively coupled by C7 to the main summer.
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