The sinewaves are generated by stepping through a stored sine table. There are three of these in the processor in different formats & amplitudes to accomodate the single, two-tone and three-tone options.

At the 9.8304 Mhz clock frequency, a 96 cycle length routine completes 25600 times per second. Incrementing the minimum of one step per cycle generates the minimum frequency of 100 Hz. Increasing this step size increases the output frequency directly. The control switches are polled once per output cycle and are serviced on change.

The serial output facility of the PIC is employed to output the sine data to the DAC. The L/R select signal is generated in software. The noise signal is generated on the fly by a 32 bit psuedo random shift register.

The sync output follows the pulse gate for pulse output modes or goes positive for 40 uS at the "zero cross" of the lowest frequency being generated. There may be an apparent touch of jitter on the sync signal depending on the output signal due to quantization of the sine index but, it's stable enough for a scope trigger. If necessary, a slight adjustment of the horizontal sweep time will produce a stable display. Note: The sync output is delayed 625 uS to match the group delay of the D/A, except for single tones above 1200 Hz where this delay would exceed a full cycle. A best approximation is employed for the higher frequency single tone sync signals. No Sync pulses will be present during continuous noise generation.

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