Sequential PROPHET-600 - Cassette;M Isc

One cycle at 261 Hz equals about 3.83 ms. which is about 76,628 ft-us periods. To be
tuned at this octave (C3), the oscillator must generate this number. To tune the next
higher octave (C4). the reference is halved to 38.314 and CV again set by successive
approximation so that one cycle produces that count. For the higher octaves. C5 - C9,
the reference count remains the same, while the Cycle Counter is reprogrammed for
2. 4, ...32 cycles. For the lower octaves. CO - C2, the biases are actually extrapolated
from the curve suggested by the higher-frequency measurements, because counting
these slow waves would take an inconvenient amount of time.
Also to save time, the whole routine is only performed on power-up. After this, since
the oscillators drift only slightly, retuning usually adjusts only the least-significant
DAC bits.
Because the oscillator signals pass through analog switches and the filter, problems in
these audio paths may appear as tuning problems. For example, if an open analog
switch disables the oscillator output to the tune comparator, the tune circuit will not
see the correct number of pulses. This failure mode causes the computer to set the
CVs for all C notes for that oscillator to maximum, while all other keys will sound in
the sub-audio range. (To isolate the problem, try using the voice defeat: hold the key
then press RECORD and PROGRAM SELECT 7.)
2-17 CASSETTE/M1SC
Referring to Sheet D (SD600-2), U326-7 latches interface data which is ac-coupled to
the TAPE output jack. TAPE IN is ac-coupled to U321 TAPE CPR, then read by
miscellaneous driver U325.
U325 also accepts the CONTROL FOOTSWITCH and OSC SCALE test point inputs.
TM600A 7/83 2-19