Sequential PROPHET-600 - POT MULTIPLEXER and ADC

The S/Hs for PW A and B CV S/Hs have some added resistors. R409-411 expand the
voltage range from below 0 to above *5, to ensure enough range to fully cutoff the
pulse width. R415 and C443 form a filter for digitization noise. This noise is the
audible result of the dynamic CV having to "staircase" rather then smoothly slew
between the strobed values. The filter removes the most objectionable part of the
staircase edges.
The MVOL CV is set to 0 during TUNE. Otherwise it tracks the VOLUME knob. The
EXT FIL EN CV is actually used as a gate which disables the FILTER CV IN during
TUNE. Otherwise it constantly strobes the FILTER CV IN to the filters.
2-12 POT MULTIPLEXER and ADC
When a program is selected, equivalent pot values are transferred from the program
area of RAM to a "Scratchpad" area, from which the values are output as appropriate
CVs. Also in Scratchpad the CPU maintains another table of the pot values
corresponding to wherever the knobs currently happen to be set.
During each loop the CPU checks either the PITCH or MOD wheel and one other knob
for motion. Because the interrupt loop is 5 ms, each wheel is sampled every 10 ms.
This rate is fast enough to detect typical real-time performance. If a difference is
detected in the PITCH wheel, oscillator pitches are raised or lowered accordingly. A
difference in the MOD wheel increases or decreases modulation amounts.
Provided no knobs are moving, each of the 25 other pots is scanned every 125 ms. If
one of these knobs is moved, the CPU reads it in successive loops, until the knob stops
moving. When the knob is moved to the programmed value, this activates Edit mode.
Once the knob has "found" the current program value, further pot motion will also
update the main Scratchpad table, thus change the sound.
See Sheet A (SD600-1). Clocked by -CS02, latch U107 provides pot addresses to
multiplexer (MUX) U108 and over TB201 to U201 (see Sheet B). These addresses
sequentially select which pot connects to the Vmux output—that goes to the ADC
comparator. Figure 2-8 shows a typical Vmux waveform.
V: IV/div
H: 2 ms/div
Figure 2-8
VMUX, PITCH WHEEL CENTERED
2-14
TM600A 7/83