| previous by date | index | next by date |
| previous in topic | topic list | next in topic |
To use only one expo, Korg has used multiplexers to feed the 6 VCO CVs to the expo, followed by a calibration voltage and then the CV for standard pitch (only for the new production voice board).
The two latter outputs are routed to two independent controllers; the last one introduces an offset to the expo input to shift the standard pitch to its desired value, while the converted calibration voltage drives a current through the photocoupler’s LED, while its LDR is placed where you would normally expect the 3300ppm tempco resistor in similar circuits.
This leads to an eady way to check whether a tuning problem is related to the photocoupler circuit at all: measure the voltage on pin 7 of IC18. Within regulation it should be somewhat between -2 and -6 volts, -3 are more normal.
When the loop breaks open due to failure of the photocoupler or related parts, it will be stuck on the maximum negative output, something around -13 volts. There must always be a small current through the LED, otherwise the loop does not work and the TUNE HIGH preset has no effect!"
2. From Chip's blog: http://synthhacker.blogspot.ca/2013/02/polysix-basic-key-assigner-timing.html
"Looking at this graphs, I see that overall timing loop is 6204 microseconds ("usec") long. This means that the basic pitch and the on/off gate of each voice only get updated every 6.2 milliseconds, which is an update rate of only 161 Hz. Frankly, I was surprised at how slow this is. But, since the synth sounds OK, I guess that it works fine. Luckily, the pitch modulations and the VCF and VCA envelopes are not at all dependent upon this loop (they have their own generators which, for the case of the VCF and VCA, are fast).
Additional examination of these graphs (and a little more probing of the synth) allows one to understand the timing cycle well enough to break it into discrete time periods. Clearly, there are six periods associated with the pitch of the six voices. I call these periods "V1" through "V6". These periods are each 676 usec in duration.
After V6, there are appears to be another voice-like period, which I call "Vx7", since it is so much like a voice, but doesn't actually sound. Interestingly, through additional experiements, I found that its voltage is always the same - it is always set to be C7, the highest note on the Polysix. Vx7 is 712 usec long. Why is it a different length than V1-V6? I don't know.
Similarly, after Vx7, there appears to be another voice-like period, which I call "Vx8". Its voltage level always appears to be identical to V1. I don't know why. I'm thinking that Vx7 and Vx8 are both used by the Polysix's pitch correction circuitry to help it stay in tune. The duration of Vx8 is only 624 usec (though the end of Vx8 is actually a bit hard to define and measure).
After Vx8 is the final period of the Key Assigner's timing cycle. I call it the "Inter-voice" (or "IV") period. During this period, the synth sets the gate signals, it drives some LEDs, it scans the keybed and some switches, and probably a few other things as well. We'll dig into that in a future post."