by cornutt » Sun Jul 05, 2009 4:58 pm
Urosh's analysis on that thread was an interesting bit of reading. I'll have to go do some more investigating re his statement about the Juno-106 (and presumably the 60 too) restarting the DCOs at note on -- I haven't picked up on this in my studies of the schematics, but it could be true. I'll have to try to devise a test to see if I can see that.
At least in regard to the 106, Urosh's statement about the DCO having a constant ramp rate is incorrect. The 106 DCO varies the ramp rate according to the frequency, just as a VCO does. The difference is that in the DCO, the ramp rate doesn't determine the frequency. The main reason the 106 does it this way is so the output level of the DCO will be constant; if you use a constant ramp rate, the output level will decrease with increasing frequency.
If the Evolver DCOs do in fact use a constant ramp rate, then there must be something to compensate the output level. If you put a VCA on the output and fed the frequency CV to it, that would do the job. If this is what DSI did, then the compensation VCAs may have a problem with control voltage feedthrough, which would cause a pop or a click when going abruptly from a low note to a high note. Or, urosh may be right about the DCO only changing frequencies at the start of a cycle, which means that if you have a very fast envelope, and you are going from a low note to a high note, you may hear up to one whole cycle of the low frequency at the start of a note before the frequency changes.
Speaking of envelopes, though: One thing people don't realize is that vintage analogs, for the most part, have very slow envelopes compared to today's synths. Everybody today wants nice sharp envelopes, but it does tend to expose glitches in the oscillator circuitry, and it isn't just DCOs that have this problem. A lot of vintage VCOs are not really capable of instantaneous frequency change; there's always a little bit of slewing, even with the portamento set to zero. I've noticed this a little bit with my EML 101; I think the integrating capacitor itself loads down the charging circuit a bit. And some of the old comparators had problems with over/undershoot. There's also the problem of what happens when you instantaneously turn on a VCA that is being fed by a free-running oscillator. If the waveform at that moment happens to be near a peak, the output will have a sharp transient where it turns on, that will be noticable if the waveform doesn't contain a lot of upper harmonics. (This is exactly what causes Hammond key click; the waveforms are almost sine waves, and the turn-on transients are very noticable.)
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