I retested the PW on my VCO and discovered that I had forgotten to ground two points. After fixing this, my own PWM works fine without pitch dropping.
I also did a more thorough test of various sync modes:
CEM Hard sync
- Datasheet says to sync against pulses, 1volt minimum, 3v maximum
In practice in my circuit
- Needs 1nF input cap
- Square wave with 50/50 duty cycle, +/-5v works fine.
- Saw wave, rising +/-5v works but only gives half sync
- Saw wave, falling +/-5v works but only gives the other half sync, and weird (but cool) behaviour when synced oscillator frequency is higher than master.
- Amplitude and offset of sync signal does not affect it.
- Strongly affected by PW, not in a good way
Conventional HS
- Datasheet says to use -negative pulses only, 8-10V
In practice in my circuit
- Saw wave, rising, +/-5v works. Falling does not work
- Square wave with 50/50 duty cycle, +/-5v works just like saw wave (but saw wave has a slight clip on top)
- Square wave with other duty cycle than 50% does not work well.
- Not affected by offset
- Affected by amplitude, +/-5v works best
- Strongly affected by PW, not in a good way
CEM Soft sync
- Datasheet says to use negative pulses, max 5V
In practice in my circuit
- Needs 1nF input cap
- Square wave with 50/50 duty cycle, +/-5v works fine
- Saw wave, +/-5v, rising and falling works fine and sound the same, has a darker tone than square wave???
- Strongly affected by PW, not in a good way
Sync circuit conclusion
- I think I should build at least the prototype voice card with selectable saw, inverse saw or square wave input.
- All waves should be +/-5v
- Waves should be tapped pre-VCA/mixer. Square should have its own square wave generator that is unaffected by PW and pulse VCA.
- I should give the option to switch between all three modes - to see how it sounds in practice
- I seem to remember that there was talk about constant frequency offset vs constant pitch offset in syncs, maybe worth investigating (but is a software issue, not hw).
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