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jeremymlong / MidiDCO

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No Real issues...just comments. #2

Open Tim-Deefield opened 2 years ago

Tim-Deefield commented 2 years ago

Hi Jeremy,

I perfboarded the hardware and it all works very well. The freq accuracy across the the octaves is amazing. I connected a guitar tuner to it and the tuning was perfect, only C8 was slightly sharp.

I did find that the PWM DAC voltage was too high and caused clipping of the bottom of the SAW. I changed the "magic" constant from 0.425 to 0.651 and that resolved the clipping. I think that maybe the tolerance of the capacitor between yours and mine may explain the variation of the "magic" value.

One other thing I did was to change the PWM LPF from a 10k/1uf to a 2 pole 10k/.1uf & 1k/.01uf filter just for faster response between note changes.

Thanks again for posting this...it has been educational and fun playing with it!

-Tim

Tim-Deefield commented 2 years ago

Hi again Jeremy,

I for got to mention...any thoughts about stacking 6 of these like a Cheetah MS6 for polyphony and spreading the MIDI notes across the 6 modules? I guess you would need a master uP for the MIDI interface and it would assign the notes to each DCO. You would have to deal with the individual gating too.

One other thing, I connected the clock out of the MidiDCO to an AS3397 chip to get the feel of using that chip and it works well too. The AS3397 gives you mutltiple waveforms, 4 Pole VCF, VCA, and Left/Right panning. The tricky part is that the Voltage Waveform (Vwf) voltage needs to be very accurate because it not only sets OSC level but also controls the shape of the waveform. A real HW DAC is needed for this task, to do the testing I just set this voltage manually. The Vwf voltage can be determined with a formula and you can set it to be a SAW, Triangle, Clipped Triangle or mixture of any of these with a SQR/Pulse. Oh and the AS3397 is basically a CEM3396 with a few additions, the interface and formulas are the same.

-Tim