tirsdag 20. januar 2015

On voltages and other practicalities

While dreaming up the OMM I have for some reason constantly kept thinking about the output from the DACs as 5V, with a OMM value of 0 corresponding to 0V and 65536 to 5V while at the same time intending to use a 1V/octave VCOs and having the pitch CV controlled from the OMM.

This, of course, leads to a severe limitation: the synth would not be able to span more than 5 octaves, and no tuning or modulation would be possible for the highest and lowest notes.

There are a few different solutions to this problem:

1) one could use a separate output as octave selector. By keeping this at 2.5V normally, we could add or subtract 2 octaves and still have 0.5V left for tuning and modulation. This will however lead to having to do some tricks when receiving notes outsidere the normal 5 octaves.

2) one can use 10V instead of 5V for the DACs. This gives us the same 10 octaves as adding an octave selector CV. However, this requires a redesign of some modules such as the VCA.

3) one can change the input resistor on the VCOs so their response is 1/2V per octave, again giving us the same 10 octaves.

I am not sure which one to choose. I will have a closer look at how other midi-2-CV converters do this.

I also need to figure out how many octaves to support. If it should be possible to do one-octave pitch bends 10 volts only allows 8 full octaves, and even then other modulations and tuning must be kept separate.

LFOs and modulation


Our numerical LFOs will have an amplitude from 0 to 64k. But when using the LFO to modulate the pitch CV, to get a vibrato effect, we want it to increase the pitch for half the cycle and lower it for the other half. In essence, we would be better off using a signed int. In other cases, and for other operators, we are only interested in the positive parts of the waveform. And in any case the DACs would normally only output a single-polarity signal (though this can be fixed).

Again, I need to check how others are doing this.

Other things to think about when it comes to LFOs is that it should be possible to adjust their starting point and direction (up/down), as well as to choose whether or not to reset the cycle at the start of a note and to sync it to an external clock.

Multiplication vs attenuation/scaling - virtual VCAs


I've realized that we need an additional function, one that treats the inputs like they are between 0 and 1 when multiplying them. This is analogous to how a VCA works, when the CV is at max, the output is the same as the input. When the CV is half of its Max, the output is attentatet by 50% etc. In other words, the output is scaled depending on the CV, between 0 and 1 times the input.

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