[sdiy] MIDI-CV convertor idea

[Gordonjcp]

On Sun, Oct 25, 2020 at 01:27:43PM +0000, Tom Wiltshire wrote:
> Hi All,
> 
> I’d like a sanity check on something I’m working on, please.
> 
> I’m designing a polyphonic MIDI-CV convertor. There are various ways this could be done, but I chose to generate the Note CVs and the Pitchbend CV separately, and then add them together. This means that the Note CV only needs to be updated when a new note arrives for that voice, and the Pitchbend CV can be updated much more frequently and will affect all voices. Otherwise every time the pitchbend alters, *all* the voices need to be updated.
> 

Doesn't that scheme kind of imply that you've got a separate DAC for each note, *and* ptichbend, and anything else?

It'd be simpler to use a single DAC and a multiplexer with a capacitor and buffer on the output, as per just about every analogue polysynth :-)

Since you're updating the mux quickly the sample-and-holds formed by the mux, capacitor and buffer don't have time to droop and there's no noticeable delay in the output.

No-one's ever called the Juno 106 slow or laggy but it only updates at around 240Hz, so any note has a good 4-5ms "latency" just before the VCA starts to crank itself open.

> Ok, so that’s the idea. Since I don’t want the Note CVs to get inverted, I either need to use an inverting mixer followed by another inverting op-amp to flip the voltage the right way up again, or I have to use a passive mix followed by some make-up gain. I decided to try the second way to save the extra op-amp.
> 
> HOWEVER…a passive mix doesn’t use a virtual ground node, so there’s the potential for bleed through from one channel to another. So I did a quick simulation of the situation to see if this was a problem, shown attached. U1 takes a 0-4V signal from the Bend DAC and turns it into a +/-83mV signal. This is then fed to both mixers. The other mix input comes from the appropriate Note DAC, also 0-4V. Each mixer is then followed by x4 make-up gain, which compensates fro the /2 loss in the mixer and adds an extra x2 to turn the Note CV range into 0-8V and the pitch bend range into the standard +/-2 semitones.

Why not just use a virtual earth mixer, and invert the CV going into it?  Just calculate 5V = C0 and 0V = C5...

-- 
Gordonjcp