[sdiy] MIDI-CV convertor idea

[Tom Wiltshire]

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.

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.

In the simulation this works perfectly. But I don’t quite believe it. I was expecting to see some limited bleed through on Output1 from the 0-4V pulse wave at NoteCV2 (not a realistic note CV, but a good stress test). There’s nothing, it’s clean as a whistle. Can anyone see anything I’ve forgotten or done wrong? Or should I believe the sim?

Thanks,
Tom




==================
       Electric Druid
Synth & Stompbox DIY
==================



-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20201025/e88d6c2f/attachment.htm>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: BleedthroughTest.png
Type: image/png
Size: 206575 bytes
Desc: not available
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20201025/e88d6c2f/attachment.png>