[sdiy] MIDI-CV convertor idea

[Richie Burnett]

Another option would be to make the output stages differential amplifiers, and feed the note CV into the non-inverting input, and the pitch-bend CV into the inverting input, in order to combine them.

You can always invert the sense of the pitch-bend CV first in software, so that bend up becomes bend down, etc.

Just an idea. 

Interested to see other's suggestions.

-Richie, 

Sent from my Xperia SP on O2

---- 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.
>
>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
>
>
>
>
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