[sdiy] MIDI-CV convertor idea

[Roman Sowa]

Hi,
1. you don't need any trick with a cascade of inverting amplifiers or 
noninverting amplifiers. Why don't you reverse the CV in firmware and 
then feed into typical one stage inverting amplifier, which also will 
have limiting resistor inside feedback loop to eliminate errors causes 
by various/unknown load?
2. And even if you use non-inverting approach, which is perfectly fine, 
there is no bleedthrough because there can't bee. See the path of a 
signal. Whatever happens on channel one, cannot pass to channel two, 
because bend CV drives the line on the way with low impedance and does 
not alow for any other source to bother it.
3. don't attenuate any CV and then add 4x make up gain, you loos 
precision this way (think noise, offset and drift)
4. like others say, if you're using one DAC and mux (your description 
does not clarify if it's multi DAC or DAC-MUX approach), then you're 
updating channels anyway at a rate at least 200Hz per channel. And 
adding PitchBend to note data may take like 2us per channel? It gives 
you flexibility to split channels, so not all of them must be affected 
by PB. In your proposal you are limited by hardware to one MIDI channel 
notes-only mode.
5. PitchBend range is adjusted by RPN with insane precision and can go 
to several octaves, it's good to have the ability to respond to that RPN

Roman

W dniu 2020-10-25 o 14:27, Tom Wiltshire pisze:
> 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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