<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><div class="">Hi All,</div><div class=""><br class=""></div><div class="">I’d like a sanity check on something I’m working on, please.</div><div class=""><br class=""></div><div class="">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.</div><div class=""><br class=""></div><div class="">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.</div><div class=""><br class=""></div><div class="">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.</div><div class=""><br class=""></div><div class="">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?</div><div class=""><br class=""></div><div class="">Thanks,</div><div class="">Tom</div><div class=""><br class=""></div><div class=""><br class=""></div><img apple-inline="yes" id="2763CA7C-35BF-42E5-8B7D-8D2A6F09664C" width="857" height="856" src="cid:AD59A743-C3BF-4129-9CB5-05F21D71BA53" class=""><br class=""><br class=""><div class="">
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