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Thread: Has anyone implimented a decent control voltage conversion yet?

  1. #1

    Has anyone implimented a decent control voltage conversion yet?

    I have been trying to setup a decent control voltage conversion into and out of microcontrollers on and off for a while now. Getting a decent and responsive signal out to audio modules and other systems has become a bit of a white whale for me. I can get a signal out of a PWM with a low pass filter but this isn't exactly as accurate as it should be. I figure I'm going to have to dive into a decent op-amp managed setup and though I should ask around before I re-invent the proverbial wheel. I doubt that reading an input is going to be much of an issue and haven't worried too much about that but getting a smooth and reliable output CV with sharp cut leading and trailing edges is a bit of a stunt. I envision reading the signal through a shmitt trigger and then gating the original low passed filter result through a 4016 or 4066. Any thoughts out there and be sure to call me an idiot if it is constructive.

  2. #2
    It depends on if you want simple CV gate outputs, or precise CV for V/Oct. tracking. For the former you can simply amplify any digital output signal to the desired voltage with an op-amp like a TL072. The latter will require a DAC for any reasonable accuracy. The DACs on the Teensy 3.x will suffice for most cases, if not then use an external DAC. Be aware that a DAC will also typically require output stage circuitry to scale the voltage correctly, and will likely need to be digitally compensated (calibrated) for good tracking across multiple octaves. Take a look at the work done by mxmxmx in the ornament and crime and temps utile eurorack modules for more info.

  3. #3
    Quote Originally Posted by recursinging View Post
    It depends on if you want simple CV gate outputs, or precise CV for V/Oct. tracking. For the former you can simply amplify any digital output signal to the desired voltage with an op-amp like a TL072. The latter will require a DAC for any reasonable accuracy. The DACs on the Teensy 3.x will suffice for most cases, if not then use an external DAC. Be aware that a DAC will also typically require output stage circuitry to scale the voltage correctly, and will likely need to be digitally compensated (calibrated) for good tracking across multiple octaves. Take a look at the work done by mxmxmx in the ornament and crime and temps utile eurorack modules for more info.
    Yep! I'm setting up a MIDI to CV converter for an analog modular system (with 3 or four Teensy based modules thrown in) so I think I'll just have to go for a DAC tacked onto a cheap and nasty STM32 Blue Pill. I wouldn't want to waste a Teensy on that but since I'm intending to make Eurorack shields for Teensys I'll still be stuck with the problem there.

    Have you heard of any 16 bit serial DAC chips out there? A quick web search isn't coming up with anything other than PWM converters. There are 12 bit devices out there but that probably won't give me the detail to render a decent scale with only 32 positions around each note, assuming a full MIDI scale of 128. At a pinch I could probably drop that down to a range covering maybe 80 or 90 notes but still that isn't much detail to fine tune with. There must be a reliable 16bit serial dac somewhere. It's a bit of an essential item. As for calibration I'd be doing it inside the code which again needs the detail of 16 bits to give the leeway.

  4. #4
    Thank you for this thread, I really learned a lot

  5. #5
    Senior Member PaulStoffregen's Avatar
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    Quote Originally Posted by RogerD View Post
    Have you heard of any 16 bit serial DAC chips out there?
    ....
    There must be a reliable 16bit serial dac somewhere. It's a bit of an essential item.
    The Ornament & Crime module uses TI's DAC8565. That chip is a little on the spendy side, but doesn't disappoint on specs.

  6. #6
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    Mutable Instruments has also open sourced their Yarns midi interface: https://github.com/pichenettes/euror...e/master/yarns

    I haven't poked around the design yet but it could provide some additional hints or ideas.

  7. #7
    Quote Originally Posted by Orhema View Post
    Thank you for this thread, I really learned a lot
    You're welcome. It was surprising how little there was out there about control voltages but since it is basically an old analog synth consideration it isn't that much of a surprise really. Sheesh! Kids today just don't know their born!!

  8. #8
    Quote Originally Posted by PaulStoffregen View Post
    The Ornament & Crime module uses TI's DAC8565. That chip is a little on the spendy side, but doesn't disappoint on specs.
    Thanks for that Paul. I wound up using an AD420 which should do the job for the 16bit for a couple of bucks more than the DAC8565, er..bit and a couple of 12bit units I got from China for next to nothing. No loss though. The DAC8565 will be well worth it for other parts of the system and will be a far simpler option than the design I came up with in the end. 4 channels on the single chip could make some very interesting things happen.

  9. #9
    Quote Originally Posted by wcalvert View Post
    Mutable Instruments has also open sourced their Yarns midi interface: https://github.com/pichenettes/euror...e/master/yarns

    I haven't poked around the design yet but it could provide some additional hints or ideas.
    Thanks for the heads up. That's my weekend taken care of.

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