VCO CV input specifications
  • Hello. My Anushri kit arrived this morning, I have not started assembling it yet. Skipping over the build/debug/test/despair stages to the using and extending phase, I have a question about the input control voltage for the vco.

    I understand that it is 1 volt/octave (or slightly different if using software tuning, but I am talking here of the hardware tuning case to use with external equipment) and that, due to the circuitry running on a single rail +5V supply, the voltage is limited to 0..5V (or -0.5 .. 5.5V as I read elsewhere, those limits being the voltages which fry the MCU if exceeded).

    However, 1 volt/octave tells me the slope but not the intercept. What frequencies correspond to 0V and 5V? Or to put it another way, what voltage gives middle C or tuning A 440Hz?

    Also, looking at the schematic and then at the MCP4822 datasheet, it seems as if the voltage range from the DAC is 0*..2.048V or 0..4.096V so actually 1.024 V/octave over 4 octaves?

    I ask because I am also making a custom controller, most of which uses MIDI but one part of which is a continuous pitch controller. While that part can communicate with a computer over USB, I thought it would also be interesting to add an MCP4822 and give it a control voltage output to use with Anushri. So I need to know what Anushri expects here so I can generate that voltage.

    Thanks for any help with this. Sorry for a complicated first post.

    • strictly, 0.010V because the output op-amp in the DAC can’t get to exactly 0. And the op-amp can get to 4.960V with a 5.0V rail although how that helps when the DAC only gets to 4.096V is not clear to me.
  • > due to the circuitry running on a single rail +5V supply, the voltage is limited to 0..5V (or -0.5 .. 5.5V as I read elsewhere, those limits being the voltages which fry the MCU if exceeded).

    Stop! This is incorrect, any voltage (well, maybe not 220V) is accepted on Anushri’s CV inputs. You are mixing things up with the Shruthi-1, in which the “CV input” ports are indeed directly hitting the MCU ADC. Anushri’s CV inputs are not processed by any ADC, they go straight into circuit nodes. Since the CV input stages are just resistors going into virtual grounds, there’s no restriction on the voltage range of the input. It’ll happily get -15V to +15V signals.

    > What frequencies correspond to 0V and 5V? Or to put it another way, what voltage gives middle C or tuning A 440Hz?

    It depends on the setting dialed in on the VCO range setting of the front panel. One way to think of it is that the external CV input “transposes” the note played internally by MIDI or the sequencer (the internal CV produced by the DAC). So if you play a middle C on the keyboard, if all transposition and sequencing options are disabled, if the VCO range setting is set to 0 semitones, the DAC will output the right voltage corresponding to a middle C (261.xx Hz); and 0V on the CV input will give 261.xx Hz, 1V on the CV input will give 523;xx Hz and so on…

    > Also, looking at the schematic and then at the MCP4822 datasheet, it seems as if the voltage range from the DAC is 0*..2.048V or 0..4.096V so actually 1.024 V/octave over 4 octaves?

    The internal DAC does not go through the same path as the external CV input. It uses a 0.5V/Octave scale to make the most of the 4.096V range of the DAC (8.192 octaves instead of 4.096 octaves).

    So the core of your problem is which voltage to generate from your custom controller. Output a CV with a 1V/Oct scale (whether straight from a DAC or through a buffer / gain stage) and you will be fine :)

  • Thanks for the correction. OK so I can input -15 to +15V without the magic smoke escaping, but the range of voltages which produces different frequencies is still 0..5V ? Or can I get more octaves (the controler covers 7) with -1 .. 6V?

    Assuming the answer to that is “no, only 0..5V” is it possible to send signals in to the 0.5V/octave path to get the 7 octaves I would like? If so, where on the schematic would be a suitable entry point?

  • > but the range of voltages which produces different frequencies is still 0..5V

    No. I’m not sure why things would work this way. This is an analog VCO, nothing involving ADC.

    Keep in mind that the VCO tracks well over 5 octaves though – the -1V .. 0V ranges and the 5V .. 6V will start getting a bit off – but if you use a continuous pitch controller anyway you’ll correct by ear…

  • Aha ok. Sorry it took me a while to get it.

    The controller is an implementation of an ondes Martenot. Mine will have a five-octave keyboard (Fatar) plus the five octave ‘ruban’ and, like the original instrument, there is a +/- 1 octave switch giving an overall range of 7 octaves. Position is indicated by small depressions (white keys) and raised knobs (black keys).

    The keyboard and the CC’s from the various sliders go out by MIDI but the continuous controller doesn’t as MIDI can’t handle that.

  • I really like the ondes martenot. Hope to see a video on youtube of yours and your Anushri hitting it off in a few days! ;)

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