CV voltage somewhere for note on/off?

edited August 2009 in WTPA1 Mods
I'm planning all these mods before even getting my WTPA heheh!

I'm thinking of adding a filter with resonance to the output of the WTPA. I've made up a simple filter module that can take about +5V CV input to open the filter fully (can be adjusted for other voltage though). Is there a point on the WTPA board that, when you press and hold the play button, or via MIDI, play a note and hold it, where that point generates a DC voltage, and then stops when you release the note? For example, a note-on indicator LED, or logic signal point? I could then use this as a CV gate signal to pass to an ADSR shaper to control the filter unit each time a note is pressed.

Any ideas? Maybe the processor can be programmed to output this out of its spare comm port?

Cheers, graham


  • Yeah, you could program it to use the spare comm pins.

    Take a look at this thread -- This addresses the reason behind why you wouldn't simply be able to use the "play" LED -- it will only be high when you're looking at the proper bank.

    However it might be a "feature" to only filter the bank you've got selected... ??
  • OK, sorry, on second thought that thread isn't -THAT- relevant... hopefully you get the idea tho
  • Yes it is - it explained a lot - for a slightly different purpose, but it tells me what can be done with it. This is a good start.
    It would probably be advantageous to separate the 2 bank status signals in this case, for exactly the reason you speculated.

    Thanks for the heads up!

    Cheers, graham
  • So the _REALLY_ classy way to do that would be by using the spare comm port to control another DAC like this one:

    A dual 12-bit DAC in an 8-pin package for $3.60. Not bad -- two CVs!
    You could even control your ADSR directly from WTPA then, or send whatever periodic CV you wanted.

  • or again,

    here's a board that's already populated with everything you need for $10...

    Of course, if you wanted +/- 12v CV or whatever, you'd need to do some more work with opamps & supply voltages
  • edited August 2009
    Thanks, Todd and smrl - that is very interesting! So Todd, are you saying that you can simply code the output of the WTPA comm port to make the DAC shape its own output voltage as an ADSR envelope for the filter board directly? If so, that would save me making the whole intermediate ADSR shaping circuit!

    So how many outputs does the WTPA comm port have? If it has more than one, does that mean I could also do the same for a VCA circuit as well as the filter, and have a different envelope on the filter to the VCA?

    If it only has one, how hard would it be to implement more comm ports in the next WTPA board revision? I can see this sort of function taking off with some people. You shouldn't have got me started heheh!

    Cheers, Graham
  • @gmeredith --
    Yup. ADSR, gates, LFOs, you name it. Digital is a slippery slope :-)

    The WTPA's "comm port" is really just a connection to the second UART on the AVR which can also be configured as an SPI module. Depending on the peripherals you are using and the control scheme, you can control lots of things with just one port.

    For instance, with the part SMRL and I both recommended you could make 2 independent control voltages. With this one:
    You could make 8, with 16-bit resolution! All using one port.

    So as of now, there's no need to expand the comm port.

  • Cool! Now you've got me thinking about way too many things to do with this! So I guess this is where the WTPA meets modular analog synth blocks really, you could control various effects or modules, depending on how you ported it. Great forethought, Todd!!

    Cheers, graham
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