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Hypothetically add Velocity to MIDI keyboard with single analog input?

ernek

I’m very new to electronics (still working on the Arduino Starter kit projects). My goal is to build a midi keyboard with velocity sensitivity. In every consumer design I’ve seen, this is done with two switches—with a slightly different travel distance—for every key by measuring the time between their closings.

All of these switches are read as distinct inputs with the help of multiplexing. It seemed to me that it might be redundant that both switches in each pair needs to be distinguished by individual key. I thought about this for a while, and thought about a way where the first switches would only tell the controller that a new key has been pressed, without revealing which one.

This might be done by wiring all of the first switches in parallel; each with its own resistor in series. The number of switches activated (likely <10) could be mapped to the current flowing to a single analog input. When this number rises by 1, a key has been pressed. The velocity and pitch could then be determined by the second switch.

I have no idea if this is feasible, or even if it would save more resources than a normal full diode matrix with double the amount of switches...?

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  • 0

    How would it work with chords? I'm not a musician, but don't musicians hold down keys for sustain, and then press more keys?

  • 0 in reply to shabaz

    Chords work fine as long as the multiplexer is fast enough to see everything. All notes that get played get passed along in real time. Sustain is handled differently depending on the system. For example on an electronic organ you can make it hold the note as long as you keep it depressed, or you can make it decay like a piano. Suppose you have your inputs set up as drum pads but you want the output to sound like a horn - you may need to do a drum roll which would get interpreted as a sustained note, which is fine because a horn can only play one note at a time and it can't do a drum roll.

    A real piano has a pedal to allow or kill sustain - a similar function can be implemented on a MIDI system.

  • 0 in reply to dougw

    Hi Doug,

     

    With the switches effectively OR'd together (which is what I believe Ernie is suggesting by paralleling them), then it will not be possible to know the velocity of any key other than the first key that is pressed, because subsequent switched won't be able to close that OR'd switch circuit because it is already closed while the first key is held down.

    For a normal chord maybe it won't matter if the musician wishes the keyboard to assume that all keys are intended to be simultaneously struck with the same velocity, but I just don't know (since I'm not a musician) if there is a requirement to have sustain while pressing additional keys. If there is, then OR'ing won't allow velocity determination for those subsequent keys. I could be talking rubbish! : ) I'm not very knowledgeable in music.

  • 0 in reply to shabaz

    Good point Shabaz. That is sort of true, but it is possible to have a unique voltage result from any unique combination of keys. The number of keys that can be managed depends on the accuracy of your A/D. For example suppose you have a 12 bit A/D and 12 buttons. If each button corresponds to a bit in the A/D, the most significant button causes a 50% change in voltage and on down the line until the least significant button only causes 1/4096 change in voltage. Each button always causes a unique change in voltage that is different from all others.

    It does get tricky when just using resistors to implement (you need a spreadsheet to figure out all 4096 combinations) - it would be a lot easier to deal with constant current sinks or sources.

    I think MIDI sustain would normally apply equally to all keys pressed at the same time, (because it is convenient) but variability here might lend uniqueness to a musician or a performance.

    Multiple simultaneous velocity is definitely a challenge with this scheme.

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  • 0 in reply to shabaz

    Good point Shabaz. That is sort of true, but it is possible to have a unique voltage result from any unique combination of keys. The number of keys that can be managed depends on the accuracy of your A/D. For example suppose you have a 12 bit A/D and 12 buttons. If each button corresponds to a bit in the A/D, the most significant button causes a 50% change in voltage and on down the line until the least significant button only causes 1/4096 change in voltage. Each button always causes a unique change in voltage that is different from all others.

    It does get tricky when just using resistors to implement (you need a spreadsheet to figure out all 4096 combinations) - it would be a lot easier to deal with constant current sinks or sources.

    I think MIDI sustain would normally apply equally to all keys pressed at the same time, (because it is convenient) but variability here might lend uniqueness to a musician or a performance.

    Multiple simultaneous velocity is definitely a challenge with this scheme.

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