Hypothetically add Velocity to MIDI keyboard with single analog input?

Question

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...?

dougw · Accepted Answer

This is possible, but some issues will need to be addressed. If the switches bounce, which is typical of mechanical switches, it will play havoc with recognizing a discrete state. he key to decent performance will be how well you can debounce the switches.

The first VCR I bought used this method on its wired remote controller. I bought it at a scratch-and-dent sale so it didn't come with a remote, but I sent away for the schematics (this was before the internet) and deduced that it used a resistor coding scheme (the remote was not included in the schematics). I built my own remote using a dozen of the cheapest switches I could find and a custom resistor network. It worked pretty well, but sometimes it would seem like the wrong button was pushed because the bounce in the switches would look like additional resistance.

I have also built a MIDI input device with 64 inputs. It used 1 multiplexed analog channel per input to obtain the note, the velocity and the duration. I use piezo sensors with special signal conditioning, but there were issues with crosstalk when I cranked up the sensitivity.

dougw · Answer

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.

dougw · Answer

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.

Hypothetically add Velocity to MIDI keyboard with single analog input?

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