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I understand why note on messages have velocities, but surely note off messages just turn them off - why do they also have a velocity?

Equally, I checked if these velocities match up to the corresponding note_on message, but they don't.

Is it a measure of how much the velocity decreases from the time you play the note to the time it stops being heard?

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Note-off velocity means the speed with which a pressed-down key is released back up. This information can be used for at least Rhodes electric piano sounds, so that the keys make a different release sound depending on if the keys are released softly or ”hard”. The difference in sound isn’t big, but there is a difference. An experienced player can notice the difference in sound, and loud key releases can be used for certain types of rhythmic “barking” comping.

If you read the MIDI Implementation Chart of instruments, you’ll notice that many, or most keyboards don’t send note-off velocity. Even the Nord Electro keyboards, which are supposed to specialize in accurate emulation of Rhodes elecric pianos and other comparable keyboard instruments. Some sample libraries have multiple layers of sampled note-off i.e. key release sounds, so by using a keyboard which doesn’t transmit note-off velocity you’ll miss out on that dimension of expression.

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    That's a new one on me! Thanks. Do you know if either Roland or Alesis use that feature? When I fire the studio up in a bit, I'll have a play! – Tim Mar 28 '20 at 8:16
  • @Tim - high-end keyboards are more likely to have it, or VSTis, as they don't have to rely on literal MIDI transmission so can pass data much quicker internally. I'd bet most DAWs would strip it & use running status unless told otherwise, so even if your keyboard sends it, it might not either pass-through or necessarily even be recorded. – Tetsujin Mar 28 '20 at 9:22
  • There's a good user-level explanation of Running Status here. I've always thought it was what confused people into believing certain sequencers were "tighter" than others. The other reason is that 99.999% of people never learned how to shift controller data out of the way, or slide slow attacks early to make this even tighter. – Tetsujin Mar 28 '20 at 9:49
  • @Tetsujin good point about Running Status that Note-Off velocity and the data reduction benefits of running status are kind of mutually exclusive, at least partly. But could that be a reason for a keyboard not sending note-off velocity? Anyway, sloppy MIDI timing can be partly the user's fault. Back when I used Cakewalk for DOS and external modules, I used to plan things like channel and note ordering, manually thin out CC data, and even re-order individual events to get important things out first... it kind of sucked but at least it taught me how hardware MIDI worked. – piiperi Reinstate Monica May 2 '20 at 9:51
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    Sloppy Midi timing is always the user's fault ;-) I spent the entirety of the 90s coaxing good performance out of barely adequate chips. I used to be responsible for most of the onboard & downloadable demos for Yamaha's keyboard & tone generator range. That teaches you how to keep your data stream lean ;) I was always rather proud of this one - soundcloud.com/graham-lee-15/… – Tetsujin May 2 '20 at 9:55
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The note off velocity is basically how "hard" you release the key. Like, if you let it up gently or suddenly. Some synthesizers/patches may use this information to change the release, cutting off the sound more gradually or suddenly.

Not all MIDI controllers will transmit this. Many will simply send a fixed velocity. Also, an annoying detail of the MIDI spec is that it's also valid to signal note off via a note on message with velocity zero. I just happen to have a MIDI processing app running in dev right now and the controller I'm using does just that.

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  • Note On = 0 is a running status message. You can, on some devices, tell it whether to use that or not. Running status streamlines transmission quite significantly by not having to send the status byte each time, like with discrete note-offs. – Tetsujin Mar 28 '20 at 9:18

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