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I was reading a review of the Akai Miniak which said

The 37 half-weighted keys are velocity and aftertouch but not pressure sensitive.

I thought aftertouch was MIDI signals of continuing pressure levels on a keyboard-key after the initial striking of the key?

I understand this is usually per-keyboard (max over all keys held down) rather than per-key.

Can someone explain the difference between "aftertouch" and "pressure-sensitive" in the review phrase? How can you have aftertouch without pressure-sensitivity?


Edit:

The way I read it, the reviewer mentions three distinct attributes of the keyboard

  • Velocity (sensitive) and
  • Aftertouch (sensitive) but not
  • Pressure sensitive

However, from Alex's answer and other subsequent research about MIDI messages, It seems that one of these three attributes must be just an alternative name for one of the others.

The Akai web page says it is velocity sensitive but makes no mention of aftertouch (or of pressure-sensitive).

So is this just an error in the review?


Edit2

The Miniak manual says

Modulation sources
- Aftertch: Polyphonic (per-note) Aftertouch, MIDI only
- Pressure: Monophonic (per-MIDI Channel) Aftertouch, MIDI only

So I guess the Miniak's keyboard is Velocity-sensitive (AKA pressure of initial impact) but does not have Aftertouch (sometimes AKA pressure-sensitivity post impact) but that you can plug into it's MIDI-In port an expensive MIDI keyboard controller that does has Aftertouch and the miniak's synth engine can use it.

I conclude the reviewer made a poor job of expressing this.

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I would have assumed aftertouch and pressure meant the same thing. –  Matthew Read Dec 15 '11 at 16:54
    
Yes, the reviewer was not using the terms correctly. After-touch and pressure-sensitivity are the same thing. –  Wheat Williams Sep 18 '12 at 21:45

2 Answers 2

up vote 2 down vote accepted

I know this is an old question but I think I can clear up the confusion here:

Two Entities: Keyboard and Sound Engine

You have to distinguish between two things:

1) The capability of a (here: Miniak's) keyboard to react to key velocity and pressure (aka Aftertouch) and generate internal modulation sources and MIDI messages from these.

2) The capability of the Miniak's sound engine to be modulated by the velocity in note on/off messages and Aftertouch messages received by either MIDI or from its own keyboard.

Definitions

  • Velocity is the speed with which a key is pressed down (note-on velocity) or with which the finger is lifted (note-off velocity). In the MIDI protocol this is expressed as the second data byte to note-on/off messages.

  • Aftertouch comes in two flavours:

    • monophonic aka Mono Pressure: applies globally to all notes played at the same time with the same amount. In the MIDI protocol this is expressed as the status byte 0xDn with one data byte (the amount).
    • polyphonic aka Poly Pressure: can be applied to each individual note. In the MIDI protocol this is expressed as the status byte 0xAn with two data bytes (the note number and the amount).

Keyboard

The Miniak's keyboard generates:

  • note-on velocity and
  • note-off velocity (not many keyboards do!)

... but does not generate mono- or polyphonic Aftertouch. Polyphonic Aftertouch is generated by very few of all keyboard models ever produced.

Sound Engine

The Miniak's sound engine can use all of these as modulation sources:

  • note-on velocity
  • note-off velocity (called VelociUp in the list of modulation sources)
  • mono pressure (called Pressure in the list of modulation sources)
  • poly pressure (called Aftertch in the list of modulation sources)

Mono and poly pressure will originate from incoming MIDI messages (or the second modwheel, see below), since, as stated above, the keyboard does not generate these.

Additional notes

  • The second modwheel of the Miniak generates mono pressure messages (internally and over MIDI) and any modulation whose amount is controlled by the second modwheel will also be controlled by incoming mono pressure MIDI messages.
  • @Alex Coplan - pressure sensitivity certainly does not mean velocity sensitivity, this usage is just plain wrong.
  • I also disagree with the notion that Aftertouch isn't very useful. It is elemental if you want to play expressive lead lines with just one hand without having to use a second hand for the modwheel.
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Thanks for this, Chris. So you disagree that "aftertouch isn't very useful" but... how COMMON is it? I mean, how much expectation is there that I will offer poly-pressure on a keyboard MIDI controller? It seems like the answer is "none at all." Right? Thanks! –  Yar Nov 16 '12 at 15:27
    
Usually, when somebody says just "Aftertouch", without qualification, they mean "Mono Pressure", and that's what I meant in my last note as well. Most good Masterkeyboards and many top-level keyboards generate Mono Pressure. Like I said, only very few keyboards generate Poly Pressure, but quite a few synths can react to incoming Poly Pressure MIDI data. So Poly Pressure can be useful as well but you don't expect to find it often in keyboards. Though the recent emergence of new types of MIDI controllers (iPad, Eigenharp, DIY projects, etc.) may change things... –  Chris Arndt Dec 10 '12 at 0:16
    
Thanks for responding, Chris. How do you expect Poly Pressure to be generated from the keys on a keyboard? As the user applies different amounts of pressure after the initial strike, there are poly pressure messages being sent out with each change in pressure? –  Yar Dec 11 '12 at 0:36
    
Yes, that's what I would expect. I have yet to come accross a keyboard that actually generates poly aftertouch, though. Here's a (prob. incomplete) list of keyboard models I know of, which can generate poly aftertouch. I haven't used any of these myself. VAX77; Kurzweil: MIDIBoard; Roland: A80/50; Ensoniq: SQ-80, VFX(-SD), ASR-10, TS-10; Sequential Circuits: Prophet-T8; Yamaha: DX1; SynthAxe; GEM: S Series workstations; Elka: MK88 Master Controller –  Chris Arndt Jan 14 '13 at 16:38

Aftertouch is when you have played a note, you're holding it, and then you vary the pressure whilst holding it (effectively bending the key).

Pressure-sensitive Velocity Sensitive is when the keyboard will record a different MIDI velocity depending on how hard you initially hit the key.

Apparently pressure sensitivity, is the force with which the note is held down after the initial impact. - Although presure sensitivity is loosely associated with velocity sensitivity.

See this wiki article.

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The article said "velocity and aftertouch but not pressure sensitive" - If "pressure-sensitive" = midi-velocity how can it be veolicity sensitive but not pressure sensitive? I'm still confused –  RedGrittyBrick Dec 15 '11 at 15:53
    
@RedGrittyBrick just looked it up on wikipedia - many people will loosely called velocity sensitivity pressure sensitivity... including myself - I thought they were one and the same, but obviously not... I'm pretty sure aftertouch is what I said it is though –  Alex Coplan Dec 15 '11 at 15:59
    
Thanks Alex, so is the reviewer writing nonsense when they list three things rather than two? Perhaps it is an editing error or something? –  RedGrittyBrick Dec 15 '11 at 16:24
    
@RedGrittyBrick - I think that they didn't need to mention that it didn't have "pressure sensitivity" - you only really need velocity sensitivity, aftertouch is nice but not hugely useful –  Alex Coplan Dec 15 '11 at 16:27

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