From: mrk@gvgspd.GVG.TEK.COM (Michael R. Kesti)
Subject: Re: Polyphonic aftertouch
Date: 31 Dec 90 05:26:25 GMT

Aftertouch, sometimes called pressure, is a MIDI control that is typically
created by keyboards that sense how hard the user continues to press the
keys after the initial presses.  It differs from velocity sensing in that
the user can alter the pressure while holding the note(s), whereas velocity
is determined, and transmitted, once per key press.  Aftertouch messages
are sent anytime that a change in pressure greater than some threshold is
detected.

There are two types of aftertouch: channel and polyphonic.  With channel
aftertouch, the instantaneous pressure value is the average (or sometimes
greatest) of the pressure of all keys currently pressed.  Polyphonic
aftertouch provides the means to transmit a unique pressure for each key.
You can see that polyphonic aftertouch has a greater potential for adding
to the MIDI clutter we have recently discussed.

The aftertouch functions are allocated unique status bytes.  The message
formats are:

	polyphonic aftertouch: 1010nnnn
			       0kkkkkkk
			       0ppppppp

	   channel aftertouch: 1101nnnn
			       0ppppppp

		where nnnn = channel number
		   kkkkkkk = key number
		   ppppppp = pressure

>> My Roland U-220 supports it, but my Yamaha V50 doesn't seem to support it.

This means that if your controller (the V50) were capable of generating
aftertouch, your U-220 would be able to use this data to modulate the
sounds being generated.  Note that auxiliary controllers, such as the J.L.
Cooper Fadermaster are capable of generating aftertouch (channel only, in
this case) and may therefore be used to provide realtime modulation control
of your U-220.

Finally, note that keyboard controllers might have neither velocity nor
pressure sensing, either velocity or pressure sensing, or velocity and
pressure sensing, although I am not aware of any that feature pressure
sensing without velocity sensing.
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Michael Kesti  Grass Valley Group, Inc. | "And like, one and one don't make
    mrk@gvgspd.GVG.TEK.COM              |  two, one and one make one."
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