Bob Ammerman wrote:
>
> Velocity sensitive keyboards often work by timing the time difference
> between two contact closures, no A/D needed.
>
> I'm guessing that each key ends up closing two contacts sequentially as it
> is pressed, and that the keys are arranged in some sort of matrix that can
> be scanned.
>
> Bob Ammerman

Most of the good midi keyboards sense velocity and aftertouch,
this is well advertised with music keyboards.

Some I have seen use two opto interruptors per key, velocity
is determined by the time between the two opto beams being
broken. The first opto is usually only digital, the second
is a crude analogue system. When the key is held down it
is against a strong spring (usually a rubber pad) that can
be compressed and this affects the second opto beam to give
more or less light, giving analogue aftertouch.

They actually gives pretty good pressure sensing, some sounds
have pitch change on aftertouch, so you can tremelo, or
volume change on aftertouch for vibrato. I have owned a
couple of these keyboards, pretty cool technology. :o)
-Roman

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