| Re; velocity sensing
Some of the others here can probably give a better technical
explanation, but the nuts and bolts of it are -
The synth measures the speed with which a key is hit. The
assumption is that when you are playing an acoustic piano, you hit
the key harder to make it louder. A consequence of hitting it louder
is that you also depress it faster.
When velocity sensing in linked to amplitude, you can control
how loud/soft the notes are by how hard/fast you hit the keys.
Without velocity sensing, you are basically just hitting
momentary contact switches (on/off). Like an organ.
Usually, velocity sensing systems have some kind of sensitivity
control, so that you have adjust the 'touch' to your style of playing.
- Karl
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| Further - the velocity information from the keyboard is encoded in the
MIDI "note on". Some keyboards also measure the velocity that you release
the key with, and this velocity data is encoded in the MIDI "note off"
message. The synthesizer can use the velocity data to influence any of
a number of parameters, e.g.:
overall volume
attack time of VCF or VCA envelope (note on)
release time of VCF or VCA envelope (note off)
The velocity data sent can be made a variety of functions of the actual
key velocity - e.g., it can be linear, or concave upwards (positive 2nd
derivative) or concave downward (negative 2nd derivative).
Finally, a keyboard can also send "aftertouch" data, which tells how hard
you are holding the key down, and which allows you to modulate the key
pressure for expressive purposes. This is less often seen than "velocity
sensing". The Roland JX-8P does both.
Synths that respond to velocity data are becoming increasingly popular,
especially as better MIDI composition software appears. For example,
the Roland TR707 and TR727 drum machines (synthesizer and sequencer
integrated into a single box) are velocity sensitive, although you can't
exploit that feature from the 707/727 itself - you need a velocity sensing
keyboard (or a PC based MIDI sequencer that allows you to control the
velocity data transmitted).
len.
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