[sdiy] MIDI velocity

[Richie Burnett]

Hi guys,

A few general questions about MIDI velocity sensitivity implementation,
(just for a DIY synth project.)

1. Is it reasonable to expect that I could use a low-end micro (e.g. PIC) to
scan a 61-key Fatar velocity sensitive keyboard with sufficient velocity
resolution to work well?  Or does it need dedicated hardware to measure the
time between top and bottom switch closures for every key in parallel
with sufficient resolution.  For instance the old E510 is quoted as having
128us resolution, and it sounds challenging to write a program to poll all
61 keys nearly 8000 times a second?  Obviously I can use a free-running
timer, and subtract time stamps obtained from the top and bottom switch
closures for each key, but it still sounds tight.  I couldn't find any
online examples of PIC based DIY velocity sensitive keyboards, and most
commercial keyboards seem to have an ASIC taking care of timing the switch
closures and reporting results to a general purpose micro to form the MIDI 
packets.

2. What does the MIDI velocity byte actually represent!? The old E510 chip
seems to contain a down-counter that starts at 127 when the top contacts
close, and counts down towards 1 (minimum allowed note-on velocity,)
stopping at whatever value it reaches when the key bottoms out and triggers
the bottom switch contact.  The value it stops at is the "velocity byte"
that's shoved in the note-on MIDI message.  But this is really a reversed
measurement of time, not velocity at all.  I would have thought that there
should be a reciprocal calculation done somewhere to convert from time to
velocity.  Is this method the commonly accepted way to implement velocity
sensitivity, or just a compromise because the reciprocal was too difficult
to do back then?  I couldn't find anything in the original MIDI
specification that describes what the velocity byte should actually
represent.  If it does actually represent time, then that's easier, but it
sounds wrong.

3. How should a receiving synth set it's voice's volume depending on
the velocity byte received?  I tried to think through the physics of someone
hitting a piano key with a specific velocity, what the energy is from
0.5mv^2 etc, and how that should influence the amplitude of the resulting
sound wave.  It seems that most synths either have a linear response or an 
x^2
response curve to velocity, but many offer a selection of different 
responses.

To add to my confusion one chip that I looked at from Dream semiconductor
mentioned a "logarithmic" timing method for measuring note on and note off 
velocity. Help!!!  Is it really just a case of everybody does it 
differently, and just go with what sounds good, or is there a definitive 
standard somewhere for how MIDI velocity *should* be implemented?

Thanks in advance for any comments and suggestions,

-Richie,