[sdiy] MIDI velocity

[Hugh Blemings]

Hiya,

Have to say - one of the most engaging threads I've seen for some time 
on any list :)

On 7/04/2016 08:21, Richie Burnett wrote:
>> What are some examples of commercial keyboards with ASIC solutions?
>
> I just looked at a few old Yamaha and Roland service manuals, like the
> D10 / D50 / D70 and alpha Juno 2 for instance.  The keyboard and MIDI
> interface is managed by an 8-bit micro, but there is always a "gate
> array" with a small (2KByte) SRAM tagged on to it that actually drives
> and senses the velocity sensitive keyboard matrix.  I don't know how
> much of an active part the gate array and the tiny RAM chip play in
> measuring the key velocities though. Maybe the gate array is purely for
> I/O address decoding and glue logic, or even just to provide sufficient
> drive current to charge and discharge the capacitances of the switch
> matrix!  I really don't know.

Yes - I have a Roland AX-1 and it uses a curious (to me) combo of a 
custom ASIC for scanning and a mask ROM 68HC11 for everything else.

So I did a bit of poking at a KX-76 a little while back and wrote about 
it here http://hugh.blemings.id.au/2015/09/26/keyboard-tinkering/

It uses the main MCU for scanning, about 1kHz update rate and appeared 
to adopt the stop scanning until key closure detected approach discussed 
elsewhere in the thread.

Two meta-things that to me come out of this thread;

Seems a common to have to solve problem - would be nice to see at least 
a basic Free software licensed implementation of this out there - 
perhaps under one of the more permissive licences so it could still be 
built into an otherwise proprietary firmware.

It may have already drifted by the list but I draw folks attention to 
this project - VAX-MIDI - it's essentially a crowd funded and Open 
version of the VAX-77 controller of some time ago.  Noteworthy perhaps 
is their method of sensing key movement - optical - it allows fairly 
trivial implementation of both velocity and polyphonic aftertouch.

http://www.vaxmidi.com/

(Not affiliated with them other than some correspondence over the years 
with the very approachable chap behind the project)

And one other thing - I've always thought it would be nice to have a 
controller that gives you the option of normal velocity sensing, or top 
of travel first motion fixed velocity for use with organ patches (where 
traditional hammonds tend to have a lighter and quicker touch than a 
synth in part due to the way the drawbar/key contacts are done)  Or a 
mode that gives you both - CH1 normal velocity, CH2 - more instantaneous 
organ style fixed :)

But I digress, again.

Cheers,
Hugh