[sdiy] Communications with voices in a polyphonic synth

[Stewart Pye]

Hi Tom,


>
>
> I've already asked about SPI after encountering problems with it. It 
> seems to me that some of the problems that I'm having are because it 
> isn't really designed for single-master/multiple-slave communications 
> over distances of a couple of feet. I've used it to talk to DACs and 
> such like, which is fine, because you've got a single master and 
> single slave and they're more or less next to each other (distances of 
> a few centimetres, not tens of centimetres).
Did you try using a line driver such as the 74AC244 on the output of the 
master and placing a termination resistor on the receiving end? I would 
have thought this would get you a few feet without problems. You could 
split it into two or three groups if you had to as well.

>
> Protocol-wise, I need to be able to send parameter and control 
> information to single voices or groups of voices. How is this 
> typically done? I can see easily enough how I can give each voice an 
> address, and then send an address message ahead of a data message (or 
> group of messages, I suppose), but how do I talk to a couple of voices 
> without repeating the message for each?

Each voice has its own unique address. You also have group addresses. 
One of the group addresses is "all". Typically all voices always listen 
to the "all " address commands. But you could also dynamically assign 
voices to groups...
Say you want a two part polyphonic synth: Send a message to each of the 
two voices you want to use to say " Hey, you with the address of x now 
belong to the group with the address of y, so you better listen to 
commands for address y as well as your own address." This way you can 
send parameter changes like envelope control to the group, but send note 
information separately (or as a group if you want a fat monosynth).


Cheers,
Stewart.