[sdiy] "Synth design questions" or "Learning from Dave Smith"

[Eric Brombaugh]

Tom Wiltshire wrote:
> 
> 1) Should the main control processor talk to the voice board digitally 
> or using control voltages?

<snip>

> I'm currently wondering about using a SPI 
> connection instead.

How about CAN? :)

No, really! It's a fairly fast 2-wire bus and a lot of the MCUs out 
there these days support it. You could go crazy and build a whole 
network with auto-discovery, etc.

Probably too complicated from a software standpoint, but the hardware 
would actually be simpler.

For a simple point-to-point connection in a fixed configuration system 
I'm with you on SPI though.

> 2) Should the front panel of a programmable synth use pots or rotary 
> encoders?

Pots are cheap and have built-in feedback. Most MCUs have ADCs with 
enough muxed channels and sufficient resolution to read a handful of 
pots to greater accuracy than you can set them.

Cheap encoders are more expensive than cheap pots and are harder to 
interface - the software requirements for polling and keeping track of 
the current state is more complex due to the debounce and quadrature 
decoding. For the reasons you describe, you can't really use encoders 
without some kind of feedback, and a ring of LEDs for for each of 64 
encoders will get complicated quickly. And the resolution sucks.

The place where encoders win over pots is when you're going to control a 
whole lot of parameters through a few knobs and you have an interface 
that gives a lot of detail about the settings (ie an LCD).

Just my opinion... YMMV.

Eric