Korg Poly800/EX800 Users

On Sat, 2009-01-10 at 04:03 -0600, Alec Jahn wrote:

> Then I ask, (again, noobness showing), what controls the clock in our
> 800s?  Something as simple as the FSB on a normal PC?

It's just a crystal.  There's nothing as complicated as the FSB in a PC
motherboard.  Put a higher frequency crystal in, and the CPU goes
faster.  Put a lower frequency crystal in, and it goes slower.

The problem comes about when you have timers controlled from the same
crystal, because they will clock correspondingly faster.  In the
Poly-800 the actual oscillators are generated by a set of programmable
dividers, so if you clocked these faster the pitch would be raised.  If
a timer was generating a periodic interrupt, these would happen faster,
so things like envelope updates would happen faster.  You could get
round this by programming the divider for a lower output rate.

> Also, could you just swap out the chip itself for a better one - a
> simple "upgrade"?  I wouldn't think it'd be that easy, yet I'm
> unfamiliar with these 'vintage' architectures in comparison to stuff
> from the last decade.

Well, if you look at any computer-controlled synth, there's generally
not a lot to it (let's leave the really serious digital behemoths out of
it for now).  In the Poly-800 you only need to program the dividers to
generate squarewaves, set a couple of analogue switches for the
square/saw outputs and chorus, and drive a DAC for the VCF controls.  It
wouldn't be hard to come up with a replacement CPU that would drop in to
replace the 8085, and feed the appropriate control signals to the
peripherals.  Well, I say "it wouldn't be hard", that's obviously
relative...

Gordon