[sdiy] Old Computers

[Johannes Öberg]

> >Anyway, from experience, if you are using an old computer like a 486,
> >you'll want DMA for the sound. LPT DAC won't sound very good,
> 
> Why?

I haven't tried doing exactly what you're suggesting, but it was
allways a problem with old trackers (Like Scream Tracker 2 that runs
through a Covox in an old 386 of a friend) that there were all sorts
of interrupts and stuff going off all the time, so you would get sound
skips if you used any sound device that had to be fed sample by
sample. With cache issues and stuff, the same code can take different
amounts of time to execute from pass to pass.

There are of course ways around this, but it gets more and more
complicated, especially if you are trying to squeeze out every last
cycle of an old 486 or something.
 
> I plan a DAC update rate of 
> 44KHz using the built in programmable interrupt timer.  This is a bit
> faster than the 31.25 KHz of the AVRSYN.

IIRC, you can't make the LPT run faster than 115 kbps, at least not
the original parallell port. That means 28khz max.
 
> The first cut will be very simple, certainly a mono-synth.  I will
> build it on a 5x86-133 which would seem to have between 2 and 4 times
> the power of the AVR used for AVRSYN.

Orangator 2.0 would run on that machine, so it certainly should work!

I'm don't know too much about the AVR, but I would suspect a 133 MHz
pentium is alot faster than 4 times the AVR with proper programming.
Especially since you'll likely will have enough memory to put many
parts of the synth engine into lookup tables.
 
> My OS will be DOS (I know, I know, but I ain't changing), it's
> something I have and I also have both C and assembler tools for DOS
> based programs.

I would also recommend DOS or a similiar non-operating-system :-) It's
the easiest way IMHO, especially regarding documentation, to get
low-level controll of the PC.

/J