[sdiy] super cheap synthisizers

Paul Maddox Paul.Maddox at Wavesynth.com
Wed Jun 12 22:58:31 CEST 2002


> Should I be doing something like this I'd probably plan on using 3 AVRs. 2
> of them for oscillators and one to control everything else. Including MIDI
> etc. The oscillators would be shaped outboard like korg once did it.
> Leaving the AVR to produce the raw clock and maybe stretch to PWM and even
> a little low speed FM. You just have to work within the limitations.

This is exactly what I did with the Monowave..
one 8515 controlling the thing
two 90s1200's , one for each oscillator, these drive a divider (4024) and
give me an 8 bit address for a waveform lookup.
The 8515 also writes an 8 bit value into a latch that then gives me a 16bit
value to drive an EPROM, holding 256 waveforms , each being 256byte wide
sample. The eproms Data outs drive a DAC...

> IF you need to store patches then you can use a tiny little I2C E2PROM.

agreed, the amount of EEPROM in the AVRs just isnt enough for this (512
bytes in the case of the 8515)
Though I use the EEPROM on the monowvae to store arp settings, midi channel,
retrig and so on.

> Of course your filter
> and VCAs are up to you. How complex or how simple you want to build them.
> Roman was telling me once about a quad VCA chip by Analog Devices. The
> controls are log but that probably doesn't matter if you wanna knock
> together a little 12 db State Variable Filter like the PAIA fatman or
> something. You'd have enough VCAs to provide cutoff, Q and one left over
> for your actual output VCA.

yep, I like the look of these chips..

> Envelopes and LFO. all done in software. If you want some kind of front
> panel you could use a 4th AVR just to deal with that. All hanging off the
> one I2C bus but you might find there are enough spare pins and horsepower
> to do that kind of thing with the same AVR.

There is ample headroom in an AVR running at 8Mhz to do front panel switch
scanning aswell..

> Of course there's an even easier way. Use a SID chip. You only have to
> drive that. And if your output stage is half way reasonable it won't be
> TOOO noisy.

hehe, but that takes all the fun out of it!!

Paul Maddox
"I've seen the light at the end of the tunnel, And its blue!'

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