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Thrashbarg
January 27th, 2007, 02:56 PM
I love analog synth music and I wanted to make some of my own. I got my 8080 out and wired up the YM2149 to my breadboard and started mucking about with passive filters and transistors.

Some of the sounds I got out of a basic square wave are simply amazing.

The YM2149 (aka AY-3-8910) has the three channels it supports attached to three separate pins, so each channel can be attached to some filters and get a different sound.

One trick I discovered was to have a high value capacitor (say 10uF) attached via a diode to the input, with the negative on ground, so it acts as a filter and changes its voltage according to the volume of the input tone. Then through another diode, I attach it to the base of a transistor in the filter stage which changes its bias and causes it to filter out different portions of the tone. This creates different sounds according to the volume of the input wave. As the note's decreasing over time the sound of the note at the output changes.

Anyway, the result is this

http://kaput.homeunix.org/~thrashbarg/fugue_shaped.mp3

This is the best sound so far. Unfortunately I lost it because of the nature of analog electronics. I adjusted the dials a bit and it's gone.

I know this isn't pure analog electronics. It's a hybrid, controlled by digital and filtered by analog.

carlsson
January 27th, 2007, 03:26 PM
Cool! Would a such custom circuit be possible to do in terms of off-shelf ICs, perhaps software programmable? Considering how common the AY/YM series of sound chips were (and I'm sure a similar trick can be applied to other chips as well), it would've been neat if any manufacturer had added this bit of extra circuitry if the sound chip itself didn't come with filters. As it was, the SID was the only home computer sound chip I'm aware of that offered filters as well as waveforms and many other features, at least until the late 1980's.

Thrashbarg
January 27th, 2007, 04:39 PM
If I got a circuit for a decent Voltage Controlled filter it could be software programmable. I prefer to have the potentiometers at my disposal though. This does mean that if I find a good sound and fiddle with it I'll loose it.

Still, it would be fun to make an analog synthesizer card out of real analog components and have it software controllable.