[sdiy] super cheap synthisizers

[harry]

Dominic Tarr wrote:

> there would be lots of ways around not having enough resolution in "sample" time, like making the first period slightly higher and the next one slighty shorter, so it works out even. I'd imagen that this would probably sound quite interesting too. <<< also as I expect to be using it mainly for bass so I don't think having only 20 steps to have a 4kHz note is much of an issue.

> oooh NOW you've got me started....   ;^P

I would NOT assume that you could make one pulse 'shorter' and one 'longer'...
if you did it evenly... it would sound like a pulse of 1/2 the frequency you intended...
with a non 50% duty cycle...

If it was a random 'short-long' it would sound like sh!t.  (well to be fair... I am known
for saying things sound like sh!t that some folks like...).  This would be 'jitter' in the
frequency and would not be a good feature.. OK to add it if you want 'that' sound..
but if you could not avoid it... really bad.

I'm with the 'one micro per voice' camp. I'm not a software expert... but I would
worry that if you had two voices from one micro with fractional detuning... they would tend to have problems as they became nearly out of phase with each other.

Some sample players have this effect. Trigger two instances of the same snare drum
on the Alesis D4... and they will flange at discrete intervals... NOT a continuous
spectrum.

OTOH I did say I'm not a software guy so there....

Now a neat, really low cost VCA for a single micro symth would be to synthesize
the envelope with a PWM output... and then chop that voltage with an analog switch
controlled by the VCO 'pulse' output. This was done on some monophonic early
Casio mini keyboards.  You can vary tone with different pulse widths... and the
envelope using DC level from the PWM out (filtered with a cap).

I've done this with a positive and and inverted envelope, and two analog switches...
the result is a bipolar wave that does not need a blocking cap to get rid of DC
offset.

> I guess I'm a bit of a newbie though, so please bear with me...
>
> does any one have some really simple designs for VCFs and VCAs lieing around?
>
> really it's the analog side of things I'm not sure about, like what would I need to do to bring the ttl level out puts up to line level (or what ever) for feeding it into filters?

TTL level is probably 10x the level you will need for a line level out, so a couple
of resistors will work. You will need to use a series capacitor to remove the DC
level or it will thump a lot...

H^) harry

>
>
> cheers - Dom

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