[sdiy] VCLFO and VCADSR details available online

[Tom Wiltshire]

On 16 Dec 2006, at 21:35, Nicholas Gregorich wrote:
> I finally got a chance to look at your PIC projects in detail.  Thank 
> you for sharing, it gets me interested in doing something similar with 
> AVRs.  I have a decent amount of embedded systems experience so it 
> makes sense for me to start on a crossover synth-uC project.
>
> Anyhow I was curious on your choice to include an output level control 
> on your LFO project, especially in the digital domain.

The reason was that it makes it much easier to add into a programmable 
system, since it saves you a VCA. I agree that with a 10-bit output, 
reducing the amplitude and hence signal to noise ratio still further 
does seem a bit odd, but in practice it actually seems to work well.
I was originally trying to copy the CEM chips (and the Env Gen isn't a 
bad copy) and they were intended for programmable systems and include 
Level CV inputs.

I've actually realised another way to add a level control to the 
outputs, which is sort-of analogue, sort-of digital. I suddenly 
remembered something I'd seen on the Juno106 schematic, where they use 
it to control the sub-oscillator level coming out of the waveshapers. 
They just use a transistor with a control voltage fed to the collector, 
and the PWM fed to the base. The circuit has the advantage of letting 
you bring the output up to a higher level without an amp, and the 
disadvantage of inverting everything, though this is easily dealt with 
in the PIC software.

Tom

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