[sdiy] square to sine

[David G Dixon]

> the surrounding circuit Dave is talking about will amplify 
> the error in sine purity, so it's rather important that it's 
> not a "crappy" sine.

I've actually been very cagey about our plans for these sine generators, but
yeah, sine purity is an issue.  In my design, I will have one spare 2164
VCA, so I might just set up a 6dB VCF for some downstream filtering of the
shaped sine.