[sdiy] square to sine

[rburnett at richieburnett.co.uk]

Hi David,

If you're going to use 2164 VCAs to get the expo conformity then I'd 
definitely go with a sinewave oscillator from the outset.  It's crazy 
messing about with shaping squarewaves or tracking filters if you don't 
have to.

The quality of the sinewave output is going to be much better if you 
can start with a half-decent sinewave oscillator, instead of trying to 
strip away the harmonics from a squarewave.  Those lightning fast edges 
of the squarewave will do their best to couple onto all signals on your 
PCB and find their way to polute the sinewave output with nasty 
high-order harmonics!

If you design a state-variable filter such that it is just slightly 
unstable to start with, and then add some level-dependent damping 
(feedback from the BP output,) you should get a very pure sinewave 
output from the LP output.  This is because any tiny amount of clipping 
that happens to control the amplitude is done at the front end, and this 
is double integrated before it arrives at the LP output.  The two lots 
of integration do a good job of attenuating what little distortion is 
introduced during any soft clipping.

That's the route I would take now, knowing a little more about your 
intentions.

-Richie,