David Van Horn wrote:

>>Is it something like two frequencies together produce frequencies
> 
> which
> 
>>are both the sum and the difference between the two inputs?  And they
> 
> go
> 
>>through a low pass filter to get the final tone? Like tuning a guitar
> 
> the
> 
>>closer you get to tuning two strings the faster the beat gets?  Or is
> 
> that
> 
>>how it might be done now?
> 
> 
> For getting the pitch itself, you use a pair of oscillators, like
> 700kHz-ish.  They sit maybe 200 Hz apart at idle more or less. So the
> mix products would be 700200, and 200 hz.

Uhh, shouldn't that be 1400200Hz, and 200Hz?
Sum and difference. 200Hz apart means 700khz and 700.2kHz as f1 & f2.

The wide difference in sum and difference products is what makes the
filtering easy. The high carrier frequency makes it more sensitive
to capacitive loading, so the higher your base frequency the wider
the pitch range for a given delta C.

I rather like the idea of an optical thermin.
They'd be just as hard to play, but would be easier for the
digital geek to build.

Ranging could be as simple as a modulated IR source with
a tuned amplifier on the photodetector so that you get a linearish
response to intensity vs hand distance. (1/R^2). Then simple math
to scale to a linear note number times 12th root of 2 note intervals.

You could even cheat and have the PIC 'fix' your bad pitch just
as modern DSPs do for today's not so good singers.


Robert

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