[sdiy] Ring Mod (was Re: Hadamard Transform Network)

[mskala at ansuz.sooke.bc.ca]

On Tue, 16 May 2017, Tim Ressel wrote:
> But you bring up an interesting point: 4QMs multiply, but they produce x+y,
> x-y tones. Anyone got the math on that?

It's a basic trig identity:

(cos a)(cos b) = 1/2 [ cos (a+b) + cos (a-b) ]

If a and b are two different multiples of t (time), then cos a and cos b
are two sine waves of different frequencies, and then you end up with the
sum and difference frequencies.

One could prove this identity with the power series expansion for cos, if
necessary.

-- 
Matthew Skala
mskala at ansuz.sooke.bc.ca                 People before principles.
http://ansuz.sooke.bc.ca/