Any guesses, or formulas welcome.
If you multiply two sources in a time domain on DSP it will be equals to a frequency shift in a frequency domain.
For example:
cos(2πf
1t) ∙ cos(2πf
2t) = 0.5 cos[2π(f
2 − f
1)t] + 0.5 cos[2π(f
2 + f
1)t]
where
f1 - frequency of source 1
f2 - frequency of source 2
you can use it to get the difference between two source.
The main issue here is that we have sum of two components f2-f1 and f2+f1 and they are mirrored from a 0 Hz, because there is no negative frequency in real world (no way to see the direction of vector rotation in quadrature if you have just a single projection on the axis) and negative frequencies are folded on positive.
This is why you can listen mirror channels on the receiver. But you can avoid such frequency mirrors agains 0 Hz border if you use multiplication for a signals represented in a quadrature because it allows to keep information about negative frequencies.
Read this article for details:
https://www.dsprelated.com/showarticle/192.php