[sdiy] 2-Channel Frequency shifting

[jhaible at debitel.net]

This is just an idea at the moment.

Has anybody tried a setup with two frequency shifters,
where a signal is shifted up, then processed by some
fixed filter, and then shifted down by the same amount
as it was shifted up?

I could imagine that there would be nice unusual VCF-like
effects if you modulate your up- and downshift (both
tracking each other), because:

a) normally fixed filters can be more complex than
   VCFs. Think of a whole Moog filterbank that is swept.
   Only that the sweeping is done via the signal, while
   the filters stay in place.

b) the up/donshift process itself would alter some
   parameters of the filter. As the shifting is a 
   linear process, and the filter slopes are dB/Oct,
   I would expect the filter slopes to be changed
   alongside with the center frequencies as well. (?)

Now such a setup would be difficult with two Frequency 
Shifters: Two FS are expensive, and you must have them
tracking each other.

So hear is the idea:

Build a 2-channel Frequency shifter. Two Dome filters,
Four Ring Modulators, but only a single Quadrature
Oscillator.

The Quadrature oscillator is used for both channels.
If you set one channel to "upshift" and the other one
to "downshift", you can build a chain with 2 FS
and a filter (bank) in between, and the pitch at the
end of the chain would be preserved in any case.

Also, such a device can also be used for "ordinary"
frequency shifting - but in true stereo, with linked
modulation, but independent L and R channels.

Interesting?

JH.






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