[sdiy] freq shift allp filter ?

[Fernando de Izuzquiza]

     Hi all, and happy new year.

     I'd like to build a frequency shifter (phase type) made out of various modules (quadrature
     osc, 90deg allpass filter and two ring mods), better than a large circuit.
     I'm not looking for linearity or precision, just one more musical tool within my modular
     syntheziser.

     I have the two ring modulators (Roman Sowa design, AD633 based) and I see there are some
     quadrature oscilators on the net (some simpler than others). But I have many questions
     about the filter...
     At first, I thought that using a cell from a phaser would just yield the 90 degrees
     shifted signal, but I readed that it will not shift the same amount for all freqs... Then
     I understood that what I need is a short delay to shift the signal 1/4 of it's period, so
     a delay capable of very short delay times was the solution... but, as far as I can
     imagine, this will only work for one frequency...

     So, dear masters, what kind of "90deg constant phase shift filter" is needed for this kind
     of freq shifting?
     And, very important (I'm not an EE), there is any circuit ready to build out there?

     (for those not familiar (ough! am I?) the circuit I'm talking about: input signal to
     filter / filter 0deg out to RMa / filter 90deg out to RMb / QOSC sin out to RMa / QOSC cos
     out to RMb / RMa and RMb out are the up and down shifted signals, QOSC freq is the shift
     amount)

     Thank you for your time and patience,

     Fernando


     f
     *
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