[sdiy] Frequency shifters, again

[jhaible at debitel.net]

> Since the filters involved are linear phase FIR types and since
> the unwanted sideband is suppressed with more then -100dB (before
> converting the floats to integer values for the .wav file)
> and since arbitrarily low frequency input signals can be processed
> this software implemetation of a frequency shifter could be
> called "ideal" or almost ideal. It just delivers a single sideband
> signal with full audio bandwith, no carrier feedthrough, no
> phase distortion, just some delay.

Excellent. If you can add some nice user interface (lots of pots),
you will have a really good FS!


> -mixing the original signal with the shiftet signal will not give
>  any interesting results. Only some amplitude modulation could be observed,
>  no interesting phasing pattern.

This is interesting.


> The absence of interesting phasing patterns makes me think
> about the role of the "dome filter" in analog FS.
> Does the pase shift which is created in that filter add
> some effects to mixing sounds? 
> If this is so (I think yes), a filter method SSB modulator
> would perhaps be not so desireable for music purposes.

I think yes, it's the dome filter. If you look at the "classic"
FS design (with dome filter), you can forget about the FS 
background completely: Just think of the two multipliers
and the quadrature LFO as a means to slowly crossfade
(or is it "vector-crossfade" ?) between two different phase
shifter channels. For a fixed position (sin of LFO = 0,
cos of LFO = 1), you just get a simple, static, phase shifted
signal. Same for the other extreme (sin = 1, cos = 0). 
Of course it is important to have the 90 degree phase
difference between the two all pass filters, in order to get
a true all pass filter response for intermediate values
(sin != 0, cos != 0) as well. But in the end it's the 
all pass filters that create the notches, when mixed with 
the straight input signal - not the modulation process.
The latter is just needed for the "right" interploation.

JH.

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