Frequency changing stuff...

[Haible_Juergen#Tel2743]

> Basically I need to get a full range audio signal, and increase it's 
frequency
> by two or three hz...
> err...
> My first thoughts, were get a computer to do it, but thats not what I 
want...

OK, what You want is a SSB (single side band) modulation with a
carrier of only a few hertz. This is possible with analogue technique
(constraints see below).
We all know the Ring Modulator which produces *two* sidebands,
one above and one below the carrier. Now we need a means to get
rid of that second band. As we have such a low carrier frequency, the
bands overlap heavily.
One possibility to remove the second band is *quadrature modulation*.
Just use two ring modulators, do a ringmodulation on the original
signals and on a 90-degree-phase-shifted carrier and signal.
As it is quite easy to produce a 0-degree/90-degree *carrier*,
this is not at all easy for a broad band signal (i.e. the audio input).
You need a *hilbert transformator* circuit - this is an all pass filter
of high order that approximates a phase shift of 90 degrees over the
whole bandwith. Now here is where the problems start.
It is not that difficult to build a good hilbert transformator for a
frequency range of 300Hz - 3kHz. Maybe an allpass of 6th or 8th
order will do (to be more exact: usually there are *two* allpass
filters on the audio path, one with a positive, and another with a
negative phase shift. Advantage: no need for a constant phase, but
"only" for a constand phase *difference* between the two paths).
I have not calculated the amount of circuitry necessary for a
20Hz - 20kHz signal, but it might be enormous.
BTW, the famous SERGE system has such a module, but it
is rather expensive even in comparison to other SERGE modules.
Does anybody know the specs of this one (Frequency Range,
Phase error)? Knox?

Hope this helped! If I'd build such a thing, I'd probably stick
to a limited frequency range to keep the whole thing handy.
Dealing with large-order filters and ultimate prcicion capacitors
is not much fun!

JH.