[sdiy] frequency shifters...

[jhaible]

> With frequency mod, a sound and it's harmonics are raised or lowered by an
> equal amount, dependant on the amplitude of the signal doing the
> controlling, basically a varying voltage. [...]

That's "pitch shifting".

>  With frequency shifting, all the fundemental and harmonicsof any given
> sound are shifted [...] by an equal frequency,

Yes. It's SSB (single sideband modulation), which is related to AM.

Now, if the local oscillator of the frequency shifter (whose frequency
determines the amound of shift) is frequency modulated by an (external)
oscillator, you can indeed get something like FM. Not sure if the spectrum
is identical with ordinary FM (such as in FM radio), but my first guess
would be that it is.

Examples (both for frequency shifter) :

(1) Local oscillator @ 100Hz (fixed) -> audio program shifted by 100Hz.
      This is SSB modulation.

(2) Local oscillator @ 0Hz, FM of Local Oscillator with external sine wave
      VCO @ 100Hz -> audio program "FM'ed" with 100Hz. (?)
      The Local Oscillator must have thru-zero capabillity for this, of
course.)

JH.