[sdiy] FM

[Grant Richter]

To get a taste what a frequency shifter sounds like, just take a 24 db/oct
lopass filter and set the cutoff to the same frequency as a sine wave
oscillator. Use the sine wave as a carrier into a balanced modulator and
filter the output with the filter. To the best of it's ability, the filter
will remove the upper side band and leave the lower single side band.

A true frequency shifter is designed to minimize the amplitude of the
unwanted side band by elaborate phase cancellation.

I haven't actually heard anything musical done with a frequency shifter. I
have serviced the Bode units and had a chance to play with them, but they
essentially render any sound "clangorous" by changing the harmonic ratios.
(IMHO) If there is some great musical quality about frequency shifters, I
fail to understand it.

It was probably once useful for "robotic" voices in the same way as vocoders
were once popular for radio station IDs.

Can anyone point out a recorded example of a frequency shifter used
musically?

>> 
>> if a ring mod gives me a signal output which is sig1+sig2 and Sig1-sig2,
>> could I not filter off the difference and just keep the sum, this wouldnt be
>> TRUE FM I know, but it ought to sound pretty close, shouldn't it?
> 
> No. You'd get an SSB-modulation (a variant of the Amplitude
> Modulation), but that's about it.
>