AW: Through-zero FM of audio signals (was: Module mania!!!)

[Haible Juergen]

	>Great idea.  Could you do this with a phase shifter?  I know
that my
	>Voodoo Vibe (a real primative phase shifter) does a pretty nice
vibrato;
	>maybe a voltage-controlled phase shifter could produce a type
of FM if
	>only the phase-shifted signal is heard. 

Yes, vibrato works great with a phaser, if you disable the direct
signal. It also works with a flanger / chorus delay line. Or take
the Hammond vibrato scanner: You have the choice between
chorus (mix of direct and delayed signal) and vibrato (delayed
signal only). 
This surely modulates the signal above and below its
original frequency, but it only works dynamically with periodic
modulation signals, and it is actually pitch shifting, not
frequency shifting. (But I don't know if you hear the difference with
very small shift (?) )

	> How about a frequency shifter (i.e. the Electronotes design)?

I have finally started to build mine. The 90-degree filter
for the input signals already works. It's the EN design, with
measured capacitors and individually calculated resistor
values to match the capacitors' tolerance. No trimming required,
and I get a perfect circle on the oscilloscope over the whole
audio frequency range.
Well, this was the easy part. The difficult one lays still ahead.
Don't expect results too soon - I won't have much time for
soldering in the next 2 months.
What I plan is using a modified version of the Bode design
for the oscillators (They are cleverly designed to cancel out
the temperature drift between fixed and variable oscillator.
The EN one is different here!), and RC4200's for the multipliers.

JH.