another BFO idea (was:RE: Soft sync and medicine to cure it.)

[Haible Juergen]

	>A talked to HAM people the other day. They spend a lot of thought
	>for their local oscillator frequency planing, because avoiding
	>locking of two neareby tuned VFOs is virtually impossible.

Now that discarding the BFO concept in frequency shifters looks more
reasonable than ever (IMO), there is another application that beckons
just for the opposite: Ordinary audio VCOs with linear thru zero
capability (LTZVCOs)

Background: There are various LTZVCO circuits published (including my
own at http://www.synthfool.com/diy/hj2vco.gif - *not* recommended for
building)
which to my knowledge all suffer from "commutating inaccuracies" when
the modulation just approaches the zero point. The direction switch might be
missed or delayed, and you get a unpleasant artefact. (It's there in my
circuit
for sure, and I remember reading similar findings about the Electronotes
LTZVCOs.)
Now, one should get rid of these problems easily with a BFO. And the locking
around 0Hz would not be a problem, because the 0Hz point would normally
not be used permanently.

In short:

FS should be able to approach 0Hz slowly / permanently
-> locking is critical -> true TZVCO better than BFO

Synth VCO will approach 0Hz dynamically
-> delays and accuracy are critical -> BFO better than TZVCO


Anybody built a BFO for use as audio range synth VCO yet ?
The critical point would be fast switching in the VCO core,
i.e. high frequency tracking I guess ...

Anybody knows if the Bode FS's BFO is accurate enough to play
tuned melodies ?

JH.