[sdiy] all analog FM synthesis?

[ASSI]

On Sonntag, 20. Februar 2005 03:02, Johannes Öberg wrote:
> I know little about
> VCO construction, but Phase Modulation the way it's done in the DX
> synths (which is really a modulated sinetable scanning routine) must
> be terribly difficult to implement in analog technology!

You'd need a quadrature VCO and a vector multiplier. Quadrature VCO are 
not too difficult to build at moderate extra expense over a "normal" 
one. The vector multiplier reduces to a dual VCA plus two mixers plus 
some signal bending at the two CV inputs if you also use the negated 
quadrature signals from the VCO (cos, sin, -cos, -sin) or if the 
quadrature VCO already has differential outputs. The CV signal bending 
is the hard part and involves folding the phase signal into wrapped 
quadrature phase signals and sin shaping of the result. If you do the 
folding with banks of biased differential pairs or OTA, it may be 
possible to incorporate a good enough "sin" shaping into the folder. 
OTOH, this particular part of the circuit may be quite a bit easier to 
implement via ADC / lookup table / DAC - that means a mixed signal uC. 
In this case, implementation would require for instance three CA3280, 
two MC1496 and an ADuC812 or similar.

> They sound quite different; FM tends to sound like disharmonic and
> "bad" wierd FX sounds, while PM sounds like electric pianos and
> metal-hit-with-a-stick-harps.

They really would sound the same as long as you do not chain 
"operators". To build sounds with higher order FM modulation chains, 
you would need to scale the operator input by f (for each of the 
frequency components involved). Even if you'd build a filter 
approximating that function, the sound would still be different due to 
the group delay.


Achim.
-- 
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