AW: vocoder freq. spacing

[Haible Juergen]

	Equal vs. unequal spacing:

	First of all, an equal spacing would be the *easiest* way
	to build a filter bank or vocoder, as it's easy to get 
	"equally spaced" components: You can use standard 
	E6 or E12 values. Calculation is then very easy. Of course
	you end up at very few (Paia: 8) or very many (Synton: 20)
	bands if you want to cover the whole audio range. So expect
	either very low-priced, or very high-prized vocoders to have
	equally spaced filter bands.

	It's absolutely right that the MAM vocoder - as well as the
	Roland vocoders - is specially tailored to speech 
	inteligibillity - so a number of 11, unequally distributed 
	bands are an optimum of excellent performance vs. 
	affordable price. Of course the building is not so easy
	as with equally spaced bands, anymore: You can't simply
	use E6 values and select them. The MAM solves this 
	problem the hard way, and the most excellent way: Each
	filter has its own trimpot and is hand-tuned (!) by their
	chief designer himself. (He won't let do this anybody else.)

	If we leave vocoder applications aside:

	Now, there's something general on equally spaced 
	filter banks: You have the choice to set the bands to
	musical intervalls that are fractions of an octave, or not.
	If you tune them in octaves, of fractions of an octave,
	and feed it with a sweeping oscillator, you will get
	a very uneven response: Every time your VCO's
	fundamental hits the resonance frequency of one band,
	many of the VCO's harmonics will hit exactly the resonance
	frequency of another band. So you have very strong 
	resonance in one moment, and almost nothing in the next 
	moment of your sweep. This behaviour may be considered
	pleasant or unpleasant, depending on your application.
	Different manufacturers took different different paths here.
	The EMS filter bank is tuned in Octaves, for example.
	The coresponding Serge Module (don't know the exact
	name) has its bands deliberately *not* set to an octave
	(was it a 7th ? - don't remember). So there would be
	a greater variety of hormonics hitting individual resonance
	frequencies.

	So, as usual, everything has its uses.

	JH.