Thomas Henry Tunable Noise Source Question

[Scott Gravenhorst]

I should explain myself better.  Aren't you guys mind readers ?
(c;

Anyway, looking at the circuit, the pitch CV input gets mixed 
with noise before going to the expo converter.
This means that there is a modulation of the CV by the noise,
but because it's expo, the modulation effect should be the
same regardless of what octave the CV is selecting.  I can
easily eliminate the expo converter since, as you say, the 
566 is linear anyway, but I can't just mix a constant amount
of noise to get the "noise melody" effect.  What happens is
that the noise becomes a less significant portion of the 
control voltage as CV increases because of the response 
of the oscillator.  In this linear system, 
a noise voltage of say, 0.1 volt will have a much larger
effect when the CV is say 0.2 volts than when it is 6.
This is because it takes a doubling in CV to double the
frequency.  At 0.2v CV, adding an AC 0.1 volts of noise
means a modulation swing from 0.15 to 0.25, This is a ratio
of about 1.67 to 1, or more than one octave of modulation.  The
same noise signal mixed with a 6 volt CV would be a swing
of 5.95 to 6.05, a ratio of  1.0168 to 1, which is a few
cents of pitch modulation.  I thought that if I were to use
the pitch CV to drive the oscillator AND a VCA, the VCA 
could pump up the noise signal so that the ratio is constant 
as it would be for expo in the original circuit.

I was hoping for a simpler/more clever way to achieve this.  
I think this is very close to the issue of LFO modulating a 
pitch CV and that expo makes this simple, whereas with linear, 
other  techniques are used.  I'm going to play around with 
injecting noise into pin 5 (a limited FM input) of the 555 
IC in the FatMan's VCO.  I did this with an LFO and it worked 
pretty well having pretty much the same audible effect on 
pitch throughout it's range.  It may be easier in the long 
run to do it this way in a linear world and just use the 566 
as a plain VCO.

=?iso-8859-1?Q?Ren=E9?= Schmitz <uzs159 at uni-bonn.de> wrote:
>><Speculation>
>>
>>The 566 VCO has a linear response.... couldn't you just remove the expo=20
>>section of Tom Henry's circuit, and simply level-shift your linear CV to=
> the=20
>>appropriate range for the 566? (I don't know if it would need a resistor=20
>>divider or some type of amplification... and I don't have a V/Hz system to=
>=20
>>experiment with)
>
>I'd rather replace the expo current source with a linear one. This should
>give you a wider sweep range.=20


>></Speculation>
>
>Bye,
> Ren=E9
>
>
>--=20
>uzs159 at uni-bonn.de
>http://www.uni-bonn.de/~uzs159
>
>=20
>
>

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