more frequency tracking..

[Scott Gravenhorst]

On Wed, 17 Jun 1998 16:24:44 -0700 (PDT), eduni at ucsd.edu (Ethan Duni)
wrote:

>>If you are using a pll, the control circuit itself already provides the
>>necessary frequency dependent voltage, by driving the loop vco.
>-i should have remembered that.. i'm new to PLL's.. at what speed will the
>phase comparator (generally?) add noticable distortion? does anyone know of
>a model that is a good compromise between speed and accuracy? as long as the
>speed is on the order of single milliseconds or so, it should be acceptible
>(no?); does there exist a phase comparator/PLL that can do that without
>distortion?  (or is the distortion introduced as a result of the changes in
>the VCO CV?)

Not sure what you mean by distortion.  The PLL will output a VCO
output signal and a VCO control signal.  For a CD4046, the VCO output
is square.  It is also possible to extract a trianglular wave from it.
The lag time (or portamento) depends on the design of the loop filter.
Mine has a pot for filter damping so the portamento introduced is
controllable.  I can get it down to practically inaudible for
approximately a 1 octave change.  Changing pitch over a range of say 5
octaves is certain to produce some audio artifact.  I think it's
really important to remember that this characteristic of a PLL
(portamento) can be an artistic asset.  I like to mix the PLL output
with the input, set the portmento on the PLL to slow and then play
alot of pitch changes.  It has a wierd dijerydoo kind of sound, but
more synthy...  You can also do other tricks like generation of
harmonics through frequency multiplication.  My PLL module generates
5ths and octaves.  A total of 7 outputs.  I used a divide by twelve
counter in the feedback loop for this.

-- Scott Gravenhorst
-- FatMan Site: www.teklab.com/~chordman