understanding some terms and theory (re wavetables)

[Paul Maddox]

Tony/All,

>
>   I'd imagine that it'd depend on the design.  For example there are a 
>few designs that use a VCO to clock the wavetable.  In this form, the 
VCO 
>drives an A/D converter, meaning there are as many samples as the bit 
>resolution of the converter.  So, 8-bit A/D, 256 samples.  Neat and 
>compact but not very practical for polyphonic machines unless you want 
a 
>hybrid analog/digital VCO scheme (and who wouldn't?!).

OUCH!!, Not a D2A, think, saw tooth going in and divirng d2a is ok ata 
fixed frequency, but as the frequency changes so does the amplitude of 
the sawtooth (if you have one that doesn't patent it quick!) and 
therefore the scaeling of the d2a needs to change.
I used a 4024, divider.. pulses go in at 128 times the output frequency 
and this drives the bottom 7 bits of my eprom with the tables on.

Why is it complex? the HARD bit is getting a stable VCO, if you use DCO 
(PPG/WALDORRF) then its no problem, simply change the output of the DCO 
accordignly.. admittedly the PPG used a clever oscilator, but its just a 
variation on a DCO.

Take a look at my webpage and download modulus 6, it has a eprom 
playback circuit, this eprom has the original waves from the ppg 2.2 
(127 of them). Im testing a high speed VCO (yes, 1v/oct) at the moment 
if it works, you have a hybrid.. BTW, this WT module has sync in, CV for 
wave selecition and 2 7-segment displays.

>   I have no idea how PPG/Waldorf did/does theirs.  Probably very 
similar 
>to standard sampling technology?
>

errrmm, nope...
what they do on top of this is interoplate (in real time it would seem) 
from one waveform to the next so you only have two waves forms, a start 
and an end on the eprom.
BTW, Im developing a module to do this from two WT modules :-)

>   Tony
>

Paul (see told you I was nuts about Wavetables) Maddox



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