[sdiy] D50 a VA or not? with pictures of sine table osc TVF emulation.

karl dalen dalenkarl at yahoo.se
Mon Dec 27 18:18:57 CET 2010


> Juergen Haible <jhaible at jhaible.de>
>That's what I conclusded from the description in the user manual. No idea >how accurate or inaccurate it is - does it fit to what you found out, >somehow?

Well, that's what i previously tried to describe!:)

I stick to my initial idea, somewhat refined, a first sine lockup table
trough a comparator to create PWM/Square or pulse as you put it if PW
changed, then add that with the second sine table osc who are phase locked
with square edges shaped by an amp envelope who triggers on square edges
to create the imaginary expo resonance curve then multiply and shift/phase
whatever to create the double freq saw, align the two amplitudes square
and sine during cycles to emulate the closing of the imaginary TVF.

> For an approximated filter function on a pulse wave, you
> don't need a real filter. You just need the step response of
> the filter you want to emulate (lookup table, surely), and
> trigger it with each upward and each downward slope of the
> ("filter incoming") slope of the oscillator.

How would that dullify a square during closing?
Some sort of crossfade between the two tables seams to
be a must , i cant see otherwise in this particular application!?
What's a step response table? Same as a sine table osc?
With slope you meant rising and falling flanks of the square?

> This only works on a single oscillator, of course - thta's
> why wach oscillator has it's own filter in the D50.

Actually it works on multiple waves, i did some emulations,
here are some bad screen dumps but you can see whats happening
anyway.  Two squares 1 octave apart:
http://img153.imageshack.us/i/res1c.jpg/

It's just a matter of align the sine table osc to the
highest frequency generated.

> That "filter" step response table method won't work on 
>saw waves, either. 

Well it does, here on two saws mixed and the sine table to
emulate as described earlier, one octave apart with resonance:
http://img574.imageshack.us/i/res2g.jpg/

And here the two saws 2 octaves apart:
http://img690.imageshack.us/i/res3u.jpg/

I downloaded the manual and at page 35 it says by changing
PW saw octave doubles so that confirms the Square-Sine Mul
part i suppose.

>Ezion said:
>However the 'filter' on the D50 _is_ available when the ringmod is used.
>The filter is placed before the ringmod.

The manual are quite crappy but it seams to confirm what you say.

> As for "VA": It predates the terminus (or at least the
> marketing use of the terminus), so it's an anachronism. But
> from it's purpose, it clearly was intended to give an
> emulation of an analog synth (plus added features like
> attack PCMs), so if it's not a virtual analog, I wouldn't
> know what else to call it.

To easy marketing ploy of relabeling a simple table readout
machine as terminus VA!:) Besides i think it was a engineering
thing, they didnt have enoug computing power to do real nice
sounding filters back in 1985-86 based on that the S550 had
filters that sounded like shit. Linear arithmetic! 

What if the S550 didn't have filters but this wierdo table emulation!:)

Reg
KD









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