[sdiy] D50 a VA or not?

[Juergen Haible]

I'm not a digital guy (and even less a guru), but I thought I'd understood 
that clever TVF in the D50 working like this:
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.
This only works on a single oscillator, of course - thta's why wach 
oscillator has it's own filter in the D50. (Clever marketing gives 2 filters 
for Single voices, or 4 filters for Dual voices, as an added benefit.)
There are no saw waves either - they look like a sine multiplied with the 
pulse waveform. That "filter" step response table method won't work on saw 
waves, either. So I think what they did is performing the "Filter" function 
on the pulse waveform *first*, and then multiply it with the sine.
Also fitting in here: No filter function availavle when Ring modulator is 
used. They use up the same multiplier for ringmod, that would otherwise be 
used for the "filter" on saw waves.
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?

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.

JH.



----- Original Message ----- 
From: "karl dalen" <dalenkarl at yahoo.se>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Monday, December 27, 2010 4:24 AM
Subject: [sdiy] D50 a VA or not?


Little are known to the public on how the D50 works, so i had this brief
conversation with a former Roland R/D manager on that item i asked as a
shot in the dark if it was just table reading for the osc's and some
kind of simple ASIC DSP for the filter TVF.

He replied that the D50 was a real VA based upon clever table manipulation
to simulate the filter effects, that was also the reason PCM samples could
not go trough the table process (TVF). (Presuming the entire wave would be
screwed up!)

So the TVF in the D50 appears not to be a filter at all, rater relabeled
to set the user into believing it was! So assuming the following (might be 
wrong or right i don't know) the D50 has two cyclic waves, Pwm and Saw, the
Pwm are created out of a sine lookup table, Xor'ing or something then goes
trough this TVF thing, then the Pwm are multiplied and shifted with the
Sine to create a double freq SAW and that's it? Since the Saw are flipped
up so are also the Pwm's lower edge resonance emulation and everything
looks neat and tidy.

So i did some initial tests, i took 2 oscillators one Square and one Sine
2x- freq and phase locked to the Square front edge who also triggered an
AD who shaped the amp of the Sine to get the log resonance curve and then
simply added these two and then swept the freq of the Sine and sure it
sounds exactly like a mid open filtered Square with resonance.

I assume some correlation between the amplitudes of the Square and Sine
during play time to satisfy the emulation of a closing filter effect
must also be in place somehow, or some phase shifting thing to get
Sine dominat in amplitude over square during lowering of center
frequency to emulate a closing filter?

So a partial in the D50 could be made up of one table osc creating
the actual tone and one sine table osc to create the resonance/filter?

1:So digital guru's what do you think about that D50 thing?

2:And what really defines à VA instrument? I had thought a mirco running
a algorithm that simulates analog functions was regarded VA, i.e computes
the function, not by fetching data from predefined tables, or similarly?


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