Who has OB-Xa ?

[Mutant]

Hi
I have a question and a small request if it is not too much trouble.

Listen to these high resonance sweeps and tell me if your unmodified OB-Xa can do the same logarithmic like attack shape ?

Yes ? No ?

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And maybe if you have time (only if you have it already connected and ready) you could record for me 2 low notes with settings like this:
VCO settings not very important, maybe just some noise if that is possible, i just need to hear/see the filter sweep.
Filter frequency: 0
Filter resonance: 100%
Filter ENV amount: enough for the resonance sweep to not go over 18-20Khz, but still fill a good part of the spectrum for us to see the attack shape on a spectrogram.

Filter ENV attack: long
Filter ENV decay: 0
Filter ENV sustain: 100% for 1 note and 0% for 2nd note
Filter ENV release: short

Asking this question because Aciddose in my thread has some doubts about OB-Xa specs and the shape of envelopes on VH-1984 and that maybe Van Halen's synth was modified in some way.
http://www.kvraudio.com/forum/viewtopic ... 4&start=30

[aciddose]

Actually if anyone does have this synthesizer, ideally we'd want:

Very high notes, as high as possible with the filter wide open and zero res.

amp attack: 0%, 50%, 75%, 90% and 100%
with decay 0, sustain 0, release 0, everything else neutral for a plain old square wave output.

This will provide a simple image of the envelope via the amplitude of the oscillator. Square is probably best.

With this we can high-pass filter the sample and take abs(), then the envelope is provided almost perfectly without needing any other tool.

The amp/filter envelopes are nearly identical in the schematic, in fact the amp envelope is ideal because it has higher output current and lower impedance, 12k rather than 16k for the filter envelope.

If my speculation is correct we should see a very linear curve at 0 attack, due to limited current available. At 50% we should see the specified asymptote of 130%. Somewhere in 75%, 95%, 100% we should see the asymptote drop to very near 100% and potentially a setting of 100% may never enter the decay stage.

Length is measured easily by looking for the initial trigger click at the beginning of the attack and the end where the decay stage occurs.

For 90% and 100%, if the envelope hangs out a long time near peak and doesn't enter the decay stage it would be awesome if we could see several samples of this. I'd expect the total length of the attack stage to be variable in a random way even while the attack control is at the same position.

Spec is 20-30s, I'd expect to see something like 25s average +/- 5 seconds or more.

[Mutant]

Won't the amp itself add its own character to the results more than the filter would ?

[penguinfromdeep]

I wish I had that synth ... I'm going to buy it one day I promise you!

[aciddose]

No, the amp/VCA shouldn't do much to the envelope shape, the result should be extremely linear. There is no exponential converter involved.

[Numanoid]

I have one, but I only use it as a doorstep

[aciddose]

Well we don't really need samples, we could take an owner's word for it. Just having someone to measure and confirm that the shape of the attack changes to more and more flat as the attack time is increased would verify that my speculation about the circuit is accurate. In which case this becomes an issue of creating a very specific "analog model" if you'd call it that, really just mimicking the same effect with a mod-matrix or similar.

I'm already certain the long attack times do not produce the 130% asymptote curve that is claimed in the datasheet for the CEM3310. I'm only unsure about what goes on across the scale, whether the asymptote changes and how it changes if so.