Technical discussion of DSI synths and their DCO pop

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Re: Technical discussion of DSI synths and their DCO pop

Postby cornutt » Sat Jul 18, 2009 5:37 am

Stab Frenzy wrote:
masstronaut wrote:Either way this does not account for 24ms of glitch. So what the heck is going on with that?

The theory that's been suggested by a few people is that the processor sends out the pitch control information after it sends out the envelope information, so the envelope opens and you hear a brief bit of the oscs at the old pitch, before it clunks up to the new pitch.


The other possibility, since it happens on low notes, is that the DCO is only capable of changing pitch at the beginning of a cycle. Going back to what Jinsai wrote, 1/.024sec is 41.7 Hz, which IIRC is a low F. If you had a free running DCO that can only change frequency at the start of a cycle (perhaps because it reloads the timer on some kind of interrupt tied to the start of the cycle), and you played a low F and then jumped it to a high note, in the worst case it would take 24 ms to respond to the frequency change. However, it does seem like there would be odd effects with modulation if this were the case. But maybe not. I'll try to rig up a simulation in Csound.

I don't think the possibility of a Hammond-like key click should be discounted yet. I just went and played with the A100 for a few minutes, and although the key click has the same probability of occurring on any note, it is far less noticable on high notes. I'm not sure why this is. It could be due to high frequency rolloff occurring inside the Hammond. But it could also be (psychoaccoustic) back masking; the high frequency tone prevents your ear from registering the transient. If that's the case, it would appear to apply to the Evolver too.

EDIT: Just thought of something else concerning that last bit... if it's a Hammond-like key click, it should be noticable when playing a series of low notes. Is that the case? Or is it only noticed when jumping from a low note to a high note? If the latter, that would seem to discount the Hammond key click theory.
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Re: Technical discussion of DSI synths and their DCO pop

Postby Jabberwalky » Sat Jul 18, 2009 6:02 am

I'm interested to see if people with an Oberheim Matrix-6 have this pop as well. I can't remember noticing then, but it's been a few years and I didn't know synths very well.
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Re: Technical discussion of DSI synths and their DCO pop

Postby cornutt » Sat Jul 18, 2009 6:25 am

Jabberwalky wrote:I'm interested to see if people with an Oberheim Matrix-6 have this pop as well. I can't remember noticing then, but it's been a few years and I didn't know synths very well.


I'll try it tomorrow.
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Re: Technical discussion of DSI synths and their DCO pop

Postby masstronaut » Sat Jul 18, 2009 3:30 pm

So actually it looks like something similar happens when changing the pitch of an analogue osc using a modulation source.

This is OSC 1 playing a triangle wave being modulated by a square LFO. You can see the glitch at every transition from low to high where the level actually seems to drop for a cycle before quickly jumping to the new pitch.

Audio

http://www.mediafire.com/download.php?uzonyy1dmml

Pic

http://www.mediafire.com/imageview.php? ... o5&thumb=6
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Re: Technical discussion of DSI synths and their DCO pop

Postby nathanscribe » Sat Jul 18, 2009 5:12 pm

This is all very interesting... I'd never really noticed till it was mentioned, and now I'm noticing it all over the place. Huh. Ignorance is bliss, they say....

masstronaut, what was the mod source used? Is this glitch found using the sequencer, external MIDI control, etc? Does it appear on all Evolver models in the same way?

By way of ambiguous testing, I bashed out a few notes yesterday using one of the brassy presets, and found it was intermittent, with some transistions from low to high showing the glitch, others not.

Does velocity have an impact on the effect?
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Re: Technical discussion of DSI synths and their DCO pop

Postby masstronaut » Sat Jul 18, 2009 5:50 pm

The mod source I used was just an LFO. One oscillator playing with the VCA level full up.

Don't know how relevant the test is really but it's interesting. I did try the same thing with a digital oscillator and of course as you'd expect you can hear a small click when the pitch changes but it's not as dramatic as with the analogue oscillator. I must admit I didn't record and look at the waveform, probably should by way of comparison. Curious what happens there though, if you look at the picture linked in my post above you can see how it actually drops in level before going up to the new pitch. On the left you see the last cycle of the lower pitch, then the transition in the centre, followed by about 20 cycles at the higher pitch.

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Main version 3.0, DSP version 3.1
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Re: Technical discussion of DSI synths and their DCO pop

Postby Stab Frenzy » Sun Jul 19, 2009 6:24 am

masstronaut wrote:Curious what happens there though, if you look at the picture linked in my post above you can see how it actually drops in level before going up to the new pitch. On the left you see the last cycle of the lower pitch, then the transition in the centre, followed by about 20 cycles at the higher pitch.

There's no drop in level, the flat section is actually higher in level than the high pitched section.

Looking at that I'm seeing (from left to right):

Roughly one cycle of the low pitched waveform.
The beginning of another cycle of the low pitched waveform which then pitches up less than a quarter of the way in to the new higher frequency.
That waveform is completed at the higher frequency and then the osc hangs at fully down for about 14ms. The waveform curves up slightly due to DC blocking caps in either the synth or the audio interface.
After the pause the waveform resumes at the higher frequency as it should. Note there is a slight DC offset towards the top of the waveform (ie the waveform isn't symmetrical around the centre point, there is more wave above it than below) dues to the DC blocking caps recharging what was discharged during the flat section.

You can see that the offset starts to go back down again from about the 727ms region and I imagine it continues going down until the waveform is symmetrical again. You can't really tell just from this little section of the waveform if the frequency will be audible, but I think there's definitely a longer very low frequency component to the artifact than just the click from the square wave.

I'm gonna have to play around with the MEK to see if I can work out if it does this all the time or just for really big jumps in pitch. It would be interesting if someone with a Mopho could try the same thing and report on whether it does it too, seeing as there's so much stuff between the oscs and the output on the Evolver that you can't be certain that it's something happening in the oscs or something in the two A/D and two D/A stages after it, or the filter or the DSP.

We can rule out my theory about the envelopes opening before the pitch changes, or the VCA opening quickly and distortion the waveform.

masstronaut: Thanks heaps for posting the clip and the picture! I'm curious if the flat section happens at the bottom of the waveform at the end of the completion of the first partial cycle of the higher pitch every time, can you possibly post a couple more of the sections were it does it? If it did it at the same point every time regardless of pitch and waveform phase it would indicate more strongly that it was something to do with the oscs rather than another point in the signal path.
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Re: Technical discussion of DSI synths and their DCO pop

Postby masstronaut » Mon Jul 20, 2009 11:30 am

Well spotted that the flat section occurs after the first cycle at the new pitch.

I haven't tested it thoroughly yet but just briefly it appears to happen very consistently on every transition from low to high. If you look at the rest of the short clip I posted you'll see that the transitions are almost identical. The length of the flat section does seem to vary along with the two pitches and/or the interval between them, not sure of the exact relationship. Also not clear if a whole cycle at the new pitch is always completed before the osc hangs in all cases.

Basically someone needs to do some more testing. :P
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Re: Technical discussion of DSI synths and their DCO pop

Postby cornutt » Tue Jul 21, 2009 2:28 am

I tried it on a Juno-106 last night, and I didn't get this effect. I did notice there's something a little funny when the VCA closes, though... :shock:
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Re: Technical discussion of DSI synths and their DCO pop

Postby mute » Tue Jul 21, 2009 5:00 am

cornutt wrote:I tried it on a Juno-106 last night, and I didn't get this effect. I did notice there's something a little funny when the VCA closes, though... :shock:


find a dying 80017a chip? :lol: :x
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Re: Technical discussion of DSI synths and their DCO pop

Postby goldphinga » Tue Jul 21, 2009 11:50 am

Its a non issue for me. My voyager pops and clicks too. Ive never noticed it as an issue on my P08. Accept these 'flaws' as part of these instruments characters and dont be so analytical.
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Re: Technical discussion of DSI synths and their DCO pop

Postby masstronaut » Tue Jul 21, 2009 2:12 pm

Instead of trying to suggest what people should or should not be interested in analysing for their own reasons how about making some recordings of your Voyager or P08 so they can be compared?
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Re: Technical discussion of DSI synths and their DCO pop

Postby cornutt » Tue Jul 21, 2009 3:39 pm

mute wrote:
cornutt wrote:I tried it on a Juno-106 last night, and I didn't get this effect. I did notice there's something a little funny when the VCA closes, though... :shock:


find a dying 80017a chip? :lol: :x


Ouch... No, I don't think so. All the voices seem to do it (and no, the chorus wasn't on). When the VCA closes abruptly, there's something that kind of sounds like a filter that is near the edge of oscillation being hit with an impulse. It's pretty subtle. I'm wondering if it has something to do with the "hidden" VCA that controls the overall otuput level. I'm going to investigate it some more and I might start a new thread if I can make any sense of it.

In the meantime... I did a quick-n-dirty test with a Matrix-6R also, and I didn't hear the effect on that synth either. But there are some things that are odd about the way the DCOs on the 6R are clocked, so I need to try it again going through all six voices and both DCOs on each voice.
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Re: Technical discussion of DSI synths and their DCO pop

Postby adamstan » Tue Jul 21, 2009 5:56 pm

When the VCA closes abruptly, there's something that kind of sounds like a filter that is near the edge of oscillation being hit with an impulse.


Maybe there is some DC Offset.
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Re: Technical discussion of DSI synths and their DCO pop

Postby goldphinga » Tue Jul 21, 2009 7:58 pm

masstronaut wrote:Instead of trying to suggest what people should or should not be interested in analysing for their own reasons how about making some recordings of your Voyager or P08 so they can be compared?


No problem. Tell me what youd like to hear and ill make some samples. It was only a suggestion btw, hardly a big deal. All im saying is that if you want to analyse it the time could be better spent just playing and making some music.
Mind you, in that respect what the h**l am i doing wasting my time on here for then!!! :mrgreen:
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