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Old 8th December 2011, 06:44 PM   #1331
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I think jitter ought to be indicated as a delta of measured variance against fixed/known peak-peak/trough-trough spacing (where it is on the time scale). My preference would be to feed an overlap of disonant frequencies (26Hz, 490Hz, 2.2kHz, 9.13kHz, 14.5kHz) in a waveform (which gives us defined spacing) then measured against the actual output result. "Ethan's feed" would be a 30 second sample and a fixed figure output of average delta referencing this test, which would indicate the frequency blend and duration.

Even a simple sine wave would be a good start. That would give us a variance that can be shown in delta @ freq. The higher the frequqncy, the higher the delta. It would be easy on my mind. Also the disonant frequencies would repeatedly produce peak-peak spreads in the supersonic range which would be interesting to see as a brutal test of a device's performance. I could envision output figures like Δj = 0.0023% @ 10kHz. I don't think this would particularly be relevant because I'm under the impression that jitter is a fixed timing error without regard to frequency. Let me see if I could come up with a better system.

We audio people like simple scales referenced to a frequency. Jitter should also be presented in an easy format.

The problem is: where do you get a waveform analysis such as I described, where the time deviance is marked and compared? Anybody know?

I've got it! The delta could could also be similar to how the color guys determine delta-E, a distance on the CIELAB color chart. In this case, it would be a cumulative (of absolute values) shift per second. It might also be good to see what the maximum spread is. I can envision output figures like Δj = 13s/S, 0.6s. I think multiple frequencies would still be required in the test to really show what's going on, but I may be wrong about that.

Last edited by ethanolson; 8th December 2011 at 07:02 PM. Reason: I'm awesomer now than when I first posted this.
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Old 8th December 2011, 07:12 PM   #1332
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Informitive jitter-related posts made while I was composing mine. Also the in my post needs to be something else because it looks too much like a margin of error.

I would assume that if we had a percentage deviation scale we'd probably want it below the same thresholds we set for distortion & noise, such as below 0.05% and really aiming for below 0.003%. Perhaps this could be added to THD+N and give a result called CD+N (cumulative distortion + noise).

OK, if you haven't figured it out by now, I'm under the impression that there's no standard established and I'm trying to form one. If there is something in place, let me know. I can't find one that's solid enough to publish, but then my search skills aren't the best.

Last edited by ethanolson; 8th December 2011 at 07:37 PM.
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Old 8th December 2011, 07:48 PM   #1333
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hmm, i actually dont like your idea, jitter is simply a word (and an overused one) used to describe a numbre of related effects that conspire to disrupt how the signal frequency and phase are related (that was hard, my apologies) . i dont think its poignant to try to reduce it to a percentage and i dont think its useful to think of it as fixed

Last edited by qusp; 8th December 2011 at 07:50 PM.
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Old 8th December 2011, 07:50 PM   #1334
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Quote:
Originally Posted by qusp View Post
hmm, i actually dont like your idea, jitter is simply a word (and an overused one) used to describe a numbre of related effects that conspire to disrupt phase. i dont think its poignant to try to reduce it to a percentage and i dont think its useful to think of it as fixed
Isn't it really just like the doppler effect in end result, with unpredictable phasing and repetition?
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Old 8th December 2011, 07:53 PM   #1335
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yes and no, because all manner of other time related and evolving effects bring it about and continue to change its severity depending on power supply noise, ground fluctuation, environmental effects, whatever the physical medium is if there is one etc etc
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Old 8th December 2011, 08:02 PM   #1336
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Absolute jitter, in picoseconds (or nanoseconds) of a digital signal is an objective measure. But it's missing the spectral/frequency component. And, perhaps worse, it's only possible on digital domain signals. It's typically only measured on S/PDIF and AES signals. What about USB? What about feeding a jitter laced S/PDIF signal to different DACs? What about all the playback-only devices like iPods, network media players, Airplay, etc? It's essentially impossible to accurately make absolute jitter measurements with all those devices, yet they can all have jitter issues.

So, IMHO, you're best off looking at jitter's effect on the analog audio performance of a device. And in that domain, as ethanolson suggests, there's no good way I'm aware to distill it down to a single objective number. Miller tried to objectify it a bit more with some success, but it's still a subjective judgement to look at J-Test (or Miller) result and try to judge if it could be problematic.

It's important to understand the distortion produced by a linear amplifier doesn't have a frequency modulation component in it but jitter does. Jitter is much closer to the old wow and flutter measurements of turntables and analog tape recorders. Because it's a different kind of distortion, it's not necessarily a safe assumption to apply the same guidelines we do with say THD.

For example, if the sum of THD+N within the audio spectrum is 80 dB or more below whatever you're listening to, odds are it's effectively inaudible. But you can't necessarily say the same thing about jitter sidebands in a J-Test. And what about the "spread" created by very low frequency jitter? Because it's a different kind of distortion, it's perceived differently.

@ethanolson, if I'm understanding you correctly, the dScope already does some of those measurements. It has a substantially more accurate and stable time base than most audio gear and it can measure both the digital signal (i.e. S/PDIF) for time variations, and also analog audio signals. The frequency resolution of the dScope is spec'd at 0.005 hz. It can, over time, track the peak frequency deviation from some center value. But the frequency at which the signal is modulated is also important.

Ultimately, when you compare the two J-Test results below, I can tell you which one I'd rather be listening to:

Click the image to open in full size.

Click the image to open in full size.
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Old 8th December 2011, 08:09 PM   #1337
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Just a friendly suggestion... is it time to create an ODA/ODAC thread for this stuff, and other discussion and updates?
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Old 8th December 2011, 08:12 PM   #1338
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Hmm, I shall look around for more papers on jitter audibility and see what comes up. Even then, as you say, the varying nature of jitter likely makes any simple judgement of audibility difficult to make.

EDIT: @sofaspud: I suspect that thread will probably come once there the details of the design are finalised.
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Old 8th December 2011, 08:20 PM   #1339
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Understood. I wasn't trying to shut y'all down. But unlike the original O2, the ODA/ODAC cat is out of the bag, so from my view there should be a place here to discuss both it and its related topics that aren't really relevant to the O2.
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Old 8th December 2011, 08:34 PM   #1340
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@qusp, wouldn't manufacturers always test at more ideal and controlled settings, and we'd all know it? Think about why THD+N is always at 1kHz. If it were at 10kHz (or worse at 20kHz @ 2V), it wouldn't sell much. Also, those characteristics can be thrown off by the environments we introduce our stuff to.

@sofaspud, yeah, though I don't know how much time I'd spend there. Maybe that would seal the deal for some.

@RS, I'm beginning to wonder if jitter needs to be plotted in 3D. I also think it's interesting that the J-Test yields so much more noise and artifact info than just the actual jitter. Is there a way to measure an analog signal's peaks on a timescale? If I had a dataset of the time peaks of a 10kHz sinewave from one channel while J-Test was running on the other channel, then we'd have some pretty awesome information, wouldn't we?!

@Willakan, we'll get it sorted. For now, I believe we have to have a standard that people understand, even if imperfect (as many of our specs already are). What I'd like to avoid is one that's considered invalid and to have objective proof of that fact. Although such a stumbling would help us along the quest.

Last edited by ethanolson; 8th December 2011 at 08:37 PM.
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