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(Not a) good explanation of jitter in TAS
andy o
post Jul 12 2009, 02:54
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QUOTE (Ed Seedhouse @ Jul 11 2009, 17:19) *
QUOTE (MichaelW @ Jul 11 2009, 16:01) *
surely the key difference is that image motion caused by hand holding powerful lenses is manifest on non-critical observation, causing obvious defects in normal use of the images.


I would certainly agree.

Yeah, I didn't say that there was gonna be more detail, I said that there wouldn't. It is a matter of not having to lug a tripod around. With photography, it even helps with a tripod and somewhat fast shutter speeds, since it can minimize the quick snappy shake of the mirror.
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MichaelW
post Jul 12 2009, 04:18
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@andy o

I wasn't arguing with you--I've been there with camera shake, and wobbly binoculars. Just pointing out a gross difference between audio jitter and image wobbles. And I use image stabilization, because a good tripod will not fit in a pocket.
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uart
post Jul 12 2009, 07:31
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QUOTE (from linked article)
I've heard this most dramatically when I reviewed Esoteric's G-0Rb, a $16,000 rubidium-based external clock. That's right: The G-0Rb is an atomic clock in your equipment rack whose sole purpose is to provide a precise clock for the digital-to-analog conversion process. With the push of a button, I was able to compare the conventional clock in the Esoteric P-03/D-03 combination with the rubidium-generated clock. Engaging the G-0Rb brought the soundstage into sharp focus, revealed the size and character of the hall through better resolution of low-level spatial cues, made instrumental timbres sound more natural and “organic,” and resulted in a wholesale increase in involvement in the musical performance.


Phew it's a good thing that blind testing has been so thoroughly discredited by the author wink.gif. Otherwise he might have been struggling a bit there.
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andy o
post Jul 12 2009, 15:47
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QUOTE (MichaelW @ Jul 11 2009, 20:18) *
@andy o

I wasn't arguing with you--I've been there with camera shake, and wobbly binoculars. Just pointing out a gross difference between audio jitter and image wobbles. And I use image stabilization, because a good tripod will not fit in a pocket.

Nah, I was agreeing with you. I thought Ed might have misunderstood my previous comment.
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EliasGwinn
post Jul 20 2009, 17:27
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QUOTE (pdq @ Jul 7 2009, 09:20) *
...because jitter is not a real problem...


Is this a common belief here at HA?

ATB,
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greynol
post Jul 20 2009, 17:36
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Why do you ask?

Can you demonstrate through a double-blind test that it is?

If so, please share.

smile.gif


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EliasGwinn
post Jul 20 2009, 17:43
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QUOTE (greynol @ Jul 20 2009, 11:36) *
Why do you ask?

Can you demonstrate through a double-blind test that it is?

If so, please share.

smile.gif


Sort of... We have an ABX box here (we are big believers in DBT testing, as well as the 'no B.S.' approach to audio technology that is embraced here at HA). The real problem with jitter audibility tests is that it is hard to impose jitter on a digital music signal. In other words, one can easily see the effects of jitter using an AP generator and an FFT. However, setting up a test to compare Beethoven's 9th w/ and w/o jitter is much more difficult (logistically).

Would you care to see FFT's of a converter w/ and w/o jitter?

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e


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pdq
post Jul 20 2009, 18:03
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I don't understand why it would be so difficult to perform an ABX test for jitter. Why not just upconvert the material to a higher sampling rate, apply jitter mathematically, and compare the before and after files?
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greynol
post Jul 20 2009, 18:09
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QUOTE (EliasGwinn @ Jul 20 2009, 09:43) *
Would you care to see FFT's of a converter w/ and w/o jitter?

If you mean "see" in a literal sense, then my answer is no.

QUOTE
8. All members that put forth a statement concerning subjective sound quality, must -- to the best of their ability -- provide objective support for their claims. Acceptable means of support are double blind listening tests (ABX or ABC/HR) demonstrating that the member can discern a difference perceptually, together with a test sample to allow others to reproduce their findings. Graphs, non-blind listening tests, waveform difference comparisons, and so on, are not acceptable means of providing support.



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greynol
post Jul 20 2009, 18:21
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Furthermore, tests for the audibility of jitter should be consistent with differences found in real-world hardware.

I doubt that you'll find anyone here who believes that there is no amount of jitter that can break the threshold of audibility in any given system.

This post has been edited by greynol: Jul 20 2009, 18:22


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Axon
post Jul 20 2009, 18:35
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QUOTE (pdq @ Jul 20 2009, 12:03) *
I don't understand why it would be so difficult to perform an ABX test for jitter. Why not just upconvert the material to a higher sampling rate, apply jitter mathematically, and compare the before and after files?
Heh. And if Hawksford's simulator is insufficient, do tell. biggrin.gif

(Personally I've had my doubt as to how accurately a bandlimited interpolation is capable of simulating a digital upsample + sigma delta modulator + zero order hold + antialias, but I can't really back that up.)

This post has been edited by Axon: Jul 20 2009, 18:37
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krabapple
post Jul 20 2009, 18:53
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QUOTE (EliasGwinn @ Jul 20 2009, 12:43) *
Would you care to see FFT's of a converter w/ and w/o jitter?

ATB,
e


Only if they're accompanied by controlled listening tests data to correlate them to audibility.

'Cos as you know, observable does not necessarily mean audible.

There's already too much audiophile hysteria and hand-waving about jitter; uncorrelated difference graphics wouldn't help the situation.

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EliasGwinn
post Jul 20 2009, 18:57
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QUOTE (greynol @ Jul 20 2009, 12:21) *
Furthermore, tests for the audibility of jitter should be consistent with differences found in real-world hardware.


Well, this is sort of a moving target, is it not? Real-world hardware can be affected by several external factors... how can one quantify what is 'real-world' and what is not?

All the best,
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greynol
post Jul 20 2009, 19:01
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Fine, then feel free to provide samples of audible jitter even if the effect has been exaggerated. They will no doubt be more useful than the garbage article referenced in this discussion.


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EliasGwinn
post Jul 20 2009, 19:09
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QUOTE (greynol @ Jul 20 2009, 13:01) *
Fine, then feel free to provide samples of audible jitter even if the effect has been exaggerated. They will no doubt be more useful than the garbage article referenced in this discussion.


Ok, I'll try to pull something together. As I mentioned, its hard to simulate jitter in a music file, and I'm sure no one wants to do a DBT with sine-waves. But I'll see what I can do.

Would you all except the following premise: if the digital cable is the only variable, any differences can be attributed to jitter?

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rpp3po
post Jul 20 2009, 19:17
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QUOTE (pdq @ Jul 20 2009, 19:03) *
I don't understand why it would be so difficult to perform an ABX test for jitter. Why not just upconvert the material to a higher sampling rate, apply jitter mathematically, and compare the before and after files?


You would need very high sample rates to apply small amounts of jitter this way and the result would not necessarily sound comparable to what a specific DAC would output from a dirty clock. Good asynchronous DACs are immune to vast amounts of jitter, anyway.

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EliasGwinn
post Jul 20 2009, 19:21
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QUOTE (rpp3po @ Jul 20 2009, 13:17) *
Good DACs are immune to jitter anyway.


True, but by this definition, there are very few 'good' DAC's.

An FFT may not be able to determine audibility, but it can determine susceptibility. If we put a DAC on an AP machine and increase jitter, the FFT will show whether the DAC is immune to the change. Most are not.

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rpp3po
post Jul 20 2009, 19:32
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QUOTE (EliasGwinn @ Jul 20 2009, 20:21) *
Most are not.


I own one whose circuit layout once filled your day, it sure is... wink.gif

But I'm wondering why not more designs employ an ASRC by default. You don't need $1000 DACs just for jitter immunity. An AD1895, for example, is just $5.36 per piece.

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EliasGwinn
post Jul 20 2009, 19:40
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QUOTE (rpp3po @ Jul 20 2009, 13:32) *
I own one whose circuit layout once filled your day, it sure is... wink.gif


?? What is it?

QUOTE (rpp3po @ Jul 20 2009, 13:32) *
But I'm wondering why not more designs employ an ASRC by default. You don't need $1000 DACs just for jitter immunity. An AD1895, for example, is just $5.36 per piece.


Well, you don't need $1000 to buy that chip, but you need $1000 to pay an engineer to design the infrastructure to execute it properly! wink.gif Not to mention all the other custom circuitry, components and board design. EDIT: (and quality control, 5-yr warranty, etc)

Just because we share a disinterest in hokus pokus electronics doesn't mean we're talking about cook-book electronics either! rolleyes.gif

All the best,
Elias

This post has been edited by EliasGwinn: Jul 20 2009, 19:42


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Soap
post Jul 20 2009, 19:48
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QUOTE (EliasGwinn @ Jul 20 2009, 13:57) *
QUOTE (greynol @ Jul 20 2009, 12:21) *
Furthermore, tests for the audibility of jitter should be consistent with differences found in real-world hardware.


Well, this is sort of a moving target, is it not? Real-world hardware can be affected by several external factors... how can one quantify what is 'real-world' and what is not?


Measurements of the amount of jitter on a production audio source would be easy enough to do, no? I think that is clearly a reasonable definition of "real-world".

Is this not a case where If X amount is audible there is no point in testing X+1? Or do different types of jitter exhibit different audio effects.


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saratoga
post Jul 20 2009, 20:37
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QUOTE (rpp3po @ Jul 20 2009, 14:17) *
QUOTE (pdq @ Jul 20 2009, 19:03) *
I don't understand why it would be so difficult to perform an ABX test for jitter. Why not just upconvert the material to a higher sampling rate, apply jitter mathematically, and compare the before and after files?


You would need very high sample rates to apply small amounts of jitter this way and the result would not necessarily sound comparable to what a specific DAC would output from a dirty clock.


Couldn't you just upsample a 1000x, apply the jitter, and then downsample? I don't see why thats so difficult, though it may use a couple seconds of your CPU time to compute.

Of course this is only as good as your model of real jitter, but I doubt a good model of clock noise is so difficult to come up with. You pick any ok one and then just increase the std deviation until you hear something . . .
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EliasGwinn
post Jul 20 2009, 20:43
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QUOTE (Soap @ Jul 20 2009, 13:48) *
Is this not a case where If X amount is audible there is no point in testing X+1? Or do different types of jitter exhibit different audio effects.


Different types of jitter DO sound different. More importantly, different converters are susceptible to jitter at different frequencies. Specifically, most are NOT immune at low frequencies (<1 kHz). Well-designed PLL's can settle high-frequency jitter, but doesn't do anything to low-frequency jitter. And, arguably, low-freq is the most detrimental jitter because it modulates the fundamentals to frequencies near-by (+/- the jitter frequency), which can cause things to sound out-of-tune (like one of a trio of piano strings is out of tune).

ATB,
e


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Arnold B. Kruege...
post Jul 20 2009, 23:31
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QUOTE (EliasGwinn @ Jul 20 2009, 12:43) *
The real problem with jitter audibility tests is that it is hard to impose jitter on a digital music signal. In other words, one can easily see the effects of jitter using an AP generator and an FFT. However, setting up a test to compare Beethoven's 9th w/ and w/o jitter is much more difficult (logistically).


Actually, its pretty easy to add jitter to a SP/DIF signal. I've done it with maybe $10 worth of parts on a protoboard plus an analog signal source for the signal that is to be the jitter.

Take a SP/DIF signal and use a high speed comparator or schmidt trigger chip to turn it into a square wave (Real world sp/dif signals are often band-limited and look a lot more like a sine wave than a square wave). Feed that through a low pass filter to create a wave with a significant rise time. Then mix in variable amounts of the analog jitter signal. Finally, use another comparator or schmidt trigger input gate to create the signal with jitter. The analog signal will slide the trigger point of the digital signal back and forth along its rise time slope, and this will change the timiing of the SP/DIF signal. The first comparator to square the input signal is optional, particularly if the SP/DIF signal coming in is very robust.

I made this work one morning and proved that it was working several ways. One of the DACs I had on hand (mid-90s era Denon DA-500) had no jitter resistance at all, and I could add clearly audible vibrato to any audio signal. I could also measure the creation of the sidebands that are chraracteristic of FM distoriton (jitter). The other DAC, a early Y2K Technics SHAC 500 surround processor (very similar to the circutis now found in just about every surround receiver) had utterly maximal jitter resistance.

With no added jitter applied, all spurious responses from the SHAC-500 were 110 dB down or more, and they stayed that way until I jittered the signal so much that the both DACs would lose lock. In the extreme case, the Deneon DA 500 would make music sound like it was being sung between buzzing lips, and the Technics SHAC 500 would sound exactly the same as if nothing unusual was happening at all.
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Arnold B. Kruege...
post Jul 20 2009, 23:34
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QUOTE (Mike Giacomelli @ Jul 20 2009, 15:37) *
QUOTE (rpp3po @ Jul 20 2009, 14:17) *
QUOTE (pdq @ Jul 20 2009, 19:03) *
I don't understand why it would be so difficult to perform an ABX test for jitter. Why not just upconvert the material to a higher sampling rate, apply jitter mathematically, and compare the before and after files?


You would need very high sample rates to apply small amounts of jitter this way and the result would not necessarily sound comparable to what a specific DAC would output from a dirty clock.


Couldn't you just upsample a 1000x, apply the jitter, and then downsample? I don't see why thats so difficult, though it may use a couple seconds of your CPU time to compute.


This is in essence how I prepared the variable jitter samples for the now-defunct PCABX web site. I used CEP 2.1 to do the upsampling and downsampling (in steps because extreme ratios can break CEPs resampling algorithm), and also used the Flanger (if memory serves) to actually apply the desired FM modulation. It was one of the time-sensitive EFX.
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rpp3po
post Jul 21 2009, 00:07
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Very interesting comments.

The problem I generally see with externally generated jitter, either through up-/downsampling or electronically, is the applicability of those results. You can take it so far, that you can hear at least something, i. e. what jitter of model m at gain g does or does not sound like. After that you will have several pairs m, g that seem to be relevant. But those then have to be translated and tested against real world implementations.

Wouldn't it make more sense to route test signals into a common DAC known to be sensible against jitter, one from a low end onboard S/PDIF source and another of very high quality, make more sense? That's about the worst it can get in real life. If that already wasn't audible, testing could end. If it was, other common DACs could be fed with the same signal.

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