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Author Topic: Jitter Specification Input Requested  (Read 9004 times)

sdevino

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Jitter Specification Input Requested
« on: April 28, 2004, 07:35:51 am »

I am a member of SC-02 committee of the AES and some discussions have developed within the group to develop some sort of standards for specifying jitter performance of audio systems and/or components.

Most of the replies and discussion are either extreme electrical designer specs (i.e. spec everything to CYA). Or audiophile responses (who simply want a means of identifying that their system is better than everyone elses  Rolling Eyes  )

While I have background as a designer, a test engineer and a user, I am more interested in the experiences of actual users and would like to represent the "users" concern in this discussion. When I buy gear or spec gear for use, I want meaningful specs and not something that is intended to fill out a marketing campaign.

So folks, I am asking for your inputs to help identify concerns of the user in terms of jitter. What information would you like the equiment vendors to supply? And why?

Thanks
Steve
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Steve Devino

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Ethan Winer

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Re: Jitter Specification Input Requested
« Reply #1 on: April 28, 2004, 11:51:47 am »

Steve,

> help identify concerns of the user in terms of jitter <

As far as I'm concerned jitter is a complete non-issue, popularized by gear makers who want an excuse to get you to buy yet more gear. As an audio pro, and a listener, jitter is the very last thing I'm concerned about. The specs I see typically put jitter 110 dB or more below the music. Who gives a flying you-know-what about anything that far below the program material? Especially since it's also masked by the program.

It amazes me when people obsess over minutiae like this, while blissfully ignoring numerous 30 dB nulls caused by standing waves in their control room.

--Ethan

oudplayer

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Re: Jitter Specification Input Requested
« Reply #2 on: April 28, 2004, 01:00:05 pm »

With all due respect, Ethan, though I acknowledge your concern about room acoustics, there's many who've discovered that there are very substantial sonic differences between AD converters, between DA converters, and that the result of using combinations of these has a large impact on the quality of mixes we make. Since some of these differing units use the identical analog stage, the difference lies somewhere in the clocking/ filtering part of the equation, and regarding the former, jitter becomes an important issue.

I'm not sure if this gets you to a "spec" or not, but an increasing number of digital products are using what Lynx calls "steady clock" (I forget the other jargon terms in use), a technology that is able to correct drifting incoming clock signals. Perhaps there could be some measure of the ability to correct incoming clock sources?
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sdevino

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Re: Jitter Specification Input Requested
« Reply #3 on: April 28, 2004, 01:23:44 pm »

oudplayer wrote on Wed, 28 April 2004 13:00


I'm not sure if this gets you to a "spec" or not, but an increasing number of digital products are using what Lynx calls "steady clock" (I forget the other jargon terms in use), a technology that is able to correct drifting incoming clock signals. Perhaps there could be some measure of the ability to correct incoming clock sources?



Sounds like marketing speak for technology that has been around since the 50's for Color TV. "Steady clock" is pure BS.

I also wnder if we really have any idea whther or not the "jitter" is the issue relative to improved audio performance of a converter. Many of us "think" it is but none of us (that I am aware of) have shown any real proof.
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Steve Devino

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Ethan Winer

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Re: Jitter Specification Input Requested
« Reply #4 on: April 28, 2004, 01:43:29 pm »

Oud,

> Since some of these differing units use the identical analog stage <

I agree with Steve that lots of things could account for the difference in the sound of one converter versus another. Again I ask you to think about the relevance of anything that's 110 dB or more below the music. How could that possibly be audible? Why must jitter be the explanation, as opposed to lower distortion? Or any of a number of more likely culprits.

All of this stuff is easily measured. And a double blind test is at least as useful to determine for once and for all what really matters and what doesn't.

--Ethan

oudplayer

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Re: Jitter Specification Input Requested
« Reply #5 on: April 28, 2004, 03:21:32 pm »

Ethan Winer wrote on Wed, 28 April 2004 10:43

 Why must jitter be the explanation, as opposed to lower distortion? Or any of a number of more likely culprits.

All of this stuff is easily measured. And a double blind test is at least as useful to determine for once and for all what really matters and what doesn't.



I apologize, as I wasn't precise enough in my first post - I'm not married to the jitter metric as the be-all and end-all in converter woes, yet notice that the use of different clock sources dramatically changes the audible characteristics of a converter (for better or worse or indifferent-yet-different). I also have the understanding that this shouldn't be the case if converters were all done "right," yet it still persists in today's technologies. If it's not jitter that's being most affected by alternative clock sources, then what phenomena/metric is it?

I'm curious about the "number of more likely culprits." What would these be, how would they be measured, and how would they map onto audible phenomenon (i.e., a higher slew rate in op-amps produces xyz audible result)?

Hopefully my DSP naivitae will assist (by provoking intelligent responses) in uncovering what is different and measurable between converters, what is significant, and the like.
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Johnny B

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Re: Jitter Specification Input Requested
« Reply #6 on: April 29, 2004, 12:59:15 am »

Steve,

May I make a few suggestions.

Have you talked to anyone at Burr-Brown
about A-to-D and D-to-A converters?

Have you talked to anyone at Audio Precision
about using their test gear?

Have you read any of Julian Dunn's white papers,
he did some work in this field and
worked for AP before his untimely demise,
but I think you may be able to downlaod his papers
off the web for free.

Have you talked to anyone at Anagram Technologies?

That's the technology that Manley Labs used in the SLAM.
Speaking of Manley, have you talked with anyone there?

My own wild ass guess is that we will get better sound when the speeds are drasticaly bumped up and the bit/word is increased to say 32 or 64 bits. I could be very wrong, look at Sony and SACD's specs. 1 bit at a million miles a second. LOL. Maybe I got it half right cuz with Sony, the speed is there.

One thing is sure, there is room for improvement, otherwise there would be no debate going on of "analog vs. digital." Although it is getting better all the time, I fear the debate will rage for some unknown amount of time.

Who knows, maybe it has something to do with the effect of heat on the sound or some other weird behavior that no one has yet noticed or measured. Science got us to the moon, but it has not yet cured the common cold nor ended the problems in the A-to-D and D-to-A debate.    

 
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Ethan Winer

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Re: Jitter Specification Input Requested
« Reply #7 on: April 29, 2004, 12:32:29 pm »

OP,

> yet notice that the use of different clock sources dramatically changes the audible characteristics of a converter <

How dramatically? This is why I suggested that a true double-blind test is the only way to really know what matters and what doesn't. If you've ever tweaked an electric guitar track to perfection only to later discover you had your fingers on the snare EQ, you'll agree that human hearing and perception are flawed at best. I'm not saying you're imagining things! Only that logic and common sense rule out artifacts that are 110 dB or more below the music.

> What would these be, how would they be measured, and how would they map onto audible phenomenon (i.e., a higher slew rate in op-amps produces xyz audible result)? <

Sure, slew rate limiting could be a factor, though you'd need an awful lot of full-level high frequency content to run into that on any gear made in the past many years. Everything in audio can be measured, and slew rate limiting simply shows up as distortion. Audio transformers are among the worst offenders, but for some reason their flaws are considered charming, warm, and "musical."

Go figure.

--Ethan

Loco

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Re: Jitter Specification Input Requested
« Reply #8 on: April 29, 2004, 02:51:44 pm »

sdevino wrote on Wed, 28 April 2004 07:35

So folks, I am asking for your inputs to help identify concerns of the user in terms of jitter. What information would you like the equiment vendors to supply? And why?


Well, I'm not sure what you can do about a converter once you find out how much jitter it carries, really. However, it would be very useful in order to compare different brands. I would say it would be important to know the jitter decorrelation between channels, since it could be the source of sloppy low end and/or loss of stereo image stability. I don't know if all of them are actually words, but you know what I mean.
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miroslav

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Re: Jitter Specification Input Requested
« Reply #9 on: April 29, 2004, 02:56:01 pm »

Ethan Winer wrote on Thu, 29 April 2004 12:32

OP,

If you've ever tweaked an electric guitar track to perfection only to later discover you had your fingers on the snare EQ, you'll agree that human hearing and perception are flawed at best.


That's choice Ethan...

...oh yeah...been there done that!  Very Happy
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Nika Aldrich

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Re: Jitter Specification Input Requested
« Reply #10 on: April 29, 2004, 03:09:43 pm »

sdevino wrote on Wed, 28 April 2004 12:35

So folks, I am asking for your inputs to help identify concerns of the user in terms of jitter. What information would you like the equiment vendors to supply? And why?


The most useful spec would be a jitter frequency chart, but it would have to be standardized such that it is taken at a given place in the system with given surrounding conditions.  In other words, it is far less useful to test a word clock master at it's crystal source than to test it at the end of a 10' cable.  On the contrary, it is far more useful to test an A/D converter's clock at the chip itself.  These are some of the problems with deriving a spec.  If I am only trying to impress people on specs I can have a great crystal and pll inside the box under test conditions, but allow it to erode quickly in the real world.  And unfortunately, jitter is so sensitive that small variations in very minor details between testing and real world can have significant variations.

The "spec" that is most useful is a frequency vs. amplitude chart of the jitter, but the testing method and protocol needs more attention.

One other minor detail - most people wouldn't know how to interpret the data anyway.

Nika.
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Re: Jitter Specification Input Requested
« Reply #11 on: April 29, 2004, 03:14:33 pm »

Ethan Winer wrote on Wed, 28 April 2004 16:51

Steve,

As far as I'm concerned jitter is a complete non-issue, popularized by gear makers who want an excuse to get you to buy yet more gear. As an audio pro, and a listener, jitter is the very last thing I'm concerned about. The specs I see typically put jitter 110 dB or more below the music. Who gives a flying you-know-what about anything that far below the program material? Especially since it's also masked by the program.


But it isn't necessarily masked by the program material - that depends on the frequency of the jitter.  50Hz jitter?  Very definitely masked on a 1KHz waveform, but very noticeable on a 60Hz waveform.  

Quote:

It amazes me when people obsess over minutiae like this, while blissfully ignoring numerous 30 dB nulls caused by standing waves in their control room.


It's all important.  And inconsistencies in the room can help expose otherwise negligable problems like jitter - and any listening room in the real world has inconsistencies.

Acoustics is not the only problem...

Nika.
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Re: Jitter Specification Input Requested
« Reply #12 on: April 29, 2004, 03:18:16 pm »

Ethan Winer wrote on Wed, 28 April 2004 18:43

Again I ask you to think about the relevance of anything that's 110 dB or more below the music. How could that possibly be audible? Why must jitter be the explanation, as opposed to lower distortion? Or any of a number of more likely culprits.


Well first of all, jitter causes distortion.  Second, how do you come up with your -110dB numbers, and at what frequencies?  And finally, do you not conceded that it is possible for signals at less than -110dBFS to be audible?  (note, if you can have a 30dB null in a room, can you have a 30dB node in a room?  hint...)

Nika.
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Re: Jitter Specification Input Requested
« Reply #13 on: April 29, 2004, 03:26:08 pm »

Johnny B wrote on Thu, 29 April 2004 05:59

My own wild ass guess is that we will get better sound when the speeds are drasticaly bumped up and the bit/word is increased to say 32 or 64 bits. I could be very wrong, look at Sony and SACD's specs. 1 bit at a million miles a second. LOL. Maybe I got it half right cuz with Sony, the speed is there.


Nope.  The converters you use at 44.1KHz are actually clocking at 2.8224MHz on the converter chip and then being downsampled to 44.1KHz, so the rate we are using is the same as SACD.  And some converters do the conversion at higher rates than that, even.

Bit/word increased to 32 or 64?  What in the world?  Why on earth?

Quote:

One thing is sure, there is room for improvement, otherwise there would be no debate going on of "analog vs. digital."


This is a misnomer.  The fact that a debate exists does not, ipso facto, yield that there is room for improvement.  People can debate about anything, whether real or not.

Quote:

Science got us to the moon, but it has not yet cured the common cold nor ended the problems in the A-to-D and D-to-A debate.


Why do you think there is still a debate in the scientific community about A/D and D/A converters?  This industry is SOOOOO far removed from actual science that we would likely never know what is known by the mathematicians and scientists.  This is NOT a rhetorical question.

Nika.
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Re: Jitter Specification Input Requested
« Reply #14 on: April 29, 2004, 08:46:01 pm »

Nika,

I know you are a very smart man and have given this area a lot of thought, but there really are a lot of respected people in the industry who believe that analog still sounds better.

I was lucky enough to spend some time in NYC
with Walter Sear, nearly everything he has is analog.
He hates the sound of CD's, but he says SACD
is getting closer to analog. I think quite
a few people would agree with him.

To me, most CD's sound brittle and there is something
to be said about the "behavior" of analog. I do not think
it is fully understood yet.

That's why I think we need more study, and more scientific
research and more experimentation. I do not feel that we are there yet. That's my take on it. I sense you will
disagree and that's fine.

When the staunch adherents to analog agree
there is no debate as to the sound quality,
that's when the debate will end.

       
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