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Author Topic: Jitter Specification Input Requested  (Read 7118 times)

Nika Aldrich

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Re: Jitter Specification Input Requested
« Reply #15 on: April 29, 2004, 08:52:34 pm »

Johnny B wrote on Fri, 30 April 2004 01:46

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.


So are we talking sounding better or more accurate?

And again, I point out that just because people debate it does not make it real.  And what, exactly, is the "A/D D/A debate" you refer to and how does the specific itteration you refer to relate to the difference in sonic character of analog and digital?

Finally, why are we again discussing analog vs. digital (tedium!) in a thread about jitter?  Smile

Nika.
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Johnny B

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Re: Jitter Specification Input Requested
« Reply #16 on: April 29, 2004, 09:15:24 pm »

Because if the A-to-D and D-to-A debate had ended there would
no need to even discuss jitter or filters or clocks.

If the the stuff really sounded as good as analog, no one would even care about jitter. Ahh, but they do care about "jitter" and "time smear" and "imaging" and a whole host of other factors.

Digital ain't perfect, it might be perfectable,
but it ain't perfect. Not yet. YMMV

 
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natpub

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Re: Jitter Specification Input Requested
« Reply #17 on: April 29, 2004, 09:26:02 pm »

Johnny B wrote on Thu, 29 April 2004 19:46


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

       


To me, isn't this is like saying when everyone agrees that bronze-wrap guitar strings sound best, then the debate is over? Would such a debate ever end, because they are merely different sounds?

The discussion at hand seemed to start about jitter and distortion in the evaluation of converters. It keeps getting dragged into competition with analog, which indeed is loaded with distortion. Some distortion we may like, some we may dislike.

Many persons who grew up trained to hear certain tape and circuit distortions as warm and beautiful do indeed prefer such sounds. Quite naturally I believe, since we from that era were conditioned to do so; we associate it with fond memories.

Currently, a generation is being largely trained to digital and file-compressed sounds. For them, that "great old sound of so-and-so" will be all or mostly digital.

I am not sure accuracy is really what people want, when they explore converters, or any medium or tool. People started down the digital path trying to get away from noise. Once they got more silence and accuracy, many did not like it.

I must agree--when I was young I really liked my Muff-Pi fuzz box, because it hid all my little errors, and made me feel like I was a rock star Razz

I just feel that different media and tools can give us a variety of sounds, and with time and study, we can learn to use each of these things to create what we wish.

KT
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sdevino

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Re: Jitter Specification Input Requested
« Reply #18 on: April 30, 2004, 09:11:55 am »

Back on topic please.

There are many members representing the various semiconductor and equipment manufacturers on the SC02 comittee. I am looking to this group to attempt to identify anthing in a jitter spec that could be useful to a recording engineer.

So far Nika has suggested a jitter spectrum in the frequency domain.

I also agree that the only jitter I care about is at the input to the converter, but I do not know enough to be able to translate what if any effect jitter on an external wordclock would have on converter performance by the time the wordclock gets passed through the PLL or other lockup crcuits to the converter input.

as far as I can tell it might matter a lot or not at all. Certainly high frequency jitter that is outside the loop bandwidth of the PLL will not matter. So why do we think we hear a better soundstage?  I think Nathan has a point since it is pretty much impossible to do a true double blind comparison.

Steve
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Nika Aldrich

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Re: Jitter Specification Input Requested
« Reply #19 on: April 30, 2004, 10:55:24 am »

sdevino wrote on Fri, 30 April 2004 14:11

Certainly high frequency jitter that is outside the loop bandwidth of the PLL will not matter.


This is a myth.  Jitter, just like audio, aliases so that variations higher than the sample rate still fold back into the legal range.  Jitter at 45.1KHz will manifest itself as 1KHz jitter in a 44.1KS/s audio system.  For this reason, jitter specs that only show the jitter frequencies in the 1fs range are unhelpful.

Isn't a big issue here that it will be relatively easy with just about any given spec to have a clock source that does very well on spec but has drastically poorer performance in certain real-world conditions?  Give me a spect that this wouldn't be the case for?  With this realization, specs are nearly useless, or no?

Nika
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Ethan Winer

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Re: Jitter Specification Input Requested
« Reply #20 on: April 30, 2004, 02:07:53 pm »

Nika,

> that depends on the frequency of the jitter <

As far as I'm concerned, if jitter is 110 dB or more below the signal, then it's not a problem.

> It's all important <

We've been through this before, so the bottom line for me is, Prove it. As Carl Sagan said, extraordinary claims require extraordinary proof. The notion that teensy amounts of noise or distortion at any frequency are a meaningful problem makes no sense to me. If you have to play extremely soft signals and then turn the monitor volume up by 40 dB past the usual speaker blowout level just to hear it means it's not a problem.

> how do you come up with your -110dB numbers<

Pohlmann's book. Most of the jitter levels in his various charts are softer than -110 dB, but that's the loudest number I found in any of the graphs so I use it to be generous.

> do you not conceded that it is possible for signals at less than -110dBFS to be audible? <

I do not concede this. To me -110 dB down is inaudible, period. Especially with a 16 bit system where anything below 96 dB is noise anyway.

> if you can have a 30dB null in a room, can you have a 30dB node in a room? hint <

I don't understand. A null occurs at a node point. What's this have to do with the audibility of noise/distortion/whatever that's 110 dB below the program?

Some day I'm going to fly out to your office, and give you the opportunity to show me the audibility of jitter in person! Very Happy

--Ethan

Nika Aldrich

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Re: Jitter Specification Input Requested
« Reply #21 on: April 30, 2004, 02:43:33 pm »

Ethan,

Rooms are inconsistent and they have the opportunity to have nulls and nodes.  When a node occurs an "artificial" amplification of certain frequencies occurs.  Just the same, where a null occurs so does an "artificial" gain reduction of certain frequencies.  For this reason it is possible to have jitter exist at a frequency that may be 110dB below program, but if program material happens to center around a null in the room and the frequency of the sideband produced by the jitter falls where the node in the room is then the manifestation of jitter in the room can be far less than 110dB below program.

The inconsistencies in rooms allows for the presentation of a lot of things that might otherwise be inaudible.  I have listened to music on highly specified systems in acoustically controlled rooms, and only upon hearing the same thing in my truck, driving on city streets did I hear a particular riff or effect.  While in theory and in practice that "effect" or whatever was below the threshold of audibility in perfect situations, in the reality of acoustical spaces I was able to hear something otherwise inaudible.

For this reason, 110dB down can still be an issue, and as I and others have heard, jitter is.

Nika.
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sdevino

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Re: Jitter Specification Input Requested
« Reply #22 on: April 30, 2004, 03:11:22 pm »

Nika Aldrich wrote on Fri, 30 April 2004 10:55

sdevino wrote on Fri, 30 April 2004 14:11

Certainly high frequency jitter that is outside the loop bandwidth of the PLL will not matter.


This is a myth.  Jitter, just like audio, aliases so that variations higher than the sample rate still fold back into the legal range.  Jitter at 45.1KHz will manifest itself as 1KHz jitter in a 44.1KS/s audio system.  For this reason, jitter specs that only show the jitter frequencies in the 1fs range are unhelpful.

Isn't a big issue here that it will be relatively easy with just about any given spec to have a clock source that does very well on spec but has drastically poorer performance in certain real-world conditions?  Give me a spect that this wouldn't be the case for?  With this realization, specs are nearly useless, or no?

Nika



Nika you missunderstood me. I am saying that HF jitter on an external clock will not make it to the ADC or DAC clock input in some applications. For instance many converter clocks are regenerated using PLL's or DDS. In these cases the external clock is used as an error or locking reference. A stable clock design would LPF the external clock such that the HF jitter is not even detected.

You are correct in that HF jitter on the clock interacts with the audio but I am not so sure what exact form it would be. I am sure that it is probably much more complex than simple aliasing. Since the jitter is probably not coherent or and could be very complex phase/frequency modulation it gets pretty dicey. ITs also really really really hard to measure jitter (and expensive).

Steve
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Nika Aldrich

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Re: Jitter Specification Input Requested
« Reply #23 on: April 30, 2004, 04:05:41 pm »

sdevino wrote on Fri, 30 April 2004 20:11

Nika you missunderstood me. I am saying that HF jitter on an external clock will not make it to the ADC or DAC clock input in some applications. For instance many converter clocks are regenerated using PLL's or DDS. In these cases the external clock is used as an error or locking reference. A stable clock design would LPF the external clock such that the HF jitter is not even detected.


Correct, and agreed.  This is part of what I was complaining about - a clock circuit is highly susceptible to the PLL on the device that follows it.  The specs on an Apogee Big Ben in relation to the jitter specs on my MOTU 2408 are highly irrelevant until I know the PLL specs on the 2408!

Quote:

You are correct in that HF jitter on the clock interacts with the audio but I am not so sure what exact form it would be. I am sure that it is probably much more complex than simple aliasing.


Actually, no.  Jitter creates very simple distortion - it's all sideband distortion with the amplitude of the distortion dependant upon three factors - the amplitude of the jitter, the amplitude of the signal, and the frequency of the signal.  The higher the frequency of the signal the greater the amplitude of the sidebands.  The higher the amplitude of the signal the higher the amplitude of the sidebands.  And the higher the amplitude of the jitter the higher the amplitude of the sidebands.  

The frequency of the sidebands is dependant on one thing - the frequency of the jitter.  1KHz jitter has sidebands removed from your signal by 1KHz.  5KHz jitter has sidebands removed from your signal by 5KHz.  1+3KHz jitter has sidebands removed by both 1KHz and 3KHz, the amplitude of each logically dependant on the variables above.  White noise jitter yields sidebands of whitenoise - or rather just more white noise.  Etc.  

If the jitter frequency is above Nyquist then the sidebands appear in an aliased way.  If you have a 5KHz tone and you have 1KHz jitter you have sidebands at 4KHz and 6KHz.  If you have a 5KHz tone and you have 23.05KHz jitter (1KHz above Nyquist) the sidebands appear 1KHz removed from a false aliased tone.  The new "center frequency" is f(Nyquist) - f(tone), or in this case 18.05KHz.  So the sidebands appear at 17.05KHz and 19.05KHz.

If the jitter frequency is above the sampling frequency then it is as if the jitter is below Nyquist.  Jitter at 45.1KHz manifests itself exactly the same as 1KHz jitter.

Make sense?  Jitter is indeed difficult to measure, but the manifestation of it is very mathematically determinable and rather simple.

Nika.
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Zoesch

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Re: Jitter Specification Input Requested
« Reply #24 on: May 03, 2004, 09:00:05 am »

This thread has me scratching my head... must be my Telco background/leanings, but why would anybody say that jitter is irrelevant? That's akin to a reel to reel manufacturer saying that tape flutter is irrelevant, and we've all heard what flutter sounds like right?

I'm with Nika, I'd like to see jitter as a measure of both parts per million as well as frequency and I want to see the PLL specs... whether or not I want them measured at the end of a cable is kinda irrelevant, I can calculate that myself and most people would be utterly confused if they saw such a measurement.
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sfdennis

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Re: Jitter Specification Input Requested
« Reply #25 on: May 03, 2004, 11:44:13 am »

Steve & All,

From an EE standpoint, I think that the jitter test suite offered by AP pretty much covers the bases, but for the everyday recording engineer such specs are pretty unusable.

As Nika summarized above, a device’s jitter profile (jitter spectrum) is going add a certain amount of noise and distortion to the recorded signal. However, a widespread understanding of the exact amounts and character of noise and distortion that will be produced by a given jitter profile is absent in the audio community. The interchange between Ethan and Nika (‘is jitter nominally -110dB FS and if so why should I care?’) bears witness to this. What people care about the most is how and by how much a jitter spectrum will mess up a signal and that is what is missing from available specs. That would be a useful presentation of jitter specs.

Luckily, there is a fairly direct relationship between jitter components of a clock signal and the resulting THD+N  produced by an ideal A/D converter at a given sampling rate and resolution. I don’t know if this is the place to get into it, but I’ll summarize for now and write more if somebody asks. You can model jitter as the sum of two kinds of components: noise and periodic errors—that’s what a jitter spectrum tells us. If you decompose the jitter spectrum into these components, then you can automatically produce a ‘worst-case’ THD+N graph over the audio band. That THD+N graph would tell you how much distortion and noise a given clock device would give you at each frequency. Using such a graph, it would be fairly easy to settle Ethan and Nika’s argument.

There are a number of wrinkles to iron out, and I don’t have all the details worked out. But I do think that THD+N graphs of jitter against an otherwise ideal A/D converter would be a lot more useful to recording engineers than today’s jitter specs.

-Dennis
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Nika Aldrich

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Re: Jitter Specification Input Requested
« Reply #26 on: May 03, 2004, 12:04:42 pm »

sfdennis wrote on Mon, 03 May 2004 16:44

Luckily, there is a fairly direct relationship between jitter components of a clock signal and the resulting THD+N  produced by an ideal A/D converter at a given sampling rate and resolution. I don?t know if this is the place to get into it, but I?ll summarize for now and write more if somebody asks. You can model jitter as the sum of two kinds of components: noise and periodic errors?that?s what a jitter spectrum tells us. If you decompose the jitter spectrum into these components, then you can automatically produce a ?worst-case? THD+N graph over the audio band. That THD+N graph would tell you how much distortion and noise a given clock device would give you at each frequency. Using such a graph, it would be fairly easy to settle Ethan and Nika?s argument.

There are a number of wrinkles to iron out, and I don?t have all the details worked out. But I do think that THD+N graphs of jitter against an otherwise ideal A/D converter would be a lot more useful to recording engineers than today?s jitter specs.

-Dennis



Dennis,

The only problem with this is that the THD+N spec would have to be curved in some way to represent how it is going to audibly transfer.  Sideband distortion of 1Hz at extremely high amplitudes is going to be practically negligable whereas sideband distortion of 1KHz at far lower amplitudes is going to significantly affect stereo image.  

I like the idea of generating a "worst case" THD+N graph, but since it is sideband distortion any frequency can impart distortion in the audible band, so we'd have to check (virtually) all frequencies.  

Also, jitter only presents noise if it is random jitter, such as clock phase noise.  I think we are generally far more concerned about the THD spec resulting from the jitter than the noise?  

Nika.
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Re: Jitter Specification Input Requested
« Reply #27 on: May 03, 2004, 12:21:20 pm »

Nika,

I was acutally suggesting a full audio spectrum plot of THD+N in the and not at a single nominal frequency. Though I expect that some folks will be interested (for silly reasons) in seeing THD+N all the way up to Nyquist and beyond. In any case, with a full audio spectrum THD+N, you'd see the sidebands if they were important and you wouldn't see them if they weren't.

As to whether random jitter is important or not, well if it was bad enough, it would show up in the graph.

BTW, it is good to banter with you again, Nika. Feels like the old days in the other place.

-Dennis
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Nika Aldrich

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Re: Jitter Specification Input Requested
« Reply #28 on: May 03, 2004, 12:37:16 pm »

sfdennis wrote on Mon, 03 May 2004 17:21

I was acutally suggesting a full audio spectrum plot of THD+N in the and not at a single nominal frequency. Though I expect that some folks will be interested (for silly reasons) in seeing THD+N all the way up to Nyquist and beyond. In any case, with a full audio spectrum THD+N, you'd see the sidebands if they were important and you wouldn't see them if they weren't.


Sure.  Makes sense.  So you recommend running a sine wave sweep (for example) through a box wherein jitter will be induced, and then measure the THD+N that is presumed to come from the jitter alone, as opposed to the conversion process that occurs along with the jitter?  It seems like this will be heavily prone to error, as it does not actually measure the jitter but rather all of the distortions present INCLUDING jitter, many of those distortions being those that the clock designer can't control.

I think we would instead need to measure the jitter at the clock and then deduce the amount of THD+N that clocking device would yield.  So how far out do we measure the jitter on the clock in order to ascertain how much THD+N it will yield?  Do we start at 1Hz or lower?  Do we measure up to 1MHz, or higher?  

Nika.
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Re: Jitter Specification Input Requested
« Reply #29 on: May 03, 2004, 01:22:31 pm »

Nika,
Well, you’ve identified my major concern with the suggestion. The way I thought about it is was you would take the jitter spectrum as you suggested early in the thread, and feed it into a program that would spit out the THD+N graph. By using a program as a ‘virtual converter’, you’ll be able to get around the idiosyncrasies of any real converter that might be used in a given test. After all, real converters have their own flaws and you wouldn’t want a clock’s jitter spec to be influenced by a particular choice of converter.

As usual, the devil is in the details. First you’d have to take the output of a real jitter spectrum measurement such as is available from the AP setup. That’s the easy part. Then a program would have to model the spectrum as a linear combination of noise and jitter frequency components. Steve’s committee will have to agree on basis functions for modeling jitter. There aren’t many reasonable choices and they are all roughly equivalent. For example, you might choose A*gaussian(mean, variance) as the template for a noise term and B*phi(omega t) as the template for a periodic term. In any case the general idea is that you would derive a function J(t), that behaved like the real device and whose terms were a linear combination of the basis functions. The value of J(t) would represent the expected error of the real clock at a given time, t.

So the error at a given sample would simply be sin(omega*t) – sin(omega*t + J(t)).  To get the THD+N for a given frequency you’d just run the program for some nominal period (say 10 seconds) at the given sampling rate and resolution, and compute the RMS error.

Lots of details to work out, and after writing this, I’m not so sure that you could get any group to converge on the myriad decisions required for this to work. There are definitely compromises to be made, and of course, the programs that did all the computations would have to be open sourced to ensure that there wasn’t any cheating. Furthermore, after all this simulation and what-not, would you have a spec that truly represented a device’s capabilities and limitations?

In spite of all these obstacles, I really do think some sort of THD+N spectral plot would be infinitely more useful to audio engineers than a jitter spectrum.

-Dennis
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