Dancing with the Jitter Bugs...

Nicholas Bedworth

WBF Founding Member
May 7, 2010
312
0
0
Maui, where else?
This is a cross post from the Tech Talk section, but I thought everyone here in the Digital section would like to see it, and make comments...

As mentioned in earlier posts, a treasure-trove of jitter related papers all stems from the work of the late Julian Dunn at his colleagues in the UK.

Juergen Reis, chief engineer and designer at MBL, kindly pointed these out to me a few months ago, for which I am most grateful. While the papers are dry and technical, they do a good job of explaining the essentials. If you go through them carefully, make notes and diagram things for yourself, a great deal can be learned.

This post is an attempt to simplify and summarize a lot of details into some general observations...

One of the most important basic points to understand is that "digital" signals are really just very strange-looking analog waveforms. All the discussions about "How can there be errors in digital systems?" confuses the idealized concept (repeat, concept) of a binary digit with the reality of its electronic representation (notice I didn't say equivalent). The concept is logical, not physical: It's either a zero or one. That's great, and looks well on the chalkboard, but try making a perfect zero or one using real circuits.

Those burdened with advanced education in the field :) will recal lthat a square wave in fact comprises a large number of component sine and cosine waves, of various frequencies and phases, all of which sum up to something that's got sharp corners and flat tops. When you start passing this "package" through real circuitry, the fun begins...

Where people often get off track is on the subject of how can one hear 100 picosecond timing errors? Well, of course you can't, directly. Human hearing can detect differences in arrival time of clicks to milliseconds and microseconds, but not picoseconds. However, the consequences of such timing errors are readily audible in many cases, that that they distort ordinary audio signals in various obvious ways.

One of the most insidious realities of jitter, in all its lovely forms, is that the distortions are seldom related to the natural harmonic structure of music. You could infer this from some of the comments, but I thought it was a good idea to mention it explicitly. A-harmonic distortions are very hard to hide, so to speak.

Another common-sense reality of jitter is that it affects higher frequencies more than lower, thus contributing to screechy violins, edgy transients and all that.

Basically the amplitude of the jitter sidebands relates to the magnitude of the timing slop, and the frequency offset of the sidebands relates to the periodicity of the timing error. More or less :)

There can be jitter in USB, in S/PDIF, in IEEE 1394/FireWire, or basically any form of interface. There are jitter artifacts caused by the bit patterns of the digital data, according to Julian. And of course during A/D or D/A sampling conversion, once again, sample clock timing errors distort the waveform.

A particularly bad situation is when the sampling clock is derived from information coming across the interface. More advanced designs use phase-locked loop devices to essentially smooth out the inevitable jitter and decouple various subsystems in the A/D or D/A.

Jitter is often pretty obvious in lower-end USB DACs, to pick on one type of component :) Sure, they can handle 96 and 192 kHz signals, the audio gets progressively worse. High levels of jitter have a lot to do with such issues.

With higher-quality gear, where extensive measures are taken to control and isolate various types of jitter, the reverse occurs: the sound keeps getting better and better with higher sampling rates, as one would expect.
 

muralman1

New Member
Jul 7, 2010
479
0
0
Sacramento Ca
I'm sorry, Nicholas, if you are talking about oversampling (Please tell me if I am wrong), over-engineering ruins my system. If there is some demerit to my sound caused by jitter, it isn't something that I can hear. all I hear are the sounds recorded.
 

Albertporter

Well-Known Member
Apr 27, 2010
185
19
1,575
Dallas, TX
www.albertporterphoto.com
This is a cross post from the Tech Talk section, but I thought everyone here in the Digital section would like to see it, and make comments...

As mentioned in earlier posts, a treasure-trove of jitter related papers all stems from the work of the late Julian Dunn at his colleagues in the UK.

Juergen Reis, chief engineer and designer at MBL, kindly pointed these out to me a few months ago, for which I am most grateful. While the papers are dry and technical, they do a good job of explaining the essentials. If you go through them carefully, make notes and diagram things for yourself, a great deal can be learned.

This post is an attempt to simplify and summarize a lot of details into some general observations...

One of the most important basic points to understand is that "digital" signals are really just very strange-looking analog waveforms. All the discussions about "How can there be errors in digital systems?" confuses the idealized concept (repeat, concept) of a binary digit with the reality of its electronic representation (notice I didn't say equivalent). The concept is logical, not physical: It's either a zero or one. That's great, and looks well on the chalkboard, but try making a perfect zero or one using real circuits.

Those burdened with advanced education in the field :) will recal lthat a square wave in fact comprises a large number of component sine and cosine waves, of various frequencies and phases, all of which sum up to something that's got sharp corners and flat tops. When you start passing this "package" through real circuitry, the fun begins...

Where people often get off track is on the subject of how can one hear 100 picosecond timing errors? Well, of course you can't, directly. Human hearing can detect differences in arrival time of clicks to milliseconds and microseconds, but not picoseconds. However, the consequences of such timing errors are readily audible in many cases, that they distort ordinary audio signals in various obvious ways.

One of the most insidious realities of jitter, in all its lovely forms, is that the distortions are seldom related to the natural harmonic structure of music. You could infer this from some of the comments, but I thought it was a good idea to mention it explicitly. A-harmonic distortions are very hard to hide, so to speak.

Another common-sense reality of jitter is that it affects higher frequencies more than lower, thus contributing to screechy violins, edgy transients and all that.

Basically the amplitude of the jitter sidebands relates to the magnitude of the timing slop, and the frequency offset of the sidebands relates to the periodicity of the timing error. More or less :)

There can be jitter in USB, in S/PDIF, in IEEE 1394/FireWire, or basically any form of interface. There are jitter artifacts caused by the bit patterns of the digital data, according to Julian. And of course during A/D or D/A sampling conversion, once again, sample clock timing errors distort the waveform.

A particularly bad situation is when the sampling clock is derived from information coming across the interface. More advanced designs use phase-locked loop devices to essentially smooth out the inevitable jitter and decouple various subsystems in the A/D or D/A.

Jitter is often pretty obvious in lower-end USB DACs, to pick on one type of component :) Sure, they can handle 96 and 192 kHz signals, the audio gets progressively worse. High levels of jitter have a lot to do with such issues.

With higher-quality gear, where extensive measures are taken to control and isolate various types of jitter, the reverse occurs: the sound keeps getting better and better with higher sampling rates, as one would expect.

Nicholas, I don't fully understand all the technical terms you describe but it reads like you're saying there can be errors in digital that alter the smoothness and texture in the high frequency region of reproduced music. The symptom you describe is one of the things I find most objectionable with old Redbook CD format.

As I've stated in other posts, I'm not totally anti digital, just frustrated by the limited quality digital formats offered to music loving consumers. I hope there are enough "audiophiles" to support ultra high quality downloads or new physical media that solve current day problems.

I would love to have digital that's equal to the quality level achieved in a studio environment on the master drive. I think it's possible but as always, it's about the money.

Thanks for your thought provoking post.
 

Phelonious Ponk

New Member
Jun 30, 2010
8,677
23
0
That brings up a question: Nicholas, is jitter lower at higher sampling rates? Or are the potential problems you're talking about independent of "hi-rez?"

P
 

Nicholas Bedworth

WBF Founding Member
May 7, 2010
312
0
0
Maui, where else?
Jitter can occur at any sampling rate... It's just the nature of what happens if the clock used to quantize any signal starts slopping around. The key thing to remember is that the magnitude of the jitter in terms of its temporal deivation from perfect relates to the amplitude of the resulting distortion, and the frequency of the jitter variation (ahead and behind perfect timing) relates to the frequency of the resulting distortion artifacts, relative to the test signal (e.g., you have a 10 kHz test signal, with distortion sidebands are 1.0 kHz to either side, at a certain amplitude above the noise floor.

These illustrations are idealized, simplified, etc., but they give you the fundamentals. The details get really complicated and are specific to a given scenario.

Lower noise floors may make the audio artifacts even more apparent. And yes, highly clock rates probably don't help. Will need to check the literature to see who has experimental data on this scenario.
 

muralman1

New Member
Jul 7, 2010
479
0
0
Sacramento Ca
Nicholas, the music on this system has no noise floor. All I get is music. Whatever jitter may be, it is certainly not audible, whether macro, or micro. That needs come with a caveat. If I were to replace my source with any number of others, there will indeed be artifacts plainly heard, especially on my system. These happen to be the players that engineers had started with oversampling, then filters to cure oversampling, then clocks to cure filters and oversampling, and now... Jitter is needed to cure filters, clocks, and oversampling. Whoopee!
 

Phelonious Ponk

New Member
Jun 30, 2010
8,677
23
0
Nicholas, the music on this system has no noise floor. All I get is music. Whatever jitter may be, it is certainly not audible, whether macro, or micro. That needs come with a caveat. If I were to replace my source with any number of others, there will indeed be artifacts plainly heard, especially on my system. These happen to be the players that engineers had started with oversampling, then filters to cure oversampling, then clocks to cure filters and oversampling, and now... Jitter is needed to cure filters, clocks, and oversampling. Whoopee!

Every system has a noise floor. If you can't hear it, that's a good thing.

P
 

Nicholas Bedworth

WBF Founding Member
May 7, 2010
312
0
0
Maui, where else?
If one is using digital sources, jitter is there regardless of filters or oversampling. Along with quantization noise, it's a fact of digital life. Presumably there are analogous gremlins in audio recording as well. Generally, there's no free lunch on the way to having a good time.

Ears and live music work pretty well, too, but then you have to deal with the musicians' union :).
 

Nicholas Bedworth

WBF Founding Member
May 7, 2010
312
0
0
Maui, where else?
@ muralman1... No, definitely not talking about oversampling.

You can hear the results of jitter (edgy strings, limited reverb, flattening of images, etc.) very easily. Jitter, while empheral, creates obvious distortion artifacts in the audio spectrum.
 

muralman1

New Member
Jul 7, 2010
479
0
0
Sacramento Ca
@ muralman1... No, definitely not talking about oversampling.

You can hear the results of jitter (edgy strings, limited reverb, flattening of images, etc.) very easily. Jitter, while empheral, creates obvious distortion artifacts in the audio spectrum.

Nicholas, I don't in the slightest doubt you are correct. As I previously wrote, there must be unmeasured distortion artifacts. When an amp building company brags every last bit of distortion is gone through cancelation, yet here, the amp exhibited substantial veiling, I have to believe there is more to distortion.

Getting back to sources, I know that, besides amps and sources, cables, and preamps can add a lot of distortion to the music. I just wonder how you differentiate between these music fowling components from source jitter?

Without concerning myself what kind of distortion I am trying to rid my system of, I succeeded to diminish audible distortion to nil. It took 8 years.
 

Nicholas Bedworth

WBF Founding Member
May 7, 2010
312
0
0
Maui, where else?
So please enligten everyone with the results of your work, and what you ended up with.

The distortion artifacts from jitter are definitely; they show up in the band frequency spectra like sore thumbs. They're aharmonic in nature to boot.

Regarding sorting out a system, the idea is to keep it as simple as possible. One places a test piece of gear into a known reference system and observes the results. There's no preamp, minimal cabling, bit-perfect input to the DAC, etc.
 

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