Interesting tidbits about "jitter" in digital audio reproduction

amirm

Banned
Apr 2, 2010
15,813
37
0
Seattle, WA
Forum members have probably heard the term "jitter" mentioned in the context of digital audio reproduction many times. Googling the terms "audio jitter" generates 780,000 hits! Yet, I doubt that some of the salient points of jitter is mentioned in the midst of heated discussions that occur around it.

This article is not designed to be complete coverage of the topic but rather, answer a question posed in another thread when I mentioned what the minimum spec must be met for a digital system to fully reproduce CD frequency response and resolution. The number is a surprising, 0.25 billionth of a second. This article is meant as an explanation of why, when we only have 44,100 samples, we need such mind numbing accuracy in timing to reproduce it.

First, an ultra quick introduction to jitter: jitter is one or more causes of variation in timing of a signal. What is a timing signal? Digital systems always operate according to a "clock." A clock is a sequence pulses which tell the system when to do its next thing. In audio, every clock tick tells the system to input or output another audio sample. Theory of digital audio reproduction assumes that the clock is infinitely precise. Unfortunately, this is never true in real life. In computers, the “Ghz” number you hear is a similar concept, determining how fast the system executes instructions.

Clock sources like any other electronic circuit or your watch, have their limit in how precise they are. Variations can occur due to many, many different causes which I won’t go through right now. One way or the other though, these variations cause the clock pulses to dance back and forth instead of precisely happening at the moment they are supposed to trigger.

You might be asking, so what that the timing changes a bit here and there. How can that change the sound? The answer lies in transforming this analogy “from time to domain” into “frequency domain.” Once there, we can then see if the spectrum of the signal changes. Here, the picture clears quite a bit. If you take a signal and move it back and forth in time using another signal (jitter), what you get in the output is your original signal, plus “sideband” shadows of it proportional to the difference between the two signals.

For example, if you try to playback a 5Khz signal, and you have a timing variation which has a frequency of 1Khz, what the system will reproduce is the 5 KHz signal, plus shadows of it at 4 and 6 KHz. These spurious frequencies are clearly distortion products as they did not exist in the original signal. And as such, can be used to compute the distortion of the system.

Question then becomes, how much jitter is audible? This can be a long, never ending debate :). What I like to drill down into here is the answer to the original question: how low does the jitter need to be for us to be able to reproduce the full resolution of audio signal. Whether you can hear distortion if your CD system is reproducing 15 or 14 bits instead of 16 due to jitter, is a different topic.

There is a great paper that answers the question hand which was published years agao in Audio Engineering Society (AES) conference proceedings: http://www.nanophon.com/audio/jitter92.pdf. Alas, it is written for industry experts so it might be hard to digest it. So here is a simplification of it.

The paper computes jitter contributions to distortion by making the simplifying assumption that it could be sinusoidal in nature. The 60 Hz hum from the power supply in your audio equipment is an example of such source. It serves to change the clock time by the frequency of power coming into your house which if you seen on a scope, looks like a sine wave. Unfortunately, many causes of jitter are of other nature and audibly, can be more disruptive. For the purposes of this article though, we won’t have to drill there.

If we glance at the formula for computing jitter using sinusoidal spectrum, we see this conclusion:
"For sinusoidal jitter of amplitude J=500ps, a 20 KHz maximum level tone will produce sidebands at -96.1 dB relative to the input tone."

Let’s put that in English. The formula is telling us that if we want to reproduce a frequency response which extends to 20 Khz (typical end of human hearing spectrum and within the capability of CD audio), then the jitter value of 500 picoseconds, or 250 picoseconds above and below zero, creates a distortion product which is weaker than the signal by 96db.

OK, that is still Greek to many :). Let’s apply one more level of translation. In digital processing, every bit of resolution is roughly equiv. to 6db of dynamic range. So a 16 bit CD audio signal has a 96 db (16 * 6) range between the quietest and loudest portions. Put another way, if you feed the system a 0 db reference signal, 16-bit signal is able to reproduce detail that is 96 db below that. Anything lower would be noise/distortion generated by the 16-bit system.

With me so far? Now let’s combine the two translations and realize that the -96db number in the paper was not picked at random, but was the lowest noise/distortion floor of 16-bit audio. Therefore, if we want to reproduce all that 16-bit audio can give us, up to the 20 KHz desirable response, then the clock timing cannot vary by more than 250 picoseconds. And therefore the minimum jitter spec must be below 250 picoseconds.

What is a picoseconds? It is an awfully small number. If you have forgotten the definition, here it is: http://en.wikipedia.org/wiki/Picosecond:

“A picosecond is 10?12 of a second. That is one trillionth, or one millionth of one millionth of a second, or 0.000 000 000 001 seconds. A picosecond is to one second as one second is to 31,700 years.”

At this point of the article, people usually ask if the above is audible. Once more, this article is not about what is audible but rather, what the system specification needs to be to reproduce what it is advertised to do. CD audio advertises 16 bits of sample resolution and 22 Khz frequency response. Therefore, its jitter spec must be below 0.25 billionth of a second. Period. The math dictates this so there is no debating it. So the precision required here is incredibly high – orders of magnitude more than anyone could imagine, not being skilled in the science.
 

DonH50

Member Sponsor & WBF Technical Expert
Jun 22, 2010
3,947
306
1,670
Monument, CO
Nice job Amir! I keep thinking about posting some plots and numbers to bring it down the earth but haven't had the time (hate it when Work interferes with Life). Only thing I would add for now is something you clearly imply: the impact of jitter depends on the signal frequency, not the clock. Having a converter running at 44.1 ksps, 192 ksps, or 1 Gsps, you still need that same 250 ps of jitter or less for that 22 kHz'ish signal tp maintain a 16-bit noise floor.

FWIWFM - Don
 

ack

VIP/Donor & WBF Founding Member
May 6, 2010
6,774
1,198
580
Boston, MA

MikeDuke

New Member
Jul 5, 2010
37
3
0
Eastern PA
Great topic. This is something that I have thought about a bit. My player, the Esoteric DV50s has a high jitter rate I guess. Stereophile measured 495pico secs.
This is what was said at the end of the measurments section.
"In almost all respects, and whether playing back CDs, SACDs, or DVDs, the Esoteric DV-50 gets a clean bill of health. However, its merely good jitter rejection gives rise to some minor concern on my part. Still, it's fair to note that Paul Bolin noticed nothing amiss in his auditioning"

But, none the less in just about every review for the DV50s, it is complemented on how good it sounds. Have I heard better yes. Do I think it's bad, no. In many reviews it was compared to players that cost more than it did and like I said, the SQ was always considered to be very good. To me, in my system the player performs very well. Now my TT does sound much better to me when I listen to it. I am trying to figure out what the jitter adds to the sound. The one way I can sort of verbalize it is that when I heard better players, or my own TT the music sounds less forced and edged. More relaxed. But if I only ever heard the Esoteric I would probably never think something was “wrong”. Again, I like the way my player sounds and I have to admit that I really like the way SACD’s sound. I was obsessed with this for a while but now I just throw in a disk and enjoy:D. Thanks for starting this.
 

MylesBAstor

Well-Known Member
Apr 20, 2010
11,236
81
1,725
New York City
Great topic. This is something that I have thought about a bit. My player, the Esoteric DV50s has a high jitter rate I guess. Stereophile measured 495pico secs.
This is what was said at the end of the measurments section.
"In almost all respects, and whether playing back CDs, SACDs, or DVDs, the Esoteric DV-50 gets a clean bill of health. However, its merely good jitter rejection gives rise to some minor concern on my part. Still, it's fair to note that Paul Bolin noticed nothing amiss in his auditioning"

But, none the less in just about every review for the DV50s, it is complemented on how good it sounds. Have I heard better yes. Do I think it's bad, no. In many reviews it was compared to players that cost more than it did and like I said, the SQ was always considered to be very good. To me, in my system the player performs very well. Now my TT does sound much better to me when I listen to it. I am trying to figure out what the jitter adds to the sound. The one way I can sort of verbalize it is that when I heard better players, or my own TT the music sounds less forced and edged. More relaxed. But if I only ever heard the Esoteric I would probably never think something was “wrong”. Again, I like the way my player sounds and I have to admit that I really like the way SACD’s sound. I was obsessed with this for a while but now I just throw in a disk and enjoy:D. Thanks for starting this.

Guess if you knew about the jitter problem beforehand, you would have never bought the Esoteric player! :)
 

Ron Party

WBF Founding Member
Apr 30, 2010
2,457
13
0
Oakland, CA
I am trying to figure out what the jitter adds to the sound. The one way I can sort of verbalize it is that when I heard better players, or my own TT the music sounds less forced and edged. More relaxed.
Welcome to the forum, Mike. Good to see another Seaton afficionado here.

Query: What makes you think *jitter* is the reason for your perceived sound difference between the Esoteric and your TT?
 

MikeDuke

New Member
Jul 5, 2010
37
3
0
Eastern PA
Thanks for the welcome. I don't if that is the difference. I am just trying to think how it would manifest itself. I have heard much better players then mine and the music is much "smoother". I am guessing that a lower jitter may play a part in that. I have no evidence to back that up though. I am just curious as to how excess jitter manifests itself when listening to music. In the reviews I have read, where the reviewer was wowed, it was typically with a player that had low jitter. The ones that they did not really like had a higher jitter. Again, I like the way my player sounds. I have not heard another high end player in my own system though. I can only compare it to other players I have heard in other systems and my TT. Not the best method I know, but what can you do.
 

MikeDuke

New Member
Jul 5, 2010
37
3
0
Eastern PA
Myles,
I really did not know much back then I took my dealer at his word when he told me outstanding the player was. Agian, I hate to be a parrot but I do like it very much. At the time I wanted a Universal player and this was the best one out there fit my budget.
 

MylesBAstor

Well-Known Member
Apr 20, 2010
11,236
81
1,725
New York City
Myles,
I really did not know much back then I took my dealer at his word when he told me outstanding the player was. Agian, I hate to be a parrot but I do like it very much. At the time I wanted a Universal player and this was the best one out there fit my budget.

Nothing wrong with liking your Esoteric :) Finding a true high-end dealer where you can trust their recommendations is a real plus!
 

Vincent Kars

WBF Technical Expert: Computer Audio
Jul 1, 2010
860
1
0
The audibility of jitter

What I have found on the Internet most of all gives me the impression that the audible threshold of jitter is not well established.
Maybe the AES has more but I have no access to their library.
What I have found up to now :

In 1974 the BBC research department concluded:
For jitter having a random, white noise, spectrum extending from 30 Hz to 16 kHz, it is estimated that impairment on critical programme would be perceptible to less than 5% of listeners provided the jitter amplitude is no more than 50 ns r.m.s.
Source: BBC

Ashihara (2005) did an experiment indicating that random jitter is not audible below the 250 ns

Adams (1994) states that jitter threshold is dependent on the combination of components used in the DAC and concludes that phase jitter has to be as low 20 ps – 1 ns to obtain signals of 16 bit quality.

Ivar Løkken, 2005 citing: Dunn, J.: “Jitter: Specification and Assessment in Digital Audio Equipment”,
AES Convention Paper 3361, October 1992.
We can see that the audibility threshold decreases from 500ns at low frequencies to as
little as 20ps at 20kHz. Especially when using formats or converters with high sample-rate this will be a major issue.

The digital equivalent of flutter is periodic jitter, which is caused by instabilities in the sample clock of the converter (Rumsey & Watkinson 1995). The sensitivity of the converter to periodic jitter depends on the design of the converter. Periodic jitter produces modulation noise. Practical research by Benjamin and Gannon involving listening tests found that the lowest level of jitter to be audible on test signals was 10 ns (rms). With music, no listeners in the tests found jitter audible at levels lower than 20 ns (Dunn 2003:34).
Source: http://en.wikipedia.org/wiki/Analog_sound_vs._digital_sound


So we can choose,
If we believe Asihara, almost all DAC’s will qualify.
If we believe Benjamin and Gannon, al lot won’t
If Dunn is right, very few will.

A bit more details: http://thewelltemperedcomputer.com/KB/BitPerfectJitter.htm
 

naturephoto1

Member
May 24, 2010
820
7
16
Breinigsville, PA
www.nelridge.com
As old as it is, I am adding an Audio Alchemy DTI PRO32 to my system particularly for Redbook CD to help to combat the jitter issue. It is scheduled for FEDEX delivery tomorrow. I will test it to try to hear any differences in performance,in the system Then, it is off for upgrade/modification and then back into my system after that. I also have a special Revelation Audio I2S digital link cable on order to run from my upgraded (modified) April Music Stello CDT 100 and the Audio Alchemy DTI PRO32. The Audio Alcehmy DTI PRO32 will then connect through an AES/EBU digital Balanced cable to my upraded (modified) Lavry DA10 DAC.

Rich
 

amirm

Banned
Apr 2, 2010
15,813
37
0
Seattle, WA
The difficulty with jitter is that the measurement by itself has no value in understanding how bad its effect might be. it is a cool term to use to show off to your friends that you know digital audio but understanding what it really means as far as audibility, requires a lot more knowledge. I hope to write up something on that soon :).
 

MylesBAstor

Well-Known Member
Apr 20, 2010
11,236
81
1,725
New York City
The difficulty with jitter is that the measurement by itself has no value in understanding how bad its effect might be. it is a cool term to use to show off to your friends that you know digital audio but understanding what it really means as far as audibility, requires a lot more knowledge. I hope to write up something on that soon :).

Now I'd read that :)
 

DonH50

Member Sponsor & WBF Technical Expert
Jun 22, 2010
3,947
306
1,670
Monument, CO
Never heard of a DAC that didn't require an output filter and a clock, at least for audio systems. I suspect both are there, internally...

There are a lot of papers on jitter and listening tests, and a lot of them leave a lot to be desired (imo). A couple of those have been discussed endlessly on other forums so I won't start again (I've been warned). Among the things to consider is that, like many other parameters, jitter is likely to be much easier to identify on simple tone and pink (white, brown) noise tests than on actual musical (or movie) material. I keep thinking I'll run some plots using some of my analysis programs and try to bring it home for people with numbers and pretty pictures, but time runs away... I will note that deterministic (signal-dependent) jitter is likely to be much more audible than random jitter (noise).

fwiwfm - Don

edit: Amir posted whilst I was typing... It appears that we are on the same page again.
 

muralman1

New Member
Jul 7, 2010
479
0
0
Sacramento Ca
My DAC is a reworked Audio Note DAC, and it does not have a digital filter, or after clock. The music and all audible building sounds appear out of blackness.
 

MylesBAstor

Well-Known Member
Apr 20, 2010
11,236
81
1,725
New York City
Never heard of a DAC that didn't require an output filter and a clock, at least for audio systems. I suspect both are there, internally...

There are a lot of papers on jitter and listening tests, and a lot of them leave a lot to be desired (imo). A couple of those have been discussed endlessly on other forums so I won't start again (I've been warned). Among the things to consider is that, like many other parameters, jitter is likely to be much easier to identify on simple tone and pink (white, brown) noise tests than on actual musical (or movie) material. I keep thinking I'll run some plots using some of my analysis programs and try to bring it home for people with numbers and pretty pictures, but time runs away... I will note that deterministic (signal-dependent) jitter is likely to be much more audible than random jitter (noise).

fwiwfm - Don

edit: Amir posted whilst I was typing... It appears that we are on the same page again.

47 Labs digital gear does the same thing as Audio Note too.
 

About us

  • What’s Best Forum is THE forum for high end audio, product reviews, advice and sharing experiences on the best of everything else. This is THE place where audiophiles and audio companies discuss vintage, contemporary and new audio products, music servers, music streamers, computer audio, digital-to-analog converters, turntables, phono stages, cartridges, reel-to-reel tape machines, speakers, headphones and tube and solid-state amplification. Founded in 2010 What’s Best Forum invites intelligent and courteous people of all interests and backgrounds to describe and discuss the best of everything. From beginners to life-long hobbyists to industry professionals, we enjoy learning about new things and meeting new people, and participating in spirited debates.

Quick Navigation

User Menu

Steve Williams
Site Founder | Site Owner | Administrator
Ron Resnick
Site Co-Owner | Administrator
Julian (The Fixer)
Website Build | Marketing Managersing