Positive Feedback: Article on Jitter.

Soundminded

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1. If jitter is an irritating problem in audio, it would be a catastrophe with video. The corresponding distortion to images would be many, perpetual, and obvious.

2. I'm always very suspicious of audiophile arguments that speak in qualitiative terms without quantitative numbers. What is the threshold of audibility of jitter? How much is typical in "bad" equipment? Ultimately errors in the digital domain and in A/D and D/A conversion will show up as non-linear distortions in the analog domain. Where are they? Why are they not measured. Where are the differences in processed analog end waveforms? What do analog distortion tests show? IMO most CDs suffer largely from linear distortion, that is FR variations that are all over the map. I have to equalize every CD individually. I attribute this to the fact that the kind of equalization that was once used to calibrate monitor speakers in recording studios during the era of vinyl LPs is no longer SOP today. Recording engineers simply use their favorite unequalized hi fi speakers as monitors and what you get is the FR of that monitor speaker...or whatever the tonal balance of the microphone used was. IMO this may explain their frequent tendency to be too bright.
 

amirm

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1. If jitter is an irritating problem in audio, it would be a catastrophe with video. The corresponding distortion to images would be many, perpetual, and obvious.
Jitter used to be visible in CRT displays. With modern fixed pixel displays however, jitter is inconsequential because we "know" the position of the pixels. If the timing says to light up pixel 2.0001, we know to ignore the fraction and activate pixel number 2.

Likewise, all digital transmission schemes including data moving around in your computer are subject to jitter. Just as video though, the timing variations are thrown away because we know where the bits need to go.

Audio works differently. We are looking at pure analog output so "when" a sample needs to materialize matters. Timing variation there modulates the output signal and creates sideband spikes that are clearly measurable. I wrote an article for the upcoming issue of Widescreen Review magazine and addresses its audibility from psychoacoustics point of view. It is a surprisingly small value.

Here is an article I wrote a while back and talks about some of this: http://www.madronadigital.com/Library/DigitalAudioJitter.html
 

Soundminded

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Any way you slice it, digital jitter would show up as non-linear distortion in the analog domain. It magnitude is insignificant especially when compared to all analog forms including magnetic tape recording, vinyl phonograph records, phono cartridges, and loudspeakers. Also vacuum tube power amplifiers. We heard the same siren song about FM distortion from Paul Klipsch for decades yet most high end loudspeakers are not horn speakers. Someone always comes up with a distortion du jour to find fault with a better technology or to justify a fix they have to sell for a problem that either doesn't really exist or is so small it is irrelevent in the overall scheme of things. Small wonder most such people do not have hard numbers to compare what is audible against what exists and explode at the mere mention of double blind testing to determine if what they say has merit to the point where expensive fixes are justified. One year it's time alignment distortion, another it's even harmonic distortion or odd, I always forget which one I'm supposed to be worried about in equipment that has less than 0.1% distortion, the next year it's something else.

BTW, -80db is not audible, it is far below the threshold of noise in virtually every listening room especially when other sources of noise from audio gear is taken into consideration. The widest dynamic range of any music would be for classical music, In the rarest of instances it is about 80 db. A one db deviation will not be audible in that circumstance.
 

rbbert

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Hmmm, you need to go read posts in some other topics here... ;)
 

amirm

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Any way you slice it, digital jitter would show up as non-linear distortion in the analog domain.
The position of a pixel in video is digital, not analog. It is at distinct spots and it cannot move. It can only do so in audio because there are no fixed point events in time and space.

It magnitude is insignificant especially when compared to all analog forms including magnetic tape recording, vinyl phonograph records, phono cartridges, and loudspeakers.
That is true but works on entirely different principal as far as audibility. The jitter in analog domain is usually at a very low frequency. As such, masking removes its audibility for the most part. What then becomes audible is due to amplitude modulation, i.e. wow and flutter. The two are not comparable for the most part unless analog "jitter" rises to higher frequencies. See a huge deep dive here: http://www.whatsbestforum.com/showt...to-Wow-Flutter&p=144993&viewfull=1#post144993

BTW, -80db is not audible, it is far below the threshold of noise in virtually every listening room especially when other sources of noise from audio gear is taken into consideration. The widest dynamic range of any music would be for classical music, In the rarest of instances it is about 80 db. A one db deviation will not be audible in that circumstance.
Putting aside the fact that we hear tones well inside the noise, the determination of noise floor of listening rooms is done incorrectly. It needs to be done relative to psychoacoustics of our hearing. Once you do that, you will find that the noise floor is far lower than people say and you state above. See this article I wrote: http://www.madronadigital.com/Library/RoomDynamicRange.html

As I noted before, I have a follow up article in the November issue of WSR on audibility of small distortions such as jitter. It is a short one but does hit on this point.
 

Gregadd

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DBT is the last refuge of...
 

Soundminded

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Wow and flutter are frequency modulation. They are heard as a perceived change in pitch, rising and falling up and down for music. A figure of 0.1% or less for magnetic tape recordings and phonograph records is excellent, below audibility for most people. My hearing is good to about maybe somewhere between a quarter and an eighth of a halftone. That's around 1% or slightly less. 80 db down is 0.000001% (times 2 considering both upper and lower resonances.) From the graph they are not really wow and flutter, they look more like sideband single frequency noise above and below the base reference frequency unless the real image is not steady state and your oscillophotograph is just a representative or worst case capture. I'll be curious to see what kind of objective DBT measurements you have by switching it in and out to indicate that it is audible cited in your upcoming article. I presume you've used the same kind of prescreened listeners Harman Kardon uses for their audiometric testing.

Tolerance for timing errors in video pixels before they become visible would depend on whether or not the jitter amplitude exceeds the time window for each pixel. That window can be determined by considering the number of pixels per line, the number of lines per image, and the number of frames per second. For example the image I'm looking at now is 1920 x 1080 x 120 or about 200,000,000 pixels per second. That's a maximum window of 5 picoseconds. New higher resolution (8x as great) and more frames per second will narrow the window even further. By comparison, the audible tolerance for jitter is enormous. I'm having a difficult time accepting your argument Amir.
 
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Gregadd

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Those who want to know what they hear rather than what they imagine?

Tim
Another data point in their objective decision making . [Amir]
To be honest they would start with a true DBT and use the results to support their argument. The"last refuge implies use of the methodology of DBT as an argument after other tactics has failed.
As in DBT has shown as opposed to DBT will show.
 

Soundminded

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Amir, I hope you have devised an audible DBT test where the change in digital jitter is the only variable. Many tests are flawed because more than one variable changes at a time. This invalidates the results because perceived difference can be caused by factors other than the one being considered. Among the many examples of this are Cheever's master's degree thesis on audibility of THD in audio amplifiers (a horribly flawed paper for many reasons IMO) and a demonstration in Japan where it appeared that ultrasonic sounds were audible which was later corrected by the eliminating the flaw in the test method and shown that it isn't (that was the expected result.)
 

Ethan Winer

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I'm always very suspicious of audiophile arguments that speak in qualitiative terms without quantitative numbers. What is the threshold of audibility of jitter? How much is typical in "bad" equipment?

Indeed. :D

If I can editorialize a bit: Years ago - before the "blogger" generation where anyone could spout off like an expert - magazine writers actually knew what they were talking about. And when they said something erroneous, a knowledgeable technical editor would catch it. Today everyone is an expert, and some just make stuff up as they go. It seems too many people lack basic logic skills to separate fact from BS.

--Ethan
 

Gregadd

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http://www.tnt-audio.com/intervis/pearsone.html
LC >
An excellent example of the power of the Internet is given by the mailing lists (not the newsgroups, too much *flames-infected*) such as analogue-addicts, for example.
What's your opinion on the Internet as a media to exchange ideas freely ? Do you believe there's a future for Internet magazines and newspapers (as long as they remain free)?

HP >
I quite dislike most of the Internet discussions on audio. They are the deaf trying to speak to the deaf. I find few stimulating ideas and almost no good criticism on the Net, where everyman becomes a "critic". And where virtually every man is unqualified to criticize.
I don't think there's any future for Internet magazines that are free. Once someone figures out how to publish on the Net profitably, then it will be a different ballgame. But some of us hate reading off a television screen and want to hold our magazines in our hands.


Harry Pearson formerly of the absoute sound.
 

microstrip

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DBT is the last refuge of...

The three letters DBT are the refuge for those who were never able to fully describe valid blind tests and want to write short posts. ;)

IMHO, the main question is not the method but its implementation and aim. Any correct method to avoid external bias is welcome, if it does not affect the test it self. Except for the tests carried by the Harman group or papers from research groups studying psychoacoustics and/or compression algorithms, I have never read about a proper documented audio blind test and its analysis.

Can any WBF member describe with detail a properly carried blind test in which he has participated, its statistical analysis and conclusions?
 

Gregadd

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Some doctors have posted a DBT protocol for medical studies. They are quite rigorous

Changing the results of a DBT to mirror the expected results? Hmmm.

I'm not an expert but, I do have a B.S. in mathematics. I can state not everything is quantifiablle
 

Soundminded

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The three letters DBT are the refuge for those who were never able to fully describe valid blind tests and want to write short posts. ;)

IMHO, the main question is not the method but its implementation and aim. Any correct method to avoid external bias is welcome, if it does not affect the test it self. Except for the tests carried by the Harman group or papers from research groups studying psychoacoustics and/or compression algorithms, I have never read about a proper documented audio blind test and its analysis.

Can any WBF member describe with detail a properly carried blind test in which he has participated, its statistical analysis and conclusions?

"Can any WBF member describe with detail a properly carried blind test in which he has participated, its statistical analysis and conclusions?"

I am sure that audiologists have a professional organization with a technical journal and peer reviewed papers. Those who study psychoacoustics using many methods like FMRI probably also perform scientifically rigorous tests to try to understand the science of sound and hearing. For audio equipment manufacturers maybe not. It's rather surprising that considering how Toole claimed to have scientifically determined what people like if not what is most accurate, Harman isn't producing speakers that dominate the marketplace. Did he draw the wrong conclusion or has Harman simply not capitalized on what it learned from him?
 

Gregadd

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rbbert

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A big part of the problem with DBT's in the audiophile arena, as we've discussed here before (at length!), is that there is little or no funding available for studies. For the most part high-end manufacturers, if they have enough money to fund studies, have no interest in doing so because there is little likelihood of any return on investment. Getting enough participants to be able to demonstrate a small difference, and even more so to "prove" no difference, is difficult at best, and that doesn't even get into the further potential obstacles of study design and analysis.
 

Phelonious Ponk

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"Can any WBF member describe with detail a properly carried blind test in which he has participated, its statistical analysis and conclusions?"

I am sure that audiologists have a professional organization with a technical journal and peer reviewed papers. Those who study psychoacoustics using many methods like FMRI probably also perform scientifically rigorous tests to try to understand the science of sound and hearing. For audio equipment manufacturers maybe not. It's rather surprising that considering how Toole claimed to have scientifically determined what people like if not what is most accurate, Harman isn't producing speakers that dominate the marketplace. Did he draw the wrong conclusion or has Harman simply not capitalized on what it learned from him?

This assumes that people listen as objectively when shopping as they do when involved in testing, that they buy speakers for sound alone, and that what they hear is not influenced by many other factors in the purchasing process. I don't think any of those assumptions are likely to be true.

Tim
 

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