The threshold of audibility of phase noise in ADC and DAC clocks ...
I would suggest you begin with a working definition of "Jitter", which you could post to the HA Knowledgebase. Is jitter "phase noise" ?
What does it sound like? What does it do to a sine wave that goes through ADCs and DACs ? Does it matter at all ?
What is the order of magnitude of jitter in PCs? and in high-end audio equipment?
There is a good article about jitter @ Digital Domain:http://www.digido.com/modules.php?name=New...icle&sid=15I'm looking forward to see such test... How many of you believe in "jitter effect"?
... total jitter figures for midrange HiFi stuff is generally somewhere between 0.1ns and 100ns ...
Would such a test be usefulWould the results of such a test be valid?What sort of test procedure would be best?
...Jitter should also be analyzed directly at the clock output in order to account for hardware induced jitter....
The results would be valid if several conditions are fullfilled.On top of my head I can think about-The kind of jitter. Some kind might be audible, some kind inaudible. We must choose the kind of jitter that should cause the biggest audible effect.-The test must be performed on a system whose jitter is small compared to the tested amount of jitter.-The samples used must be sensitive to jitter.
I would dare say that none of us HA readers has the equipment to directly measure jitter with any degree of precision.
...I do have access to such equipment when I am at University, and hopefully I can get another student to teach me to use it. Real measurements would be invaluable but, as Pio2001 says, if the jitter is data dependent (and it seems to be in many cases) it becomes a much harder problem.Gigapod - you sound like you know quite a lot about jitter/phase noise - thanks for your input so far. Keep it coming
Quote from: dariju on 27 December, 2006, 11:07:49 AMThere is a good article about jitter @ Digital Domain:http://www.digido.com/modules.php?name=New...icle&sid=15I'm looking forward to see such test... How many of you believe in "jitter effect"?Having just read the article you linked to, I found it a little too much "audiophile"-minded and not very clear about the technical issue of jitter. Quoting: " ...The sonic results of passing this signal through processors that truncate the signal at -110, -105, or -96 dB are: increased "grain" in the image, instruments losing their sharp edges and focus; reduced soundstage width; apparent loss of level causing the listener to want to turn up the monitor level, even though high level signals are reproduced at unity gain..."I know quartz oscillators have a small amount of phase noise (jitter).How much does the phase noise in a 12MHz quartz oscillator driving a DAC at 48kHz affects the final waveform?
...Here is a more scientific approach by someone who has designed ADC and DAC for MRI and military devices and makes studio quality ADC and DAC’s.http://www.lavryengineering.com/white_papers/jitter.pdfunfortunately jitter is very real.
Cabbagerat suggested in a post in another thread that most manufacturers don't quote jitter figures for their audio equipment gear because the effects of jitter can ultimately be measured in noise and THD figures. I think this is correct; I would only add that providing jitter figures directly would be a) difficult (because jitter is difficult to measure and where exactly do you measure it ?) and b) meaningless, as you can only hear the side effects of jitter as added noise or distortion.
...The problem here is that THD does not measure how bad the distortion sounds. In general, higher THD is worse, but a tube amp fan will tell you that 2% THD doesn't always sound bad. That's well accepted, but it means that audiophiles (and others) will claim that the distortion introduced by jitter is particularly bad....
...C134 JITTER SIMULATION IN HIGH RESOLUTION DIGITAL AUDIO, Hawksford, M.O.J, 121st AES Convention, San Francisco, October 2006, paper 6864
Uncorrelated jitter, although it can result in modulation noise, is generally believed to be more benign compared to jitter that has a correlation with the audio data or has a relationship to a periodic signal such as mains hum.
The second one of those paper is fascinating. It's the mathematical treatment of jitter that I have been looking for for a while. One excellent quote from the paper is:QuoteUncorrelated jitter, although it can result in modulation noise, is generally believed to be more benign compared to jitter that has a correlation with the audio data or has a relationship to a periodic signal such as mains hum.Much of the complex maths in the paper is derivation of a simplification of the standard bandlimited interpolation formula, with added jitter factors. This optimisation might be necessary for real-time processing, but for offline processing simple truncation can make performance of the unsimplified algorithm acceptable on modern processors. Sections 7 and 8 (one analyzing LPCM and one sigma-delta) are interesting, as are his conclusions.
...For all its complex maths, there's an unfortunate lack of audibility data in the paper, or even a summary of previous such data, to indicate how 'important' this ends up being to the consumer .... so what does it do towards addressing the subject of this thread?
well, let me offer this 2005 paper for consideration, thenhttp://www.jstage.jst.go.jp/article/ast/26/1/50/_pdfAudibility threshold for timing jitter, for 'golden eared' listeners in a two-alternative forced-choice paradigm using their preferred listening environment and samples: 250 ns.
For tests, why not generate a simple program that can simulate jitter?