jitter reduction?

Jitter studies must be taken with grain of salt. The systems and recordings used for these are all inferior IMO. I have been reducing jitter in my own and other products for many years. Each time I think the incremental jitter reduction will be inaudible, I'm surprised to find that it is audible, at least in my system, which has world-class resolution and extension. For instance, an obvious change can be heard between the excellent Superclock4 and the new Ultraclock when used for clocking USB interfaces, CDP's or upsampling DAC's. Both of these have miniscule jitter, on the order of 10-100's of picoseconds.

Most jitter circuits in CDP's and DAC's have limited effect. None of them completly eliminate jitter. This is impossible. Most use either a PLL scheme or asynchronous upsampling (ASRC). The chips that do these inject their own jitter, and usually are not immune to incoming jitter either, although they do recude overall jitter.

The goal should be making it inaudible. Again, if your system has a lot of other sibilance and noise, then you may not hear the benefits of low jitter. It is just one more thing that needs to be addressed when creating a highly resolving sytem.

Here are some more papers to read:
http://www.positive-feedback.com/Issue22/nugent.htm
http://www.positive-feedback.com/Issue14/spdif.htm

Steve N.
Empirical Audio

Jitter is definitely audible..it doesn't take a big buck system to hear it either. Just a very simple one will allow you to hear the differences in lowering the jitter.One that doesn't use a processor as a preamp would be a good start. I find most or all the preamp processors I've heard mask or strip the music of its essence...all the while adding noise. I doubt anyone using one of these for two channel music could hear any differences in jitter.

As stated above...more focus in the sound stage is one of positive effects of lower jitter. Having auditioned one of Steve's Dacs...this guy knows what he's talking about!

Just using your Mac's HD as a transport will be better than the average CD transport.

Dave's assertion that a "DSP preamp will mask or strip the music of its essence while adding noise" so as to make jitter inaudible in two channel music does not seem to be supported by the authors (they tested a lot of players and interfaces). To me this sounds like an "urban legend" - there are a lot of DSP's amps out in the market today in HT setups and I don't believe there is such a universal problem with them all. Anyway, I won't get drawn into a discussion on opinions to which there is no conclusion, instead I leave you to make up your own mind and suggest you get the paper ($20 at AES website), I believe you will find it quite readable and interesting.

I can't send you the paper as you need to buy it from the AES, however, I can extract the conclusion;

The effect of clock jitter in the digital interface was studied extensively. Measurements of the jitter spectrum of numerous digital audio sources, primarily DVD players, were conducted. A wide range of performance was found. The jitter spectrum of a typical source can be characterized as a white noise floor with one or many sinusoidal jitter components with a magnitude in the range of 10 ps to 10 ns rms. The effect of jitter induced in the interface was studied and found not to be a significant factor for short interconnection runs likely to found in a domestic environment. Several DACs and their DIRs were measured in order to characterize the sensitivity to distortion induced by jitter. These results were compared to each other and to results derived from simulations. Most DACs were found to be similar to each other and to the simulation in terms of susceptibility to jitter induced distortion. That distortion is approximately -107+201og(F)+201og(J) dBr for sine wave signals at F kHz with J ns rms of clock jitter. Up-Down threshold and AB comparison listening tests were conducted to determine the threshold of audibility for jitter-induced distortion. The threshold of audibility for pure tones was found to be about 10 ns rms at 20 kHz and higher at lower frequencies. For nearly all program material no audible degradation was heard for any amount of jitter added below the level at which the DIR lost lock. Certain program material was found in which an audible degradation due to jitter was heard. The threshold of audibility for these programs was generally found to be in the range of 30 ns rms to 300 ns rms for sinusoidal jitter. Finally, the audible degradation was found to correspond to measurable
changes in the spectrum of the program material.

The influence of jitter in causing audible distortion was found to be less than anticipated by the authors, and less than that predicted by both the technical and consumer audio press. Jitter induced by the digital audio interface was not found to be an audible problem for any of the program material auditioned.

It should not be assumed that jitter-induced distortion is a non-issue. Distortion induced by jitter is a real phenomenon and work to reduce its effects should continue. Although the threshold of audibility was found to be relatively high in the authors' experiments, the effect of all distortions in the audio chain is cumulative and it is reasonable to reduce them to the lowest practical levels. Manufacturers of DACs may find the methodology for evaluating jitter susceptibility presented in this paper useful in characterizing and presenting meaningful jitter specifications for their products.

Acknowledgments
The authors would like to express their appreciation to Edmund Chu, who designed and built the wide bandwidth AB comparator system used in the listening tests. We would also like to thank our many colleagues at Dolby Laboratories who participated in long and difficult listening tests.

As Dave and Steve have pointed out it is indeed a complex issue. Dave claiming to be able to hear jitter quite easily because of his highly resolving non-DSP system. The extensive lab listening tests done by the authors was only a miniscule percentage of the population - so it certainly leaves a "crack" in the door open to the "golden eared" with highly resolving systems. The authors of the paper (Dolby Labs) say they were surprised at how high the level of jitter needs to be in order to be above the threshold of audibility, however, they sensibly recommend maintaining jitter as low as possible, "it is reasonable to reduce them [jitter] to the lowest practical levels."

On reducing jitter as being a good thing everyone is in 100% agreement!

I'm not saying DSPs are bad. They have their place in audio. For HT.. DSPs are great! None of them I've heard do well with 2 channel music. I owned Meridian and friend owns Lexicon. Dealer friend carried McIntosh and now carries Anthem DSPs. I've heard quite a few over the years.

I'm not totally against DSP. I've recently been playing with the idea of buying a second pair of speakers that use active crossovers. I would of course still use the analog inputs of the active crossovers and a analog style preamp.

The Dacs in these units at least IMHO still leave something to be desired in musical enjoyment. Now if someone can talk Steve into modding the digital crossovers and offering it at an affordable price for the average Joe(like myself)..we're in business! LOL

All I can say is you'll have to put the papers down some day and listen for yourself.From your statement "there are a lot of DSP amps out in the market today in HT setups"...you are correct..key word being HT.

Good listening

Shadorne, thanks for your above post, gotta love objective measurement.

The threshold of audibility for pure tones was found to be about 10 ns rms at 20 kHz and higher at lower frequencies.

I would like to seek your opinions on the above, though.

1. I am gong to assume that Dolby typically has experienced (read, greybeards;)) and to listen for audibility at 20kHz and on a pure test tone, would seem to be counterintuitive. Firstly, high frequency hearing goes down with age and you really have to crank a 20kHz tone up, to hear it, were they trying to perform a "worst case" secenario? But the above makes sense as they do say it is higher at lower frequencies.

2. Secondly, why do it with a pure test tone, when we react to complex waveforms in another manner, and music is, of course made of complex waveforms so I am surprised they didn't use a beat frequency oscillator or any other complex waveform. Do you have input on that?

3. If the threshold of audibility was indeed higher at lower frequency, why not take most of the tests at 1kHz, right in the middle of the curve?

Not attacking you nor their tests at all. I have been an executive for so long, and off the test bench, that I am just curious if you knew?

Now, unless I am missing something, when I look at Atkinson's Miller Suite data,it seems about 250pS PEAK is where he draws the line on good/bad DAC performance, which means, if I am understanding this correctly, that virtually any audiophile DAC is an 100X better approximately than what they say is the hearing threshold..Yes? No?

Thanks,
C