jitter reduction?

One of the principle benefits of using a computer transport is that virtually all of the jitter associated with a traditional transport is eliminated. The other main problem area is associated with the SPDIF implementation.

Yes, you can buy various reclockers but IMO (and the experience of virtually everyone else on these forums) it is unnecessary.

What will greatly affect the quality of your playback is how you rip (import) your CDs. Use Apple Lossless with Error Correction on (under Preferences/Advanced/Importing) and all other sound features (Preferences/Playback) off.

I ran a Toslink from my G5 to my TriVista for a long time with excellent results.

Here is a paper on the subject:
Eric Benjamin and Benjamin Gannon, "Theoretical and Audible Effects of Jitter on Digital Audio Quality", 105th AES Convention, San Francisco, September 1998. This paper concluded that the threshold of audibility of jitter on normal music signals is around 20nS.

Since the average cheap CD player is around 1 ns then in theory one should be fairly safe even with a portable player for next to nothing. In practice this is not good enough for most audiophiles however, and jitter has become a hotly contested subject. Many people claim to hear jitter and can tell you what it sounds like.

While I have not yet incorporated my Mac into my audio system, I can state that I have heard great improvement with the Camelot Dragon Pro II when it comes to jitter reduction. Some of the notable names, Genesis Lens and Monarchy, work but the Camelot gear was clearly better.

I discovered that with higher quality transports with their supirior powe supplies and build quality—I use a Wadia now—I did not feel the need for additional jitter reduction, though on even some top notch transports, the Camelot Dragon Pro II did add some focus.

I find that the most noticible effect of jitter reduction is improved bass. Inner detail gets better. You begin to hear individual voices and instruments more clearly.

I am interested to incorporate my computer into my system, but after consultations with other audiophiles, I expect that like the rest of any good system, the quality of the sound will depend on the quality of the DAC, cables, power supply and other components. I know you can't cut corners.

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

Gmood1 - I have a Proceed AVP. No problem hearing the effects of jitter on this one. It beats the Lexicon hands down. It's so good that I have not bothered to mod it. I dont listen to music on it anymore, only movies.

As for the crossover, I do mod the DEQX, but it is expensive. In the future, I wil be doing fewer mods, not more. My own products take up most of my time now.

Steve N.

Chris,

The authors found experimentaly through listening tests that pure tones made it easier to detect jitter. They looked at all frequencies not just 20 Khz. The paper is a mix of mathematical modelling and lab measurments and lab listeting tests. Jitter creates sideband intermodulation distortion - new frequencies appear as the jitter frequencies modulate the musical or primary signal. According to the paper, the best way to hear distortion is to get the sideband distortion in the ears sensitive 1 to 4 Khz range whilst keeping the music outside this range so there is as little masking as possible.

I'd strongly recommend to download and read it if you are interested - refreshingly absent of any formulas and complex mathematical jargon that you often see in AES papers. They used headphones in listening tests so loud speaker distortion was not an issue (most loud speaker distortion would probably make it even harder to hear jitter)

The important thing to walk away with is that it still recommends that you should always keep jitter as low as possible as distortion is cumulative...what might not be audible distortion from one root cause can accumulate with other non-linearities to become audible.

Shadorne, thanks.
c

"They used headphones in listening tests so loud speaker distortion was not an issue (most loud speaker distortion would probably make it even harder to hear jitter"

Headphones???

I used to believe that headphones were better than speakers, but not anymore. I have attended headphone shows and tried the best of the best. My ribbons sound better and give more detail and 3-D imaging than the best headphones available. The sibilance from jitter I have found to be more obvious in a system that images properly. Headphones dont image.

It's like trying to identify a flaw in a 2-D photograph versus a 3-D live image. Much easier to pick out the flaw in a 3-D presentation. IME the brain uses its best sensing capability in stereo, both audio and visual.

Steve N.

I don't doubt that a ribbon would likely make jitter distortion all the more audible. I am just surprised that a ribbon would be regarded as a reference source for listening for jitter distortion. Dome tweeters in general have much lower distortion. Given distortion is cumulative would it not be better to use a dome tweeter?

I can easily hear the effects of jitter with either dome tweeter or a good ribbon. The point that I was making is that true stereo improves your odds for hearing jitter easily using lots of different source material.