Jitter : Better Sound?

Robert Katz Leave a Comment

On Tue, 30 Apr 1996, Bob Katz wrote:
I have observed that I can copy a CD into the DAW, and make a new CD that sounds better than the original.

Steve Potter wrote:
Bob, how would you define ‘sounds better’?

And I reply:

In summary, let’s compare two presentations of the same material, which is otherwise identical data. The one which is “warmer, wider, deeper”. (Pick one, two or three) is the one with less jitter. It makes sense logically, and it has been borne out by measurements.

How can a listening test be objective? How can we separate “different” from “better”?

This is a very tough question to answer. For example, regarding listening tests for jitter, no one that I know has reached the point of being able to say: “100 picoseconds of white noise jitter mixed with 25 picoseconds of signal-correlated jitter sounds better than 100 picoseconds of signal-correlated jitter mixed with 25 picoseconds of white noise jitter”.

No, we have not reached the degree of sophistication where we can judge “better” in that fashion.

However, a number of experienced listeners have been able to use the judgemental term “better” in ways that correlate quite well with the measurable physical phenomena that are under investigation.

For example:

First: one definition of “better” is the source which sounds closer to the analog master tape, or the live source if it is available.

Second: Those of us who have been chasing the jitter phenomenon have begun to educate our ears and recognize the sonic differences that different types and degrees of jitter cause. Please note that in every case I use the same D/A converter to monitor the different sources under test at repeatable monitoring gains.

If you wish to begin entering down this jittery road, I suggest you start listening to the easiest form of jitter to recognize, one which every user of Workstations can hear every day, while monitoring through a high-resolution playback system. (A high-resolution playback system consists of a good 24-bit D/A converter, wide range monitors in an acoustically treated room, power amplifier with wide dynamic range, and a quiet listening environment).

The easiest form of jitter to recognize is signal-correlated jitter. Signal-correlated jitter adds a high-frequency (intermodulation) edge to musical sounds when monitored on a susceptible D/A Converter. Obviously, a theoretically perfect D/A would not be susceptible to jitter. We must not blame the message directly…just that the message is contaminated with jitter.

Every day I load into the workstation connectors while listening through the desk, I hear (and am bothered by) the most primitive form of proof that correlated jitter is audible: The sound is different during the load-in than during the immediate playback from the hard disk! It also sounds demonstrably worse during the load in than during the playback.

This is attributable to the intermodulation of Sonic’s master clock by signal-correlated jitter during the loadin. During playback, since the source is no longer feeding digital audio, even though the DAW is still locked to the external clock, the external clock is much more stable from the DAW’s point of view. Its PLL is no longer modulated by the digital audio that is combined with the external clock. This well-known phenomenon, known as signal-correlated jitter, has been documented by researcher Malcolm Hawksford. See AES preprint titled “Is the AES/EBU S/PDIF Interface Flawed” AES Journal 1995. This form of jitter is quite audible.

After an engineer learns to identify the sound of signal-correlated jitter, then you can move on to recognizing the more subtle forms of jitter and finally, can be more prepared to subjectively judge whether one source sounds better than another.

Here are some audible symptoms of jitter that allow us to determine that one source sounds “better” than another with a reasonable degree of scientific backing:

It is well known that jitter degrades stereo image, separation, depth, ambience, dynamic range.

Therefore, when during a listening comparison, comparing source A versus source B (and both have already been proved to be identical bitwise):

The source which exhibits greater stereo ambience and depth is the “better” one.

The source which exhibits more apparent dynamic range is the “better” one.

The source which is less edgy on the high end (most obvious sonic signature of signal correlated jitter) is the “better” one.

Does this help you?

Seems like this could be very subjective. I could almost certainly agree on ‘sounds different.’ To be fair, I have not been involved in the kind of research you have done in this area but I still feel there is a lot of subjectivity involved.

I recently attended the NAB convention and watched some demos of video line doublers and quadruplers. While in some ways they ‘improved’ the projected image, I could not flatly say that they made every aspect of the picture ‘better.’ It was decidedly ‘different’ but among the group I was with we couldn’t all agree on what aspects we liked and didn’t like about the ‘improved’ picture.

That’s for sure. Well, line doublers actually alter the data which is sent to the monitor, so, unlike with audio jitter reduction units, the data is changed, and you get into very valid subjective questions. The question of “better” is definitely a slippery subject. I don’t pretend to have all the answers, but to me, the audio copy which sounds most like the original points the direction of the degradation. Then, I can relate an experience with two different jitter reduction units, both of which produced excellent-sounding outputs, but both sound very different from one another. One has a slightly “flabby” bass, the other a tighter bass. At least both of them sound “better” than the jittery copy as monitored without the jitter reduction. So, when comparing two different D/A converters or two different jitter reduction units, even more subjective judgment enters into the picture. I agree with Steve Potter that “Better” is a very complex subject! Here’s a followup on the maillist from Peter Cook of the CBC:

Date: Wed, 1 May 1996
From: Peter G. Cook
Subject: Re: DEFINITIONS OF “better” versus “worse” sound

A fine mini essay Bob. Perhaps you could add this on your web pages.

At 08:21 1/5/96, Bob Katz wrote:

Therefore, when during a listening comparison, comparing source A versus source B (and both have already been proved to be identical bitwise):

The source which exihibits greater stereo ambience and depth is the better one.

The source which exhibits more apparent dynamic range is the better one.

The source which is less edgy on the high end (most obvious sonic signature of signal correlated jitter) is the better one.

The better one, and it is better, is also easier to listen to. . . less fatiguing. I would also add to this that the low end just “feels” bigger and more solid. This is perhaps a psychoacoustic affect more than a measurable one. It may be that the combination of a less edgy high end and greater depth and width makes the bass seem better.

All of this makes sense if thought of in terms of timing (that is what we’re talking about isn’t it ;-]). With minimal jitter nothing is smeared, a note and all its harmonics line up, the sound is more liquid (a term probably from the “audiophile” crowd but one which accurately describes the sound none the less), and images within the soundstage are clearly defined.

-Peter

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