Listening for jitter, s'phile test CD2 - ???

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I have stereophile test CD#2, track 26 is a jitter demonstration track. It has a 11Khz tone, then this 11Khz tone "being jittered at a frequency of 4Khz", then returning to the pure tone.

I don't hear a difference, and when I look at the analog output of the DVD player, I don't see a difference on my crummy o'scope.

Does this mean that my DVD player is 'jittery'? I also tried a DVD player at work, and my old CD changer - same results.

Thanks,
Matt
 
How fast is your scope? You can measure the jitter by examining the word clock inside the DAC or CD player. The word clock will be a 44.1kHz square wave into the DAC chip. If you have a digital scope, it may be able to automatically measure the jitter. If not, you can measure the jitter yourself by untriggering the scope and examining the eye pattern. The width of the crossover in the center of the pattern is the jitter range. You want to minimize this width.

See these articles:

http://www.tek.com/Measurement/cgi-...opes/jitter/selection_guide.html&FrameSet=mbd
http://www.tek.com/Measurement/App_Notes/55_15631/eng/
http://www.tek.com/Measurement/App_Notes/55_13769/eng/index.html
 
External measurement?

Is there any way to measure the effect of the jitter on the analog output? If there isn't a difference on the analog output, it doesn't seem that it's a big deal.

BUT, if there is supposed to be a high level of jitter (or 'artificial' jitter) on this track, and I can't hear it or measure it on the analog output, there must be something else wrong with the system.(?)

Any thoughts?

Thanks,
Matt
 
You should be able to measure this on the analog outputs as well. Attach your probe to the analog output. Set your scope to 50µs/div, and turn up the persistence. The unmodulated waveform should have a thinner trace than the modulated one. If there is any difference in the actual analog output, you will definitely see it on the scope.

If you don't see any difference, that would imply that the jitter induced inside your CD player is more than the jitter intentionally induced in the recording, or perhaps that your scope has poor timing.
 
Could anybody explain exactly what it means that the track
has jitter in this case? Maybe I am missing something obvious,
but as far as I can see there are two possibilities:
1) The signal is jittered by displacing pits and lands in the
pressing matrix, or
2) The original analog signal is AD converted using a jittery clock.

In case 1, a good CD player should be able to reclock the signal
so the jitter does not affect the end result, I think. Hence, the
difference should be heard only on some CD players.
In case 2, the signal recorded on the CD is jitterfree but the
sample values are wrong due to the errors in the AD conversion.
The sound would be affected, but I cannot see how one could
measure this jitter in the CD player, since the jitter has already
been transformed from time error to amplitude error before
pressing the CD.

Answers, comments, corrections welcome.
 
from the CD notes:

This is the wording from the CD notes:

To demonstrate this, track 26 offers first a pure tone at 11kHz - followed by the same tone with the effect of the data words representing the tone being jittered at a frequency of 4kHz. Each data word should be precisely spaced at 22uS intervals; the uncertainty in the data word timing is 10nS peak to peak. This is a little higher than that encountered in typical CD players, but it has been exaggerated to make the effect clearly audible.

(If the two halves of track 26 sound identical, then it is likely that the difference is being obscured by high levels of jitter in your player and/or processor.)


My words:
It looks like the problem is with my system. Anyone else have this CD?

Thanks again,
Matt
 
Matt MacBeth said:
I understand it as jwb said it:

This test signal is probably synthesized digitally. The peaks of the 11kHz signal will be modulated in time at 4kHz.

Matt

Not sure what it would mean to modulate the peaks, but i
assume you mean that the 11kHz frequency is modulated
by a 4kHz one. This would correspond to my case 1, since it
doesn't really matter if the signal is synthesized or sampled.

So, the purpose of the test then must be to check how good
the CDP is at reclocking the signal?
 
You are right jwb. I got lost in my own two cases, I am afraid. :)

It corresponds to case 2, not case 1, but that should mean, as
I said previously, that you can hear the jitter, but not measure
it as jitter, except for looking at the final analog output from
the CDP.

Interesting test. It is similar to those test LPs recorded with
varying levels of wow and flutter. It should not be much of
a problem to generate such test CDs then.
 
So you want to hear jitter?

I don't have this particular CD, but the jitter track seems pretty limited to me. Real life jitter will be much more random in nature.

Endulge me while I again engage in some armchair engineering that I haven't (at least yet) personally attempted.

How about replacing your player's oscillator with one of the new spread spectrum devices? This is perhaps the ultimate antithesis of a clock upgrade!

The reason for the existence of spread spectrum oscillators is to reduce EMI emissions. The closer a digital circuit is to having a perfectly stable and jitter free clock, the more likely it is to radiate or conduct ECI higher emissions at the clock frequency or one of its harmonics.

By taking a single frequency clock and spreading its spectrum (which is essentially producing jitter by intention), a noise peak occuring at a particular frequency can be reduced. There will be increased noise just above and below the original frequency, but the peak amplitude can often be reduced enough so that it doesn't exceed regulatory requirements.

In a way, this is kind of playing games with statistics and the current state of EMI regulations. Any single oscillation at the original frequency will emit just as strongly as a single oscillation of a fixed frequency oscillator. Because the regulations use averaging, spread spectrum clocks result in lower readings.

As long as all of the set-up and hold timing requirements aren't violated, most digital circuits simply don't care about jitter. Data is data and bits is bits. It can be mechanically stored and then servoed into FIFO buffers to be reclocked. It can be broken up into packets and then reassembled. These are all much more severe timing aborations than will ever be caused by jitter.

The only time that jitter is a concern is when a conversion to or from analog is involved. With analog, time stability is of the utmost importance.

So why would anybody ever consider putting a spread spectrum oscillator with intentional jitter into their music system?

It seems to me that this would be a good method to evaluate jitter recovery mechanisms at or just before the DAC. If the system can recover a stable clock (this can be objectively measured), and sound good (subjective evaluation) when being fed a spread spectrum clock, then I'd say it had excellent jitter rejection!

Of course, once the evaluation was finished, I'd want to get rid of the spread spectrum device and replace it with a high quality stable oscillator.

Regards,
Brian.:cubist:
 
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