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Listening for jitter, s'phile test CD2 - ???
Listening for jitter, s'phile test CD2 - ???
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Old 12th November 2002, 04:23 PM   #1
Matt MacBeth is offline Matt MacBeth
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Default Listening for jitter, s'phile test CD2 - ???

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
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Old 12th November 2002, 05:04 PM   #2
jwb is offline jwb  United States
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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-b...l&FrameSet=mbd
http://www.tek.com/Measurement/App_Notes/55_15631/eng/
http://www.tek.com/Measurement/App_N...eng/index.html
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Old 12th November 2002, 05:10 PM   #3
Matt MacBeth is offline Matt MacBeth
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Default 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
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Old 12th November 2002, 05:20 PM   #4
jwb is offline jwb  United States
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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.
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Old 12th November 2002, 05:30 PM   #5
Matt MacBeth is offline Matt MacBeth
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I'll give that a shot. Thanks for the help.

Matt
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Old 12th November 2002, 05:50 PM   #6
Christer is offline Christer  Sweden
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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.
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Old 12th November 2002, 06:00 PM   #7
jwb is offline jwb  United States
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This test signal is probably synthesized digitally. The peaks of the 11kHz signal will be modulated in time at 4kHz.
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Old 12th November 2002, 06:00 PM   #8
Matt MacBeth is offline Matt MacBeth
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Default 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
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Old 12th November 2002, 06:07 PM   #9
Christer is offline Christer  Sweden
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Maybe I am just stupid, but I think it is still unclear. The notes
quoted from the CD are not quite clear, and they could
actually mean either of my cases 1 and 2. Or perhaps it some
3rd case that I didn't think of?
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Old 12th November 2002, 06:17 PM   #10
jwb is offline jwb  United States
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Default Re: from the CD notes:

Quote:
Originally posted by Matt MacBeth
Anyone else have this CD?
No, but if you ripped it to a WAV file and put it on an FTP or HTTP server somewhere, I might download and burn it.

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