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Old 14th March 2004, 04:22 PM   #31
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Location: Ramsey, Cambridgeshire, UK
Default Measuring Clock Jitter

Hi JohnW,

I have added a X22 amplifier on the output (NE5534) which does add its own noise because of the gain as can bee seen by the noise floor rising to -125dB, but the jitter signal can now be seen much more easily.

I am not sure how much noise the intergrator would add but as it has a gain of around -1.08 at 20Hz, it should be quite small.

I have added a plot of the circuit with a X22 amp on the output - a quieter amp such as a AD797 would give improved noise floor results.

I am still not sure why you suggest integrating the output - could you explain further?

Regards

David
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Old 14th March 2004, 05:16 PM   #32
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Default Measuring Clock Jitter

Hi JohnW,

I have fed a couple of clocks into the monostable with the x22 amp and have got some plots which do actually measure some difference between the two clocks, so with a bit of development it may still give better results.

My sim is a little temperamental also.

Regards

David
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Old 14th March 2004, 08:23 PM   #33
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Default Measuring Clock Jitter

Hi All,

There is of course the mixer approach as has been pointed out by HpW of HP Works but it does require two clocks and to remove any beat tones and a PLL is also needed. This may prove interesting reading or stimulation for a design.

http://www.wenzel.com/documents/measuringphasenoise.htm

and

http://www.wenzel.com/pdffiles/lowamp.pdf

Regards

David
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Old 14th March 2004, 08:42 PM   #34
Giorgio is offline Giorgio  Italy
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Guys,
I just wanted to discuss some jitter aspects, including jitter mesurements. This thread is really a pleasure to read, for me.

I just wanted to open up a little more the jitter measurement discussion, in a direction already tauched by some one of the members.

I have ben working a little by myself in the last months, and came up with a tentative preliminary version of an article for TNT. The article is part of a series about digital inteconnections, and the specific topic is a discussion of the standard SPDIF connection, in my case through an electrical interconnect, with reclocking just before getting ot of the transport. But after some measures, the major discussion has moved to what happens at the receiver side.

The results are in some extent rather extreme, in my view, and I do not feel so comfortable with them, to the extent that I am setting up a system similar to David's one just to confirm all these data.

I just add a link to this pre-release version of the article, it is hosted on a blind link of the TNT server, just to make it available to all of you that might be interested to discuss results.

The other measurement system I have been setting up should help me in confirming these results in the next few days. I should be now ready to start serious measures.

If you also can help me in discussing the results and checking if what I think I have seen is real, I would really appreciate.

By the way, I have found out recently an article on TNT of someone else that used something very similar to my setup.

What I really find strange, is that the results seem so immediate and easily available, but I have never heard anyone discussing anything similar. So there might really be something wrong in the approach (yes, but whose approach???).

The link is www.tnt-audio.com/clinica/spdif_reclocking_e.html

Thank you all in advance for your patience
(I am afraid you'll need plenty... trust me, if you cannot understand is only because you have not seen the size of the article...)

Giorgio
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Old 3rd June 2004, 12:40 PM   #35
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What an interesting thread.

I'll ask some neophyte questions:

- Do I understand correctly that the often used Miller Audio Research gear (and the way it is often interpreted for its results) does not cut it at near 100ps anymore?

- It is possible to build a rather inexpensive jitter measurement device that is quite accurate by using a sound card + software analyzer + simple schematic device as listed in this thread?

- Is the accuracy of such measurements limited by the accuracy of the clock on the sound card?

- Is it possible/worthwhile to replace the clock of a say, RME DIGI 96/8 sound card (used on HpWorks page as an example) with a low jitter clock such as TentLabs Xo2? This would have to be on a separate board, in a separate PCI slot, using +5V/+12V feeds from a normal/noisy computer PSU.



Somewhat off-topic...

I will also offer the following document links to those who may not yet know about them (probably not very useful to the discussers in this thread, but perhaps other lurkers like me):

Jitter Theory - Part 1 'Interface Jitter' (Julian Dunn, Audio Presicion, Newsletter vol 14, no. 1)
http://www.aesitalia.org/Materiale/a...er%20Theory%20(1).pdf

Jitter Theory - Part 2 (Julian Dunn, Audio Presicion, Newsletter vol 15, no. 1)
http://www.aesitalia.org/Materiale/a...er%20Theory%20(2).pdf

TOWARDS COMMON SPECIFICATIONS FOR DIGITAL AUDIO INTERFACE
JITTER (Julian Dunn, et. al., Nanophon)
http://www.nanophon.com/audio/towards.pdf

Jitter: Specification and assessment in digital audio equipment (Julian Dunn, Nanophon)
http://www.nanophon.com/audio/jitter92.pdf

Diagnosis and Solution of jitter-related problems in digital audio (Julian Dunn, Ian Dennis, Prism Sound)
http://www.nanophon.com/audio/diagnose.pdf

On Jitter (Dan Lavry, dB Technologies)
http://www.lavryengineering.com/white_papers/jitter.pdf

Converting between RMS and Peak-to-Peak Jitter at a
Specified BER (Maxim Integrated Products)
http://pdfserv.maxim-ic.com/arpdf/AppNotes/3hfan402.pdf

Jitter and Its Effects (Benchmark Media Systems)
http://cmd.bweb..http://www.benchmar...d/jittercu.asp

Why Jitter is Important (Audio Precision, offline, but copy without images available from the Internet Archive)
http://web.archive.org/web/199712240...ions/jan96.htm

Some jitter measurements of transports and cd-rs
http://members.chello.nl/~m.heijlige...as/jitter.html

Verifying Jitter Measurement Accuracy
http://www.techonline.com/community/tech_group/29107

Jitter - Understanding, measuring, eliminating (Johnnie Hancock, Agilent)
http://www.highfrequencyelectronics....04_Hancock.pdf

Measuring Clock Jitter Spectra (Paul Winser, London DIY Hifi Circle)
http://www.earthcurrents.com/london-live/cd-jitter.pdf
http://www.earthcurrents.com/london-...er-spectra.pdf

Why Jitter matters in high resolution digital audio systems (Peter Schut, Axon)
http://www.broadcastpapers.com/audio/Jitter1-rev1.PDF

Theoretical and Audible Effects of Jitter on Digital Audio Quality _ Eric Benjamin and Benjamin Gannon, Dolby Laboratories, Inc., San Francisco, CA, USA
http://www.aes.org/publications/prep...lists/105.html

Some of the stuff is quite old and some implementations may be considered outdated, but for leaning they are helpful (imho).

I cannot vouch for the accuracy of the above papers, but they've been helpful to me in understanding jitter, measurements, effects and audibility.

cheers,
halcyon
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Old 3rd June 2004, 02:21 PM   #36
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Default Since I missed this one first time around.......

I would imagine that the jitter of a TOSLINK would be around 1 nSec or so. A really lousy setup. The rise and fall times are slow, and asymetrical. So, I would expect it to sound like crap.

And it does.

As for fiber....this applies to laser and glass transmission........

Fiber was designed to start working at distances where copper craps out........IOW at 1 km or so. It will work at shorter distances, but suffers from pulse distortion due to mode hopping in the laser. Which translates into jitter.

It also sounds like crap.

Unless done right. Which no high-end company ever figured out. Eventually, even Stereofile had to publish results that showed glass fiber setups had lots of jitter. (Some of us tried to tell them......but we didn't count.)

But don't take my word for it......ask Lucent (or whatever the old ATT Labs company is called), who made the laser parts that Wadia and Krell used. They will tell you: designed for links greater than 1 km.

Jocko
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Old 3rd June 2004, 03:49 PM   #37
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Default Re: Since I missed this one first time around.......

Quote:
Originally posted by Jocko Homo
Fiber was designed to start working at distances where copper craps out........IOW at 1 km or so. It will work at shorter distances, but suffers from pulse distortion due to mode hopping in the laser. Which translates into jitter.
Actually fiber optics in computer networks go much further than 1km, but that's a completely different story.
Not the crap transmitters, receivers and cheap plastic used in (audio) optical cables.
Actually, you shouldn't pass around 15 meters.
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Old 15th February 2013, 01:48 PM   #38
Bazukaz is offline Bazukaz  Lithuania
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Hi,

I have some questions to experts of digital audio here.
To me the jitter introduction by optical transmission lines is quite new; I was always thinking that it is caused by D/A converters only.
I am a computer tech guy and view TOSlink cable as a data cable to transfer digital information. So, in order to play it, the digital receiver must recover data bit-to-bit, and map incoming signal to it's own clock for D/A conversion? Perhaps some recovery algorithms are used to correct for dropped bits like its common in computers?
So how can this affect sound quality at all? Do I misunderstand how it works?

Regards,
Lukas.
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Old 15th February 2013, 02:24 PM   #39
DF96 is offline DF96  England
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The bits are not the problem. Timing is the problem. Most DACs derive their output data timing from a PLL locked to the incoming data timing. The PLL filters out high frequency jitter, but can't (in fact, mustn't) fiddle with low frequency jitter. It appears that basic optical transmission systems introduce more jitter than basic coaxial cable systems. Hence for short distance cable is better. For long distances optical is better, but if you want to preserve timing then you need something better than the basic setup.
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Old 15th February 2013, 02:39 PM   #40
oshifis is offline oshifis  Hungary
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There is ususally a sample-and-hold function in the D/A process. Each sample is held constant until the next conversion. This results in a stepped signal, which in turn will be filtered to remove unwanted components. Now, even if the individual samples are 100% accurate in their amplitude, the inaccuracy in the conversion timing (that is jitter) results in deviation from the original energy of each sample. I use the loosy word "energy" to describe the area of the sample (integral of amplitude over a sample period). The net result is that the signal does not match the original analog waveform after filtering. It can be perceived as noise and/or distortion.
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