How many forms of Jitter can you identify?


Just a few days ago I was read about a clock upgrade for a CD player that was said to reduce jitter by having a more accurate clock. Today, I'm reading about a Mikrosmooth CD polishing kit that claims to reduce jitter.

I'm absolutely positive that it isn't possible to apply a product to a CD that will have any effect on the digital clock on the CD player. I'm also skeptical that it can make any difference in how the CD player spins the disk so how does it reduce jitter? At most it should a good cleaning product that could allow the laser to read the disk better eliminating any error correction from being necessary.

What am I missing?
mceljo
I see it very possible that a good, clean, as transparent as possible disc will help the laser pickup work at its best and read the light pulses induced by pit-to-land and land-to-pit transitions with very good temporal accuracy. And since jitter is temporal INaccuracy, this enhanced transparency will reduce jitter.
There is a commercial being broadcast nowadays on TV advertising a product claiming that it will restore one's car's headlamps transparency by removing the layer of oxidation that tends to develop and makes them slightly opaque.
Something like that.
I doubt that such a product will improve a new, or well maintained disc, but may be worth a try for older ones.
There is a good summary of ten potential contributors to jitter in this paper by Steve Nugent of Empirical Audio, about a third of the way down the page.

Note that items 2 and 3, which pertain to the pits on the cd itself, and the electromechanical systems that spin the disk and read the data, are "not an issue ... for newer players that completely buffer the data at high-speed from a CDROM reader to a memory buffer."

IMO, assuming everything involved is reasonably well designed, by far the most major contributor to jitter is the S/PDIF or AES/EBU interface that is commonly used between source and dac. Making the timing that is applied to the dac chip itself ultimately dependent on a clock supplied by a different component, which is multiplexed together with the audio data into a single signal that is then transmitted between components, the clock subsequently having to be extracted from that signal, is an inherently compromised concept IMO. Several different items in Steve's list, btw, pertain to that interface.
At most it should a good cleaning product that could allow the laser to read the disk better eliminating any error correction from being necessary.
Assuming the player is reasonably well designed, error correction is not an issue, because it results in bit perfect recreation of the original data. An issue only arises when the data being read is so badly messed up, for instance due to a severe scratch on the disk, that the errors cannot be corrected. In that case error interpolation, or conceivably outright muting, will occur, which will affect sonics as it represents an approximation at best.

Best regards,
--Al
Well, shortly after I clicked "submit" I came to the realization of my "senor moment" when I totally forgot about the buffering that completely eliminates any possibility of jitter being generated by pit/land reading itself.
What can I say, I wish I could withdraw my post.
But since that's not possible, here I am again, standing corrected and humbled by Al's perfect answer.
The worst forms of jitter are non-random (by far)!

If a disc wobbles while it spins then this may cause cyclical adjustments to the pick up laser servo and these repetitive draws on power may induce variations in the clock through the power supply.

IMHO, the most accurate or reliable approach is an asynchronous setup where the output clock is not synchronized to the input.
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I'm absolutely positive that it isn't possible to apply a product to a CD that will have any effect on the digital clock on the CD player. I'm also skeptical that it can make any difference in how the CD player spins the disk so how does it reduce jitter? At most it should a good cleaning product that could allow the laser to read the disk better eliminating any error correction from being necessary.

What am I missing?
Mceljo

not true! Shadorne has hit upon the correct explanation altho' he gave yet another cause for jitter creation:
If a disc wobbles while it spins then this may cause cyclical adjustments to the pick up laser servo and these repetitive draws on power may induce variations in the clock through the power supply.
That's why that they have disk cutters (which re-cuts the outer edge of the disk with a sharp knife so as to smooth it & reduce/eliminate disk wobble).

You get the same random/spikey draws of current from the power supply when the laser light (trying to read the disk) reflects at funny angles off a dirty disk & after repeated reflections inside the laser transport box reaches the optical reader circuit. These multiple reflected rays of light are not the data from the disk but the optical reader knows no better & there are draws of current from the power supply as it tries to read this "data".
A power supply with lots of glitches tranfers these glitches to all the circuits are it is powering. So, in EE parlance we say that amplitude modulation (AM) on the power supply has created AM & PM (phase modulation) in the circuits being powered by that supply. Both AM & PM create jitter in the digital (music) signal being processed. This is one of the reasons you see multiple supplies in digital (CDP, transport, DAC, re-clockers) equipment & often it is prominently advertised. They are trying to prevent one corrupt sub-system from corrupting another...
07-19-11: Ericjcabrera
my buddy has a furutech demag that we use in addition to cleaning discs, and both of us, on both of our systems, can hear sonic improvements, sometimes significant on some cd's. how demagnetizing cd's affects player performance is beyond me, but i do know it works.
the CD disk is made of plastic & some organic dies that hold the digital data. The plastic structural material is not non-magnetic. Thus, when the laser light hits the plastic underside to read the data, over time the plastic gets mildly magnetized. For example, how many times have you experienced plastic packaging material "stick" to your fingers due to electro-static charge on it??? I'm sure many times...
The de-mag unit un-does this magnetization so that there is minimal/zero interaction between the mild magnet that the CD disk has become & the magnets inside the laser transport box.
The CD disk is made of plastic & some organic dies that hold the digital data. The plastic structural material is not non-magnetic. Thus, when the laser light hits the plastic underside to read the data, over time the plastic gets mildly magnetized. For example, how many times have you experienced plastic packaging material "stick" to your fingers due to electro-static charge on it???
Bombaywalla, I'm not sure I follow that. Static electrical charges and magnetization are two different things. And why would exposure to light cause either one?

Good point by Shadorne. Seconding Bombaywalla's comment about that point, it should be kept in mind that the extent to which that effect may be significant will be highly dependent on the design of the particular component.

Best regards,
-- Al
Bombaywalla, I'm not sure I follow that. Static electrical charges and magnetization are two different things.
Al, it true that these are 2 different effects but they are interactive & influence each other. Static electric charge does create an electric field around it & this electric field can be/is influenced by magnetic materials used inside the transport box. Over time some of the magnetizm is transfered over to the CD disk (just like when you keep a ferro-magnetic material that is not originally a magnet in contact with a magnet over a longish period of time that ferro-magnetic material becomes a weak magnet).
(I'm sure that you'll agree that static charge does create an electric field around it).
And why would exposure to light cause either one?
the laser light is highly focused beam of photons impinging on the CD with very high intensity. Over time this will transfer electrons (or bleach electrons) to mildly charge the underside of the disk one way or the other. Then we are back to the above argument about statically charged CD disk being influenced by the magnets inside the transport box....

There is yet two other items that will mildly magnetize the CD disk that I did not touch upon earlier - (1) the inks that are used to print the CD label contain ferro-electric substances like iron-oxide (FeO2) & (2) the impurities in the Aluminum used to create the dye. You can read more on the Acoustic Revive website as to what A-R think about this topic. Here is that website:
http://www.acoustic-revive.com/english/rd3/rd3_01.html
Bombaywalla, thanks for the explanation.

The logical follow-up question would be how significant is all of this from a quantitative standpoint? The data on page 3 of the reference you provided addresses that question, although the various terms and quantities are not clearly defined. My interpretation of it is as follows:

1)Looking at the first two of the four tabulations, on average there were 0.00 uncorrectable errors in each 1 second block, both before and after demagnetization.

2)Looking at the last two of the four tabulations, the WORST CASE (across some unspecified number of disks, trials, etc., and averaged in some way) was that there were 0.22 uncorrectable errors per 1 second block, or in other words 1 interpolated sample approximately every 5 seconds (which would comprise approximately 450,000 16-bit samples for the two channels).

And what they are saying is that demagnetizing would reduce that worst case number of interpolated samples by about one-third.

The bottom line on audibility, IMO: I'll give it a "definite maybe," but from a personal standpoint I don't consider any of that to be sufficiently compelling to inspire me to go out and buy one.

Best regards,
-- Al
Al, no problem.

yeah, it's hard to say how quantative this particular effect is. It is a mechanism of jitter generation & it might not be significant in & of itself but combined with all the other hash in the signalling chain, it probably does contribute to the overall digital nature of CD playback.
In the A-R website they did show some plots of amplitude of wanted signals increasing after de-magnetizing & their claim was one could hear sounds not heard before during playback. Never tried one myself so I cannot comment but they are showing data to that effect.....
So, like many things - best to attack the jitter issues by addressing the largest perpertrators leaving only the minor contributors to be attacked some later day time &/or budget permitting.
I can't really identify jitter, but I know the heebie jeebies when I hear 'em.
When you listen to speed thrash metal, jitter does not stand a chance.
Many of Julian Dunn's classic AES papers on jitter can be found here, and are IMO required reading on the subject:
http://www.nanophon.com/audio/
Two big conceptual errors I see very commonly are the assumption that any intrinsic jitter related to retrieval of information off of a CD actually occurs through the forward signal/data path, and that any sonic artifact associated with parts upstream of the DAC must be classifiable as jitter.

In reality, CD players, transports, and DACs are a menagerie of true mixed-signal design problems, and there are a lot of different noises sources living in close proximity with suceptible circuit nodes. One oft-overlooked source is crosstalk from the disc servomechanism into other parts of the machine . . . analog circuitry, S/PDIF transmitters, PLL clock, etc., which can be dependent on the condition of the disc.

One easy way of measuring this on the test bench is to have two versions of the same test-tone CD, one pristene, the other scratched. A conventional distortion analyzer is used to null out the the tone(s), and then an FFT (or visual 'scope analysis) is used to analyze the residual. One would be suprised at some of the nasty things that sometimes come up out of the noise floor when the focus and tracking servos suddenly have to work really hard to read the disc.

But that said, demagnetizing a CD is of course silly. If it's scratched or hazy, I use a 3M product called "Finesse-It", which is sold as a professional polishing compound for automotive finishes. Sometimes spot pre-treatment with a Mr. Clean Magic eraser on particularly tough scratches helps, then Finesse-It to remove the scratches left by the eraser.
Kirkus well said I agree fully. CD, DVD and Blu-ray and SACD are all made from materials that are non-magnetic.

De-magnetizing appears to be unlikely to do anything.
Only 2 forms I know are Jitterbug and Derek Jitter.

This digital stuff is complicated.
There was or is a tavern called Jitter's in Wisconsin. I believe many had just that the next morning after a night out.