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Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ.

There is more to it than mere data retrieval.

209.215.72.176

I don't think anayone is arguing about the data retrieval anymore, this has been checked, and the data comes of the disc pretty much intact in terms of the ones and zero's.

What follows is all just theory, so don't take me to task because I do not have a million dollar optics lab to check these things out completely.

What may be at issue is how much the laser bouncing around inside the disc will cause the transistion from a land to a pit, and then from a pit to a land to vary.

Laser light is coherent. The very principle of the disc read is based on a cancellation effect, so that when the laser is reading the pits, there is a large difference in the output reaching the detector diode.
However, that large difference is based on a near perfect cancellation effect from the laser light being received approx. 180 degrees out from the nominal reflections.

Now there is no doubt that the light bounces around inside the disc.
Take a small powerful penlight, and shine it into the shiny side of the disc, and look at the edges from the edge on. They light up.

Even more convincing, is use a laser pointer (the usual cautions apply do not look directly into the beam!) and hold it shiny side next to a white surface, and shine the laser onto the disc inbetween the disc and the white surface. If you angle the disc slightly, so as to clear the main reflection, you will usually be able to see at least three or four weaker reflections off to the sides. This shows that the laser is bouncing back and forth between the aluminized surface, and the clear surface of the disc. There may also be some reflections from the beam passing through the aluminized layer, and bouncing off the laquer/ink label area, and back through the aluminization.

Interestingly enough, some of the CD optical treatments reduce these reflections in strength.

I don't recall having ever seen this discussed anywhere, so this may be the first exposure of this info re the laser penlight reflections and optical treatments.

The "green" ink is just what we see at visible light wavelengths, and does not take into account that the particular dye used may be very efficient at absorbing infrared wavelengths as well. Another dye, that looked black, might be better, or just as good.

The question will inevitably arise: how do all these reflections affect the data? Well, I think it has been established that no data is irrecoverably lost or altered. But since we are talking about coherent laser light, if the light bouncing around inside the disc happens to arrive at a moment when the transistion from a pit to a land is being made, then it is conceivable that it might either ADD to the strength of the detected beam, or SUBTRACT, just a little, not enough to cause the bits to be altered, but enough to alter the detection point.

Remember, this is coherent light, the interference may not have to actually shoot into the detector diode, just interfere and weaken or strengthen the beam that is aimed at the detector as it travels through the disc interior.

This covers the mechanisms that might occur inside the disc, and how it could have an effect, and why green ink and optical treatments might affectt he laser beam read.

This still leaves the question of how altering the read detection points will affect the sound.

At this point, it would help to check out this tome (paying particular atention to the parts about LIM and the PS):
http://members.nbci.com/Jon_Risch/jitter.html

Ok, back now?

The read of the data off the disc has been jittered by the laser light bouncing around inside, and when this jittered read signal is poassed on, it has dirtied the PS variations, causing a more "fuzzed" read pattern to be superimposed on the PS, and hence, the rest of the CD player/transport.

That jitter read off the disc can affect the sound is shown by the various attempts by CD manufactuers and music distribution companies to reduce or eliminate poor timing consistency of the pits, e.g., JVC's XRCD process, and Sony Music's efforts ot reduce jitter incorporated during glass matering.
See:
http://www.digido.com/jitteressay.html
where some of this is duscussed, and
http://www.AudioAsylum.com/audio/general/messages/977.html

Over the lat few years, some major artists have refused to release their latest album, due to horrid sound from the test pressings off the glass master. The latest one was Steely Dan's most recent album, and Roger Nichols refused to let them out until they found out what the problem was. Turned out to be mastering jitter, and when they reduce that trhough a low pseed master and clean mastering techniques, the CD was found to be acceptable.

With decent CD-R burners, burned at low speed, one can often clean up a commercial CD that has mastering jitter in it. The latest example is the Beatles CD of hits, it sound's tad bright and aggressive, made a good copy, and it is so much smoother and clearer.

So if the CD itself can cause degraded playback due to mastering jitter, it is not a big stretch to see that laser light bouncing around inside the disc could affect the sound too.

Jon Risch





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