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More likely scenario?

Posted by Jon Risch (B) on April 25, 2010 at 17:48:45

In Reply to: Stupid tweak I knew wouldn't work - I was wrong posted by bevo on April 24, 2010 at 20:21:29:

The laser read mechanism in a CD/DVD player is at the back end of a lens assembly, which focuses the laser beam bounced off the disc read layer (which is NOT the surface). Thus the reflected beam shines back through the lens and is focused onto a phototransistor.

This phototransistor has a certain range of sensitivity to light in terms of strength AND in terms of wavelength (or the color of the light).

Obviously, it will respond to the wavelength of laser light used for the laser beam sent by the CD player, but it is likely that it also will react to some extent to light that is at the right angle. Light shining inside the player that manages to bounce around and enter the lens assembly could also reach the phototransistor.

Any light other than the reflected laser light will possibly change the threshold of detection for the phototransistor, perhaps making it easier to trigger the phototransistor, or the circuitry following the phototransistor, so that the transitions that the laser reads off of the disc become more defined or clear.

Of course, the opposite could be true, additional light into the phototransistor might make things worse in some cases, due to taking the phototransistor out of it's optimum range of detection, etc.

My thoughts about this are that the laser beam strength is limited by FCC mandate, and the practicalities of a solid-state laser LED, thus, the phototransistor may not be getting an optimum amount of light to trigger as cleanly as it is possible. Thus, it is more likely that added light, a sort of "bias light" for the phototransistor, would be much more likely to help things than to hurt.

How the different wavelengths interact is not likely to be a factor, as a very precise wavelength would be needed to "cancel" or to do other things in terms of affecting the original laser light. I can see where the laser beam could interfere with itself, after all it IS coherent light, and could conceivably cancel itself (in fact, the way most of the read mechanisms work, the distance in depth between the CD pits and lands is 1/2 wavelength of the laser or very close, and so, the pits and lands represent a reinforcement or a cancellation of the reflected laser light). But I don't easily envision a mechanism where the laser beam could be affected directly by a different wavelength altogether.

Several things would fall out of the validity of this theory:

One, you would want the extra light source to be absolutely stable and steady, thus the use of a DC powered LED rather than an AC powered incandescent light bulb. This would help make sure the improved detection would remain stable and optimal. An AC powered light source would be going ON and OFF (or modulating it's strength) about 120 times a second.

Two, you would want to try different strengths of light, and different frequencies (wavelengths) would require different strengths due to the differing sensitivity of the phototransistor at different wavelengths.

You would want to shine the light into the laser read lens as stably as possible, this would mean that you would probably not want to bounce it off of the spinning disc if at all possible, and instead, shine it directly into the read lens assembly.

A quick check with several different CD's and CD-Rs showed me that blue and green LED light tends to shine THROUGH the data metallization layer.
This means that external light might be best shined through the disc, more directly into the read lens. Red didn't shine through as easily, and keeping in mind that the laser beam for CD's is an infra-red wavelength (just below, or shorter than red).

Of course, CD's with a painted label would mess with this angle of injecting light, so the best way would seem to be to bounce light off the entire inside area surrounding the disc area, hoping to get as stable an amount of injected light to the read lens as possible.

With this in mind, it is likely that multiple light sources of sightly different wavelengths might be a more optimal approach.

Anyway, this theory provides for a mechanism where added external light could provide a laser read lens phototransistor "bias light" to more cleanly sense the land to pit to land transitions, thus, reducing jitter.


Jon Risch

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