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Old 11-22-2010, 05:55 PM #1
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IanR IanR is offline
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Lightbulb How do CD players work?

Hi,

I was wondering if someone might be able to clear up a few things on the use of lasers CD players please?

I thought read the bits from a CD was fairly simple, although I keep finding different explanations as to how they are read - which one is true please?

I understand the basic concept of CDs being made up of a single spiralling track containing 'pits'.

1) Scattering Theory
Light is scattered by pits so little laser light arrives back at the detector, giving a 0. The track area with no pit, referred to as 'land' reflects light well giving a 1.

2) Polarising Theory
Light light passes through a diffraction grating, which converts the light into a central peak plus side peaks, but, as was stated before, only the three central beams are strong enough to matter in the mechanism. The three beams go through a polarizing beam filter, which reflects all but the horizontal polarization. The emerging light (now horizontally polarized) is then collimated by a lens, and is passed through a plate that converts it into circularly polarized light.

The circularly polarized light is then focused onto the disk by the objective lens. If the light strikes "land" it is simply reflected back into the objective lens. If it strikes a bump-like feature, only the center beam (of the three beams) is reflected by the feature, and the rest are still reflected by the land. The features are 1/4 of wavelength high, and therefore the reflected light rays have a 180 degrees phase difference; and hence interfere destructively completely. No light is received back in that case.
If any light is received back, it is passed through the wave plate again. Since it is going the reverse direction, it will be polarized vertically. When the vertically polarized light hits the polarizing filter this time, it will be reflected (since the filter reflects all non-horizontally polarized rays). Thus, it will reflect though the focusing lens and be imaged on the photodetector array. The array treats this as a 1 bit, and if it receives no light due to destructive interference due to a hit of a feature, it reads a 0 bit. The digital information thus gathered is then sent to the Digital to Analog converter and is played through the speaker systems (or headphones).
From: 2.972 How a CD Player Works

3) Data read by transitions
Digital data are carved into the CD-ROM as pits (low spots) and lands (high spots). As the laser shines into the moving pits and lands, a sensor detects a change in reflection when it encounters a transition from pit to land or land to pit. Each transition is a 1. The lack of transitions are 0s.
From: CD-ROM - Computer Dictionary Definition

Any comments / information greatly appreciated!

Thanks

Additional Comment: Some resources mention "optical bits", where differ from digital bits. Are these perhaps involved in the answer, e.g. the method of data bit reading depends on processing of optical bits?



Last edited by IanR; 11-22-2010 at 07:46 PM.
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