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Compact Disc Players
Page 3 of 3 (24 Questions and Answers)

1242. How can I clean a dirty CD which has a very difficult to remove stain? Which materials are best for cleaning? — AM, Mexico
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Most CD's are made from polycarbonate plastic (though other plastics with the same index of refraction are occasionally used). Polycarbonate is a pretty tough material, so it should survive most common stain or gum removing solvents. Try your favorite solvent on an unimportant CD first; such as one of the free discs that come occasionally in the mail. However, if the stain molecules have diffused into the plastic and have become trapped within the tangle of plastic molecules, you're probably out of luck. Removing such a stain will require wearing away some of the plastic. Since the disc's surface finish must remain smooth and the thickness of the disc shouldn't change much, serious resurfacing is likely to make the disc unplayable. Also, stay away from the printed side of the disc—it has only a thin layer of varnish protecting the delicate aluminum layer from injury. Solvents can wreck this side of the disc. Finally, if the stain is a white mark (or a scratch), you may be able to render the disc clear again by filling the tiny air gaps that make it white with another plastic. I'll bet that a clear furniture polish or liquid wax will soak into the white spot, replace the air, and render the disc clear and playable.

1295. How does a CD player pause a CD if the CD continues to turn? — BC, Oxon, England
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A CD player reads ahead of the sound it is playing so that it always has sound information from at least one full turn of the disc in its memory. It has to read ahead as part of the error correcting process—the sound information associated with one moment in time is actually distributed around the spiral rather than squeezed into one tiny patch. This reading ahead is particularly important for a portable CD player, which usually saves several seconds of sound information in its memory so that it will have time to recover if its optical system is shaken out of alignment. When you pause the CD player, it reads ahead until its memory is full and then lets its optical system hover while the disc continues to turn. When you unpause the player, it uses the sound information it has saved in its memory to continue where it left off and its optical system resumes the reading ahead process.

1296. In instructions for cleaning CDs, it always specifies cleaning the CD by wiping radially from the center out. Why does it matter? — AB, Toronto, Canada
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Whenever you wipe a CD to clean it, there is a chance that you will scratch its surface. If that scratch is wide enough, it may prevent the player's optical system from reading the data recorded beneath it and this loss of data may make the CD unplayable. It turns out that tangential scratches are much more serious than radial scratches. When the scratch is radial (extending outward from the center of the disc to its edge), the player should still be able to reproduce the sound without a problem. That's because sound information is recorded in a spiral around the disc and there is error-correcting information included in each arc shaped region of this spiral. Since a radial scratch only destroys a small part of each arc it intersects, the player can use the error correcting information to reproduce the sound perfectly.

But when the scratch is tangential (extending around the disc and along the spiral), it may prevent the player from reading a large portion of an arc. If the player is unable to read enough of the arc to perform its error correcting work, it can't reproduce the sound. That's why a tangential scratch can ruin a CD much more easily than a radial scratch can. That's why you should never wipe a CD tangentially. Always clean them by wiping from the center out.


1382. How does a phonograph work? — MS
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A phonograph record represents the air pressure fluctuations associated with sound as surface fluctuations in long, spiral groove. This groove is V-shaped, with two walls cut at right angles to one another—hence the "V". Silence, the absence of pressure fluctuations in the air, is represented by a smooth portion of the V groove, while moments of sound are represented by a V-groove with ripples on its two walls. The depths and spacings of the ripples determine the volume and pitch of the sounds and the two walls represent the two stereo channels on which sound is recorded and reproduced.

To sense the ripples in the V-groove, a phonograph places a hard stylus in the groove and spins the record. As the stylus rides along the walls of the moving groove, it vibrates back and forth with each ripple in a wall. Two transducers attached to this stylus sense its motions and produce electric currents that are related to those motions. The two most common transduction techniques are electromagnetic (a coil of wire and a magnet move relative to one another as the stylus moves and this causes current to flow through the coil) and piezoelectric (an asymmetric crystal is squeezed or unsqueezed as the stylus moves and this causes charge to be transferred between its surfaces). The transducer current is amplified and used to reproduce the recorded sound.


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