April 10, 1995

One Minute Papers - Questions and Answers

Why do CD's skip?

CD players must position their optical system very precisely, relative to the spinning disk itself. It uses very sophisticated electromechanical devices to keep it in place. But if you jar a player violently enough, it will lose its position and the audio may suffer. Most modern CD players save a short amount of information so that they are reading ahead of where they are playing. Even if they lose the track for a few hundredths of a second, they have enough music saved up that they can keep playing continuously. But if the upset is severe enough, they will run out of saved music and will go silent for a moment or two.

Why do you need to separate the different polarizations of light?

Any light wave can be described in terms of horizontally and/or vertically polarized light. For most things, these two polarizations are unimportant. But when light reflects from surfaces or passes through certain materials, these polarizations become important. The charges in surfaces and materials do not always respond equally to the two polarizations of light. The two polarizations may even travel through very different paths (e.g. in the polarization beam splitter).

How does a laser diode work?

A laser diode resembles a light emitting diode, in which electrons flowing across a p-n junction (in a diode) find themselves in conduction levels of the p semiconductor, with lots of excess energy. These excited electrons give up their excess energy by emitting light and they drop down into empty valence levels with much less energy. In a laser diode, the region in which this energy release occurs is a very narrow channel with mirrored ends. Instead of emitting their light spontaneously, the electrons experience stimulated emission. Light bounces back and forth between the ends of the channel and is amplified as it passes new excited electrons. Because all of the light produced by a laser diode emerges from one end of this very narrow channel, it experiences severe diffraction and spreads out into a wide, cone-shaped beam. To convert this cone of light into a narrow beam, a converging lens is usually attached to the diode laser's housing and this lens bends the beam into a fine pencil of light. Most laser diodes operate in the red or infrared portion of the spectrum, although some laser diodes that emit blue light have recently been developed.

How does alternating current affect the laser? Does it make the laser reverse?

A diode laser will only emit light (lase) when current flows through it in the proper direction. It is, after all, a diode and only conducts current in one direction. But small fluctuations in current do affect the light emission. If you run a modest current through a laser diode, so that it emits a steady stream of light, and then begin to modulate that current up and down slightly, the light emitted by the laser will modulate up and down slightly, too. In this manner, you can send sound or other information over a laser beam. This technique is useful as a private means of communicating over long distances. Only someone who can "see" the blinking laser beam can detect the information that it contains.

What does a cleaning CD do?

The optical system of the CD player must be very clean. If the final lens has dust on it, the photodiode will not see the full range of light and dark patches that it expects. A cleaning CD presumably cleans this final lens, although I'm not sure how. In principle, the whole CD player should be pretty resistant to dust problems because the laser beams are large except when they focus on the CD itself.

How are the binary numbers represented in the ridges of the CD?

In principle, the binary numbers could be written as the presence or absence of ridges (i.e. a 1000 nanometer long ridge could be a 1 while a 1000 nanometer long flat area could be a 0). However, this technique has technical problems. The main problem is that the number "0" would be a long flat region (16 adjacent flat regions would be one 16000 nanometer flat region). If the flat region became too long, the CD wouldn't be able to follow the track any more. So an encoding scheme is used to make sure that ridges and flat areas are never too long. They use a length encoding scheme, where ridges of different lengths correspond to a short group of binary bits. Furthermore, a very extensive error correcting arrangement makes sure that the music can be read even if a great many bits are unreadable. About 25% of the CD's surface is dedicated to this error correcting information.

Will light going in two directions in the same space create destructive interference?

No. Although the electric fields do add to one another at each moment, those electric fields go in and out of phase with one another very rapidly as the waves pass one another. The end result is that they do not interfere with one another at all. An object with these two waves passing through it will interact with each wave independently, as though the other wave were not present. The only exceptions occur when the two waves are extraordinarily bright, so that each wave actually modifies the nature of the object. In that case, one wave can affect the other through their mutual interactions with the object.

Do you know anything about a special kind of digital tape that could replace the CD?

Digital audio tapes have been around for a few years. These tapes store sound as digital information on a tape. Because of the digital recording and playback, the reproduction is almost perfect. The digital process involves an enormous amount of information each second; too much to be recorded in the conventional method used in cassette tapes. Instead, I think that a helical technique is used, in which information is written as diagonal stripes across the length of the passing tape. By writing a closely-spaced series of these stripes, the DAT (digital audio tape) player uses much more of the tape's surface than a standard cassette and stores much more information on that surface. I doubt that DAT tapes will replace CD's because CD's are so easy to mass produce. DAT tapes must be recorded one at a time.

Although I have heard that CD players are on average better at reproducing sound, I have also heard that the best sound quality can still be had from high end phonographs. To what extent is this true?

The digitization process does introduce some distortions into the sound signal, including aliasing (confusion about high frequencies) and quantization error (round-off errors in recording the softest sounds). However, these distortions should be so small or at such high frequencies that they should be inaudible. Still, there are always some audiophiles who can hear (or claim to hear) these imperfections.

Why do some CD players sound better than others even if the CD is seriously scratched on the bottom half?

At this point, there should be very little difference between CD players that are playing perfect CD's. They all create almost distortionless reproductions of the original sound. However, different players use different tracking techniques and optical systems and thus have different abilities to recover from imperfections in the CD.

What is the deal with the new mini disc players?

I only know how the prerecorded mini disc players work: they work a lot like CD players. However, they use a much smaller disk, made possible by intelligent data reduction. Instead of using 16 bits to represent each current measurement, the mini disk uses a variable number of bits. The recording equipment determines how many bits are needed to represent the sound accurately and eliminates unnecessary (or inaudible) details in the current measurements. The optical systems in mini disc players are the same as CD players.

To what extent are laser disc players similar to CD players?

Optically, they are identical. However, the laser disc player uses an analog recording technique (non-digital) to recreate the video signal. I think that the lengths of ridges (or perhaps pits) in the aluminum surface are used to control the analog signal strength.

How does the player (laser) read the whole disk at once?

It doesn't. It reads only a tiny portion of the disk at any given moment. The disk spins and the reading system slowly works its way from the center of the disk towards its edge, following a spiral path around the disk.

Why can't CD's be recorded onto other CD's?

Most of the CD's you encounter are prerecorded. These CD's were mass-produced from a master, using plastic molding techniques, followed by metal deposition and painting. Recordable CD's, which are used now in CD-ROM applications, are written by an intense laser beam, which alters the reflectivity of the CD spot by spot to create a disk that behaves just like a prerecorded CD. However, once a CD has been "written", it cannot be cleaned for rewriting. At present, recordable CD's can only be written once. There are some new optical and magneto-optical techniques around that allow erasure, but I don't think these techniques have appeared in CD's yet.

Why can't I record song directly onto CD's, like I can onto a tape?

To record CD's, you need a much more powerful laser and a blank recordable CD. Both of these items cost lots of money. Reading a CD does not alter the CD but writing it does. You need more laser power and a special CD disk. If you tried to record a normal CD, you would not be able to restructure its aluminum layer. You would not "erase" the old material on it and would not "write" new material onto it.