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How CD players work

How they work

Today CDs are everywhere. Whether they are used to hold music, data or computer software, they have become the standard medium for distributing large quantities of information in a reliable package. CDs are so easy and cheap to produce that America Online sends out millions of them every year to entice new users. Anyone with a computer and CD-R drive can now create their own CDs and put anything they want on them. We will also look at all the different forms CDs take, as well as what the future holds.

Understanding Analog and Digital Recording. When CDs were first introduced in the early 1980s, their single purpose in life was to hold music. In order to understand how a CD works, you therefore need to first understand how digital recording and playback works and the difference between analog and digital technologies.

Thomas Edison is credited with creating the first device for recording and playing back sounds in 1877. His approach used a very simple mechanism to store an analog wave mechanically. In Edison's original phonograph, a diaphragm directly controlled a needle, and the needle scratched an analog signal onto a tin foil cylinder:

You spoke into Edison's device while rotating the cylinder, and the needle "recorded" what you said onto the tin. That is, as the diaphragm vibrated so did the needle, and those vibrations impressed themselves onto the tin. To play the sound back, the needle moves over the groove scratched during recording. During playback, the vibrations pressed into the tin cause the needle to vibrate, causing the diaphragm to vibrate and play the sound. This system was improved by Emil Berliner in 1887 to produce the gramophone, which is also a purely mechanical device using a needle and diaphragm. The gramophone's major improvement was the use of flat records with a spiral groove, making mass production of the records easy. The modern phonograph works the same way, but the signals read by the needle are amplified electronically rather than directly vibrating a mechanical diaphragm.

What is it that the needle in Edison's phonograph is scratching onto the tin cylinder? It is an analog wave representing the vibrations your voice creates. For example, here is a graph showing the analog wave created by saying the word "Hello"

This waveform was recorded electronically rather than on tin foil, but the principle is the same. What this graph is showing is, essentially, the position of the microphone's diaphragm (Y axis) over time (X axis). The vibrations are very quick - the diaphragm is vibrating on the order of 1,000 oscillations back and forth per second. This is the sort of wave scratched onto the tin foil in Edison's device. Notice that the waveform for the word "hello" is fairly complex. A pure tone is simply a sine wave vibrating at a certain frequency, like this 500 hertz wave (500 hertz = 500 oscillations per second):

You can see that the storage and playback of an analog wave can be very simple - scratching onto tin is certainly a direct and straightforward approach. The problem with the simple approach is that the fidelity is not very good. For example, when you use Edison's phonograph there is a lot of scratchy noise stored with the intended signal, and the signal is distorted in several different ways. You can also see that if you play a phonograph repeatedly, eventually it will wear out. When the needle passes over the groove it changes it slightly (and eventually erases it).

In a CD (and any other digital recording technology) the goal is to create a recording with very high fidelity (very high similarity between the original signal and the reproduced signal) and perfect reproduction (the recording sounds the same every single time you play it no matter how many times you play it). To accomplish these two goals, digital recording converts the analog wave into a stream of numbers and records the numbers instead of the wave. The conversion is done by a device called an analog-to-digital converter. Then to play back the music, the stream of numbers is converted back to an analog wave by a digital-to-analog converter (DAC). The analog wave produced by the DAC is amplified and fed to the speakers to produce the sound.

The analog wave produced by the DAC will be the same every time, as long as the numbers are not corrupted. The analog wave produced by the DAC will also be very similar to the original analog wave if the analog-to-digital converter sampled at a high rate and produced accurate numbers.

You can understand why CDs have such high fidelity if you understand the analog-to-digital conversion process better. Let's say you have a sound wave, and you wish to sample it with an A-to-D converter. Here is a typical wave (assume here that the ticks on the horizontal axis represent 1/1000ths of a second):

When you sample the wave with an analog-to-digital converter you have control over 2 variables. The first is the sampling rate. The rate controls how many samples are taken per second. The second is the sampling precision. The precision controls how many different gradations (quantization levels) are possible when taking the sample. In the following figure, let's assume that the sampling rate is 1,000 per second and the precision is 10:

The green rectangles represent samples. Every 1/1000th of a second the A-to-D converter looks at the wave and picks the closest number between 0 and 9. The number chosen is shown along the bottom of the figure. These numbers are a digital representation of the original wave. When the DAC recreates the wave from these numbers, you get the blue line shown in the following figure.

You can see that the blue line lost quite a bit of the detail originally found in the red line, and that means the fidelity of the reproduced wave is not very good. This is the sampling error. You reduce sampling error by increasing both the sampling rate and the precision. In the following figure, both the rate and the precision have been improved by a factor of 2 (20 gradations at a rate of 2000 samples per second):

In the following figure the rate and the precision have been doubled again (40 gradations at 4,000 samples per second):

You can see that as the rate and precision improve, the fidelity (the similarity between the original wave and the DAC's output) improves. In the case of CD sound, fidelity is an important goal so the sampling rate is 44,100 samples per second and the number of gradations is 65,536. At this level the output of the DAC so closely matches the original wave form that the sound is essentially "perfect" to most human ears.

One thing about the CD's sampling rate and precision is that it produces a lot of data. On a CD the digital numbers produced by the analog-to-digital converter are stored as bytes, and it takes two bytes to represent 65,536 gradations. There are two sound streams being recorded (one for each of the speakers on a stereo system). A CD can store up to 74 minutes of music, so the total amount of digital data that must be stored on a CD is:

44,100 samples/channel/second * 2 bytes/sample * 2 channels * 74 minutes * 60 seconds/minute = 783,216,000 bytes

That is a lot of bytes! To store that many bytes onto a cheap piece of plastic tough enough to survive the abuse most people put a CD through was no small task, especially when you consider that the first CDs came out in 1980.

Understanding the CD. To fit that much data onto a disk only 12 centimeters in diameter means that the individual bytes have to be fairly small. By looking at the physical construction of the CD you can learn how small they are.

A CD is a fairly simple piece of plastic about 1.2 millimeters thick. Most of the CD consists of an injection-molded piece of clear polycarbonate plastic. During manufacturing this plastic is impressed with microscopic bumps arranged as a single, continuous, extremely long spiral track of data. We will return to the bumps in a moment. Once the clear piece of polycarbonate is formed, a thin, reflective aluminum layer is sputtered onto the disk, covering the bumps. Then a thin acrylic layer is sprayed over the aluminum to protect it. Then the label is printed onto the acrylic. A cross section of a complete CD (not to scale) looks like this:

A CD has a single spiral track of data circling from the inside of the disk to the outside. The fact that the spiral track starts at the center means that the CD can be smaller than 12 centimeters if desired, and in fact there are now plastic baseball cards and business cards that you can put in a CD player. CD business cards hold about 2 megabytes of data before the size and shape of the card cuts off the spiral.

What the picture on the right does not even begin to impress upon you, however, is how incredibly small the data track is. The track is approximately 0.5 microns wide, with 1.6 microns separating one track from the next. The track consists of a series of elongated bumps 0.5 microns wide, a minimum of 0.97 microns long and 125 nanometers high. Looking through the polycarbonate layer at the bumps, they look something like this:

You will often read about "pits" on a CD instead of bumps. They are pits on the aluminum side, but on the side the laser reads from they are bumps.]

The incredibly small dimensions of the bumps makes the spiral track on a CD extremely long. If you could somehow lift the data track off a CD and stretch it out into a straight line, it would be 0.5 microns wide and almost 5 miles long!

To read something this small you need an incredibly precise disk-reading mechanism.

Understanding the CD player. The CD player has the job of finding and reading the data stored as bumps on the CD. Because the bumps are so small, the CD player is an exceptionally precise piece of equipment. The drive consists of 3 fundamental components:

Inside the CD player there is also a good bit of computer technology to form the data into understandable data blocks and send them either to the DAC (in the case of an audio CD) or to the computer (in the case of a CD-ROM drive)

The fundamental job of the CD player is to focus the laser on the track of bumps. The laser beam passes through the polycarbonate layer, reflects off the aluminum layer and returns to an opto-electronic device that detects changes in light. The bumps reflect light differently than the "lands" (the rest of the aluminum layer), and the opto-electronic sensor can detect that change in reflectivity. The electronics in the drive interpret the changes in reflectivity to read the bits that make up the bytes

The hard part is keeping the laser beam centered on the data track. This centering is the job of the tracking system. The tracking system, as it plays the CD, has to continually move the laser outward. As the laser moves outward, the spindle motor slows the speed at which the CD is revolving so that the data coming off the disk maintains a constant rate.

Understanding CD formats. If you have a CD-R drive and want to produce your own audio CDs or CD-ROMs, one of the great things going in your favor is the fact that software handles all the details for you. You can say to your software, "Please store these songs on this CD" or "please store these data files on this CD-ROM" and the software will do the rest. Because of that you don't need to know anything about CD data formatting to create your CDs. However, CD data formatting is complex and interesting, so here is a bit of detail.

To understand how data is stored on a CD, you need to understand all of the different conditions the designers of the data encoding methodology were trying to handle. Here is a fairly complete list:

There are several different formats used to store data on a CD, some widely used and some long-forgotten. The two most common are CD-DA (audio) and CD-ROM (computer data).

DVD - The Next Generation in digital storage A new entry in the data storage market is the DVD. A DVD is very similar to a CD but has a much larger data capacity (about 4.7 billion bytes, or approximately 7.5 times more data than a CD), giving a DVD enough capacity to store MPEG2-encoded movies. When you think about CDs what you think about is "music, and they also happen to be able to store 650 megabytes of data". When you think about a DVD what you think about is "movies, and they also happen to be able to store 4.7 gigabytes of data per side".

A DVD is physically very similar to a CD. It is the same diameter. It is the same thickness. It uses the same technology of a laser beam bouncing off of a reflective layer interrupted by bumps. The main thing that gives a DVD so much more capacity is the fact that the bumps are much smaller. DVD tracks are separated by 0.74 microns (compared to 1.6 microns for a CD), and the track width and minimum pit length are reduced by about the same percentage as well. There are also some data encoding differences which mean that fewer bits are wasted on overhead.