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How CD-ROM Drives Work?
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This article was published
in the online version of IngEEEnious, the magazine of IEEE chapter
of Fr. C.R.C.E
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DOWNLOAD IN PDF FORMAT (28 KB)
Thomas Edison is credited with creating the first device for recording and playing back sounds way back in 1877. From then music evolved, means of storing music also evolved. Today, people use Compact Discs (CDs) to store not only music, but also data or computer software.
CDs were a breakthrough in storing music, because they used digital recording techniques. Edison's phonograph, on the other hand, used analog recording methods, in which the diaphragm directly controlled a needle, and the needle scratched on the analog signal into a tin foil cylinder. Playback was achieved by the needle vibrating due to these scratches, thereby causing the diaphragm to vibrate and produce the sound.
The phonograph and the more recent gramophone were replaced with the magnetic tape, in which, the vibrations were recorded onto a magnetic media. This cassette or tape dominated for a few decades until the advent of the CD.
The goal of any recording technology 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 time you play it no matter how many times you play it). This is where the analog methods discussed above receive a setback. Thus, when the CDs were introduced in the early 1980s, their single purpose in life was to hold music.
Let us see how the CD overcomes the drawbacks of analog recording. In digital recording, the analog wave is converted into a stream of numbers and this number is recorded 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). This wave is amplified and fed to the speakers to produce the sound. The analog wave will be the same every time, as long as the numbers are not corrupted. It will be very similar to the original analog wave if the analog-to-digital converter sampled at a high rate and produced accurate numbers. On the CD, the digital numbers are stored as bytes, and it takes two bytes to represent 65,536 gradations. There are two sound streams being recorded (one for each speaker on a stereo). Thus, a CD can store upto 74 minutes of music or 783,216,000 bytes!
Now imagine fitting all this onto a piece of plastic 12 centimeters in diameter. A CD is a piece of polycarbonate plastic about 1.2 millimeters thick. During manufacturing, this is impressed with microscopic bumps arranged as a single, continuous, spiral track of data. A thin, reflective layer of aluminum is sputtered onto the disc and an acrylic layer is sprayed over it to protect it. The actual data track on the CD is approximately 0.5 microns wide, 1.6 microns separating one track from the next one. The track consists of a series of elongated bumps 0.5 microns wide, a minimum of 0.97 microns long and 125 nanometers high. The data rack can stretch almost 5 miles long.
To read this information, the CD drive has the following components: a drive motor to spin the disk (200 to 500 rpm), laser and lens system to read the bumps, a tracking mechanism to move the laser assembly to pint of reading data. Above all, it also has a DAC to reconvert original wave. 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 from the lands (the rest of the aluminum layer) and the sensor detects that change in reflectivity. The electronics in the drive then interpret the changes in reflectivity to read the bit that make up the bytes. The tracking system has to align the laser in such a way that the data coming off the disk maintains a constant rate.
Today, CDs are also used to store data and are available in various speeds: 8X, 16X, 24X, 32X, 48X; the 'X' representing the speed of an audio CD drive (the one discussed above). There are two main formats used to store data on the CD: the CD-DA (audio) and the CD-ROM (data). In addition, the CD is also being used to store motion video. The main problem here, however, is the size of the video. It often requires more than one CD to store video.
A new entry in the data storage market is the DVD (Digital Versatile Disc). With the advent of data compression techniques, a DVD can store MPEG-2 encode movies. A DVD is physically similar to a CD. The main difference is that the bumps are smaller to a CD. The main difference is that the bumps are smaller. DVD tracks are separated by 0.74 microns and track width and minimum pit length being reduced by same percentage. This gives the DVD a whooping 4.7 gigabytes of data per side. Newer data encoding methods are further shrinking the size required on the storage medium.
A DVD can store 8 hours of music per side or upto 133 minutes of high resolution video (in MPEG-2 format), soundtrack presented in upto 8 languages using 5.1 channel Dolby digital surround sound and subtitles in upto 32 languages.
We surely have come a long way from Edison's phonograph. Who knows what's in store for us in the next millenium?
