How Compact Discs Work

Today CDs are everywhere. They are used to hold music, data or computer software. They have become the standard medium for distributing large quantities of information. CDs are very easy and cheap to produce. Lets look how CDs and CD drives work and at some different forms of CDs.

Analogue and digital recording

When CDs come out in the early 1980s, their single purpose in life was to hold music. So to understand how a CD works, we need to understand how digital recording and playback work.

Thomas Edison created the first device for recording and playing back sounds in 1877. He used a very simple mechanism to store an analogue wave. In Edison's original phonograph a diaphragm[1] controlled a needle and the needle scratched an analogue signal onto a thin foil cylinder. During playback, the vibrations pressed into the tin cause the needle to vibrate, causing the diaphragm to vibrate and play the sound. Modern phonographs work in the same way, but the signals read by the needle are amplified electronically. The problem with the simple approach is that the fidelity is not very good and if a phonograph is plaid repeatedly, eventually it will wear out.

In a CD the goal is to create a recording with very high fidelity[2] and perfect reproduction. To accomplish these two goals, digital recording converts the analogue wave into a stream of numbers and records the numbers instead of the wave. The conversion is done by a device called an analogue-to-digital converter. Then to play back the music, the stream of numbers is converted back to an analogue wave by a digital-to-analogue converter (DAC). The analogue wave produced by the DAC is amplified and fed to the speakers to produce the sound.

When you sample the wave with an analogue-to-digital converter there are 2 variables. They must be controlled. 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[3] are possible when taking the sample.

In the case of CD sound 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.

Understanding the CD

To fit 74 minutes of music (that are 782,216,000 bytes) onto a disk with only 12 centimetres in diameter[4] means that the bytes have to be fairly small.

A CD is a simple piece of plastic about 1.2 millimetres thick. It consists of an injection-molded[5] piece of clear polycarbonate plastic. During manufacturing this plastic is impressed with microscopic bumps arranged as a single extremely long spiral track of data. Once the clear piece of polycarbonate is formed, a thin, reflective aluminium layer is sprayed onto the disk, covering the bumps. Then a thin acrylic layer is sprayed over the aluminium to protect it and the label is printed onto the acrylic.

A CD has a single spiral track of data circling from the inside of the disk to the outside. 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 bumps, 0.5 microns wide, a minimum of 0.97 microns long and 125 nanometres high.

You will often read about "pits[8]" on a CD instead of bumps. They are pits on the aluminium side, but on the side the laser reads from they are bumps. If you could somehow lift the data track off a CD and stretch it out into a straight line, it would be almost 7.4 kilometres.

Understanding the CD player

The CD player has the job of finding and reading the data stored an bumps on the CD. Because the bumps are so small, the CD player is an exceptionally[9] precise piece of equipment. The drive consists of 3 fundamental components:

A drive motor to spin the disk

A laser and a lens system to focus in on the bumps and read them

A tracking mechanism that can move the laser to follow the spiral track

Inside the CD player there is also a good bit of computer technology to form the data into understandable data blocks and send them to the DAC.

The laser beam passes through the polycarbonate layer, reflects off the aluminium layer and returns to an opto-electronic device. The opto-electronic device detects the changes in light that the bumps make to the laser. The hard part is keeping the laser beam centered on the data track. This centering is the job of the tracking system.

Problems and their solutions

Because the laser is tracking the spiral of data using the bumps, there can be no gaps in the data track where there are no bumps. To solve this problem data is encoded using EFM (eight-fourteen modulation). 8-bit bytes are converted to 14 bits.

Because the laser wants to be able to move between songs, there needs to be data encoded within the music telling the drive "where it is" on the disc. This problem is solved using the "subcode data". Subcode data can encode the absolute and relative position of the laser in the track and can also encode things like song titles.

Because the laser may misread a bump, there need to be error correcting codes to handle single-bit errors. To solve this problem, extra data bits allow the drive to detect single-bit errors and correct them.