Compact Discs - Advanced Guide
The principles
Compact discs store digitally encoded information in a single, continuous spiral of data. An analogue-to-digital conversion process converts the original audio signal into binary data, which is stored on the CD. A CD player reads this, feeding the signal to a digital-to-analogue converter (DAC) that then feeds an amplifier, which in turn feeds speakers to create audio.
The same process is used on DVD players (see our Guide to DVD players) but CDs got there first, launching in the US in 1983.
How it works
The critical elements when transforming analogue sound into digital data are the sampling rate and the sampling precision.
The sampling rate dictates how often the original audio signal is referred to when creating a digital facsimile. The sampling precision dictates how accurate the facsimile will be at each sampling point.
Lets look at the process. Analogue audio takes the form of a wave pattern. An analogue-to-digital converter (ADC) looks at this wave and constructs a digital version that could be visualised as a bar graph.
It is obvious that if a low sampling rate is employed (represented by the horizontal axis), or if insufficient sampling precision is employed (represented by the vertical axis), the resultant bar graph will be a poor representation of the original wave.
The conversion process used for CDs tackles this issue by sampling at a rate of 44.1kHz that is, 44,100 times per second. Sampling is performed with 65,536 gradations. In other words, each sample of data can have one of 65,536 possible values, and 44,100 samples are conducted per second for each audio channel.
This creates a digital version of the original analogue sound wave that is virtually indistinguishable, certainly to the human ear. It also creates a lot of data! Each sample point requires two bytes of data to represent it, and a 74-minute, two-channel album requires over 780Mb of information.
There is an obvious question: why go to such lengths just to create an imperfect copy of the original, when analogue recording formats can get much closer to the original sound?
The answer is simply that digital data is less susceptible to errors when being read (and CDs employ sophisticated error-correction processing to deal with those that do occur). Where a vinyl album will wear with each playing and is easily compromised by dust particles or scratches, CDs protect their binary data far better and can reproduce exactly the same signal every time they are played.
The data is stored on a CD in the form of microscopic bumps on a reflective surface. The bumps (sometimes referred to as pits, although from the side the laser reads them they actually appear as bumps rather than indentations) are a mere 0.5 microns wide (a micron is a millionth of a metre) and 125 nanometers high (a nanometer is a billionth of a metre).
The grooves in the spiral are tightly packed, with just 1.6 microns between them. Reading this tightly packed data obviously requires an extremely precise laser, and those used in CD players are close to infrared wavelengths (DVD players use red lasers, while the forthcoming HD DVD and Blu-ray players utilise blue lasers, with even shorter wavelengths).
The polycarbonate layer of a CD holds the bump and land structure (lands are the areas between the bumps), and a coating of aluminium makes it reflective. This is then topped with a protective acrylic layer, on top of which sits the label.
The disc is spun by the disc motor and the laser pickup is moved across the underside of the disc by the tracking motor. This sophisticated mechanism must be able to move the laser pickup in extremely small steps in order to faithfully follow the data track. At the same time, the disc motor must vary its speed to keep the relative velocity (the speed of the disc as it passes the laser pickup) constant as the pickup moves from the inside to the outside of the disc.
If all of this works smoothly, the laser pickup is able to distinguish between the reflections from bumps and lands and construct a stream of binary data that is passed on to a DAC.
The process of sound reproduction is then passed on to an amplifier and speakers (see our Guides to Amplifiers and Speakers), but the really hard work has already been done!
Compact Disc/CD Glossary
Blu-ray A rival to HD DVD, offering more storage capacity but still delivering high-definition movies. Already launched (as recorders) in Japan, but still some way off for the rest of the world
HD DVD A high-definition version of DVD technology, due to launch in the US in late 2005
Article from HomeCinemaUK.com
