How Do CD's Work?
Before the onset of digital technology, musical recordings were processed for playback on LP records or cassette tapes. With LP records, the music was stored on a plastic material imprinted with a variation of bumps.
These bumps could then be interpreted as sounds by the playback equipment. Cassette tapes used a form of varying magnetic patterns which when read by the playback devise, would be replicated as sound. Both these media used analogue technology, which required specific materials and had limitations relating to storage capacity and long term durability.
How a CD Stores Sound:
All replicated CD’s originate from one original ‘master’ disc. The technology used to replicate CDs consists of a digital process known as ‘sampling’, which converts a musical recording into a series of digital numbers. Based on the results of the sampling process a master disc is then created. For durability, the master disc will often be created using a heavy duty glass underside, unlike the plastic copies that will later be manufactured.
Based on data collected from the original recording, tiny bumps (referred to as pits), are etched onto the surface of the disc using a laser. These pits represent zero digits. Pits typically measure 100 nm deep by 500 nm wide with lengths that could be between 850 nm to 3.5 µm.
Areas without bumps, known as lands, represent the number one digits. Each track on the CD results from the sampling process and if broken down, it would appear to be a string of millions of zeros and ones. This pattern of zeros and ones burnt onto the reflective surface of the disc is known as a binary digital format. If the human eye were able to view the string of digits, it would show a continuous spiral of approximately 3 to 5 billion pits.
If the spiral could be untangled, the information would stretch for 6 km, approximately 10 miles. An individual pit requires an area approximately two millionths of a millionth square meter, an amazing concept to imagine.
After the master disc reaches the final production stage, it is used to stamp millions of plastic duplicates for distribution. When copies of the master disc are created, they feature a negative or opposite image of the bumps and lands. CDs played at home are actually the opposite format from the original master.
Playing a CD:
CD playback equipment uses a fine laser to read, or ‘measure’ the zeros and ones on the disc surface approximately 50,000 times per second. CD players contain an optical chip that reads the recorded sound data.
The chip consists of three photosensitive rectangles that are designed to process the information. An electronic tracking system is used to ensure that the laser bean remains cantered in the middle section of the track. Data is read by a semiconductor laser which focuses a 780 nm wavelength through the bottom of the polycarbonate layer.
As the laser reads the disc, the difference in height between the pits and lands causes the laser to be reflected with different levels of intensity. The CD player then analyses the intensity of the reflected light and interprets this as a one or a zero.
Therefore, a replicated CD is just a collection of processed numerical data, where the combination of burned and unburned areas can be used to relay instructions to the speakers. The resulting sound waves then reproduce the original recording.
Materials Used for CD Reproductions:
A typical manufactured CD consists of two primary components:
- The base: This consists of high quality polycarbonate plastic, weighing approximately 15 to 20 grams.
- A thin layer of aluminium or gold used to create the reflective surface onto which the data can be burnt.
A CD also features the following separate components (starting from the centre outward):
- the spindle or hole,
- the clamping ring,
- the stacking ring,
- the mirror band,
- the data or information area and the rim.
The reflective surface of the CD is protected with a film of lacquer applied directly on top of the reflective layer. The label is printed using either a screen printing or offset printing method and is applied after the lacquer is processed.
Although CDs typically require less care than the earlier LP and tape recording systems, damage from extended use or environmental exposure can harm the playback quality. For readability purposes, pits are placed closer to the label side of the disc rather than the clear side.
If scratches, defects or dirt accumulate on the label side, repairs may be more difficult. However, slight damage to the clear side of the CD can be repaired using refractive plastic or gentle polishing.
