What is it?
Compact disk - Read Only Memory. The familiar silver platter contains a large number of tracks - just like a hard disk -
but instead of holding bits of information by magnetising a tiny island (See: What is a Magnetic Disk?), the bit is held by a
tiny hole or depression burnt in by a laser. So digital audio or indeed any data can be recorded on the disk. Pre-recorded CDs are Write-once/ Read Many or Read Only Memory.
The Science/ Technology
Lasers
A laser beam is used to burn a hole to signify a bit of value ‘one’. LASER means Light Amplification by Stimulated
Emission of Radiation. The useful characteristic of laser light is its coherent light of a single colour (monochrome) emerging in a single direction. In other words all the photons of the
electromagnetic radiation or light (usually infrared) are not only of the same frequency, but also all in phase (coherence). Lasers can emit a pulse of light of immense power for a very small
period of time - just perfect for burning up to several billion holes in a spinning CD! The precision of a laser is also used to read back the recorded data by shining a
narrow, constant beam of low power and measuring the strength of the reflection from the CD.
CDROMs
Physically, a CDROM is a 12cm optical disk with a spiral track starting from the centre. An audio CDROM uses a 16 bit
word to represent each sample and the sampling is at 44.1kHz (giving a theoretical maximum frequency response of 22 kHz). Twenty 16 bit samples are packed into a data frame, plus a control
byte, then an additional 388 bits for control and error correction complete the data frame. Up to 99 music tracks are allowed. Single speed CDROMs have a data transfer rate of 150kbytes/s, and a
typical data latency time of 600ms. If a CDROM is used for data on a computer, higher spin rate CDROM drives are used to increase the data transfer rates e.g. a 24 speed drive gives a rate of
3.6Mbytes/s
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