THE COMPACT DISC – Talk for Cheltenham U3A S+T group 2010.

References to diagrams in the text are to the accompanying visual slides.


The compact disc is used to store digital data which can take the form of audio, photographs, text, or pictures. Originally developed in 1965 by James Russel at Pacific NW Labs in California, the rights were sold to Sony and Phillips. It took until 1980 until those companies commercialised the product and the reasons are principally that the components and technological components were too expensive to produce on a commercial basis.

As we look in a bit more detail at how the CD is made and how it works, you will see the importance of very small architecture high speed processing at the heart of CD players. Equally, the importance of lasers, originally in 1965 using precious materials, but by 1980 lasers were available as semi conductor devices albeit at low power. The CD is itself constructed in polycarbonate, a material in common use in spectacles, visors, bullet proof glass and jet fighter canopies, racing cars, and so on. So in every sense the CD is an amalgam of many technologies, involving the use of some complex scientific principles, and engineered to tolerances of the highest order.

Since this talk forms part of the familiar things set, I am going to concentrate on the CD as an audio device; domestically that is where we encounter it mainly. I shall briefly include the CD ROM which say comes with a computer and holds the operating system, your Mac or Windows 7 operating system may well accompany your PC on a number of CDs or DVDs (a similar process)

The CD came to replace slowly at first and by 2010 almost entirely the gramophone record or vinyl disc as we called them after 1960. That technology lasted from 1870 via Edison, and cylinders, and beeswax discs, through RCA Victor, to 78, 45, 33 rpm machines. The underlying principle , which is different to CDs is that they were storage devices of analogue data. That analogue data set was in practice a permanently etched record of the amplitude and frequency changes recorded after audio had caused a diaphragm to vibrate, causing sympathetic electrical impulses to create a recording. Initially I think Edison used tin foil. Here is a record under the microscope. See accompanying visual slides.

Let’s get the terminology right here.

Analogue - is a continuously variable waveform which denotes some intelligence (music, speech) The audio spectrum is about 300Hz to 20 KHz, but few in his room will have that range.

Here is a drawing of it.

Now we have the digital data ready. Lets make a CD.

Diagram of CD layers.
A. A polycarbonate disc layer has the data encoded by using bumps.
B. A shiny layer reflects the laser.
C. A layer of lacquer helps keep the shiny layer shiny.
D. Artwork is screen printed on the top of the disc.
E. A laser beam reads the CD and is reflected back to a sensor, which converts it into electronic data

A CD is made from a 1.2mm thick polycarbonate plastic weighing 15-20grammes. From the inside out there is 1. a spindle. 2. transition clamping, 3 .clamping area. 4. transition band. 5. data. 6. rim.

A thin layer of aluminium, for a commercial quality CD, or more rarely gold, is applied to the surface to make it reflective, and covered with a layer of protective lacquer. The data bits are stored as a series of tiny indentations called pits and encoded in a spiral track moulded into the top of the polycarbonate layer.

Each pit (which represents a data bit) is circa 100 nm deep by 500 um wide, 850nm in length. The distance between tracks is 1.6um.

To read. A semi conductor laser, operating at around 780nm wavelength, near infra red, is fired through the underside of the CD polycarbonate layer. The pits are relected as ridges and read as a different light intensity. A photodiode reads the reflected light and those readings are read as binary digits. Non return to zero encoded. The convention is that a pit to land is a 1, or 0, no change is a 0. There can not be more than 10 consecutive zeros. These data are then reverse 14 – 8 bit decoded, then Reed Solomon decoded, revealing a pure stream of binary at 44.056kbps.

The data spiral track starts in the middle and ends at the rim, the laser tracks across the radius of the CD which spins at 500 rpm at the centre and 200 rpm at the outside. The tracking motor aligns the tracking pickup and controls the turning motor. The CD mechanism is actually one revolution ahead of the listener; it reads, decodes, and outputs later.

Two flip charts of how it works

The CDs which you make at home on your computer or CD writer use CD blanks which have in lieu of the aluminium track, a coating of dye which when heated (for 1 or zero) reflects a different intensity colour. A rewritable CD also uses a dye coating but is reset by a laser wash and can then be rewritten.