CD

CD

The CD – which is short for Compact Disc – started a real revolution in the audio world by making digital audio accessible to a wider public, beginning in the year 1982. A CD is a digital optical disc that can store around 700 MB of data. CDs were originally developed for musical purposes but were quickly adapted to store data as well. Since the 700 MB limit exceeded the storage capacity of hard drives at that time, CDs were very popular for all kind of data storage, so much so that around 200 billion CDs have been sold as of the year 2007. In the early CD era only CD-ROMs – Read Only Memory – were available but in the 90s it became possible for consumers to write their own CDs using a CD burner and a dedicated CD-R or CD-RW.

History

In 1970 James T. Russell, an American inventor, patented a system to record digital information on an optical foil. Sony and Philips licensed Russell’s patent and started developing the CD. Philips had already been active in the field for some time together with Pioneer and MCA Inc. which resulted in the Laserdisc. In 1974 Philips started to develop an analog optical disc that would have a better sound quality than vinyl. It became quickly apparent that a digital format would achieve better results. In 1977 Philips established a laboratory that would develop the CD as we know it today. This laboratory set the diameter of a CD to 11.5 cm, the diagonal of a compact cassette.

Sony and Philips joined forces 1979 to design a new digital audio disc. They came up with the Red Book CD-DA standard, the audio format for audio CDs, which uses PCM to encode audio. The sample rate was matched to the PAL and NTSC video standards, although ultimately a sample rate of 44.1 kHz was chosen because it was easier to remember than 44.056 Hz. The first Philips CD players had a 14 bit DA-convertor and used oversampling to make 16 bit audio possible.

Red Book

The Red Book document contains all the specifications for CD audio, and is the first book in the Rainbow Books. The 16 bit/44.1 kHz audio can be accompanied by CD text. With this text it is possible to provide metadata on a CD. Generally the CD text is stored in the lead-in area of a CD allowing it to have a size of roughly 5 Kb. Although two channel audio is the most common on CDs, the Red Book standard allowed an option to provide four channel audio. Single channel audio however, wasn’t allowed. In case of a mono signal, the two stereo channels would carry identical audio.

Technique

Digital information is presented in binary code: a series of 0’s and 1’s. On CDs, these digits are represented as pits and bumps on a polycarbonate disc layer. This polycarbonate layer has another reflecting layer on top of it that reflects the laser. This way, a laser can read these pits and bumps on the polycarbonate layer and convert them into audio. A pit on a CD is approximately 100 nm deep. The disc spins at 500 RPM when reading the inside of the disc’s circle and at the outside around 200 RPM, resulting in a scanning speed of about 1.2 meters per second.

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