Trademark term for the Sony-Philips digital audio optical disc storage system. The disc is made of plastic, with a top metallized layer, and is read by reflected laser light. Variations (such as the 3" disc) are reserved for special applications. The system stores 75 minutes (maximum) of digital audio and subcode information, or other non-audio data, on a 12-centimeter diameter optical disc.

The specifications for the compact disc and compact disc players were jointly developed by Sony, Philips, and Polygram. This specification is contained in their standards document referred to as the Red Book.

According to popular folklore, the maximum length of the compact disc was set at 75 minutes to allow a recording of Beethoven's ninth symphony to fit on one CD. (Actually, most of the recordings that I have listened to last for about 65 minutes, I'm not sure which orchestra plays such a slow version).

Compact disc data encoding scheme

When Philips introduced the compact disc in 1982, commercial laser technology wasn't nearly as well developed as it is nowadays. The people at Philips' NatLab research center had to develop various elaborate schemes to be able to read the data off the CD surface with one of those crude devices. Some of those schemes are still being used today for reasons of backward compatibility, even though they limit the storage capacity of a CD dramatically. The most elaborate space wasting hack they had to pull is 8 to 14 modulation. It simply means that every byte on a CD gets stored as a 14 bit word. That means you can actually store 3/4th more data on your CD's than you thought!

Why is this necessary? Actually, it isn't, but back then it was. The data on a Compact Disc surface are represented by microscopic holes. These are known as pits. The absence of a pit is known as a land in CD jargon. Contrary to common misconception, a land/pit does not represent a bit on the CD, but a transition between a land and a pit represents a one, and no transition (two consecutive lands or pits) represents a zero. They had to do that, because a transition between two states during one clock cycle was far easier to detect than the value itself.

This representation of data presented a new problem. What if you would want to encode a byte with two consecutive ones? That would mean a transition from land to pit and from pit to land (or vice versa) in an incredibly small amount of space. It would form either a very small land or a very small pit. Either way, this would have been expensive to read and to manufacture because it required greater precision of the equipment.

What they did instead, was to assign each byte a code with no consecutive ones in it before putting it on the cd. When a player reads the data back it needs to decode the data again. In order to do this, you need to assign each byte value to a new 14 bit word. Anything less than 14 bits for the new values won't do because there won't be enough combinations to make without consecutive ones to make up for the 256 different byte values. You can count that if you like, but you can easily prove it. I will leave that as an exercise to the reader, though.


One of the relatively unknown trivia about the design of the CD is how Philips decided on the size of the hole in the middle. The answer is so simple it is almost laughable. One of the engineers simply picked a Dutch 10 cent coin (dubbeltje) out of his pocket and used it as a mask. This means that if you put a dubbeltje in the center hole it fits but doesn't fall through. Amazing.

Unfortunately, with the Euro now being the official currency in the Netherlands this gets more and more difficult to demonstrate, because 10 cent coins are not in circulation anymore. So if you come across one, save it so that you can show this trick to your grandchildren when they admire your antique CD collection.

I haven't been able to verify this for sure anywhere (thanks stupot for getting me to research this), but my story is supported on where it says:

Did you know?
The size of the compact disc, the playing length and the size of the hole in the middle were determined during negotiations between Philips and Sony in the 1980s. They figured that the size of a (Heineken?) beer coaster would be "in the ballpark". The hole in the middle is exactly the size of a Dutch dime. The playing length was determined by the Japanese. The wife of Akio Morita, the president of Sony, played classical music, among which Beethoven's 9th (72 minutes). This had to fit on the CD. The inner diameter in combination with the playing length determined the eventual outer diameter of 12 cm.

There are also mentions on the following site:


This blurb, or variations of it, appeared in the booklets of many CD's until record companies were certain that consumers were comfortable dealing with compact discs.

The Compact Disc Digital Audio System offers, on a small, convenient sound-carrier, state-of-the-art sound reproduction. The Compact Disc's superior performance is the result of laser-optical scanning combined with digital playback, and is independent of the technology used in making the original recording. Recording technology is identified by a three-letter code:

DDD = digital tape recorder used during session recording, mixing and/or editing, and mastering (transcription).

ADD = analog tape recorder used during session recording; digital tape recorder used during subsequent mixing and/or editing and during mastering (transcription).

AAD = analog tape recorder used during session recording and subsequent mixing and/or editing; digital tape recorder used during mastering (transcription).

For best results, apply the same care in storing and handling Compact Discs as you would with conventional records. No cleaning will be necessary if the Compact Disc is always held by the edges and is replaced in its case immediately after playing. Should the Compact Disc become soiled with fingerprints, dust or dirt, it can be wiped (always in a straight line, from center to edge) with a clean, soft, dry, lint-free cloth. No solvent or abrasive cleaner should ever be used. If you follow these suggestions, your Compact Discs will provide a lifetime of pure listening enjoyment.

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