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Compact Disk Digital Audio (CD-DA)

The first CD format was of course that which defined the audio CD used in all regular CD players, called CD Digital Audio or CD-DA for short. The specifications for this format were codified in the first CD standard, the so-called "red book" that was developed by Philips and Sony, the creators of the original compact disk technology. The "red book" was published in 1980, and actually specifies not just the data format for digital audio but also the physical specifications for compact disks: the size of the media, the spacing of the tracks, etc. In a sense, then, all of the subsequent standards that came after CD-DA build on the "red book" specification, since they use the same specifications for the media and how it is read. They also base their structure on the original structure created for CD audio.

Data in the CD digital audio format is encoded by starting with a source sound file, and sampling it to convert it to digital format. CD-DA audio uses a sample rate of 44.1 kHz, which is roughly double the highest frequency audible by humans (around 22 kHz.) Each sample is 16 bits in size, and the sampling is done in stereo. Therefore, each second of sound takes (44,100 * 2 * 2) bytes of data, which is 176,400 bytes.

Audio data is stored on the disk in blocks, which are also sometimes called sectors. Each block holds 2,352 bytes of data, with an additional number of bytes used for error detection and correction, as well as control structures. Therefore, 75 blocks are required for each second of sound. On a standard 74-minute CD then, the total amount of storage is (2,352 * 75 * 74 * 60), which is 783,216,000 bytes or about 747 MB. From this derives the handy rule of thumb that a minute of CD audio takes about 10 MB, uncompressed.

Using special software, it is possible to actually read the digitally-encoded audio data directly from the CD itself, and store it in a computer sound format such as a WAV file. However, while every CD-ROM drive will play standard "red book" digital audio, not every drive will allow you to read the digital audio data directly, which is sometimes called CD-DA extraction. Sometimes the reasons are technical, but more often they are political; some drive manufacturers intentionally program their drives not to allow data extraction, in order to avoid possible copyright infringement by their owners! See this page for more information of interest regarding CD-DA.

Next: CD-ROM Digital Data (CD-ROM, ISO 9660, "High Sierra")

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