Home » CD Replication: Audio CD (Red Book Standard)

CD Replication: Audio CD (Red Book Standard)

The following text is an edited extract from original article at http://en.wikipedia.org/wiki/Compact_Disc

The logical format of an audio CD (officially Compact Disc Digital Audio or CD-DA) is described in a document produced in 1980 by the format's joint creators, Sony and Philips. The document is known colloquially as the "Red Book" after the colour of its cover. The format is a two-channel 16-bit PCM encoding at a 44.1 kHz sampling rate per channel. Four-channel sound is an allowed option within the Red Book format, but has never been implemented. Monaural audio has no existing standard on a Red Book CD; mono source material is usually presented as two identical channels on a 'stereo' track.

The selection of the sample rate was primarily based on the need to reproduce the audible frequency range of 20Hz - 20kHz. The Nyquist–Shannon sampling theorem states that a sampling rate of double the maximum frequency to be recorded is needed, resulting in a 40 kHz rate. The exact sampling rate of 44.1 kHz was inherited from a method of converting digital audio into an analogue video signal for storage on U-matic video tape, which was the most affordable way to transfer data from the recording studio to the CD manufacturer at the time the CD specification was being developed. The device that turns an analogue audio signal into PCM audio, which in turn is changed into an analogue video signal is called a PCM adaptor. This technology could store six samples (three samples per each stereo channel) in a single horizontal line. A standard NTSC video signal has 245 usable lines per field, and 59.94 fields/s, which works out at 44,056 samples/s/stereo channel. Similarly, PAL has 294 lines and 50 fields, which gives 44,100 samples/s/stereo channel. This system could either store 14-bit samples with some error correction, or 16-bit samples with almost no error correction.

There was a long debate over whether to use 14 bit (Philips) or 16-bit (Sony) quantization, and 44,056 or 44,100 samples/s (Sony) or around 44,000 samples/s (Philips). When the Sony/Philips task force designed the Compact Disc, Philips had already developed a 14-bit D/A converter, but Sony insisted on 16 bit. In the end, 16 bits and 44.1 kilosamples per second prevailed. Philips found a way to produce 16-bit quality using their 14-bit DAC by using four times oversampling.

The partners aimed at a playing time of 60 minutes with a disc diameter of 100 mm (Sony) or 115 mm (Philips). Sony vice-president Norio Ohga suggested extending the capacity to 74 minutes to accommodate Wilhelm Furtwängler's 1951 performance of Beethoven’s 9th Symphony.

The extra 14 minute playing time subsequently required changing to a 120 mm disc. Kees Immink, Philips' chief engineer, however, denies this. According to a Sunday Tribune interview, the story is slightly more involved. At that time (1979) Philips owned Polygram, one of the world’s largest distributors of music. Polygram had set up a large experimental CD plant in Hanover, Germany, which could produce huge amounts of CDs having, of course, a diameter of 115 mm. Sony did not yet have such a facility. If Sony had agreed on the 115 mm disc, Philips would have had a significant competitive edge in the market. Sony decided that something had to be done. The long playing time of Beethoven's Ninth imposed by Ohga was used to push Philips to accept 120 mm, so that Philips’ Polygram lost its edge on disc fabrication.

The 74-minute playing time of a CD, which was much longer than the 15 to 20 minutes per side possible with long-playing vinyl albums, was often used to the CD’s advantage during the early years when CDs and LPs vied for commercial sales. CDs would often be released with one or more bonus tracks, enticing consumers to buy the CD for the extra material. However, attempts to combine double LPs onto one CD occasionally resulted in an opposing situation in which the CD would actually offer fewer tracks than the LP equivalent. An example is the 1987 album Kiss Me, Kiss Me, Kiss Me by The Cure, which states in the CD liner notes: "The track Hey You!!! which appears on the double album and cassette has been omitted so as to facilitate a single compact disc." The 2006 re-release of this album saw the re-inclusion of the missing track.

Another example is the original late-1980s Warner Bros. Records reissue of Fleetwood Mac's Tusk album, which substituted the long album version of "Sara" with the shorter single version. Enough complaints were lodged to eventually convince Warner Bros. to remaster the album in the mid-1990s with the original contents intact.