Thursday, May 03, 2007

The TAS blog - Red Book CD history

The Absolute Sound's editor Robert Harley has his own blog. I copy hereby pretty fascinating blog entry about history of Red Book CD-format.

"April 17 - The World’s Most Expensive Book—the CD “Red Book”

Audiophiles correctly use the term “Red Book” to describe the CD format, but incorrectly to refer to any 44.1kHz/16-bit digital audio, or even to pulse-code modulation (PCM, the CD’s encoding format) in general.

So, what exactly is the “Red Book,” and why would we describe an audio format with such a name?

When Sony and Philips jointly developed the CD format in the late 1970s and early 1980s (Philips contributed the optical system, Sony the electronics and error correction, broadly speaking), they needed a specification that described the format in detail for CD factories. When a CD replicator bought a license to manufacturer CDs (for $5000), it received a copy of this 8.5″ by 11″ document, which happened to have a red cover. (Each CD format has its own book and colored cover. The book that described the CD-ROM format, for example, has a yellow cover and is called the “Yellow Book.”)

I had a copy of the “Red Book” on my desk when I worked in CD mastering. It describes the disc’s physical parameters, encoding and decoding scheme, optical system, and types of data errors and maximum allowable error rates, among other things. The reference to 44.1kHz/16-bit linear PCM encoding occupies a miniscule fraction of the book’s contents, yet the term “Red Book” is now firmly entrenched as meaning 44.1kHz/ 16-bit digital audio.

The Red Book contains some interesting provisions that aren’t widely known. For example, the CD was designed to carry 4-channel audio as an option, but at a slower sampling frequency and shorter word length. This option was, obviously, never used. Another unused option is the provision for graphics, text, or other data in the disc’s “subcode.” The CD has eight subcode channels, designated P–W, with each having a data rate of a relatively slow 7.35 kilobits per second. The “P” channel simply “goes high” (binary 1) for two seconds before the start of each track to allow cheap CD players to find track starts; the Q channel contains all the timing and track information that you see on your player’s display; the R–W channels are reserved for other use, such as graphics or text. No CD players that I know of have ever been able to access the R–W subcode. I’ve seen R–W subcode graphics displayed experimentally and can tell you why they never caught on—it takes an agonizingly long time (because of the slow data rate) to “paint” a single still graphic on a video display.

It was widely reported that the CD’s specifications were influenced by conductor Herbert von Karajan, who told Sony’s president that this new format would need to hold Beethoven’s Ninth Symphony in its entirety. I don’t know if this is true, but I do know that the Red Book doesn’t specify the CD’s maximum playing time. Rather, it specifies the disc’s rotational speed (linear velocity), track pitch (the space between tracks) inner starting radius, and outer ending radius. (CD’s are read from the inside out, with the disc speed varying from about 500 rpm on the inner radius to about 200 rpm at the outer radius, which maintains a constant linear velocity as seen by the playback laser.) From these parameters one can infer the maximum playing time. For many years after CD’s introduction, 74 minutes was considered the upper limit of the CD format’s capacity. But by mastering a CD at the edges of the allowable parameters (slowest possible linear velocity, soonest starting radius, latest ending radius, narrowest track pitch), one can make a CD with more than 80 minutes of playing time that still conforms to the Red Book specification."

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