Once More Into The Fire

Red Book

First up is Red Book. As I mentioned earlier, it is the standard by which audio data is encoded. Phillips and Sony released the “red book” specification on Compact Disc Digital Audio System, and was adopted by the Digital Audio Disc Committee. It was later ratified as the IEC 908 standard for digital audio on compact discs. The IEC 908 standard primarily specifies the layout and data structure of the information area of the disc, which begins at the groove 46 mm from the center of the disc, extending to 117 mm at the edge of the disc. A Red Book compliant CD would story audio data in sectors, each valued at 2352 bytes, precisely 1/75th of a second of audio recorded at stereo 44KHz and 16bits in 2’s complement PCM format. (So it’s not actually the WAV that gets recorded. It’s the WAV converted into a PCM format. PCM, by the way, stands for Pulse Code Modulation. For the pedantic, the 2352 bytes is not the amount of data that’s actually on the CD. The 2352 bytes is the size after the data has undergone EMF demodulation and CIRC error correction, and after the subcoding stuff.)

Beginning at 46mm is the Lead-In Area. This area stores meta-data that describes what kind of data is stored on the CD, the TOC, logical block addresses, etc. (Meta-data is information that describes characteristics of other data. For example, if my vitals were listed in a big book, that would be the data. Information such as the book is red, the ink that prints the data is blue, the data is written in Spanish, is meta-data.) By the way, if you are interested, the Lead-In area extends from 46mm from the center to 50mm from the center.

After the Lead-In is the program area, where the actual audio is stored. According to Red Book standards, there must be a pre-gap before the actual data. This pre-gap is simply 150 sectors of blank data (or two seconds). There is a pre-gap in every track. Without getting into the gruesome details dealing with subcodes, simply note that there is a way to change the length of the pre-gap, and also to store non-blank data in the pre-gap. Keep this last statement in mind, as it will become useful later in the article when we discuss the trick behind Hidden Track CDs.

I know that I just went through a lot of technical detail. So here is a quick summary:

  • Red Book is the standard used to encode audio data.
  • Red Book specifies that there is a Lead-In which describes what is stored on the CD
  • There is a two second pre-gap between each audio track, and also prior to the first track (this is why some CD players will display -00:02 seconds. It’s the pre-gap time).
  • Pre-gap lengths can be changed through changing something called subcodes.
  • After the pre-gap comes the actual data stored in PCM format (converted from WAV), in sectors that each stores 1/75th of a second of audio data.

Know Your File Format

We are almost through the boring part of the article. I promise! But before we can move on to the fun stuff, it is probably in our best interest to take another quick look at the different file formats that can be used on a CDR.

ISO-9660: Probably the most popular file system. It was created as an international standard for writing CD Recordables. The Level I specification allows for the classic 8.3 filenames found in DOS. There is a Level II specification which allows for long filename support. The disadvantage is that it is not supported under MS-DOS. A Level III specification is also defined for noncontiguous storage of files, but it’s not that important.

Rock Ridge: Rock Ridge is an ISO 9660 extension that allows for Unix’ish file system support. This file system allows for mixed case, long filenames. It also allows for symbolic linking between files.

HFS: Hierarchical File System is a Macintosh file system. Unfortunately, it is not ISO9660 based, which means it is not readable on Windows and Unix machines.

Joliet: A file system created by Microsoft (who else but Microsoft) to support long filenames. Under non-windows systems, the short filenames are displayed.

Romeo: An Adaptec Easy CD Creator file format. But, it never really caught on.

A fairly nice chart detailing all the file systems can be found on Adaptec’s web site at http://www.adaptec.com/tools/compatibility/cdrecfilename.html

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