Chip's CD Media Resource Center:
CD-DA (Digital Audio) 4

CD Audio Logical Structure

The data on a disc is organized into three regions:

radius 22-23mm: Inner buffer zone. radius 23-25mm: Lead-in area. radius 25-58mm: Program area. radius 58-58.5mm: Lead-out area. radius 58.5-59mm: Outer buffer zone. radius 59-60mm: Rim area.
	Illustration from Disctronics

The Lead-in area of an audio CD contains the Table of Contents, listing up to 99 digital audio tracks and is also where the player synchronizes itself to the disc being played. The Lead-out area contains simple codes which allow the audio CD player to recognize the end of the disc. The Program area is where all the digital audio goes. Note that the Lead-out area and the outer buffer zones are both defined as being at least 0.5mm wide (measured radially). The Rim area is the unrecorded part at the edge of the CD, and comprises all the region between the outer buffer zone and the physical edge of the disc.

The combination of Lead-in area, Program area, Lead-out area, and outer buffer zone is called the Information area. This comprises the total area in which anything is recorded on the disc.

The diagram is slightly misleading in that the program area does not have to run all the way to radius 58mm. If it ends at, say, 45mm, then the space between the end of the Program area at 45mm and the start of the Rim area at 59mm will be divided between the Lead-out area and the outer buffer zone. Each must be at least 0.5mm wide.

The standard track pitch is 1.6 microns, meaning that center of one "groove" must be that far apart from the center of an adjacent groove (to revive the phono disc analogy). Again, there really is no groove, it's just a spiral track of pits and lands. I found it awkward to know what to call one revolution of a continuous spiral, which is why I resorted to the analogous term "groove". But ECMA-130 (the CD-ROM spec) provides a formal solution: they are defined as "Physical Tracks, each of which forms a 360 degree turn of a continuous spiral line." I'll use that term from now on.