Chip's CD Media Resource Center:
CD-ROM 2

CD-ROM Logical Structure

A CD-ROM can contain both [digital] audio tracks and digital data tracks. Audio tracks have the same structure as for CD-DA, whereas digital data tracks have a more elaborate logical structure using Sectors. This was imposed by the dual necessities of (1) much more stringent error detection and correction and (2) easy and precise random access to data on the CD.

A Sector is the same size (98 frames) as a subcode block, however it is not identical and the new term Section is introduced as synonym for subcode block. More on this later.

Layout of the Disc -- How to Recognize What Kind of Track?

Like the audio CD, the surface of a CD-ROM is divided into several zones and areas. The combination of the Lead-in Area, User Data Area (same as the Program Area) and Lead-out Area are known collectively as the Information Area, because that is where information is recorded. The CD-ROM specification requires a single data or audio track in each of the Lead-in and Lead-out areas. I suppose the CD-DA standard of putting digital silence in those areas, along with particular subcode information, would qualify, so this isn't as much a change as I first thought.

In order to facilitate the transition between tracks [I think], data tracks may be separated by specially defined Pauses called Pre-gap and Post-gap.

Pause

"A part of an Information Track on which only control information but no user data is recorded." [ECMA-130 clause 20]
A Pause is indicated by sections where INDEX=0 in the q subcode channel. I'm not sure what, if anything, prevents one from recording user data during a Pause. In fact I think I've read something in the CD-R FAQ about this.

Pre-gap

The Pre-gap and Post-gap are two types of Pauses. Pre-gap is a Pause at the beginning of a data track. If it doesn't apply to audio tracks, I think it is because you cannot legally have a pause at the beginning of an audio track, since the TOC is supposed to point to the first INDEX=1 sector of the track. This needs clarification.

Pre-gap basically provides a transition between two data tracks that may be in different formats. It is divided into two intervals. The first pre-gap interval is at least 1 second (75+ sections) "coded as the preceding track," meaning that the data format remains unchanged: specifically, the q-channel Control field and the Sector Mode byte should be identical to those of the preceding track.

The second pre-gap interval is at least 2 seconds (150+ sections) in which the q-channel Control field and the Sector Mode byte are identical with those of the user data of the current track. In other words, if the Control field or the Sector Mode byte are changing for the new track, then give the reader 2 seconds warning.

Post-gap

Post-gap is a Pause at the end of a data track. It must be at least 2 seconds long (150+ sections) during which the q-channel Control field and the Sector Mode byte must not change.

Here are some rules for applying Pre- and Post-gaps. The precise reasons for these are not immediately self-evident.

• A digital data track in the Lead-in area must end with a Post-gap.
• A digital data track in the Lead-out area must not begin with a Pre-gap.
• If the first track in User Data area is a data track, then "it shall start with a Pause of 150 Sections (2 s) and shall be coded as the second interval of a Pre-gap." [ECMA-130, clause 20.2]
• Otherwise, a data track "shall begin with a Pre-gap if the preceding track is a Digital Data Track with a different Sector mode or if it is an Audio Track."
• If a data track is followed by an audio track, then it must end with a Post-gap. "This rule applies also to the last Digital Data Track in the User Data Area, which is followed by the Lead-out Track."

Are digital data tracks indexed in the TOC like audio tracks? I don't know...