Pregap

A CD can contain audio before the nominal start point of track 01. In a real CD player, you would have to start playing then disc, and then seek/scan backwards to go all the way back to true beginning of the disc's audio. Some discs use this area to hide another song or interlude. Normally, though, if this area exists at all, it only contains a fraction of a second of silence.

In CUETools and CUERipper, the terms HTOA (Hidden Track One Audio) and Pregap refer to this portion of the disc.

Each track can have a "pause" section

The audio on a CD is in one continuous stream. Track boundaries are defined in a Table Of Contents (TOC) in the lead-in area at the beginning of the disc, and also in a timecode stream (part of the disc's "Q" subchannel) that the drive or player has access to.

Every track is also divided into sub-tracks called indexes, with index 01 being the only mandatory index. Where index 01 begins, the track nominally begins. For example, when you jump to track 04, you're going to what the TOC says is the start of the index 01 portion of track 04. Indexes higher than 01 are uncommon, but are sometimes used for interludes or multi-part songs.

In the timecode stream, a portion of a track can optionally be designated index 00. This comes before index 01 and is normally used for a brief pause (a time when silence plays) between songs. However, it doesn't have to be silent; it could contain applause, the introduction of the next song, a musical interlude, or anything else. Although the pause may be between songs, it is not actually between the divisions we call tracks; it's just a designated portion of a track.

When a disc plays in a real CD player (not a computer's optical drive), and the index 00 section of a track is reached, you will see the track number advance, and the track timer will restart with a negative number and then count up to zero (-0:03, -0:02, -0:01, 0:00), at which point it has reached the index 01 portion.

Evolution of the terminology

The name of the pause/index-00 portion of a track evolved into "gap" or "pregap" over time:

• In the early 1980s Red Book standard which defines the physical & logical content of CDs (e.g. IEC 60908), the terms gap or pre-gap are nowhere to be found. However, each track's index 00 portion, as defined in the "Q" subchannel data stream and flagged in the "P" subchannel, is called a pause, and every audio track can have one.

• In the 1990s CD-ROM standards which define how computers read & write CDs (e.g. ECMA-130 and SCSI MMC), pause is still the preferred term for audio tracks. The term pre-gap refers only to the pause before a CD-ROM data track. Also, the term is only used as part of the binary-format cue sheet which is sent to the drive before burning a recordable CD.

• In the mid to late 1990s, free tools for reading and writing CDs on Unix-like operating systems (Linux, Solaris, BSD, etc.) began referring to any index 00 portions as pre-gaps, even if they were on audio tracks.

• Around the same time, the developer of the MS-DOS and early Windows CD-ROM burning software CDRDAO and CDRWIN introduced the text-based cue sheet (CUE) format which CUETools specializes in handling. The idea was to allow users to easily author some of the data which would then be incorporated by the burner into the binary cue sheet; it basically contains everything except the actual audio. Among the things you can put in the text cue sheet are the mandatory starting points of all tracks and their indexes, and the optional PREGAP command, which defines an amount of extra silence to insert as the index 00 portion for any type of track (audio or data).

• Since the late 1990s, the audio CD ripping software Exact Audio Copy (EAC) allows for the user to choose how to handle index-00 portions of tracks—e.g., when saving tracks as separate files, append these portions to the previous track's file like normal, or omit them entirely. To simplify the options, it uses the term gaps in all cases.

• Presumably, the popularity of EAC and CUETools in the 2000s and beyond has led to many people just merging the two points of view: the index-00 portions are generally called "gaps", and the one for track 01 is specifically called the disc's "pre-gap". However, some software continues to prefer the term "pre-gap" for all of them, and this is not an incorrect point of view.

Incidentally, track 01's pregap is not the same as the disc's 2-second lead-in area. The lead-in is not part of the audio program area where the tracks live, and it is not normally accessible.