The pregap on a Red Book audio CD is the portion of the audio track that precedes ‘index 01’ for a given track in the table of contents (TOC). The pregap (‘index 00’) is typically two seconds long and usually, but not always, contains silence. Popular uses for having the pregap contain audio are live CDs, track interludes, and hidden songs in the pregap of the first track. On certain CDs, such as ‘Think Tank’ by Blur, the pregap contains a hidden track. The track is truly hidden in the sense that most conventional standalone players and software CD players will not see it. Such hidden tracks can be played by playing the first song and ‘rewinding’ (more accurately, seeking in reverse) until the actual start of the whole CD audio track. Not all CD drives can properly extract such hidden tracks. Some drives will report errors when reading these tracks, and some will seem to extract them properly, but the extracted file will contain only silence.

The pregap was used to hide computer data, tricking computers into detecting a data track whereas conventional CD players would continue to see the CD as an audio CD. This method was quickly made obsolete in late 1996 when an update to Windows 95 made the pregap track inaccessible. It is unclear whether or not this change in Microsoft Windows’ behavior was intentional: for instance, it may have been intended to steer developers away from the pregap method and encourage what became the Blue Book specification ‘CD Extra’ format.

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