Pregap

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The part of the program on an audio CD that lies between the end of a track and the track start mark of the next track is referred to as the pregap (German literally Vorlücke ) . This is usually used to mark the pause between two sections (e.g. two songs) of the program. For technical reasons, there is also a pregap before the first track start mark.

Details

Usually the program on a CD is divided into several tracks; a track on a CD contains a song from an album, a sentence from a concert, a section from an audio book, etc. In the following, for the sake of simplicity, only song is mentioned .

A track on an audio CD can be divided into several (up to a hundred) parts by so-called indices. The obligatory index 1 defines the actual beginning of the song as the track start marker. All other index numbers (0, 2 to 99) are optional. (Note: Most CD players are not able to display or select the index numbers.) If there is an index 0, this marks the end of the previous track; This is usually used to mark the break between two songs. The area from index 0 to index 1 is also known as the pregap . During a pregap, the remaining time until the beginning of the following song is normally displayed by the CD player as a countdown. A standard practice that is sometimes used when mastering a CD is to insert two second long pregaps that contain only silence; an early version of the Red Book standard even made this mandatory. The pregaps can also contain normal audio data, which is often the case with live albums. When the CD is played normally from beginning to end, the entire program on the CD is played from the beginning of the first song (i.e. index 1 of track 1) to the end of the CD including all pregaps in between. An exception is the pregap before the first track, which is not reached in this way and therefore usually does not contain an audio program (even with live albums). In contrast to the other pregaps, it is compulsory, lasts at least two seconds and enables the CD player to let all the necessary control processes (such as speed, tracking and focus) settle in when playback starts and thus to use it cleanly at the start of the actual program. This pregap can only be heard by rewinding before the actual start.

Problems reading and burning CDs

Although the pregap formally belongs to the following track (whose index it is 0), most CD rippers add it to the end of the previous track. If a ripper creates a separate audio file for each track, these do not begin with a pause, but with index 1, i.e. the actual beginning of the song, and end with the end of the pause for the following track. The exception is the last track, which of course cannot have a break for the next track. If a CD contains audio data in the first pregap, i.e. at index 0 of track 1, it will be lost when ripping. Many burning programs have a standard setting that automatically inserts a pregap of two seconds of silence between two tracks. A careless combination of the two described behaviors results in a CD copy, the pauses of which are extended by two seconds each and sometimes contain disturbing silence (due to the missing seamless transition between two program parts); Any information from the first pregap is also missing on the copy.

Unusual use of the pregap

Hidden audio tracks

On certain audio CDs, the pregap of the first song contains a hidden track . Since playback normally starts with index 1, this hidden track is actually hidden; many CD players or audio programs do not recognize it. Such CDs with an unusually long pregap in front of the first track can be identified and copied using special programs such as Exact Audio Copy or Cdrtools . Furthermore, they are noticeable (as well as those with data in the pregap) due to a large difference between the total time and the remaining time display immediately after playback has started. The audio material can still be heard by rewinding. In the English language Wikipedia there is a list of CDs that contain audio material hidden in the pregap (see web links).

Computer data in the pregap

Another application of the pregap is to store computer data together with audio material on a CD (even a session) (“i-Trax”). In this case, this data can also be accessed with CD-ROM drives that are only capable of single sessions , whereas an audio CD player plays the CD like a normal audio CD.

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This method has become obsolete with the proliferation of multi-session drives. In 1996 caused Microsoft through an update for Windows 95 in the driver SCSI1HLP.VXDthat the Pregap track was no longer accessible; under Windows XP , this behavior also shows. It is unclear whether this change in the behavior of Windows was intended by Microsoft: it is conceivable that Microsoft intended to dissuade developers from using this method and instead rely on the standardized "CD Extra" format, which is described in the Blue Book Standard is described.

Under Windows, software like IsoBuster can still be used to access the data part of such a CD.

Web links