off in the decoding process. Other formats may require extra metadata for the player to achieve the same. The popular MP3 format defines no way to record the amount of delay or padding for later removal. Also, the encoder delay may vary from encoder to encoder, making automatic removal difficult. Even if two tracks are decompressed and merged into a single track, a pause will usually remain between them. CD recorded in TAO mode Audio-CDs can be recorded in either disc at once (DAO) or track at once (TAO) mode. The latter is more flexible, but has the drawback of inserting approximately 2 seconds of silence between tracks. Ways to eliminate the gaps Precise gapless playback As opposed to heuristic techniques, what is often meant by precise gapless playback, is that playback timing is guaranteed to be identical to the source. By this definition, a precise gapless player is not allowed to introduce either gaps or overlaps (crossfading) between successive tracks, and is not allowed to use guesswork. Apart from accounting for playback latency, the preciseness here lies in treating lossless data as-is, and removing the correct amount of padding from lossy data. This is not possible for file formats with loosely defined encoder specifications and no metadata and therefore no way for encoders to record the duration of extraneous silence. Approximate methods Heuristics are used by some music players to detect silence between tracks and trim the audio as necessary on playback. Due to the loss of time resolution of lossy compression, this method is inexact. In particular, the silence is not exactly zero. If the silence threshold is too low, some silences go undetected. Too high, and entire sections of quiet music at the beginning or end of a track may be removed. Digital signal processing (DSP) algorithms can also be used to crossfade between tracks. This eliminates gaps that some listeners find distracting, but also greatly alters the audio signal, which may have undesirable effects on the listening experience. Some listeners dislike these effects more than the gap they attempt to remove. For example, crossfading is inappropriate for files that are already gapless, in which case the transition may feel artificially short and disturb the rhythm. Also, depending on the length of untrimmed silence and the particular crossfader, it may cause a large volume drop. These methods defeat the purpose of intentional spacing between tracks. Not all