Gapless playback

Gapless playback is the seamless playback of digital audio formats. It allows live music or consecutive tracks to be heard exactly as they are mastered, without gaps between tracks.

Contents

1 Why gaps occur 2 Optimal solution 3 Alternative solutions 4 Gapless solutions 5 See also 6 External link

Why gaps occur

Most lossy audio compression schemes involve a frequency/time domain transform, so the playtime of the audio data may not be equal before and after the compression. Most audio playback software will also close the audio output stream when switching tracks, introduce gaps or making existing gaps larger. Unless the beginning of the next track is buffered and introduced immediately when the current track ends, gaps will occur.

Some compression methods such as the popular MP3 can create problems because they introduce small gaps on decompression. Even if two tracks are decompressed and merged into a single track, a gap will remain between them. More recent audio formats have been designed to address this problem, and will produce gapless audio if played back correctly.

Optimal solution

It is possible to store metadata in the audio to explicitly declare the playtime, and/or the delays introduced in the encoding process. This information can be used to ensure that playtime will remain constant after decoding. The audio playback software must be able to recognize the metadata, and trim the decoded audio as necessary.

The software can then take care to keep the output stream open between tracks. It must also buffer the beginning of the following track in the same way it buffers the current track during normal playback.

If the compression method supports gapless playback, the software properly decodes the audio data and metadata, the next track is buffered and ready to play, and the output stream remains open between tracks, optimal gapless audio is achieved. A collection of consecutive tracks will then play in the same way they were mastered, allowing the listener to hear their album as the author intended.

Alternative solutions

Digital signal processor (DSP) plugins can be used to detect silence between tracks and trim the audio as necessary on playback. This is not an optimal solution because it does not produce results identical to the source. Sometimes an artist may intentionally leave silence at track boundaries for dramatic effect; removing this silence also removes that effect.

It can also be difficult to properly implement silence removal. If the silence threshold is too low and the track contains decoder artifacts, the software may not recognise some silences. Conversely, if the threshold is too high, the software may remove entire sections of quiet music at the beginning or end of a track.

DSP plugins can also be used to crossfade between tracks. This eliminates gaps that some listeners find distracting, but also greatly alters the audio data and is not always desirable. In particular, when tracks are meant to be played together and perform the transition at high volume, crossfading results in a large volume drop.

Both of these alternate solutions are typically used to address compression methods that do not support the metadata for gapless playback. Like the optimal solution, they still require buffering and not closing the output stream; however, they require more computations, making them less efficient. In portable digital audio players, this can mean a reduced playing time on batteries.

Due to the drawbacks of the alternative solutions above, some listeners dislike their negative effects more than the gap they attempt to remove.

Gapless solutions

• foobar2000: Optimal gapless playback
• Otachan's in_mpg123: A gapless MP3 decoder for Winamp

See also

• Ogg Vorbis
• Gapkiller

External link

• MP3 players: Buyer Beware, a description of gapless playback in digital audio players