Definitions

Gapless playback

Gapless playback

Gapless playback is the uninterrupted playback of consecutive audio tracks without intervening silence or clicks at the point of the track change. Gapless playback is common in compact discs or gramophone records, but is not always available with other formats that employ compressed digital audio. This may be a source of annoyance to listeners of music where tracks segue into each other, such as some classical music (opera in particular), progressive rock, and electronic music.

Causes of gaps

There are two main reasons why gaps occur during playback: compression scheme artifacts, and delayed output.

Compression scheme artifacts

Most lossy audio compression schemes add a small amount of silence to the beginning of a track. One reason that this happens is because many such schemes involve a time/frequency domain transform (such as an MDCT) which can introduce gaps called encoder delay. These gaps can be enlarged at decode time when a reverse-MDCT is performed, because the reverse transform will also introduce gaps (decoder delay) of its own. Another factor is that transforms act on the data in units of fixed-size blocks. In order for the audio signal to be encoded in its entirety, small amounts of silence are appended to the input before the transform. If the amount of padded silence is not accounted for, the padding will be decoded together with the audio data, also introducing gaps between tracks. Because of the introduction of such gaps, the playtime of the audio data is often slightly increased.

This issue is technical but also standards-related. The popular MP3 standard, for example, 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 gap will usually remain between them. More recent compressed audio formats (such as Ogg Vorbis) have been designed to address this problem, and can therefore produce gapless audio if played back correctly.

Delayed output

Even when the audio file itself does not contain undesirable gaps, software/firmware/hardware design often adds gaps during playback. In some cases, software closes and re-opens the output stream when switching tracks, causing the hardware to create a very short "click". This problem is solved in more sophisticated designs of gapless playback.

A different design problem relates to software/firmware/hardware which are not ready to seamlessly move to the next track by the time the current track is complete. In this scenario, the listener is left waiting in silence as the player locates the next file, reads it, decodes the first blocks if necessary and then starts loading the buffer for playback. The gap can be as much as half a second, or even more — very noticeable in "continuous" music such as certain classical or dance genres.

Many older audio players on personal computers do not implement the required buffering to play gapless audio. Some of these rely on third-party gapless audio plug-ins to buffer output. Some newer players and newer versions of old players now support gapless playback directly.

Precise gapless playback

When gaps are caused by silence introduced during the compression process, it is possible to store metadata in the file that explicitly declares the amount of delay or padding that was introduced. The audio playback software must be able to recognize the metadata and trim the decoded audio accordingly or else the information is just ignored. Typically uncompressed audio formats don't require this because the start and end of the original audio data is clearly defined.

That alone may not address the issue of introduced gaps. Ensuring the audio hardware itself is not stopped and started between tracks such that a click is added may also be necessary and it may help to process the next track while the current one is running so that the data is available as a continuous stream.

For those seeking precision this may be the ideal solution because there is no guesswork being performed by the software: the playback timing would be identical to the source.

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 always 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 recognize 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 cross-fade 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, cross-fading 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. Another problem is that the solutions above do nothing to prevent the output stream from being closed and reopened at track boundaries; some measures can be taken to simulate a gapless output stream, but they are not always successful and side-effects may occur.

Another alternative is to ignore track boundaries, encoding a single collection of tracks as a single compressed file, relying on cue sheets (or something similar) for navigation. While this method results in gapless playback within the collection of tracks with consecutive playback, it can be unwieldy due to the possibly large size of the resulting compressed file. Furthermore, unless the playback software or hardware can recognize the cue sheets, navigating between tracks may be difficult.

Last of all, with some implementations, it is possible to add gapless metadata to existing files. If the encoder is known, it is possible to guess the encoder delay. Assuming the compression was performed on CD audio to create the files, the original playback length will be an integer multiple of 588 samples. Thus the total playback time can be guessed also. Adding such information to audio files will work with implementations which recognize metadata.

Format support

Since lossless data compression excludes the possibility of the introduction of padding, all lossless audio file formats are inherently gapless.

These lossy audio file formats have provisions for gapless encoding:

Some other formats do not officially support gapless encoding, but some implementations of encoders or decoders may handle gapless metadata.

  • LAME-encoded MP3 can be gapless with players that support the LAME Mp3 info tag.
  • AAC in MP4 encoded with Nero Digital from Nero AG can be gapless with foobar2000, latest XMMS2, and iTunes 7.1.1.5 onwards.
  • AAC in MP4 encoded with iTunes (current and previous versions) is gapless in iTunes 7.0 onwards, 2nd generation iPod nanos, all video-capable iPods with the latest firmware, and recent versions of foobar2000.
  • iTunes-encoded MP3 is gapless when played back in iTunes 7.0 onwards, 2nd generation iPod nanos, and all video-capable iPods with the latest firmware.
  • ATRAC on both MiniDisc and NW WalkMans is gapless through the use of time codes

Player support

Optimal solutions:

Alternative or partial solutions:

  • Amarok for Linux
  • Herrie for Linux almost never has to perform flushing of buffers, which makes audio output almost always gapless.
  • Windows Media Player: Has supported gapless ripping and playback of WMA since Windows Media 9. Available on all current Windows machines.
  • XMMS2 - has native support for gapless MP3 / Ogg Vorbis and FLAC

References

External links

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