Variable Bitrate Compression

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See also: Audio Formats | Compression

AKA: VBR - Variable Bitrate Recording

A type of lossy media compression that changes quality over the course of the song or video by using a quality rating based on changing original file quality; more data is used for increasing detail, less for decreasing. A variable bitrate file with a wide frequency sequence would be larger in size while a low-frequency sequence would mean a sudden drop in size.

Variable Bitrate Compression is different from Fixed Bitrate Compression, which keeps the same bit rate throughout, good for streaming media.

Benefits

  • A long period of silence in audio or a blank screen would mean a very small file. No audio data is wasted on silence or on simple tones and no visual data is wasted on a still screen or blackness.
  • Although variable speed compressed files are difficult to stream continuously over most Internet connections, those that download the entire file before viewing are unaffected.
  • More efficient processing means a better use of sound quality and file size.
  • The format is ideal for audio recorded before the 60s/70s as the range of audio is usually much lower and can be compressed even smaller than expected with no perceivable quality loss. Smaller size means more files on your portable audio player, more hard drive space, or faster send over the Internet.

In terms of pure quality, variable encoding is usually far better than fixed but some variable encoding has a quality rating as well, allowing for much more lossy encoding. At low bitrates, this can be useful for spoken word audio as there are often pauses and silence between sentences.

Drawbacks

  • Encoding using a variable bitrate can take considerably longer than using an equivalent fixed bitrate
  • Must be re-encoded in order to be streamed. This is considered another generation and will involve a loss of quality.

Audio Encoders

  • Ogg Vorbis - free and open source - a patent-free algorithm for encoding audio.
  • LAME - free and open source - a variable bitrate MP3 encoder, generally considered the best of the MP3 encoders.
  • Xing MP3 Encoder - commercial encoder. Also an MP3 encoder - very fast but sacrifices quality.

Commons Questions

So should I encode files Variable or Fixed?

Unless you need to stream files from your Web site (see below for more) or don't like having files turn out an unpredictable size, variable is strongly advised.

How do I know if I have a variable bitrate MP3/OGG?

In some players, such as Winamp or XMMS, a bit readout is available on the player. If the bitrate changes during the course of the song, you have a variable-bitrate encoded file. The command file also recognizes the bitrate, and other details, of a MP3 or OGG.

But why are most files on the Internet encoded at a fixed bitrate?

This is because Variable Bitrate can still be poor-quality. Variable Bitrate files are more efficient, not necessarily better. If they have a low average "midpoint," or setting of average quality, the overall value of the file is decreased. Many variable files can be of lower actual quality overall than a 192-encoded MP3. Therefore, high, fixed-bitrate files are more often distributed. In this case, 192 is a chosen standard in quality but some also exchange in 256.

Is audio streaming in variable bitrate recommended?

No. For audio streaming, fixed-bitrate is usually better to avoid short breaks in the music as the higher-quality sections are downloade. However, from a bandwidth vs. quality point of view, the "Average Bitrate Compression" on some encoders may be a better option. This is a bitrate compression somewhere between variable and constant bitrate. The stream will maintain an average bitrate, but sometimes the bitrate will be higher sometimes lower, saving bits when the signal is easy to encode and spending some extra bits when the signal is difficult. The average bitrate as well as the minimum and maximum bitrates can be set in the bitrate group.

Check your individual software's documentation for instructions.