Variable bitrate

From Wikipedia, the free encyclopedia

Variable bitrate (VBR), or less commonly variable bit rate, is a term used in telecommunications and computing that relates to the bitrate used in sound or video encoding. As opposed to constant bitrate (CBR), VBR files vary the amount of output data per time segment. VBR allows a higher bitrate (and therefore more storage space) to be allocated to the more complex segments of media files while less space is allocated to less complex segments. The average of these rates is calculated to produce an average bitrate for the file that will represent its overall sound quality.

MP3, WMA, Vorbis, and AAC audio files can optionally be encoded in VBR. Variable bit rate encoding is also commonly used on MPEG-2 video.

[edit] Pros and cons

The basic argument for using VBR streams is quite simple and compelling — it produces a better quality-to-space ratio compared to an equivalent CBR algorithm.

However, there are some disadvantages:

• Encoders and decoders are typically more complex
• Possibly due to the above, VBR support is not as ubiquitous as CBR. For instance, many commodity set top box players support CBR but not VBR playback of mp3 files.
• mp3 Files encoded with VBR may not play on VCD/DVD players.
• mp3 Files encoded with VBR often have noise associated with them when there is a single tone, or minimal tones (for example audiobooks and acoustic music). When burning mp3 files encoded with VBR to a cd, there can be sounds that mimic a "digital chirp" during the quiet portions of the song or when there is only speaking.

However many programs do support VBR encoded mp3 files such as the newer versions of Adobe Flash (formerly Macromedia Flash), therefore greatly benefiting any Flash file made for web site use. VBR is not widely supported only because it is newer, so as time goes by VBR will become increasingly wider in use and support.

• Some MP3 Players support VBR such as the Aigo A208.