Real Media Variable Bitrate (RMVB) is a file format developed and owned by RealNetworks®. It's designed to be compatible with RealPlayer®, the company's proprietary media player software. RMVB files are encoded with a variable bitrate (VBR) format, as opposed to a constant bitrate (CBR). The VBR format allocates different output rates and storage space based on segment complexity in a music file; more complex data gets higher bitrates and more storage. The CBR format allocates constant rate and space throughout a music file. VBR typically produces higher quality than CBR. The RMVB codec, or digitizing compression device, was the first codec designed by RealNetworks® that supports variable bitrates.
VBR encoding creates smaller files by adjusting recording quality based on a file's complexity. During the portions of a movie with fast-moving action or complex images, for example, the bitrate is high to maintain a clean and crisp picture. During portions of the video where there may be little movement, or just a blank screen, the bitrate is decreased. An RMVB file will typically look as clear and defined as a high-end video-streaming file such as Audio Video Interleave (AVI), but will only be a fraction of the size.
The RMVB format is different from other RealPlayer® codecs because it is not a streamed format. Any RMVB file downloaded to a computer is located entirely on that computer's hard drive; no internet connection is required to view the content once it has finished downloading.
While the RMVB format is exceedingly popular among Internet users because of its small size and above-average video quality, the RealPlayer® itself is more controversial. The program has been criticized as being spyware and adware. It is also commonly criticized for associating itself with too many file types and for being excessively difficult to remove once it is installed.
As a response, independent developers have designed and released freeware that makes the RealPlayer® codec compatible with other media players, such as the VLC player and Windows Media Player Classic. RMVB converters have also been released independently of RealNetworks®, and allow users to convert the files to other popular file formats such as AVI, which do not require the RealPlayer®.