Constant bitrate is a tool used in digital telecommunication signals, such as when transferring audio files off of the Internet. A constant bitrate file is encoded to produce a file which plays back at the exact same bitrate for its entire duration. The biggest advantage to a constant bitrate file type is that it allows for steady playback of streaming multimedia, as the bitrate will never fluctuate, reducing any potential lag and jitters from the server's end of the transmission. Although this type of file is ideal in those circumstances, it is disadvantageous for storing more complex types of files, as the steady bitrate may be overburdened or under-utilized depending upon variations in the file.
A constant bitrate file is like sand trickling through an hourglass: it will always progress at exactly the same rate. Contrast this with an opposite file type, the variable bitrate file. In a variable bitrate file, the "sand" is clumpy, resulting in sometimes small granules of information trickling through and sometimes larger, more complex chunks.
As stated, one of the best uses for a constant flow of bitrate is during playback of a multimedia file. Compressing everything within the video or audio file to a single playback enforces consistency across the entire file, forcing the images and tones to become substantially similar to one another. In a multimedia file encoded in a variable bitrate format, the quality of the file can change drastically from moment to moment as the bitrate peaks and falls like a roller coaster. Although a file using a constant bitrate will not always have the optimal image quality — as some images might have to be reduced in appearance to "fit" the selected bitrate — at least the entire presentation will be even and fluid to the end-user.
It might seem as though a file with a bitrate that is constant is always preferable, but this is not always the case. Certain circumstances tend to favor the ability to mold the bitrate within a specific range of values. Consider storing a multimedia repository of popular paintings. While some paintings in the collection are hopelessly complex, requiring a high bitrate to capture their true essence, others are much more simple, necessitating a much lower bitrate to keep the overall file size low. In cases like this, files using a bitrate that remains constant would generally provide too much or too little storage space for each virtual painting image.
Although one solution might be to increase the bitrate's "ceiling," allowing for even the most complex paintings to be stored with impunity, this is less than optimal from a programming point of view. Higher bitrate files require more storage space on the hard drive, as each element on the file is given more room to "breathe" by the higher bitrate. The more space that is wasted by files whose complexity fails to warrant the chosen high bitrate, the more inefficient the solution becomes.