What Is a Variable Bitrate?

Variable bitrate (VBR) is one method of encoding audio that uses a variable, or changing, rate of bits to encode the audio from a recording to a digital medium. This changing rate of bits allows fewer bits for simple portions of a song and more bits for complex portions. Contrasted by constant bitrate (CBR), variable bitrate uses fewer bits overall but also has a higher quality of sound than the CBR method of encoding. While better, it will take much longer for the encoding because it is more complex and, if the audio is very complex, the song may become memory-heavy.

In a variable bitrate file, the number of bits used to encode the song changes, depending on what is occurring with the file. The common range for a VBR file is from 128 kilobytes per second (Kbps) to 320 Kbps. If the song has a simple portion, such as silence at the beginning or a very simple drumbeat without accompanying instruments, then a low bitrate will be employed. When all the instruments are played at once, more bits will be used for a higher-quality sound.

By using bits that adjust to the audio’s need, variable bitrate is able to deliver a higher-quality sound. This is because the encoding method is able to fit the song’s needs, rather than place an arbitrary bitrate constraint over the song. The overall amount of memory and bits used are typically less than constant bitrate, but that is not guaranteed because some songs will need more bits than others.

While variable bitrate offers high-quality music, it is not the best method. There is a lossless audio compression format that delivers the highest-quality music with no quality lost from the recording. The problem with this encoding method is that each song will be around 20 megabytes (MB), which makes storing these songs on a computer or CD very difficult. This lossless method is rarely used unless the user is archiving the song.

The contrast to variable bitrate is constant bitrate. This method of encoding uses a constant rate of bits for all portions of the song. When the song is silent, or the song is playing a complex portion, the same amount of bits will be used. This method is usually lower in quality and may take up more memory, but it takes much less time to encode.

There are a few problems with the variable bitrate encoding format. The encoding is more complex, and the computer has to perform more calculations to accurately encode the song in this format, so the encoding itself will take much longer than CBR. If the song is consistently complex, it may become very memory-heavy, which can make it hard to store on the computer.