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What Is a Constant Bit Rate?

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  • Written By: Alex Newth
  • Edited By: Angela B.
  • Last Modified Date: 21 November 2014
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A constant bit rate (CBR) is an audio- and file-encoding technique in which the amount of bits per file has no variation. This is opposed to variable bit rate (VBR), where the bits can be changed to account for different file types and quality levels. While many people complain about the bit rate of constant bit rate, it is useful for those who want a consistent quality level and those who need to keep track of the amount of bits used in files. The major complaint about CBR is that it uses an amount of bits that is not conducive to file quality.

Constant bit rate means that, when a file is encoded, it will be encoded at the same bit rate, so the amount of bits depends on the size of the file, not the quality or file type. While this is not entirely true — some file types will contain more bits that other file types — this amount differs by an insignificant amount. With CBR, the quality level is the same for an audio, video or document file, regardless of whether a portion of the file is complex or simple.

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One pro of using constant bit rate is that operators and users are granted the most compatibility, especially with online use. Some online streaming programs and websites allow VBR, but all of them allow the use of CBR. By using a CBR stream, the user will be able to send the file over any streaming system, without worrying about having to exclude any websites.

Another pro is that users can accurately estimate the amount of bits per file when using constant bit rate. This advantage will not be useful for common users, but professional users who are hired to encode files, and users who optimize their computers, will find this useful. By knowing the amount of bits in a file, the user will be able to maximize the amount of files available on the computer.

Quality is also consistent among constant bit rate encoded files. This can be good, or bad, depending on the file. For example, if there is a complex portion of a song, such as a part where there is an intricate guitar solo, the amount of bits used for this portion will be the same as the silence that normally occurs in songs at the beginning and end. While this can be a problem, most users and listeners do not notice the difference much. People who argue for VBR files say that CBR does not use enough bits for complex portions, and too much for simple portions.

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