Constant bitrate (CBR) is a term used in

telecommunications Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than tha ...

, relating to the

quality of service Quality of service (QoS) is the description or measurement of the overall performance of a service, such as a telephony or computer network, or a cloud computing service, particularly the performance seen by the users of the network. To quantitat ...

. Compare with variable bitrate. When referring to

codec A codec is a device or computer program that encodes or decodes a data stream or signal. ''Codec'' is a portmanteau of coder/decoder. In electronic communications, an endec is a device that acts as both an encoder and a decoder on a signal or ...

s, constant bit rate

encoding In communications and information processing, code is a system of rules to convert information—such as a letter (alphabet), letter, word, sound, image, or gesture—into another form, sometimes data compression, shortened or secrecy, secret ...

means that the rate at which a codec's output data should be consumed is constant. CBR is useful for streaming multimedia content on limited capacity channels since it is the maximum bit rate that matters, not the average, so CBR would be used to take advantage of all of the capacity. CBR is not optimal for storing data as it may not allocate enough data for complex sections (resulting in degraded quality); and if it maximizes quality for complex sections, it will waste data on simple sections. The problem of not allocating enough data for complex sections could be solved by choosing a high bitrate to ensure that there will be enough bits for the entire encoding process, though the size of the file at the end would be proportionally larger. Most coding schemes such as Huffman coding or run-length encoding produce variable-length codes, making perfect CBR difficult to achieve. This is partly solved by varying the quantization (quality), and fully solved by the use of padding. (However, CBR is implied in a simple scheme like reducing all 16-bit audio samples to 8 bits.) In the case of

streaming video Video on demand (VOD) is a media distribution system that allows users to access videos without a traditional video playback device and the constraints of a typical static broadcasting schedule. In the 20th century, broadcasting in the form of ...

as a CBR, the source could be under the CBR data rate target. So in order to complete the stream, it's necessary to add stuffing packets in the stream to reach the data rate wanted. These packets are totally neutral and don't affect the stream.

See also