CBR - Constant Bit Rate

By: Preethi Ramkumar

Meaning of CBR – “Constant Bit Rate” is a term used in telecommunications, relating to the quality of service compared with variable bit rate. When referring to codecs, constant bit rate encoding 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 would not be the optimal choice for storage as it would not allocate enough data for complex sections (resulting in degraded quality) while wasting data on simple sections. 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.)

The formal abbreviation for “bit per second” is “bit/s” (not “bits/s”). In less formal contexts the abbreviations “b/s” or “bps” are often used, though this risks confusion with "bytes per second" (“B/s”, “Bps”). Even less formally, it is common to drop the “per second”, and simply refer to “a 128 kilobit audio stream” or “a 100 megabit network”. “Bit rate” is sometimes used interchangeably with “baud rate”, which is correct only when each modulation transition of a data transmission system carries exactly one bit of data (something not true for modern modem modulation systems, for example).

Experts and audiophiles may detect artifacts in many cases in which the average listener would not. Some musicians enjoy the distinct artifacts of low bitrate (sub-fm quality) encoding and there is a growing scene of net labels distributing stylized lobit music. The bit rates in this section are approximately the minimum that the average listener in a typical listening or viewing environment, when using the best available compression, would perceive as not significantly worse than the reference standard: [edit] Audio 4 kbit/s — minimum necessary for recognizable speech (using special-purpose speech codecs) 8 kbit/s — telephone quality 32 kbit/s — MW quality 96 kbit/s — FM quality 128 kbit/s — CD quality 256 - 320 kbit/s - Studio quality

Other Related Definitions:

“…Constant bit rate (CBR) encoding is the default method of encoding with the Windows Media Format SDK. When using CBR encoding, you specify the target bit rate for a stream, and the codec uses whatever amount of compression is necessary to achieve it. With CBR encoding the bit rate and size of the encoded stream are known prior to encoding. For example, if you are encoding a three minute song at 32,000 bits per second, you know that the file size will be about 704 kilobytes (32,000 bps x 180 seconds / 8 bits per byte / 1,024). You also know that the bandwidth required to stream the encoded content is about 32,000 bits per second. ” [Microsoft]

“…Constant Bit Rate - This is best used from a streaming server NOT to be confused with Progressive Download (http). On streaming servers one would need to have very tight control over the bandwidth in use at any moment. CBR does that. With CBR, the encoder decides what data to "throw out" in order to keep the constant bit rate. Complex video such as dissolves, camera motion, fast action suffers most as far as image quality, whereas "talking head" video would suffer less loss in quality. Complex video needs more data to look good ” [Flip4mac]

“…Constant Bit Rate (CBR) - A type of encoding that maintains a fixed bit rate throughout a file, so that data is sent in a steady stream. But because more complex passages may be encoded with fewer than necessary bits, and relatively simple passages may be encoded with more bits than are necessary, CBR can potentially result in lower-quality sound. See also: Variable Bit Rate (VBR).” [ Hewlett-Packard Development Company]

Related Links:

Flip4mac - Working with Compression Settings
cisco - constant bit rate (CBR) for the ATM circuit emulation service
Service.real - Constant and Variable Bit Rate Encoding
Arstechnica - Constant bitrate MP3s are the most common type of MP3
Tangentsoft - Videographica: MPEG Encoder Modes Explained
Building the CBR component - A Constant Bit Rate (CBR) Component
Uottawa - Connection Admission Control for Constant Bit Rate Tra.c at a Multi-Bu.er Multiplexer using the Oldest-Cell-First Discipline
Bitrates - Bit Rates and Sound Quality
Scientificatlanta - Deploying Dedicated Constant Bit Rate Service Applications Over HFC
Purdue - Simplest of all: constant bit rate (CBR)
Varasto - Modeling of MPEG video
Hydrogenaudio - Forums, article

Technical Resources:

Microsoft - Constant Bit Rate (CBR) Encoding

Products and Solutions:

Digitalprosound
IBM
Mp3developments
Mp3tunes
Roadrunnerbusiness
Tripod
Videobench
videohelp

Blogs, News, feeds…

http://www.cisco.com/warp/public/121/cbr.html
http://www.mp3machine.com/news/842/
http://www.findarticles.com/p/articles/mi_qa3751/is_199907/ai_n8865318

Books:

CCIE: Cisco Certified Internetwork Expert Study Guide by Rob Payne, Kevin Manweiler, Rob Payne, Kevin Manweiler
Microsoft Windows Media Resource Kit by Microsoft Corporation, Bill Birney, Tricia Gill, Microsoft Corporation
Modern Recording Techniques by David Miles Huber, Robert E. Runstein

See Also:

CBR Resources

 

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