The bit rate is a data transmission rate and designates the output amount of information units in digital multimedia formats in the ratio of a data amount to a time , typically measured in bits per second , abbreviated as bit / s or bps . With the progress of technology are sorted by topic increasing multiples usual, from kbit / s or kbps (1000 bits / s), further comprising Mbit / s (1,000,000 bits / s) and Gbit / s (1,000,000,000 bits / s ).
The bit rate in audio and video compression can either be constant ( constant bit rate , CBR) or variable ( variable bit rate , VBR). With VBR, the bit rate is dynamically adapted to the data to be encoded. For example, with MPEG video compression, the video bit rate is reduced in quiet scenes, while it is increased in action-packed scenes. This allows optimal use of the storage space and a higher image quality than would be achievable with CBR with the same storage requirement.
Constant bit rate
Constant bit rate ( CBR , at ITU-T also deterministic bit rate, DBR ) is a compression method to transmit or save audio and video data at a constant data rate, regardless of the complexity of the signal. The same amount of data is always generated per unit of time. Constant bit rates are often found in multimedia streams , as the transmission capacities are limited and the maximum possible quality is achieved with CBR. However, with CBR, transmission volume is also “given away” if the actual bit rate is higher than that required for complete reconstruction. This often occurs with videos in quiet scenes and with audio in quiet passages. An extreme example of this waste is the coding of silence in front of hidden tracks (hidden tracks) at a constant bit rate (of course chosen so high that it could also code the music in an acceptable quality).
- An example of a method that works with CBR is Video CD .
- CBR is a quality of service in the Asynchronous Transfer Mode transmission technology .
Variable bit rate
The counterpart to the constant bit rate is the variable bit rate ( VBR , at ITU-T also statistical bit rate, SBR ), which is also a compression method and generates audio and video data of constant quality. This is usually preferable for archiving. A variable bit rate can also be selected for streaming . In order not to overload the limited transmission capacity, modern codecs such as Vorbis offer the option of setting a maximum bit rate. The advantage here is the saving of valuable transmission volume where it is possible or useful. Even short jumps in the bit rate beyond the transmission capacity can be handled thanks to buffering . The process has established itself in most areas, as it offers higher quality with less storage space overall than compression with a constant bit rate.
In contrast to coding with a constant bit rate, individual time segments are compressed to different degrees depending on the complexity of the underlying material in order to achieve consistent quality with the lowest possible data volume.
The difficulty of the method is to automatically find the passages that require greater attention, to allow them a more generous volume of data, but without wasting resources too freely. VBR has had a reputation for delivering unpredictable quality for years, but is now considered mature and is now preferred over CBR.
Before coding, it cannot be determined how large the file will be, as the file size depends directly on the complexity of the data. VBR encoding is therefore not suitable for achieving a specific file size. In this case, coding with an average bit rate (ABR, see below) is preferable.
All common codecs / methods ( MP3 , Ogg Vorbis etc.) offer VBR for audio compression . Similar processes can be found in the compression of image files and videos. Where JPEG still compresses the entire image “constantly” strongly, JPEG 2000 works in a more targeted (“variable”) manner by first dividing the image and compressing the various areas to different degrees, if necessary even storing it without loss. MPEG4 is also characterized by this feature of adapted, variable compression.
In the case of lossless compression methods, a variable bit rate is mandatory, since if a constant bit rate was selected, it would have to be as high as that of the source in order to be able to encode all possible patterns. In this case, however, there is no longer any compression.
Average bit rate
The average bit rate ( ABR ) is a compression method that encodes the source material with a variable rather than a constant bit rate in order to be able to use the available storage space more efficiently and thus increase the quality. In order to achieve the desired average bit rate as precisely as possible, some codecs offer a compression process with two passes . The material is first analyzed and only coded in the second run.
It is closely related to the variable bit rate, but in contrast to this it is based on a specified bit rate in order to be able to better calculate the resulting data size. The deviation tolerance can be precisely defined.