The bit rate is the ratio of an amount of data at a time, typically measured in bits per second, abbreviated as bit / s or bps. It denotes the output quantity of information units in digital, bound to a constant output time multimedia formats on a timeline.
The bit rate for audio and video compression can either be constant ( constant bit rate CBR) or variable ( variable bit rate, VBR). VBR is dynamically adapted to the bit rate of the data to be encoded. For example, in the MPEG video compression is in quiet scenes, the video bit rate reduced while it is raised in action-packed scenes. This allows an optimal use of storage space and a higher image quality than would be achievable at the very same memory requirements with CBR.
- 2.1 Technical Background
- 2.2 applications
Constant bit rate
Constant bit rate ( constant bit rate, CBR, even deterministic bit rate in ITU -T, DBR) is a compression method for audio and video data to be transmitted with a constant data rate or to save, regardless of the complexity of the signal. Per unit of time, the same amount of data is always generated.
The counterpart of this method is the variable bit rate (variable bit rate, VBR ). It is usually preferable for the archiving. Constant bit rate is often found in multimedia streams, since the transmission capacity is limited and by the CBR here maximum quality is achieved. However, "given away " at CBR also transfer volume, namely, when the actual bit rate is higher than that for the complete reconstruction necessary. This often occurs with videos in quiet scenes and during quiet sections of audio. An extreme example of this waste is the encoding of silence before the hidden item ( Hidden Track ) at a constant bit rate (of course, so high that they could also encode the music at a decent quality).
Even when streaming you can choose a variable bit rate. In order not to overload the limited transmission capacity, modern codecs like Vorbis offer the ability to set a maximum bit rate. It is advantageous to the saving of precious transmission volume where it is possible or useful. Even short jumps in the bit rate of the transmission capacity addition are manageable due to buffering.
- An example of a method that works with CBR, is video - CD.
- CBR is one QoS class in transmission technology Asynchronous Transfer Mode.
Variable bit rate
Variable bit rate (variable bit rate, VBR, and statistical bit rate in ITU -T, SBR) is a compression method, which creates audio and video data with consistent quality. The method has prevailed in most areas, since it is a higher quality at lower overall memory consumption offers than the compression at a constant bit rate.
In contrast to the coding with constant bit rate individual time sections can be compressed to different extents in order to achieve consistent quality at the lowest possible data volume can here depending on the complexity of the underlying material.
The difficulty of the process is automated, the passages to find that require greater attention to give them a more generous data volumes, but without revealing to waste resources. VBR was therefore used to be known to deliver unpredictable quality, but is now considered mature and is now preferred over CBR.
Before the encoding can not be determined how big the resulting file is because the file size is directly dependent on the complexity of the data. VBR encoding so is not suitable to achieve a certain file size. In this case, a coding with an average bit rate (ABR, see below) is preferable.
In audio compression all popular codecs / procedure (MP3, Ogg Vorbis, etc. ) VBR offer. Similar methods can be found in the compression of image files and videos. Where the entire JPEG image still " constant " highly compressed JPEG 2000 works targeted ( "variable" ) by first dividing the image, and the different areas vary greatly compressed, if necessary even stores lossless. Also DivX and Xvid are characterized by this feature of the adjusted variable compression.
In lossless compression techniques, a variable bit rate is mandatory because when choosing a constant bit rate, it would have to be as high as that of the source in order to encode all possible patterns. In this case, no compression occurs.
Average bit rate
The average bit rate ( average bit rate, ABR ) is a compression method, which does not encode the source material with a constant but a variable bit rate to use as the available space more efficiently and therefore be able to increase the quality. In order to achieve the desired average bit rate as exactly as possible, some codecs have a compression process with two passes. Initially, the material is analyzed and encoded before the second run.
It is closely related to the variable bit-rate -oriented, in contrast to the latter, at a predetermined bit rate in order to calculate so the resulting data size better. The Tolerance can be defined precisely.