Static bit rate (CBR) and dynamic bit rate (VBR)

CBR：

namely Constants Bits Rate, Fixed bit rate , It's static （ Constant ） Bit rate means .CBR It is a compression method with fixed sampling rate .

（1） advantage ： Fast compression , Can be supported by most software and devices .
（2） shortcoming ： Large space , The effect is not very ideal . Has been gradually VBR Way to replace .

Fixed bit rate is a term used to describe the quality of communication service . The opposite is variable bit rate or variable bit rate （variable bit rete, abbreviation VBR） When multimedia communication is carried out in a bandwidth limited channel CBR It's very useful , At this time, the highest bit rate is limited ,CBR It is easier to use such channels . however CBR Not suitable for storage , because CBR It will result in insufficient bit rate to encode complex content parts ( This leads to a decrease in quality ), At the same time, some bit rate will be wasted in the simple content part .

The output of most coding schemes is a variable length codeword , For example, Huffman coding or run length coding （run-length coding）, This makes it difficult for the encoder to be perfect CBR. The encoder can be quantized by adjusting （ Then adjust the coding quality ） To partially solve this problem , If you also use filler codes to perfectly achieve CBR.（ occasionally ,CBR It also refers to a very simple coding scheme , Let's take a 16 Bit precision audio data stream is sampled to get a 8 Bit precision data stream ）.

VBR：

VBR（Variable Bit Rate）, Also known as dynamic bit rate coding , When using this method , You can choose from the worst sound quality / Maximum compression ratio to best sound quality / Various transition stages between the lowest compression ratios , stay MP3 When the document is encoded , The program will try to maintain the quality of the entire selected file , Different bit rates suitable for different parts of the music file will be selected for encoding .
（1） advantage ： It can make the whole song roughly meet our sound quality requirements .
（2） shortcoming ： The compressed file size cannot be estimated during encoding

difference ：

VBR With the traditional CBR Coded MP3 Different , Conventional CBR（Constant BitRate） Static bit rate ,CBR The agreement is dead MP3 The sampling rate of is a fixed value . a MP3 A fixed value from beginning to end, such as 128KBit/s Compress . and VBR A new , The compression method of the whole process dynamic adjustment technology . When in the bass segment ,VBR A lower bit rate such as 32KBit/s Compress the sound quality ; A higher bit rate will be used when in the high range, such as 224KBit/s Compress the sound quality ; At the very high end, the highest 320KBit/s Compress .VBR MP3 When the file size is controlled , Maximize the MP3 The sound quality of .
  Different
  (1)CBR： Fixed bit rate FRAME（ frame ） It's also fixed in size , Just know the total length of the file , And the frame length can be adjusted by playing each frame 26ms calculated mp3 Total playback time , You can also control fast forward by counting the number of frames , Fast rewind and slow play .
  (2)VBR：VBR yes XING The algorithm introduced by the company , So in MP3 Of FRAME There will be “XING” This keyword ( Now many popular small software can also be used VBR Compress , Whether they abide by this Agreement , I don't know ), It's stored in MP3 The first one in the file is valid FRAME in , It identifies this MP3 File is VBR Of . At the same time, the first FRAME There's... In it MP3 Of documents FRAME Total number of , This makes it easy to get the total playback time , As well as 100 Bytes store the total playback time 100 Time phased FRAME Of INDEX.
 
  Use... In streaming playback schemes CBR Coding is the most effective . Use CBR When coding , The bit rate remains basically constant during the flow and is close to the target bit rate , Always within the time window determined by the buffer size .CBR The disadvantage of encoding is that the quality of the encoded content is unstable . Because some pieces of content are harder to compress than others , therefore CBR Some parts of the flow are inferior to others . Besides ,CBR Coding will result in different quality of adjacent streams . Usually at lower bit rates , The change of quality will be more obvious .
 

 VBR A suitable application scenario is media storage , Instead of network transmission . When planning to provide content for users to download 、 Play content locally or on devices with limited reading speed （ Such as CD or DVD Player ） When playing on , Please use VBR code .（ Peak values can also be used when planning to stream content VBR Coding mode ） When the encoded content is mixed with simple data and complex data （ for example , Video switching between fast motion and slow motion ） when ,VBR Coding is very advantageous . Use VBR When coding , The system will automatically allocate fewer bits for simple parts of the content , Thus, a sufficient number of bits are reserved for generating high-quality complex parts . This means constant complexity （ For example, news broadcasting ） Will not benefit from VBR code . Use for mixed content VBR When coding , With the same file size ,VBR The output of the code is better than CBR The output quality of the code is much better . In some cases , And CBR Encoded files of the same quality VBR Code file , It may be only half the size of the former . 

Because the current video coding standards are based on block coding hybrid coding framework , So a rate control algorithm can be used in any protocol （ Of course, sometimes you need to make small changes ）, It's just a question of whether the control effect is good , No one. RC Which agreement is exclusive .