What is CBR? (constant bit rate)
CBR is an encoder to convert (compress) an audio file to mp3 format. In CBR encoding, the bit rate is kept constant throughout the file: the same number of bits are allocated to encode every second of audio, and within frames of the audio data they occur at regular and predictable intervals, thus that the full file size for a given audio length is predictable.
Therefore, CBR is the “opposite” of VBR. However, in some formats there may be some variability in the number of bits that contain the actual audio information from frame to frame.
This concept manifests itself in the MP3 bit repository. In MP3 CBR, even if the frames are of a fixed size, the audio data is not necessarily sequentially distributed among them; the audio for a frame may use fewer bits than the frame has, so this frame “adds” spare bits to a “bucket” that can fill in the allocated bits for the next block.
So the effective bit rate may vary slightly in CBR MP3, although there is a fixed number of frames in all audio. The degree of variability in the entire MP3 file is not as great as that of the VBR, but it is not negligible; a CBR encoder using repository inefficiently is more likely to produce a lower quality file than a VBR. Unlike VBR, the perceived quality of decoded audio will vary depending on the CBR file.
This is because CBR encoding is similar to the ABR form of VBR encoding in that it is typically based solely on the target bit rate and analysis of the input audio; often no attempt is made to use the lowest possible bit rate that will maintain a certain level of output quality. Technically, CBR implementations always involve predicting product quality, but they rely on fixed algorithms, rather than trial and error verification of results, as is done in VBR. Who should use this encoder: CBR is useful for people interested in maintaining maximum compatibility, especially with some streaming applications and some hardware decoders that do not support VBR.
CBR is also useful for people who want to be able to get accurate estimates of the bit rate or approximate length of a decoded audio file without scanning and partially decoding the entire file. VBR advocates are very vocal against CBR and often say that no one should use CBR when given a choice. Some reasonably argue that the goal of using a compression algorithm, especially in a lossy codec like MP3, is to store as many bits as possible while maintaining a certain level of quality, so CBR tends to use more bits than necessary. on simple passages, and using too little for complex passages is wasteful and should lead to worse results (at least in complex comps) than VBR.
However, these arguments need to be carefully refined and it would be incorrect to conclude that there are quality differences between CBR and VBR. In general, for most types of compression, considering identical input, encoding techniques, and reasonable goals for VBR quality and bit rate limits, VBR will almost always produce results of equal perceived quality. or better than CBR for files of the same size. This has been shown in numerous hearing tests.
CBR may exceed the quality of VBR if the comparison is not limited to the average bit rate or if the VBR encoding method does not take into account the actual output quality. For example, a 256 kbps CBR MP3 containing moderately complex audio is likely to sound noticeably better than a similarly encoded VBR, which averages 128 kbps, although VBR can use up to 320 kbps in some frames.
And even when VBR measures the quality of the output signal, there is a margin of error, especially when using psychoacoustic models of perception, so the encoder (even the highly respected LAME) can accidentally compress some segments, depending on the characteristics. audio, placing restrictions on quality and bit rate. At high bitrates, the quality difference between typical CBR and VBR files is close to zero, so for some CBR users it is quite acceptable, especially if the maximum savings in accommodation or hard drive space is not important. At low average bit rates, the quality difference between CBR and VBR is more pronounced with the same input signal, so VBR is often more desirable. At high bit rates, the quality difference between typical CBR and VBR files is close to zero, so for some CBR users it is quite acceptable.
