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Audio Coding Part5

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Audio Coding Part5

VBR

About VBR

VBR Encoding

VBR: An interesting feature of MP3 files is that they can be read and played, which is also in line with the most basic features of streaming media. That is, the player can play without first reading the entire content of the file and play where it reads, even if the file is partially damaged. Although mp3 can have a file header, it is not very important for mp3 format files. Because of this feature, each frame of an MP3 file can have a separate average data rate without a special decoding scheme. That is why there is a technology called VBR (Variable bitrate, dynamic data rate), which allows each segment or even each frame of an MP3 file to have a separate bitrate, the advantage of this is that the sound quality is guaranteed to the maximum. . File size is limited. The advantages of this technology are obvious, but it is really difficult to use, because it requires the encoder to know how to assign the bitrate to each segment, which is like a dummy for encoders without waveform analysis. As such, VBR technology didn’t seem glamorous as soon as it appeared.
Experts have found that the human ear has a protective effect through long-term acoustic research. The sound signal is actually a type of energy wave, which propagates in air or other media. The most direct response of the human ear to the amount of sound energy, that is, the volume or pressure of the sound, is to hear the size of the sound. We call it the volume, which means the volume. The unit of energy is the decibel (dB). Even sounds of the same volume can be perceived by people as different in size due to their different frequencies. The 500 Hz frequency is most easily heard by the human ear. No matter whether the frequency is increased or decreased, even if the volume is the same, everyone will feel the sound become smaller. But when the volume drops to a certain level, the human ear cannot hear it, and each frequency has a different value.
You can see that this curve basically forms a V. When the frequency exceeds 15000 Hz, the human ear will feel that the sound is very small. Many people who are not very good at hearing cannot hear the frequency of 20000 Hz at all, no matter how loud it is… When the human ear hears two sounds with different frequencies and different volume at the same time, the one with the lower volume will also be ignored. For example, it is hard to hear the sound of the computer cooling fan during the day, but it becomes a noise source at night. According to this principle, the encoder can filter out many inaudible sounds to simplify information complexity and increase the compression ratio without significantly reducing sound quality. This shading is called the simultaneous shading effect. However, sound A is protected by sound B. If A is within the protection range centered on B, the protection will be more obvious. This range is called the critical bandwidth. The critical bandwidth of each frequency is different and the higher the frequency, the larger the critical bandwidth.
Frequency (Hz) Critical Bandwidth (Hz) Frequency (Hz) Critical Bandwidth (Hz)
Based on this effect, the experts designed a mental model of human hearing. After this model was imported into mp3 encoding, it led to a momentous revolution in sound quality. gradually eluted. At this point, the VBR technology, which has been buried for a long time, shines brightly, and with the use of the psychological model, it can perform powerful temptation and lethality.
For a long time, many people have a bad impression of MP3. More and more people think that the best sound quality of WMA is better than MP3. This statement is not correct. At medium and high bit rates, properly encoded MP3 is much better than WMA. It’s close to CD quality, with not-so-great hardware support, not many people can tell the difference between the two, it’s not a fairy tale, though you used to be able to easily tell the difference between MP3 and CD blindly. listening, but now cannot guarantee that it can distinguish correctly. Because MP3 is an excellent codec that was buried before.

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What is CBR? (constant bit rate)

What is CBR? (constant bit rate)

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.

CBR

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.