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An overview of the principle of MP3 encoding

Audio compression consists of two parts: encoding and decoding. Encoding is the conversion of digital audio data in a wave file into a highly compressed form (called a bitstream); decoding is the reconstruction of the bit stream into a wave file.

Audio compression can be divided into lossless compression and lossy compression. Lossless compression is to minimize the redundancy of audio data to reduce its volume. Once the audio signal is encoded and decoded, it must be consistent with the original signal. The compression ratio of lossless compression is relatively limited, but now the best APE can achieve 50% compression ratio (I use Monkey’s Audio 3.97 to compress WAV in extra high compression mode, and the compression ratio can reach a minimum of 52%). Lossy compression is the use of all means, including the methods used in lossless compression, to lose all losable data in order to reduce volume. After audio compression, decoding sounds at least the same as the original, and the compression ratio of lossy compression can be greatly improved. MP3 is lossy compression, and the compression ratio is 12:1 (128 kbps).
MP3 files are made up of frames and frames are the smallest unit of MP3 files. What is a painting? Do you remember how the original animation was made? Different continuous images are alternated for dynamic effects, each image is a “frame”, the difference is that frames in MP3 record audio data instead of graphic data. The MP3 frame rate is about 30 frames per second.
Each frame is made up of frame header and frame data. The frame header records the basic information of the frame, including the bit rate index and the sample rate index (this is very important to understand the ABR and VBR encoding methods). Frame data, as its name suggests, records the main audio data.
The above is the basis of MP3 encoding, but in fact, the early encoders are very imperfect, the compression algorithm is almost rudimentary, and the sound quality is not ideal. The sound quality of MP3 has reached the current level with two leaps: the introduction of the human auditory psychological model (perceptual model) and the application of VBR technology.
◆Human auditory psychology model
Some important principles will be briefly introduced below:
1) The minimum hearing threshold
The hearing range of the human ear is the frequency range of 20Hz-20k Hz, but the sensitivity of the human ear to sounds of different frequencies is different, and the intensity of sounds of different frequencies to reach a level that can be heard for the human ear is different. Then, through calculation, the sound that exists in the music file but cannot be heard by the human ear can be removed. Through this principle, we can also build a model to allocate most of the data space to the 2kHz to 5kHz range, where the human ear is most sensitive, and allocate less space for the rest of the frequency;
2) The masking effect of the human ear
The masking effect occurs when a strong signal masks weak signals at adjacent frequencies. From life experience, in a quiet room, a needle can be heard when it falls to the ground, but in the street, even if the volume of the mobile phone is at maximum, it may not be heard when there is a call. done, and the sound of the mobile phone exists. Yes, the reason is that it is obscured by the louder sounds around. With the results of the investigation of the shading effect, the encoder can calculate the shading from the strong signal to the nearby weak signal according to the established mathematical model, so as to retain the sound that can attract people’s attention.
The human ear also has a pre-masking effect and a post-masking effect: because the human needs a certain amount of time to process the sound signal, the weak signal before or after the strong signal will be masked. . The front shading effect time is only 2-5ms, and the back shading time is relatively long, about 100ms. Using this, we can reduce the resolution before and after the strong signal;

Factors to consider before converting to MP3

MP3 Encoding Settings

Introduce
The MP3 format is the most popular lossy audio format in use today and has been around for over a decade. Its success is mainly due to its universal compatibility. Even with such an achievement, there are still some rules that you need to know before creating an MP3 file. The following factors will give you an idea of ​​how to adjust your encoding settings for best results.

Audio source quality

To choose the best encoding value, you must first consider the nature of the audio source. For example, if you use analog video tape to encode a low-quality recording and use the highest possible encoding settings, you are wasting a lot of storage space. If you are converting a 96kbps bitrate MP3 file to a 192kbps bitrate file, you will not be able to improve the quality. The reason is that the original speed is only 32kbps, so going over that number will only increase the file size, not the resolution of the sound.

Here are some typical bitrate settings you might want to try:

32 kbps – voice/analog tape recording.
128 to 192 kbps – Rip CD tracks.
192 to 320 kbps: Record complex audio sources (audio sources that contain a wide range of frequencies, such as classical music).
at a loss

The MP3 format is a lossy format and conversion to another lossy format (including another MP3) is not recommended. Even if you try to convert at a higher bitrate, you will still lose quality. Generally, it’s best to leave the original file as it is, unless you want less storage space and don’t mind lowering the audio resolution.

CBR and VBR
Constant bit rate ( CBR ) and variable bit rate ( VBR ) are two options you can choose from when encoding MP3 files, and both options have their pros and cons. Before deciding whether to use CBR or VBR, you should first consider how you will hear the audio. CBR is the default setting and is universally supported by all MP3 decoders and hardware devices, but will not produce the most optimized MP3 files. Alternatively, VBR produces an MP3 file optimized for file size and quality. VBR is still the best solution, but it is not always compatible with older hardware and some MP3 decoders.