MP3 file format
1. Overview:
the MP3 file is made up of a frame, the frame is the smallest unit of the MP3 file. The full name of MP3 should be MPEG1 Layer 3 audio file. MPEG
(Moving Picture Experts Group) translates into Chinese as Moving Picture Experts Group, which refers to the compression standard for moving video and audio.
sound part of the MPEG1 standard, also called MPEG. The audio layer is divided into three layers based on compression quality and encoding complexity, namely
Layer-1, Layer2 and Layer3, which correspond to the three sound files of MP1, MP2 and MP3 respectively, and use different
levels of audio files according to different purposes. The higher the level of MPEG audio encoding, the more complex the encoder and the higher the compression ratio. The compression ratio of MP1 and MP2 is 4:1 and
6:1-8:1 respectively, while MP3 compression ratio is up to 10:1-12:1, i.e. one minute of CD-quality music requires 10MB
of storage space without compression, but only about 1 MB after encoding with MP3 compression. However, MP3 uses a lossy compression method for audio signals. In order to reduce the degree of sound distortion, MP3 adopts “sensory encoding technology”, that is, the spectrum analysis of the audio file is done during encoding, and then the noise level. I know
filtered with a compressed file filter can achieve a sound effect closer to the original sound source during playback. 2. The whole structure of MP3 files: MP3 files are roughly divided into three parts: TAG_V2 (ID3V2), Frame, TAG_V1 (ID3V1)
ID3V2 contains information such as author, composer, album, etc. The length is not fixed, which expands the amount of ID3V1 information.
A series of frames, the number of which is determined by the file size and frame length. The length of each FRAME of the
frame
may or may not be fixed, and is determined by the bitrate
.
Each frame is divided into two parts: frame header and data entity Header
from
box Records the bit rate, sample rate, version, and other mp3 information, and each box is independent of each other.
ID3V1 contains the author, composer, album and other information, and the length is 128BYTE. 3. FRAME format of MP3: each FRAME has a FRAMEHEADER frame header, the length is 4BYTE (32 bits), there may be two CRC check bytes after the frame header, the existence of these two bytes depends on the frame information. FRAMEHEADER Bit 16 , if 0 there is no control after the frame header, if 1 there is control. The control value is 2 bytes, followed by FRAMEHEADER, followed by the frame entity data. The format is as follows: FRAMEHEADER CRC (free) MAIN_DATA 4 BYTE 0 OR 2 BYTE The length is calculated from the frame header 1. The FRAMEHEADER frame header format is as follows: AAAAAAAA AAABBCCD EEEEFFGH IIJJKLMM
The meanings of the 13 characters in the table header are as follows:
Sign Length Position Description
(bit) (bit)
A 11 (31-21) Frame Sync (all bits set)
B 2 (20,19) MPEG audio version
00 – MPEG version 2.5
01 – reserved
10 – MPEG Version 2
11 – MPEG Version 1
C 2 (18,17) Layer Description
00 – reserved
01 – Layer III
10 – Layer II
11 – Layer I
D 1 (16) Protection bit
0 – CRC protected (16-bit CRC follows header)
1 – Not protected
E 4 (15,12) Rate index bits
bits V1,L1 V1,L2 V1,L3 V2,L1 V2,L2 V2,L3
0000 free free free free free free free
0001 32 32 32 32 32 8 ( 8)
0010 64 48 40 64 48 16 (16)
0011 96 56 48 96 56 24 (24) 0100
128 64 56 128 64 32 (32)
0101 160 80 64 160 80 64 (40)
0110 1 2 6 2 1 2
112 56 (56)
1000 256 128 112 256 128 64 (64)
1001 288 160 128 288 160 128 (80)
1010 320 192 160 320 192 160 (96)
1011 352 224 192 352 224 112 (112)
1100 384 256 224 384 384 256 128 (128)
1101 416 320 256 416 320 256 (144)
1110 448 384 320 448 384 320 (160)
1111 bad bad bad bad bad
NOTES: All values are in kbps
V1 – MPEG Version 1
V2 – MPEG Version 2 and Version 2.5
L1 – Layer I
L2 – Layer II
L3 – Layer III
