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Описание формата MP3 (eng)

MPEG Audio Layer I/II/III frame header

There is no main file header in an MPEG audio file. An MPEG audio file is built up from a succession of smaller parts called frames. A frame is a datablock with its own header and audio information.

    In the case of Layer I or Layer II, frames are some totally independent items, so you can cut any part of MPEG file and play it correctly. The player will then play the music starting to the first plain valid frame founded. However, in the case of Layer III, frames are not always independant. Due to the possible use of the "byte reservoir", wich is a kind of buffer, frames are often dependent of each other. In the worst case, 9 frames may be needed before beeing able to decode one frame.

    When you want to read info about an MPEG audio file, it is usually enough to find the first frame, read its header and assume that the other frames are the same. But this is not always the case, as variable bitrate (VBR) files may be encountered. In a VBR file, the bitrate can be changed in each frame. It can be used, as an exemple to keep a constant sound quality during the whole file, by using more bits where the music need more to be encoded.

    The frame header is 32 bits (4 bytes) length. The first twelve bits (or first eleven bits in the case of the MPEG 2.5 extension) of a frame header are always set to 1 and are called "frame sync".

    Frames may have an optional CRC checksum. It is 16 bits long and, if it exists, follows the frame header. After the CRC comes the audio data. By re-calculating the CRC and comparing its value to the sored one, you can check if the frame has been altered during transmission of the bitstream.

    Here is a presentation of the frame header content. Characters A to M are used to indicate different fields. In the table below, you can see details about the content of each field.

AAAAAAAA AAABBCCD EEEEFFGH IIJJKLMM

Sign Length
(bits)
Position
(bits)
Description
A 11 (31-21) Frame sync (all bits must be set)
B 2 (20,19) MPEG Audio version ID
00 - MPEG Version 2.5 (later extension of MPEG 2)
01 - reserved
10 - MPEG Version 2 (ISO/IEC 13818-3)
11 - MPEG Version 1 (ISO/IEC 11172-3)

Note: MPEG Version 2.5 was added lately to the MPEG 2 standard. It is an extension used for very low bitrate files, allowing the use of lower sampling frequencies. If your decoder does not support this extension, it is recommended for you to use 12 bits for synchronization instead of 11 bits.

C 2 (18,17) Layer description
00 - reserved
01 - Layer III
10 - Layer II
11 - Layer I
D 1 (16) Protection bit
0 - Protected by CRC (16bit CRC follows header)
1 - Not protected
E 4 (15,12) Bitrate index
bits V1,L1 V1,L2 V1,L3 V2,L1 V2, L2 & L3
0000 free free free free free
0001 32 32 32 32 8
0010 64 48 40 48 16
0011 96 56 48 56 24
0100 128 64 56 64 32
0101 160 80 64 80 40
0110 192 96 80 96 48
0111 224 112 96 112 56
1000 256 128 112 128 64
1001 288 160 128 144 80
1010 320 192 160 160 96
1011 352 224 192 176 112
1100 384 256 224 192 128
1101 416 320 256 224 144
1110 448 384 320 256 160
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

"free" means free format. The free bitrate must remain constant, an must be lower than the maximum allowed bitrate. Decoders are not required to support decoding of free bitrate streams.
"bad" means that the value is unallowed.

MPEG files may feature variable bitrate (VBR). Each frame may then be created with a different bitrate. It may be used in all layers. Layer III decoders must support this method. Layer I & II decoders may support it.

For Layer II there are some combinations of bitrate and mode which are not allowed. Here is a list of allowed combinations.
bitrate
single channel
stereo
intensity stereo
dual channel
free
yes
yes
yes
yes
32
yes
no
no
no
48
yes
no
no
no
56
yes
no
no
no
64
yes
yes
yes
yes
80
yes
no
no
no
96
yes
yes
yes
yes
112
yes
yes
yes
yes
128
yes
yes
yes
yes
160
yes
yes
yes
yes
192
yes
yes
yes
yes
224
no
yes
yes
yes
256
no
yes
yes
yes
320
no
yes
yes
yes
384
no
yes
yes
yes

F 2 (11,10) Sampling rate frequency index
bits MPEG1 MPEG2 MPEG2.5
00 44100 Hz 22050 Hz 11025 Hz
01 48000 Hz 24000 Hz 12000 Hz
10 32000 Hz 16000 Hz 8000 Hz
11 reserv. reserv. reserv.
G 1 (9) Padding bit
0 - frame is not padded
1 - frame is padded with one extra slot

Padding is used to exactly fit the bitrate.As an example: 128kbps 44.1kHz layer II uses a lot of 418 bytes and some of 417 bytes long frames to get the exact 128k bitrate. For Layer I slot is 32 bits long, for Layer II and Layer III slot is 8 bits long.
H 1 (8) Private bit. This one is only informative.
I 2 (7,6) Channel Mode
00 - Stereo
01 - Joint stereo (Stereo)
10 - Dual channel (2 mono channels)
11 - Single channel (Mono)

Note: Dual channel files are made of two independant mono channel. Each one uses exactly half the bitrate of the file. Most decoders output them as stereo, but it might not always be the case.
    One example of use would be some speech in two different languages carried in the same bitstream, and then an appropriate decoder would decode only the choosen language.
J 2 (5,4) Mode extension (Only used in Joint stereo)

Mode extension is used to join informations that are of no use for stereo effect, thus reducing needed bits. These bits are dynamically determined by an encoder in Joint stereo mode, and Joint Stereo can be changed from one frame to another, or even switched on or off.

Complete frequency range of MPEG file is divided in subbands There are 32 subbands. For Layer I & II these two bits determine frequency range (bands) where intensity stereo is applied. For Layer III these two bits determine which type of joint stereo is used (intensity stereo or m/s stereo). Frequency range is determined within decompression algorithm.

Layer I and II Layer III
value Layer I & II
00 bands 4 to 31
01 bands 8 to 31
10 bands 12 to 31
11 bands 16 to 31
Intensity stereo MS stereo
off off
on off
off on
on on

K 1 (3) Copyright
0 - Audio is not copyrighted
1 - Audio is copyrighted

The copyright has the same meaning as the copyright bit on CDs and DAT tapes, i.e. telling that it is illegal to copy the contents if the bit is set.
L 1 (2) Original
0 - Copy of original media
1 - Original media

The original bit indicates, if it is set, that the frame is located on its original media.
M 2 (1,0) Emphasis
00 - none
01 - 50/15 ms
10 - reserved
11 - CCIT J.17

The emphasis indication is here to tell the decoder that the file must be de-emphasized, ie the decoder must \'re-equalize\' the sound after a Dolby-like noise supression. It is rarely used.

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