it just informs you how to read the MPEG headers and the MPEG TAG. MPEG Version 1, 2 and 2.5 and Layer I, II and III are supported, the MP3 TAG (ID3v1 and ID3v1.1) also..
MPEG Audio Compression Basics
MPEG standards MPEG-1, MPEG-2 and MPEG-4 are known but this document covers first two of them. There is an unofficial MPEG-2.5 which is rarely used. It is also covered.
MPEG-1 audio (described in ISO/IEC 11172-3) describes three Layers of audio coding with the following properties:
one or two audio channels
sample rate 32kHz, 44.1kHz or 48kHz.
bit rates from 32kbps up to 448kbps
Each layer has its merits.
MPEG-2 audio (described in ISO/IEC 13818-3) has two extensions to MPEG-1, usually referred as MPEG-2/LSF and MPEG-2/Multichannel.
MPEG-2/LSF has the following properties:
one or two audio channels
sample rates half those of MPEG-1
bit rates from 8 kbps up to 256kbps.
MPEG-2/Multichannel has the following properties:
up to 5 full range audio channels and an LFE-channel (Low Frequency Enhancement <> subwoofer!)
sample rates the same as those of MPEG-1
highest possible bitrate goes up to about 1Mbps for 5.1
MPEG Audio Frame Header
An MPEG audio file is built up from smaller parts called frames. Generally, frames are independent items. Each frame has its own header and audio informations. There is no file header. Therefore, you can cut any part of MPEG file and play it correctly (this should be done on frame boundaries but most applications will handle incorrect headers). For Layer III, this is not 100% correct. Due to internal data organization in MPEG version 1 Layer III files, frames are often dependent of each other and the y cannot be cut off just like that.
When you want to read info about an MPEG file, it is usually enough to find the first frame, read its header and assume that the other frames are the same This may not be always the case. Variable bitrate MPEG files may use so called bitrate switching, which means that bitrate changes according to the content of each frame. This way lower bitrates may be used in frames where it will not reduce sound qualit y. This allows making better compression while keeping high quality of sound.
The frame header is constituted by the very first four bytes (32bits) in a frame. The first eleven bits (or first twelve bits, see below about frame sync) of a frame header are always set and they are called "frame sync". Therefore, you can search through the file for the first occurence of frame sync (meaning that you have to find a byte with a value of 255, and followed by a byte with its three (or four) most significant bits set). Then you read the whole header and check if the values are correct. You will see in the following table the exact meaning of each bit in the header, and which values may be checked for validity. Each value that is specified as reserved, invalid, bad, or not allowed should indicate an invalid header. Remember, this is not enough, frame sync can be easily (and very frequently) found in any binary file. Also it is likely that MPEG file contains garbage on it's beginning which also may contain false sync. Thus, you have to check two or more frames in a row to assure you are really dealing with MPEG audio file.
Frames may have a CRC check. The CRC is 16 bits long and, if it exists, it follows the frame header. After the CRC comes the audio data. You may calculate the length of the frame and use it if you need to read other headers too or just want to calculate the CRC of the frame, to compare it with the one you read from the file. This is actually a very good method to check the MPEG header validity.
Here is "graphical" presentation of the header content. Characters from A to M are used to indicate different fields. In the table, you can see details about the content of each field.
AAAAAAAA AAABBCCD EEEEFFGH IIJJKLMM
A 11 (31-21) Frame sync (all bits set)
B 2 (20,19) MPEG Audio version ID
00 - MPEG Version 2.5
01 - reserved
10 - MPEG Version 2 (ISO/IEC 13818-3)
11 - MPEG Version 1 (ISO/IEC 11172-3)
Note: MPEG Version 2.5 is not official standard. Bit No 20 in frame header is used to indicate version 2.5. Applications that do not support this MPEG version expect this bit always to be set, meaning that frame sync (A) is twelve bits long, not eleven as stated here. Accordingly, B is one bit long (represents only bit No 19). I recommend using methodology presented here, since this allows you to distinguish all three versions and keep full compatibility.
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. If the correct fixed bitrate (such files cannot use variable bitrate) is different than those presented in upper table it must be determined by the application. This may be implemented only for internal purposes since third party applications have no means to findout correct bitrate. Howewer, this is not impossible to do but demands lot's of efforts.
"bad" means that this is not an allowed value
MPEG files may have variable bitrate (VBR). This means that bitrate in the file may change. I have learned about two used methods:
bitrate switching. Each frame may be created with different bitrate. It may be used in all layers. Layer III decoders must support this method. Layer I & II decoders may support it.
bit reservoir. Bitrate may be borrowed (within limits) from previous frames in order to provide more bits to demanding parts of the input signal. This causes, however, that the frames are no longer independent, which means you should not cut this fil es. This is supported only in Layer III.
More about VBR you may find on Xing Tech site
For Layer II there are some combinations of bitrate and mode which are not allowed. Here is a list of allowed combinations. bitrate allowed modes
32 single channel
48 single channel
56 single channel
80 single channel
224 stereo, intensity stereo, dual channel
256 stereo, intensity stereo, dual channel
320 stereo, intensity stereo, dual channel
384 stereo, intensity stereo, dual channel
F 2 (11,10) Sampling rate frequency index (values are in Hz) bits MPEG1 MPEG2 MPEG2.5
00 44100 22050 11025
01 48000 24000 12000
10 32000 16000 8000
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 fit the bit rates exactly. For an example: 128k 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.
How to calculate frame length
First, let's distinguish two terms frame size and frame length. Frame size is the number of samples contained in a frame. It is constant and always 384 samples for Layer I and 1152 samples for Layer II and Layer III. Frame length is length of a frame when compressed. When you are reading MPEG file you must calculate this to be able to find each consecutive frame. Remember, frame length may change from frame to frame due to padding or bitrate switching.
Read the BitRate, SampleRate and Padding (remember if padding is used in a frame it is four bytes long for Layer I or one byte long for Layer II and Layer III) of the frame header.
For Layer I files us this formula:
FrameSize = 12 * BitRate / SampleRate + Padding
For Layer II & III files use this formula:
FrameSize = 144 * BitRate / SampleRate + Padding
Layer III, BitRate=128000, SampleRate=441000, Padding=0 ==> FrameSize=417 bytes
H 1 (8) Private bit. It may be freely used for specific needs of an application, i.e. if it has to trigger some application specific events.
I 2 (7,6) Channel Mode
00 - Stereo
01 - Joint stereo (Stereo)
10 - Dual channel (Stereo)
11 - Single channel (Mono)
J 2 (5,4) Mode extension (Only if Joint stereo)
Mode extension is used to join informations that are of no use for stereo effect, thus reducing needed resources. These bits are dynamically determined by an encoder in Joint stereo mode.
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 algorythm. 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
K 1 (3) Copyright
0 - Audio is not copyrighted
1 - Audio is copyrighted
L 1 (2) Original
0 - Copy of original media
1 - Original media
M 2 (1,0) Emphasis
00 - none
01 - 50/15 ms
10 - reserved
11 - CCIT J.17
MPEG Audio Tag ID3v1
The TAG is used to describe the MPEG Audio file. It contains information about artist, title, album, publishing year and genre. There is some extra space for comments. It is exactly 128 bytes long and is located at very end of the audio data. You can get it by reading the last 128 bytes of the MPEG audio file.
AAABBBBB BBBBBBBB BBBBBBBB BBBBBBBB
BCCCCCCC CCCCCCCC CCCCCCCC CCCCCCCD
DDDDDDDD DDDDDDDD DDDDDDDD DDDDDEEE
EFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFG
A 3 (0-2) Tag identification. Must contain 'TAG' if tag exists and is correct.
B 30 (3-32) Title
C 30 (33-62) Artist
D 30 (63-92) Album
E 4 (93-96) Year
F 30 (97-126) Comment
G 1 (127) Genre
The specification asks for all fields to be padded with null character (ASCII 0). However, not all applications respect this (an example is WinAmp which pads fields with <space>, ASCII 32).
There is a small change proposed in ID3v1.1 structure. The last byte of the Comment field may be used to specify the track number of a song in an album. It should contain a null character (ASCII 0) if the information is unknown.
Genre is a numeric field which may have one of the following values:
0 'Blues' 20 'Alternative' 40 'AlternRock' 60 'Top 40'
1 'Classic Rock' 21 'Ska' 41 'Bass' 61 'Christian Rap'
2 'Country' 22 'Death Metal' 42 'Soul' 62 'Pop/Funk'
3 'Dance' 23 'Pranks' 43 'Punk' 63 'Jungle'
4 'Disco' 24 'Soundtrack' 44 'Space' 64 'Native American'
5 'Funk' 25 'Euro-Techno' 45 'Meditative' 65 'Cabaret'
6 'Grunge' 26 'Ambient' 46 'Instrumental Pop' 66 'New Wave'
7 'Hip-Hop' 27 'Trip-Hop' 47 'Instrumental Rock' 67 'Psychadelic'
8 'Jazz' 28 'Vocal' 48 'Ethnic' 68 'Rave'
9 'Metal' 29 'Jazz+Funk' 49 'Gothic' 69 'Showtunes'
10 'New Age' 30 'Fusion' 50 'Darkwave' 70 'Trailer'
11 'Oldies' 31 'Trance' 51 'Techno-Industrial' 71 'Lo-Fi'
12 'Other' 32 'Classical' 52 'Electronic' 72 'Tribal'
13 'Pop' 33 'Instrumental' 53 'Pop-Folk' 73 'Acid Punk'
14 'R&B' 34 'Acid' 54 'Eurodance' 74 'Acid Jazz'
15 'Rap' 35 'House' 55 'Dream' 75 'Polka'
16 'Reggae' 36 'Game' 56 'Southern Rock' 76 'Retro'
17 'Rock' 37 'Sound Clip' 57 'Comedy' 77 'Musical'
18 'Techno' 38 'Gospel' 58 'Cult' 78 'Rock & Roll'
19 'Industrial' 39 'Noise' 59 'Gangsta' 79 'Hard Rock'
Any other value should be considered as 'Unknown'
MPEG Audio Tag ID3v2
This is new proposed TAG format which is different than ID3v1 and ID3v1.1. Complete tech specs for it may be found at http://www.id3.com/.