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 they 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 quality. 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
| Sign | Length (bits) |
Position (bits) |
Description | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| 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 eleve 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. |
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| C | 2 | (18,17) | Layer description 00 - reserved 01 - Layer III 10 - Layer II 11 - Layer I |
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| D | 1 | (16) | Protection bit 0 - Protected by CRC (16bit crc follows header) 1 - Not protected |
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| E | 4 | (15,12) | Bitrate index
NOTES: All values are in kbps MPEG files may have variable bitrate (VBR). This means that bitrate in the file may change. I have learned about two used methods: 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.
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| F | 2 | (11,10) | Sampling rate frequency index (values are in Hz)
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| 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. It is calculated in slots. One slot is 4 bytes long for Layer I, and one byte long for Layer II and Layer III. 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 of the frame header. For Layer I files us this formula: FrameLengthInBytes = (12 * BitRate / SampleRate + Padding) * 4 For Layer II & III files use this formula: FrameLengthInBytes = 144 * BitRate / SampleRate + Padding Example: |
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| 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) |
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| 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.
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| K | 1 | (3) | Copyright 0 - Audio is not copyrighted 1 - Audio is copyrighted |
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| L | 1 | (2) | Original 0 - Copy of original media 1 - Original media |
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| M | 2 | (1,0) | Emphasis 00 - none 01 - 50/15 ms 10 - reserved 11 - CCIT J.17 |
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