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|Sign|| Length |
| Position |
|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 |
NOTES: All values are in kbps
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.
|F||2||(11,10)||Sampling rate frequency index |
|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.
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.
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.
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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