mp3 frame

[fabiodelorenzo]

This smaller pieces of the audio is then put into something called 'frames', which is a little datablock with a header.

The header is 4 bytes, 32 bits, big and begins with something called sync.
This sync is, at least according to the MPEG standard, 12 set bits in a row.
Some add-on standards made later uses 11 set bits and one cleared bit.
The sync is directly followed by a ID bit, indicating if the file is a MPEG-1 och MPEG-2 file. 0=MPEG-2 and 1=MPEG-1



The total size in bytes for any given frame can be calculated with the following formula:

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FrameSize = 144 * BitRate / (SampleRate + Padding)

The layer is defined with the two layers bits:

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0 0   Not defined
0 1   Layer III
1 0   Layer II
1 1   Layer I

With this information and the information in the bitrate field we can determine the bitrate of the audio (in kbit/s) according to this table.

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0 0 0 0  --------
0 0 0 1  32 32 32 32 32 8
0 0 1 0  64 48 40 64 48 16
0 0 1 1  96 56 48 96 56 24
0 1 0 0  128 64 56 128 64 32
0 1 0 1  160 80 64 160 80 64
0 1 1 0  192 96 80 192 96 80
0 1 1 1  224 112 96 224 112 56
1 0 0 0  256 128 112 256 128 64
1 0 0 1  288 160 128 288 160 128
1 0 1 0  320 192 160 320 192 160
1 0 1 1  352 224 192 352 224 112
1 1 0 0  384 256 224 384 256 128
1 1 0 1  416 320 256 416 320 256
1 1 1 0  448 384 320 448 384 320
1 1 1 1  ----------------

The sample rate is described in the frequency field. These values is dependent of which MPEG standard is used according to the following table.

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Frequency MPEG-1      MPEG-2
Value
0 0       44100 Hz    22050 Hz
0 1       48000 Hz    24000 Hz
1 0       32000 Hz    16000 Hz
1 1

See
http://id3.org/mp3Frame
http://en.wikipedia.org/wiki/MP3
http://www.mp3-tech.org/programmer/frame_header.html