== How is MP3 built? ==
Most people with a little knowledge in MP3 files know that the sound is divided into smaller parts and compressed with a psycoacoustic model. This smaller pieces of the audio is then put into something called 'frames', which is a little datablock with a header. I'll focus on that header in this text.

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 layer is defined with the two layers bits. They are oddly defined as [[BR]]
|| 0 0 || Not defined ||
||0 1 || Layer III ||
||1 0 || Layer II ||
||1 1 || Layer I ||


[[BR]] 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. [[BR]]
|| Bitrate[[BR]]value || MPEG-1,[[BR]]layer I || MPEG-1,[[BR]]layer II || MPEG-1,[[BR]]layer III || MPEG-2,[[BR]]layer I || MPEG-2,[[BR]]layer II || MPEG-2,[[BR]]layer III ||
|| 0 0 0 0 ||<-6> ||
|| 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 ||<-6>||