Elementary Stream - General Layout of MPEG-1 Audio Elementary Stream

General Layout of MPEG-1 Audio Elementary Stream

The digitized sound signal is divided up into blocks of 384 samples in Layer I and 1152 samples in Layers II and III. The sound sample block is encoded within an audio frame:

  • header
  • error check
  • audio data
  • ancillary data

The header of a frame contains general information such as the MPEG Layer, the sampling frequency, the number of channels, whether the frame is CRC protected, whether the sound is the original:

Field Name # of bits Description
sync word 12 0xFFF
ID 1 '1'=mpeg1 '0'=mpeg2
layer 2 '11'=1 '10'=2 '01'=3
no protection 1 '0'=Protected by CRC (16bit CRC follows header)
'1'=Not Protected
bit rate index 4
sampling frequency 2 kHz '00'=44.1 '01'=48 '10'=32
padding 1
private 1
mode 2 '00'=Stereo '01'=joint stereo '10'=dual channel '11'=single channel
mode extension 2
copyright 1 0=none 1=yes
original or copy 1 0=copy 1=original
emphasis 2

Although most of this information may be the same for all frames, MPEG decided to give each audio frame such a header in order to simplify synchronization and bitstream editing.

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