Elementary stream

Encyclopedia : E : EL : ELE : Elementary stream

Defined by MPEG communication protocol. An elementary stream contains only one kind of data, for example audio or video. The output of a video encoder is an elementary stream. The output of an audio encoder is also an elementary stream. Sometimes referred to as "elementary", "data", "audio", or "video" bitstreams or streams. The format of the elementary stream depends upon the codec or data carried in the stream.

General layout of MPEG-1 audio 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[link]:

Field Name	# of bits	Description
syncword	12	0xFFF
ID	1	'1'=mpeg1 '0'=mpeg2
layer	2	'11'=1 '10'=2 '01'=3
protection	1
bitrate 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=origininal
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.

Elementary stream

General layout of MPEG-1 audio stream

See also