2.3. MPEG-2 transport stream Video and audio encoders transmit signal in the main stream. Raw uncompressed audio and
video parts of the signal, known as presentation units, are located in the encoder for receiving
video and audio access units. Video access unit can be I, P and B coded picture. Audio access
units are containing encoded information for a few milliseconds of sound window: 24 ms (layer
II), and 24 or 8 ms in the case of the layer III. The video and audio access units form the
elementary streams in a respective manner. Each elementary stream (ES) is then divided into
packets to form a video or audio packetized elementary stream (PES). Service and other data are
similarly grouped into their PES. PES packets are then divided into smaller 188-bit transport
packages [2, 10].
To gain access to the transfer of MPEG-2 signal, data streams must be multiplexed. With
multiplexing, the following is obtained:
• portable data stream (TS = transport stream)—designed to transmit signals to terrestrial, cable
and satellite connections,
• programming data stream (PS = program stream)—designed for storing digital data on DVD
or other storage space.
Multiplexing of audio and video signals is necessary in order to enable their joint transmission,
and properly decode and display. The multiplexing hierarchy determined by MPEG-2
standard can be divided into:
• basic data stream (ES = elementary stream),
• packetized basic data stream (PES = packetized elementary stream),
• portable (TS) or program data stream—PS (Figure 2).
Programming flow obtained by multiplexing includes packages resulting from one or more
elementary streams belonging to one program. It can contain one stream of the video signal,
and more data streams of an audio signal.
All packages have certain common components that are grouped into three parts: header, data
and control data [10, 11].
Packets of the program stream have a variable length, which causes difficulties when the
decoder needs to recognize the exact beginning and the end of the package. To make this
possible, the packet header contains information of the length of the package. PES packet can
vary in length up to a maximum of 64 KB, while the typical length is about 2 KB. The part that
follows the header contains the access unit as parts of the original elementary stream. At the
same time, there is no obligation to equalize the start of access units with a start of information
part (payload). According to that a new access unit can start at any point in the information
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part of PES packets, there is also the possibility that a few small access units can be contained
in one PES packet.
