The MPEG Data Stream - Digital Video and Audio Broadcasting Technology - page 35

3.1 The Packetized Elementary Stream (PES)
All elementary streams in MPEG are first packetized in variable-length
packets called PES packets. The packets, which primarily have a length of
64 kbytes, begin with a PES header of 6 bytes minimum length. The first 3
bytes of this header represent the “start code prefix”, the content of which
is always 00 00 01 and which is used for identifying the start of a PES
packet. The byte following the start code is the “stream ID” which de-
scribes the type of elementary stream following in the payload. It indicates
whether it is, e.g. a video stream, an audio stream or a data stream which
follows. After that there are two “packet length” bytes which are used to
address the up to 64 kbytes of payload. If both of these bytes are set to
zero, a PES packet having a length which may exceed these 64 kbytes can
be expected. The MPEG decoder then has to use other arrangements to
find the PES packet limits, e.g. the start code.
After these 6 bytes of PES header, an “optional PES header” is transmit-
ted which is an optional extension of the PES header and is adapted to the
requirements of the elementary stream currently being transmitted. It is
controlled by 11 flags in a total of 12 bits in this optional PES header.
These flags show which components are actually present in the “optional
fields” in the optional PES header and which are not. The total length of
the PES header is shown in the “PES header data length” field. The op-
tional fields in the optional header contain, among other things, the “Pres-
entation Time Stamps” (PTS) and the “decoding time stamps” (DTS)
which are important for synchronizing video and audio. At the end of the
optional PES header there may also be stuffing bytes. Following the com-
plete PES header, the actual payload of the elementary stream is transmit-
ted which can usually be up to 64 kbytes long or even longer in special
cases, plus the optional header.
In MPEG-1, video PES packets are simply multiplexed with PES pack-
ets and stored on a data medium (Fig. 3.5.). The maximum data rate is
about 1.5 Mbit/s for video and audio and the data stream only includes a
video stream and an audio stream.
This “Packetized Elementary Stream” (PES) with its relatively long
packet structures is not, however, suitable for transmission and especially
not for broadcasting a number of programs in one multiplexed data signal.
In MPEG-2, on the other hand, the objective has been to assemble up to
6, 10 or even 20 independent TV or radio programs to form one common
multiplexed MPEG-2 data signal. This data signal is then transmitted via
satellite, cable or terrestrial transmission links. To this end, the long PES
packets are additionally divided into smaller packets of constant-length.