Glossary
Glossary–6
MTD200 MPEG Test Decoder User Manual
PES Header
Each PES packet in the transport stream starts with a PES header. The PES
header contains information for decoding the elementary stream. The
presentation time stamp (PTS) and decoding time stamp (DTS) are of vital
importance. The beginning of a PES header and thus also the beginning of a
PES packet is indicated in the associated TS packet by means of the set
payload unit start indicator. If the PES header is to be scrambled, it is
scrambled at the transport–stream level. It is not affected by scrambling at
the elementary–stream level (see Conditional Access).
PES Packet
The PES packet (not to be mixed up with TS packet) contains a packetized
elementary stream (PES) as transmission unit. In a video stream, for
example, this is a source–coded picture. The length of a PES packet is
normally reduced to 64 Kbytes. Only if a video picture requires more
capacity may a PES packet be longer than 64 Kbytes. A PES header
precedes each PES packet.
Presentation Time Stamp (PTS)
The PTS is a 33–bit value in the PES header and represents the output time
of the content of a PES packet. The value refers to the 33 most significant
bits of the associated program clock reference. If the order of output does not
correspond to the order of decoding, a decoding time stamp (DTS) is
additionally transmitted. This is the case for video streams containing
differential pictures.
Program Association Table (PAT)
The PAT is a PSI Table. It lists all the programs contained in a transport
stream and refers to the associated PMTs containing further information
about the programs. The PAT is transmitted in TS packets with PID 0x0000
and indicated by table_ID 0x00.
Program Clock Reference (PCR)
The PCR is a 42–bit value contained in an adaptation field and helps the
decoder to synchronize its system clock (27 MHz) to the clock of the
encoder or TS multiplexer by means of PLL. In this case, the 33 most
significant bits refer to a 90–kHz clock while the 9 least significant bits
count from 0 to 299 and thus represent a clock of 300
×
90 kHz (= 27 MHz).
Each program of a transport stream relates to a PCR which is transmitted in
the adaptation field by TS packets with a specific PID. The presentation time
stamps (PTS) and decoding time stamps (DTS) of all the elementary streams
of a program refer to the 33 most significant bits of the PCR. According to
MPEG2 PCRs have to be transmitted at intervals of max. 100 ms, according
to the DVB regulations at intervals of max. 40 ms.
Summary of Contents for MTD200
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Page 29: ...Introduction to Digital TV Transmission Technique 1 14 MTD200 MPEG Test Decoder User Manual...
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Page 174: ...Maintenance and Checking 6 6 MTD200 MPEG Test Decoder User Manual...
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