Page 116
NewS
tream
User Guide and Technical Manual
Term
Defi nition
PCR
Program Clock Reference
To enable a decoder to present synchronized content, such as audio tracks
matching the associated video, at least once each 100 millisecond, a Program
Clock Reference [PCR] is transmitted in the adaptation
fi
eld of an MPEG-
2 transport stream packet. The PID with the PCR for an MPEG-2 program
is identi
fi
ed by the PCR_PID value in the associated Program Map Table.
The value of the PCR is employed to generate a system_timing_clock in the
decoder. The STC or System Time Clock decoder provides a highly accurate
time base that is used to synchronize audio and video elementary streams.
Timing in MPEG2 uses this clock as a reference.
PES
Packetized Elementary Stream
PES is a speci
fi
cation in the MPEG-2 parameter that de
fi
nes carrying of
elementary streams [usually the output of an audio or video encoder] in
packets within MPEG program stream and MPEG transport stream. The
elementary stream is packetized by encapsulating sequential data bytes from
the elementary stream inside PES packet headers.
PID
Packet ID
Each table or elementary stream in a transport stream is identi
fi
ed by a 13-
bit packet ID [PID]. A demultiplexer extracts elementary streams from the
transport stream in part by looking for packets identi
fi
ed by the same PID. In
most applications, time-division multiplexing is used to decide how often a
particular PID appears in the transport stream.
PMT
Program Map Table
A PMT contains information about programs. For each program, there is one
PMT. While the MPEG-2 standard permits more than one PMT section to be
transmitted on a single PID, most MPEG-2 users such as ATSC and SCTE
require each PMT to be transmitted on a separate PID that is not used for
any other packets. The PMTs provide information on each program present in
the transport stream, including the program_number, and list the elementary
streams that comprise the described MPEG-2 program. There are also
locations for optional descriptors that describe the entire MPEG-2 program, as
well as an optional descriptor for each elementary stream. Each elementary
stream is labeled with a stream_type value.
QPSK
Quadrature Phase-Shift Keying
Phase-shift keying [PSK] is a digital modulation scheme that conveys data
by changing, or modulating, the phase of a reference signal or carrier wave.
Quadrature Phase-Shift Keying [QPSK] transmits twice the data rate in a
given bandwidth compared to Binary Phase-Shift Keying [BPSK] - at the
same Bit Error Rate [BER]. The engineering penalty that is paid is that QPSK
transmitters and receivers are more complicated than the ones for BPSK.
However, with modern electronics technology, the penalty in cost is very
moderate. QPSK is also known as 4 - QAM.
Roll-off
Roll-off describes the steepness of a transmission function with frequency, in
connection with
fi
lter circuits in the transition between a passband and a stop
band. It is most typically applied to the insertion loss of the network. It is usual
to measure roll-off as a function of logarithmic frequency, consequently, the
units of roll-off are either decibels per decade [dB/decade], where a decade is
a 10-times increase in frequency, or decibels per octave [dB/8ve], where an
octave is 2-times increase in frequency.
