Definition of Terms - Digital Video and Audio Broadcasting Technology - page 561

Payload Unit Start Indicator
The payload unit start indicator is a 1 bit flag in the second byte of a TS
header. It indicates the beginning of a PES packet or of a section of PSI or
SI tables in the corresponding TS packet.
PCMCIA (PC Card)
PCMCIA is a physical interface standardized by the Personal Computer
Memory Card International Association for the data exchange between
computers and peripherals. A model of this interface is used for the com-
mon interface.
PCR Jitter
The value of a PCR refers to the exact beginning of a TS packet in
which it is located. The reference to the 27 MHz system clock yields an
accuracy of approx. ±20 ns. If the difference of the transferred values devi-
ates from the actual difference of the beginning of the packets concerned,
this is called PCR jitter. It can be caused, for example, by an inaccurate
PCR calculation during transport stream multiplexing or by the subsequent
integration of null packets on the transmission path without PCR correc-
tion.
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.
PES Packet
The PES packet (not to be confused with TS packet) contains a trans-
mission unit of a packetized elementary stream (PES). In a video stream,
for example, this is a source-coded image. The length of a PES packet is
normally limited to 64 kbytes. It may exceed this length only if a video
image requires more capacity. Each PES packet is preceded by a PES
header.
Plesiochronous Digital Hierarchy (PDH)
The Plesiochronous Digital Hierarchy was originally developed for the
transmission of digitized voice calls. In this method, high-bit-rate trans-
mission systems are generated by time-interleaving the digital signals of
low-bit-rate subsystems. In PDH, the clock rates of the individual subsys-
tems are allowed to fluctuate and these fluctuations are compensated for by
appropriate stuffing methods. The PDH includes E3 and DS3, among oth-
ers.