The MPEG Data Stream - Digital Video and Audio Broadcasting Technology - page 49

PCR
every
~40 ms

MPEG2
encoder
42 Bit

MPEG2
decoder

Video Video

PCR PCR

MPEG2 TS

Audio Audio

42 Bit

Counter

Counter

Load

Copy

27 MHz
STC

+ +

27 MHz
STC

Numerically
controlled
oscillator
(NCO)

Fig. 3.18. Program Clock Reference

To accomplish this, reference information is transmitted in the MPEG
data stream (Fig. 3.18.). In MPEG-2, these are the “Program Clock Refer-
ence” (PCR) values which are nothing else than an up-to-date copy of the
STC counter fed into the transport stream at a certain time. The data
stream thus carries an accurate internal “clock time”. All coding and de-
coding processes are controlled by this clock time. To do this, the receiver,
i.e. the MPEG decoder, must read out the “clock time”, namely the PCR
values, and compare them with its own internal system clock, that is to say
its own 42 bit counter.
If the received PCR values are locked to the system clock in the de-
coder, the 27 MHz clock at the receiving end matches the transmitting end.
If there is a deviation, a controlled variable for a PLL can be generated
from the magnitude of the deviation, i.e. the oscillator at the receiving end
can be corrected. In parallel, the 42 bit count is always reset to the received
PCR value, a basic requirement for system initialization and in the event of
a program change.
The PCR values must be present in sufficient numbers, that is to say
with a maximum spacing, and relatively accurately, that is to say free of
jitter. According to MPEG, the maximum spacing per program is 40 ms
between individual PCR values. The PCR jitter must be less than ± 500 ns.
PCR problems manifest themselves in the first instance in that instead of a
color picture, a black/white picture is displayed. PCR jitter problems can
occur during the remultiplexing of a transport stream, among other things.
The reason is that e.g., the order of the transport stream packets is changed