Digital Signal Processing Reference
In-Depth Information
42 bit-long counter which provides the System Time Clock (STC). For
each program, a separate system time clock (STC) is used. To be able to
link the MPEG-2 decoder to this clock, copies of the current program sys-
tem time are transmitted about every 40 ms per program in the adaptation
field. The PMT of the respective program carries information about the TS
packets in which this clock time can be found.
The STC reference values are called Program Clock Reference (PCR).
They are nothing else than a 42 bit copy of the 42 bit counter. The MPEG-
2 decoder links itself to these PCR values via a PLL and derives its own
system clock from them.
PCR
Optional
adaptation
field
Header
Fig. 11.7.
PCR value
If the repetition rate of the PCR values is too slow, it may be due to the
fact that the PLL of the receiver has problems in locking to it. MPEG-2
specifies that the maximum interval between two PCR values must not ex-
ceed a period of 40 ms. According to the DVB Measurement Guidelines, a
PCR_error has occurred if this time is exceeded.
The timing of the PCR values with respect to one another should also be
relatively accurate, i.e. there should not be any jitter. Jitter may occur, for
example, if the PCR values are not corrected, or are corrected inaccurately,
during remultiplexing.
If the PCR jitter exceeds ±500 ns, a PCR_accuracy_error has occurred.
PCR jitter frequently extends into the ±30 Μs range which can be handled
by many set top boxes, but not by all. The first indication that the PCR jit-
ter is too great is a black/white picture instead of a colour picture. The ac-
tual effect, however, depends on how the set top box is wired to the TV re-
ceiver. An RGB connection (e.g. via a SCART A/V cable) is certainly less
critical than a composite video cable connection.
A PCR_error occurs when
•
the difference between two successive PCR values of a program
is greater than 100 ms and no discontinuity is indicated in the
adaptation field, or
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