Pcr_Error, Pcr_Accuracy_Error (2Nd Priority) - Tektronix MTD200 User Manual

PCR_error, PCR_accuracy_error (2nd priority)

MTD200 MPEG Test Decoder User Manual
In every transport stream, coded time values obtained from the system time are
transmitted to enable the MPEG-2 decoder to link its own timing to the system
timing of the coder to allow decoding (PLL). Each program contained in the
transport stream may have its own independent program system timing
(referenced in PMT). The program map table (PMT) reveals for all programs
contained in the transport stream in which packets (PIDs) the PCR (Program
Clock Reference) values are transmitted.
PCR values are transferred in the optional Adaptation Field with a width of 42
bits. The 42 bits contain two parts: a PCR base with 33 bits and the PCR
extension with 9 bits. The following formulae hold for the bit structure :
PCR base (i)
PCR extension (i) =
PCR (i)
A 42-bit PCR value coded this way starts again from count 0 after the elapse of
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2 * 300 clocks (corresponds to a time period of approximately 26.5 hours at
27 MHz).
The MPEG-2 standard also tolerates discontinuity of PCR values following one
another, provided this is indicated by the discontinuity indicator in the optional
adaptation field (AF) of the same packet. This method is primarily used for the
suppression of PCR error messages when changing programs preceded by
remultiplexing of the transport stream.
In the DVB Measurement Guidelines (ETR 290) the preconditions for a
PCR_Error message are set as follows (abbreviated designations in parentheses
are the text displayed in the monitoring report):
PCR ERROR is signaled if
the difference of two consecutive PCR values of a program is > 100 ms and no
discontinuity is indicated in the optional adaptation field (DISCONTINUITY)
the interval between two packets with PCR values of a program is > 40 ms
(UPPER DISTANCE)
The above mentioned intervals are given in the DVB Measurement Guidelines
(ETR 290) and are preset in the MPEG test decoder, but they can also be
modified according to individual requirements in the MONITORING / LIMITS
menu (refer to MONITORING / LIMITS Menu on page 4–15).
= ( system clock frequency * t(i) DIV 300 ) % 2
( system clock frequency * t(i) DIV 1 ) % 300
= ( PCR base (i) * 300 ) + PCR extension (i)
Measurement Functions
33
or
2–13