(5) PMT_error: For each program, a Program Map Table (PMT) is transmitted at maximum intervals of 500 ms.
The PIDs of the MAPs are listed in the PAT. The PMT contains the respective PIDs of all elementary streams
belonging to this program. If a PMT referred to in the PAT is missing, the set top box or decoder will be
unable to demultiplex or decode them. If a PMT is listed in the PAT but is either missing, errored or
scrambled, will lead to the error message “PMT_error”.
A “PMT error” occurs when
•
A PMT listed in the PAT is missing,
•
A section of the PMT is not repeated after 500 ms or less,
•
A PMT is scrambled,
•
The table ID is not 2.
(6) PID_error: The PIDs of all elementary streams of a given program are contained in the associated
program map table (PMT). The PIDs are pointers to the elementary streams: they are used to access to
the corresponding packets of the elementary stream to be decoded. If a PID is listed in a PMT but not
contained in the transport stream, the decoder will be unable to access the corresponding elementary
stream.
A “PID_error” occurs when
•
Transport stream packets with a PID referred to in a PMT but not contained in the transport
stream or
•
Their repetition rate exceeds a user-definable limit which is usually 500 ms or less.
9-3.2 Priority 2 Parameters
(1) Transport_error: Every MPEG-2 transport stream packet contains a bit called Transport Error Indicator
which is transmitted right after the sync byte. This bit flags any errors in the transport stream packets at
the receiver. During the transmission, bit errors may occur for various reasons. If error protection (at least
Reed Solomon in DVB and ATSC) is no longer able to repair all errors in a packet, this bit is set indicating
this packet can no longer be utilized by the MPEG-2 decoder and must be discarded.
A transport_error occurs when:
•
The transport error indicator bit in the TS header is set to 1.
(2) CRC_error: During transmission, all tables in the MPEG-2 transport stream, whether they are PSI tables or
other private tables according to DVB (SI tables) or according to ATSC (PSIP tables), are protected by a
CRC checksum. It is 32 bits long and transmitted at the end of each sector. Each sector, which can be
composed of many transport stream packets, is thus additionally protected. A CRC error has occurred if
these checksums don’t match the content of the actual section of the respective table. The MPEG-2
decoder must then discard this table content and wait for this section to be repeated.
A CRC_error occurs when:
•
A table (PAT, PMT, CAT, NIT,...) in a section has a wrong checksum which doesn’t match its
content.
(3) PCR_repetition_error: The PCRs are used to re-generate the local 27 MHz system clock. If the PCRs are
not transmitted with sufficient regularity, then this clock may jitter or drift. The receiver/decoder may
even go out of lock. In DVB, a repetition period of not more than 100 ms is permitted, previously a max of
40ms was recommended.
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 Header Optional adaptation field, or
•
The time interval between two packets with PCR values of a program is more than 40 ms.
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