Program-specific information (PSI) is metadata about a program (channel) and part of an MPEG transport stream.
The PSI data as defined by ISO/IEC 13818-1 (MPEG-2 Part 1: Systems) includes four tables:
- PAT (Program Association Table)
- CAT (Conditional Access Table)
- PMT (Program Map Table)
- NIT (Network Information Table)
PSI is carried in the form of a table structure. Each table structure is broken into sections. Each section can span multiple transport stream packets. On the other hand, a transport stream packet can also contain multiple sections with same PID. Adaptation field also occurs in TS packets carrying PSI data. The PSI data will never be scrambled so that the decoder at the receiving end can easily identify the properties of the stream.
The sections comprising the PAT and CAT tables are associated with predefined PIDs (Program Identifier) as explained in their respective descriptions below. There may be multiple independent PMT sections in a stream; each section is given a unique user-defined PID and maps a program number to the metadata describing that program and the streams within it. PMT section PIDs are defined in the PAT, and are the only PIDs defined there. The streams themselves are contained in PES packets with user-defined PIDs specified in the PMT.
PSI structure
Table Sections
Descriptor
PAT (Program association specific data)
The PAT is assigned PID 0x0000 and table id of 0x00. The transport stream contains at least one or more TS packets with PID 0x0000. Some of these consecutive packets form the PAT. At the decoder side the PSI section filter listens to the incoming TS packets. After the filter identifies the PAT table they assemble the packet and decode it. A PAT has information about all the programs contained in the TS. The PAT contains information showing the association of Program Map Table PID and Program Number. The PAT should end with a 32-bit CRC
PMT (Program map specific data)
This table contains PID numbers of elementary streams associated with the program and it has information about the type of these elementary streams (video, audio, etc.). In addition it may also contain an ECM (entitlement control messages) stream for any other stream that is encrypted. These messages provide the information used in the cipher key selection stage.
CAT (Conditional access specific data)
- Table ID value is 0x01.
This table is used for conditional access management of the cypher keys used for decryption of restricted streams. This table contains privately defined descriptors of the system used and the associated EMM PID. It is used by a network provider to maintain regular key updates.
NIT (network information specific data)
This optional table may group transport stream identifiers into a network, providing access parameters and other details. ITU-T Rec. H.222 and ISO/IEC 13818-1 do not define the NIT structure. The European Broadcasting Union DVB specification ETSI EN 300 468 (DVB-SI) does. The purpose of mentioning but not defining this table in H.222 is that it has reserved program number 0 (zero) in the PAT. The Table ID extension is used to identify the local network together with a directory listing of transport streams. Descriptors are used to list the modulation, source of those streams and programs. The original network identifier is meant to allow transport streams and programs on foreign networks to be included in the local network which allows no remapping of transport and program IDs that may be duplicated between networks. The DVB specification defines the transport packet identifier as 16 and the table identifier of the local network of transports as 64. A table identifier of 65 is for a foreign network of transports. The network identifiers are maintained via DVB Services who have separated the identifiers into two unique, yet unnecessary groupings of Network_ID and Original_Network_ID. The two groupings have a large number of overlapping entries.
PSI labels
Table Identifiers
Each table in a transport stream is identified by an 8-bit table identifier.
SCTE Specific tables:
0xCO table id is used by Programme Information Message
0xC1 table id is used by Programme Name Message
It is not necessary that pid of PMT pointed by PAT will contain a table with table id 0x02 Ignoring table id while reading PMT could have bad consequence
Program Element Descriptor Tags
Each descriptor in a transport stream table is identified by an 8-bit descriptor tag.
Elementary stream types
Each elementary stream in a transport stream is identified by an 8-bit elementary stream type assignment.
References
External links
- tektronix Poster [1]
