Tuesday, October 28, 2008

Program and System Information Protocol (PSIP)

The Program and System Information Protocol (PSIP), also known as A/65, describes specific channel and program event information in terrestrial and cable transport streams. The current time, channel name (station ID), virtual major-minor channel mappings, parental ratings, and program schedule are some of the information that PSIP carries.

PSIP tables:
  • System Time Table (SST) - GPS time, UTC leap second offset, daylight savings time info
  • Master Guide Table (MGT) - general information about the other tables; PID mapping
  • Virtual Channel Table (VCT) - channel name, major-minor channel numbers, other stream components
  • Rating Region Table (RRT) - static codes for geographical ratings such as MPAA in the U.S.
  • Event Information Table (EIT) - event times and program names like a TV guide
  • Extended Text Table (ETT) - optional text messages (channel info, program descriptions, ...)
  • Directed Channel Change Table (DCCT) - optional perform an automated channel retune
  • Directed Channel Change Selection Code Table (DCCSCT) - optional genre codes and criteria for DCCT

The STT uses a system time is a 32-bit unsigned integer number of GPS seconds since 00:00:00 UTC, January 6th 1980. The UTC leap second offset is a correction to the GPS time. The daylight savings time structure contains information of the current status and when the next transition is to occur.

The VCT comes in both Terrestrial (TVCT) and Cable (CVCT) versions. CVCT uses two parameters that TVCT doesn't have; Path Select and Out of Band (OOB). The Path Select is for the rare case of having two physical cables. The Out of Band channel is a cable-operator supplied data channel that requires a dedicated OOB tuner. The OOB channel has supplemental PSIP information such as program guides and a navigation map. Also, depending on the carrier, the CVCT modulation field data may be in error and 8-VSB, 64-QAM, and 256-QAM may be encountered. Note that CVCT (cable) is not required by the FCC to carry the first 4 EITs which contain the next 12 hours of program guides.

The PSIP text strings use Huffman data compression to reduce the size which lowers the transmission bandwidth requirements.

An automatic scan can go through all the possible channels retrieving and storing the appropriate channel name and major-minor channel identification data. A standard 6 MHz analog channel can have between 19.4 Mbps (8-VSB) to 38.8 Mbps (256-QAM) of bandwidth which is enough for several HD and SD channels. PSIP helps the digital TV and the consumer manage all of this channel information.


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