Although proper implementation of the DTV Program and System Information Protocol (better known as PSIP) at the television-station level is not particularly complex, neither is it straightforward. Broadcasters have voiced their need for simplified information regarding the implementation of PSIP Standard A/65A. To this end, the ATSC is developing a draft Recommended Practice to explain the operator-oriented elements of PSIP and to provide practical examples of typical station operation. The draft document will also provide guidelines that should allow designers of PSIP-related hardware and software to prepare clear operating instructions for the users of such equipment.
PSIP is the glue that holds the digital television signal together. Although PSIP is a voluntary standard of the ATSC, and the FCC only requires parts of the standard, it is, in fact, a requirement in terms of actual real-world operation. In most locations, multiple DTV stations can be received — in some cases, from multiple markets. The PSIP protocol was developed with these real-world situations in mind.
PSIP is a small collection of tables designed to operate within every transport stream (TS) for terrestrial broadcast of digital television. Its purpose is to describe the information at the system and event levels for all virtual channels carried in a particular TS. Additionally, it may incorporate information for analog channels as well as digital channels from other transport streams.
There are two main categories of information in the ATSC PSIP Standard: system information and program data. System information allows navigation and access of the channels within the DTV transport stream, and program data provide the information necessary for efficient browsing and event selection. Some tables announce future events and some locate the digital streams that make up an event. The PSIP data are carried via a collection of hierarchically arranged tables. Figure 1 shows the primary components and the notation used to describe them. The base tables are:
- System Time Table (STT)
- Rating Region Table (RRT)
- Master Guide Table (MGT)
- Virtual Channel Table (VCT)
The Event Information Tables (EITs) are a second set of tables whose packet identifiers are defined in the MGT. The Extended Text Tables (ETTs) are a third set of tables and, similarly, their PIDs are defined in the MGT.
The System Time Table is a small data structure that fits in one transport stream packet and serves as a reference for time-of-day functions. Receivers can use this table to manage various operations and scheduled events, as well as to display the time of day.
The Rating Region Table transmits the rating system in use for each country using the ratings. Provisions have been made for multi-country systems.
The Master Guide Table provides indexing information for the other tables that comprise the PSIP Standard. It also defines table sizes necessary for memory allocation during decoding, defines version numbers to identify those tables that need to be updated and generates the packet identifiers that label the tables.
The Virtual Channel Table, also referred to as the Terrestrial VCT (TVCT), contains a list of all the channels that are or will be online, along with their attributes. Among the attributes are the channel name and number. This table is critically important as it contains the set of data that enables a receiver to tune and locate the service being broadcast. The VCT is essentially a list containing information about each service that a broadcaster creates (or has announced that it will create) within the DTV major channel assignment, as well as information about the broadcaster's associated analog channel.
There are several Event Information Tables, each of which describes the events or television programs associated with the virtual channels listed in the VCT. Each EIT is valid for a time interval of three hours. Because the maximum number of EITs is 128, up to 16 days of programming may be advertised in advance. At minimum, the first four EITs must always be present in every transport stream, and 24 are recommended.
Basic PSIP requirements
The three main tables (VCT, EIT, STT) contain information that makes it easy for suitably equipped receivers to find the components required to present a program (event). Although receivers are expected to use stored information to speed channel acquisition, sometimes parameters must change. The VCT is the table that must be accurate each instant because it provides the actual connection path. If nothing has changed since an EIT was sent for an event, the anticipatory use of the data is expected to proceed. When there is a change, the new parts would be used. These relationships — and the tables that carry them — are designed to be kept with the DTV signal when it is carried by a cable system.
There are certain “must have” items and “must do” rules of operation. If the PSIP elements are missing or wrong, there may be severe consequences, depending on the type of receiver. The following are key elements that must be set and/or checked by each station:
Transport Stream Identification (TSID)
The station must set the TSID correctly in all three locations (PAT, VCT common information, and virtual-channel-specific information).
System Time Table (SST)
The station should check the STT time daily and lock it to house time.
Short Channel Name
This is a seven-character name that the station can set to any desired virtual-channel name. For example, WNABSD1, KNABSD2, WNAB-HD, KIDS, etc.
In most cases, the previously assigned, paired NTSC channel is the major channel number.
The service type selects DTV, NTSC, audio only, data, etc., and must be set as operating modes require.
This parameter must be set appropriately.
The Source ID is a number that associates virtual channels to events on those channels. Typically, it is automatically updated by PSIP equipment or updated from an outside vendor.
Service Location Descriptor (SLD)
The PIDs identified here and in the PMT must be the same for the elements of an event/program. Some deployed systems require separate manual setup, but PIDs assigned to a VC should not change (unless there is a good reason).
It is recommended that broadcasters set up a minimal set of three days of tables. The recommended table cycle times result in a minimal demand on overall system bandwidth.
Most common mistakes
Experience has shown that certain errors are common in many PSIP implementations. Typically, these problems include the following:
- Missing tables, specifically the STT and EIT.
- Major channel number set to the DTV RF channel number, rather than the associated (legacy) NTSC channel number.
- TSID set to 0 or 1, the NTSC TSID, or to another station's TSID; or it is not the same in the three required places.
- System time missing or set to 00:00:00 on 1/6/1980
Some receivers react to these errors by not tuning to that station.
About the VCT
Broadcaster must input essential station-specific VCT information to allow viewers to tune programs properly.
Because the VCT also allows each minor channel to be assigned a permanent short name and channel name, and since each minor channel will keep the same TSID, carrier frequency (zero or not), and modulation mode over time, the PSIP encoder-system software should allow the user to create a local look-up table that associates each minor-channel number with these fixed values. This allows the user to create new VCTs simply by entering the number of the minor channel he or she wants to include in the new VCT. The PIDs for each minor channel should not be changed unless the nature of the virtual channel changes, because it will likely increase the time it takes for the receiver to tune to the station.
Even though the station can permanently assign the TSID and other parameters for each virtual channel, the minor channels that the station is using may change over time. When a program on a new virtual channel is announced in the EIT, the PSIP standard requires that the VCT contain EIT VCT information, and vice versa. Because of this, it is recommended that broadcasters update the VCT first to reflect a change in the channel lineup and then use the appropriate source_id to construct the EITs.
A broadcaster can transmit a new VCT containing updated information at any time by increasing the version_number by one. However, since a VCT normally describes the channels in the same transport stream, if virtual channels are added to the VCT at arbitrary times they will not be detected by the receiver until it is tuned to that particular transport stream. For this reason, it is highly recommended that broadcasters add channels in advance (with the program_number set to 0) to give receivers the opportunity to scan the frequencies and detect the channel presence. This is one reason for programming three days of EITs for transmission. The system design assumes the receivers scan all RF channels at least once just after being turned off. For sets that are never turned off or that experience extended power outages, filling three days worth of EITs once a day should reduce the risk of not having information at the time of tuning.
To underscore the PSIP implementation challenge facing broadcasters, WHD-TV — the model station project sponsored jointly by CEA, NAB and MSTV — has released a report on current PSIP implementation issues. The report, dated Nov. 14, 2001, can be obtained from the MSTV Web site at www.mstv.org.
Jerry Whitaker is technical director of the Advanced Television Systems Committee, Washington, DC. The author wishes to acknowledge the work of the ATSC Specialist Group on Data Multiplex/Transport, which developed the draft Recommended Practice upon which this article is based.