This years NAB demonstrated that our march towards DTV continues. Some might argue that is too strong a verb, but most would certainly agree that is not
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This years NAB demonstrated that our march towards DTV continues. Some might argue that “march” is too strong a verb, but most would certainly agree that “crawl” is not strong enough. Now that the majority of stations have a DTV presence, many DTV facilities are beginning to evolve beyond the minimally required ATSC stream. One area that is receiving increased attention is the collection of “tables” carried in the ATSC stream known as Program and System Information Protocol, or PSIP. (See Figure 1.)

PSIP is intended to provide three important services for the broadcaster and viewer. The first is to preserve a station's branding as they move from NTSC to ATSC. There is a table that is part of the PSIP data called the Virtual Channel Table (VCT). This table's purpose is to list the DTV's virtual channels and link them to the DTV channel's analog equivalent. Thus, the VCT contains a major channel number, which is the current NTSC channel, and one or more minor channel numbers, which list the virtual DTV channels implemented via ATSC in the actual DTV channel. The linkage that VCT provides is intended to preserve branding across a station's analog and digital spectrum. This helps to maintain the current channel branding because DTV receivers will electronically associate the two channels, making it easy for viewers to tune to the DTV station even if they do not know the channel number. This allows the viewer, via the set-top box, to navigate between a station's current analog and its various DTV channels or services. PSIP identifies the associated major and minor channel numbers and indicates to the receiver whether multiple program channels are being broadcast and, if so, how to find them. In addition, the protocol identifies whether the programs are closed captioned and if data is associated with the program, and conveys V-chip information.

VCT actually exists in two versions: one for terrestrial and a second one for cable applications. The existence of a cable version probably causes some concern among many broadcasters as it implies cable will be tearing apart terrestrial multichannel ATSC streams and reassembling them based on the cable operator's agenda.

A second aspect that PSIP brings to the DTV table is an aid to channel navigation: the electronic program guide (EPG). This is accomplished via at least four Event Information Tables (EITs), which list TV programs (events) for the virtual channels described in the VCT. An STB, so designed, can use EIT information for actual and virtual channel tuning. The VCT and EITs can work in concert to provide this service via another PSIP table, the Master Guide Table (MGT). The MGT defines sizes, PIDs and version numbers for all of the relevant tables that comprise PSIP.

Since we're mentioning PSIP tables, let's discuss the other five tables that comprise PSIP as well (see Table 1):

System Time Table (STT) carries time information needed for any application requiring synchronization.

Rating Region Table (RRT) defines rating tables valid for different regions or countries.

Extended Text Table (ETTs) carries longer text messages for describing events and virtual channels. Each EIT has a flag that indicates whether there is an associated ETT, as does the VCT.

Data Event Table (DET) announces the data portion of a video/audio/data event.

Directed Channel Change Table (DCCT) instructs the receiver to change channels based on viewer preferences, demographics or geographical location. This table from the broadcaster works in conjunction with a Directed Channel Change Selection Code Table in the STB. This table defines the classification scheme used by viewers to express preferences during receiver setup.

DTV also allows the announcement of data-enhanced services via the PSIP tables. Thus, via the program guide, the viewer would see which DTV program streams were enhanced with additional data, such as stats for sports programming or Web page content to supplement the audio and video streams. Additionally, PSIP can announce the existence of stand-alone data. It can be used to indicate such things as times when computer software or electronic versions of the local newspaper are available for downloading.

The basic goal of PSIP is that it be simple enough that every receiver can implement at least a rudimentary use of the data available via PSIP. The ATSC also wanted PSIP to be extensible for higher end products, and to present a small change in tuning concepts for the viewer. Some broadcasters have already found that incorrect implementation of PSIP can render some or all of their NTSC and DTV services unavailable to DTV receivers. Mis-programming of PSIP data has even allowed DTV stations to inadvertently “hijack” virtual channels of other DTV stations in early DTV receivers. Along those lines an interesting ability of PSIP is to tie the virtual channels of separate DTV physical channels together. In the DTV realm duopolies will be able to combine the two stations seamlessly under a single brand.

PSIP, although not explicitly required by the FCC, is an ATSC requirement. Most DTVs to date have met this requirement with static information. At this most simplistic operational level, VCT and MGT tables are manually entered into the ATSC mux and forgotten. This allows rudimentary branding by telling the STB the NTSC and DTV channel association. But if STBs are ever to display worthwhile electronic program guides, all the aforementioned tables will have to be filled by the broadcaster, and at an ever-changing rate. Dynamic PSIP requires a separate subsystem, either a separate stand-alone box or modules added to a vendor's mux that accept a constantly updated data stream — from companies such as Tribune Media Services, or organizations such as the PBS National Database, and from traffic and even automation systems — to continually update the PSIP tables.

Like most things in life, there is a lot of differentiation between static and dynamic PSIP. Some vendors offer various stages of dynamic PSIP. It is possible to implement systems that only dynamically update the EIT tables. The basic recommended program schedule presented to the viewer is 12 hours. That requires four EITs. There are offerings that allow you to begin with these four, and progressively add tables to allow your presented EPG to be extended out to cover multiple days. This a la carte approach allows ETTs to be added when desired, to elaborate on the basic program guide presented via the EITs. Then only DCCT need be added, enabling directed channel changes to provide full PSIP capability.

This year's NAB offered a selection of dynamic PSIP solutions, including Digital Vision's PSIP generator, which allows the broadcaster to define its analog and digital services, and then import program-listing data as a single operation. Additional features include compression of textual descriptions, support of caption and multiple audio services, and maturity rating advisory codes. The new Directed Channel Change (DCC) specification is implemented to support switching between HD and multicast modes. Manual editing and live updating of event information allow the operator to handle last-minute changes in schedule. The generator formats a full complement of MPEG and PSIP tables with descriptors and outputs these in real time as a transport stream via a DVB ASI port, suitable for connection to most third-party multiplexers.

Harris also offers a dynamic PSIP solution, the PSIPplus, which allows automatic importation of data from listing services, traffic/automation systems and other databases. It is compatible with Flexicoder and Unicoder encoding systems. The system incorporates proprietary versions of Triveni's technology. Both companies allow multiple levels of PSIP generation. Harris and Triveni refer to the first step up from Static PSIP as Light PSIP. This approach uses four dynamic EITs, as opposed to static, non-changing EITs for Static PSIP. The next level up is Basic PSIP, which provides 24 EITs and 24 ETTs to provide full program information for 72 hours. Harris provides two products at the Basic PSIP level. The PSIPplus Basic product allows input of program information through a manual user interface. The PSIPplus TMS product allows automatic import of programming information from a listing service. Full implementation of PSIP is available using the PSIPplus Pro, which provides all tables needed for receiver tuning, full channel and program descriptions, and an EPG with programming data populated for up to a full 16 days.

Thales offers the PEARL Editor, which provides a GUI to edit parameters in the PSIP configuration and display playlists. It stores and retrieves configuration and accepts incremental changes via an XML file interface. The PEARL Scheduler deals with the dynamic aspect of PSIP: It generates all the table updates, whether those updates occur naturally (e.g. EIT shift every three hours) or result from a configuration change. The PEARL Output Module stores, packetizes and carrousels all the PSIP tables into a compliant ATSC transport stream. Thales also supports Directed Channel Change. A single PEARL system can generate and cross-carry the information for several streams simultaneously.

Triveni offered its PSIP bit stream generators, GuideBuilder and GuideBuilder Pro, which automatically convert program information from program listing services or traffic systems to the ATSC A/65 PSIP format. This information is then fed to a multiplexer in a digital broadcast environment. Triveni is another vendor that offers a series of migration steps from static to dynamic PSIP. They offer Lite, Basic (Editing and Listing) and Pro versions of GuideBuilder. Their Basic PSIP offerings are the GuideBuilder Editing version that allows input of programming information through a manual user interface, and the GuideBuilder Listing version that allows automatic import of programming information from a listing service. From this level a user can move up to full PSIP with GuideBuilder Pro.

PSIP vendors like to point out that the broadcaster who implements full PSIP now will face a few calls from viewers as the new service shakes out. But broadcasters that wait to implement full PSIP until a much wider rollout of DTV receivers could face a magnitude of calls from viewers complaining about the lack of EPG content or startup errors that occur with most new services. A logical step for STBs down the road will be integration in the receiver of all the receivable PSIPs in an area into a single EPG. At that point the DTV station with full PSIP implementation will fare better than the one using only the basic tables.

Jim Boston is a West Coast consultant.