Continuity, noun, 1. an uninterrupted connection or union [ant : discontinuity] 2. a detailed script used in making a film in order to avoid discontinuities from shot to shot 3. the property of a continuous and connected period of time [syn : persistence].
For decades, the broadcast television business has operated under the assumption that the primary business of a television station is to deliver a continuous programming stream. The goal is to capture the interest of a viewer, and then to hold that interest across the discontinuities that are inevitable with both advertiser-supported- and non-commercial television. Those discontinuities may include: commercials and promotional announcements, fund-raising drives, the changing menu of programs throughout the broadcast day, and unscheduled interruptions for breaking news and emergency warnings.
Three decades ago, TV remote controls were uncommon. Channel surfing required getting up off the sofa and turning a mechanical tuner to choose from a handful of stations. This was the golden age of network television, when families gathered in front of the TV and watched one station all evening based on the popularity of an anchor program.
Decades later, the primary program stream is still king, and continuity is as important as ever, at least to those who manage TV operations. Many stations have made the operational transition from analog to digital, virtually ignoring the fact that the new digital world has little in common with the analog world they once dominated.
Today, most broadcasters manage their stations as if they were still broadcasting to that captured '70s audience. But channel surfing is now an uncontrollable epidemic. The discontinuity of a commercial makes those remote controls light up, with no guarantee that the viewer will come back after the break. And dare I even mention TiVo?
With DVDs, NVOD, VOD and Personal Video Recorders (PVRs), the viewers are becoming the program directors, watching what they want, when they want, with no regard for continuity.
Baseband or splicing?
With digital television, the equivalent of an analog vertical interval switch becomes a complex compression management task. Gone are the days of virtually instant channel change. With DTV, a channel change can take several seconds depending on the design of the decoder.
The reality of MPEG-2, MPEG-4/AVC and other interframe encoding techniques is that one cannot simply switch between two MPEG elementary streams at random points. I-frame access points may only occur every 15 to 30 frames. Two operational philosophies now exist to manage digital operations and continuity.
One approach is to handle master control operations using baseband (uncompressed) digital streams. Continuity is managed in exactly the same manner as in the good old days, as vertical interval switching is still possible with baseband digital signals. The output of the digital master control switcher is then fed to a real-time MPEG-2 encoder.
The other approach is based on the splicing of MPEG streams to maintain continuity. Splicing is possible as long as the streams use the same format and access points are managed properly. It is important to note that when the video format changes — for example, when an SD program is followed by an HD program — continuity is lost using both of these approaches, as consumer decoders must lock up to the new format.
Set-top master control
Both of these approaches are patches that are unfortunately necessary because the most desirable solution was not economically practical with first-generation implementations of digital decoders and set-top boxes. The proper place to handle traditional master control functions is in the consumer decoder/set-top box, which is rapidly becoming a miniaturized TV station, complete with video server, mixer and downstream keyer.
I recently subscribed to the digital HD tier of Cox Cable, opting for a set-top box with HD PVR capabilities. The Scientific-Atlanta Explorer 8300 HD box includes a large-capacity hard disk drive, two HD-capable MPEG-2 decoders, an SD-quality MPEG-2 encoder to convert analog tier programs to digital for recording and sophisticated local image processing capabilities. The box handles a wide range of interformat conversions, allowing it to simultaneously output both SD and HD in a choice of formats. It offers picture-in-picture, a variety of aspect ratio accommodation modes and generates high-quality on-screen graphics, keying them properly over any video format.
This box, and many others with integrated PVR functionality, has virtually everything needed to become a miniature TV station. Standards, such as MPEG-4, already define everything that is needed to handle multistream processing and video composition in a compliant decoder.
Let's consider the enhanced functionality that would result by turning the decoder into the master control switcher:
- Cross dissolves and fadesA fade can easily be handled in a decoder by signaling the box to mix the decoded stream and locally generated black. A dissolve can be handled by using a decoder for each stream, with the mixing taking place after decompressing the sources.
- Switching between multiple eventsIf a station decides to put several programs in its multiplex, there is no way to switch between sub-channels without discontinuities as the decoder acquires a new stream. But, with two decoders, the desired new stream can be opened and switched to with a vertical interval cut or a fade through black. A station could run several commercials during a break and let metadata, such as the decoder location (e.g., zip code), determine which commercial to display. Or during a newscast, the decoder could be instructed to choose the appropriate sub-stream for localized news segments targeted as a station's sub markets.
- Localized video overlayLocal ad tags and other information can be sent as efficient vector graphics and inserted over the video stream in the proper location. A logo bug can be inserted locally in the proper position for both 4:3 and 16:9 displays independent of the source format.
Perhaps the time has come for broadcasters to deal with the future — future where managing discontinuities will be placed in the hands of consumers. Worrying about maintaining the continuity of your primary program may be meaningless, in a world where the name of the game will be convincing the program director — sitting in his or her recliner — to program the TV to capture some of your programming for both live and asynchronous consumption.
Craig Birkmaier is a technology consultant at Pcube labs, and he hosts and moderates the OpenDTV forum.
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