One of Divicom's authors. Mark Magee.............................
tvbroadcast.com
Multiplexed Transmissions
Up to this point, we've been concentrating on options for a single program stream feeding one channel. For a more detailed view of required and optional data service options at the back end of a DTV plant, let's look at some possibilities offered by Mark Magee of Divicom in a paper presented at the SMPTE Advanced Imaging Conference in Toronto. The full paper is on the Web at www.divi.com (click "technology").
Data Multiplexing
Magee's minimal system diagram (see Figure 6) begins at the encoder with a single channel NTSC analog or 480i digital component signal. The encoder is feeding an SD signal into the system. Its output is multiplexed with required Program and System Information Protocol (PSIP) data from the PSIP Server and--possibly--optional information from a Data Server. The PSIP information has been defined by the ATSC to help the receiver's decoder navigate through the multiple video, audio and data services that coexist in DTV channels. While much of the PSIP information may come from an affiliated broadcast network or an outside service, it will have to be modified and updated to reflect local conditions.
"Initially," said Magee, "PSIP could be supported by a standalone computer, but at some point, the PSIP server should be tied into the station's automation computer".
The data server in this scenario is optional, but with single channel video and audio occupying only a fraction of the 19.3 Mbps ATSC payload, there is plenty of room for experimental broadcast data services.
Multiple Channels
This simple, single-channel arrangement can be scaled up in modular fashion to accommodate multiple SD channels and HD. If the original encoder and multiplexer were chosen with future expansion in mind, additional encoders and other data sources can be added without replacing previous purchases. No changes are required at the transmitter or in the STLs. Figure 7 shows a mixed HD, SD expansion integrated with station automation.
In this example, the primary SD and HD channels are fed from separate master controls and the secondary SD services receive their feeds from a simplified multichannel switcher. One for one redundancy is provided for the multiplexer, PSIP server, data server and primary SD channel encoder. For the secondary services, one backup encoder serves three active channels. HD isn't backed up. (Of course, redundancy depends on your comfort level and your budget. Likewise, the number of channels and whether or not HD is included depends on your business plan.)
In a future evolution, the separate SD and HD Master Controls would probably merge into a single, multi-channel control room. The individual HD and SD routers could become a single 1.5 Gbps unit carrying a mixture of compressed and uncompressed signals. Or, it might be a narrower bandwidth unit in the 270 Mbps to 540 Mbps range, capable of routing only compressed HD along with compressed and uncompressed SD. Switched computer networks such as Fiber Channel or ATM are also options.
While acknowledging that multiple routing schemes will probably coexist in future plants, Magee said he believes video routing and control systems will borrow heavily from computer networking. "The economies of scale of that industry produce an irresistible force," he stated. "Perhaps even the venerable 75 ohm cable will fall victim at some point."
Statistical Multiplexing
Multi-channel scenarios lend themselves to statistical multiplexing (statmuxing). It improves video quality by managing bit rates in the various channels based on need at the moment. Rather than having a fixed bit allocation for each channel, a statistical multiplexer can dynamically assign rates: fewer bits to the channel(s) with easily compressed material, more to the one(s) with difficult scenes.
Of course, if all channels are showing car chases at the same instant, there may not be much spare bandwidth to share, but that's usually not the case. There's bound to be a still graphic or a talking head somewhere. Depending on the encoders and multiplexers selected, adding
statmuxing may be as simple loading a software option.
The Value Of Bandwidth
Statmuxing plays directly into Mike D'Amore's concept of "the value of bandwidth." He remarked that "At the affiliate level, stations will learn that bandwidth has value."
If you simply simulcast one channel of SD, your DTV bandwidth has a zero or even a negative value (aside from protecting the license). "If I invest money and get little back, I have very little value." he said.
For D'Amore, "statmux[ing] and dynamic bandwidth management are the goal to [shoot] for." They represent an "optimum bandwidth utilization" in which the number of bits used is based on program content. "Transmit HD when it makes sense," he added. "When you're not doing HD, transmit multiple SD channels and/or more data services."
As Sarnoff Labs demonstrated during a display of a BMW commercial at NAB a few years back, there are openings for "opportunistic data" even when HD is theoretically filling the channel. Bits can be diverted from video to data services as the demands of video content vary.
Some Parting Thoughts
As complicated as all this seems, broadcasters can take comfort in the knowledge that, as long as their design supports their business plan, it'll be hard to make a serious mistake. Equipment manufacturers seem committed to producing gear for all of the most requested formats. As Panasonic's Livingston related, "We're format agnostic."
The DTV compliance requirement for receivers to decode all 18 of the ATSC video formats means that viewers will be able to see whatever broadcasters transmit. Certainly in the early years, the consumer will be hard pressed to notice the difference between 720p, 1080i or upconverted images.
And everyone agrees that receivers will improve. Livingston cautioned that we shouldn't use today's CRTs to judge the ultimate quality of HDTV. Remember, the capability to transmit 1080 progressive lurks out there at the high end of the quality scale. Progressive scan will probably have some advantage when progressive displays become affordable and as-yet-undesigned consumer devices begin to scale and recomposite images from multiple sources.
But it all goes back to the business plan. As Sony's Lude advised, "All systems have to be designed holistically." Broadcasters need to weigh such factors as application, quality, latency and price--as they relate to their specific station. It's important to talk to those broadcasters that are already getting their digital feet wet. Make friends at the Model Station and others who will go on the air early. Remember, copying someone's good idea is the sincerest form of flattery.
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