For years, broadcasters have told consumers seeking the best high-definition picture quality to use an antenna to receive signals over-the-air, instead of relying on a cable or satellite operator to deliver the programming.
That's because many multichannel operators have recompressed, or “rate-shaped,” broadcasters' HD video to reduce the bandwidth needed to pass along the signal. For example, a cable operator might receive an 18 megabit-per-second HD stream at the headend and recompress it to 15 Mbps before passing it down the pipe. So, receiving the signal directly over-the-air would ensure that the viewer got exactly what the station transmitted.
But increasingly it is local broadcasters, not multichannel operators, that are degrading their hi-def picture quality. The culprit is multicasting, i.e., transmitting digital subchannels alongside the primary HD stream within a station's 19.4 Mbps digital TV pipe. While such subchannels are finally starting to gain traction and generate meaningful revenues (see Station to Station, June 22), many are doing so at the expense of the high-definition images that were the primary impetus for the DTV standard. Engineers and HD aficionados note, with considerable irony, that while there is far more HD content available today, the relative picture quality may not be as good as the first HD broadcasts more than a decade ago.
Multicasting has long been a part of the digital TV system. Most engineers say it is perfectly feasible to simultaneously transmit one high-quality HD stream alongside one or two standard-definition streams, particularly for ABC and Fox affiliates that use the 720-line progressive HD format instead of the more bandwidth-intensive 1080-line interlace format favored by CBS and NBC. But some stations are now broadcasting three subchannels alongside their main HD feed. Others have gone as far as multiplexing two high-definition feeds, along with a third standard-def feed, in their DTV channel.
Encoder vendors may have their own guidelines for how many bits a given picture format should receive to maintain quality. But it is up to individual stations to decide how hard to crank the compression dial.
“Everybody has a particular picture quality in their brain that they think is acceptable,” says Matthew Goldman, VP of technology for Tandberg. “It's a gray area. Some customers are adamant: 'It has to be pristine.' Some will say, 'You know what, this is still really good quality, and we need to do this to provide multiple services.”
The pressure on picture quality is bound to increase later this year when some stations launch mobile DTV services to be received by cellphones and other portable devices. Engineers estimate these services will consume 3.5 to 6 Mbps of the DTV pipe, due to the robust forward-error-correction technology used in the proposed ATSC-M/H standard. So, stations that want to offer mobile streams will look to minimize their HD bitrate as much as possible.
Live and let live
Perhaps the best example of the current HD squeeze is Live Well HD, a new high-definition, lifestyle-focused subchannel launched by the 10 ABC owned-and-operated stations in late April. ABC O&Os like WABC New York had already been transmitting two 480i standard-def subchannels—generally a local news channel and an AccuWeather channel—alongside their primary 720p HD stream, with no major impact to HD picture quality. But when ABC stations replaced the news channel with Live Well HD and began transmitting it as a second 720p HD program stream at the same time they were broadcasting National Basketball Association playoff games, the primary HD picture suffered significantly.
Viewer complaints flooded into stations as well as enthusiast Websites such as the AV Science Forum, where HD experts accused ABC of destroying its HD picture quality and derided both HD streams as looking no better than widescreen SD.
Since then, ABC engineers have tweaked the compression parameters on their Harris NetVX encoders, devoting more bits to the primary HD channel and reducing the bits that Live Well HD gets. From watching WABC, these changes appear to have eliminated major problems on the primary ABC program stream. But Live Well HD is still subject to frequent compression artifacts. Moreover, many HD purists say ABC's HD primary service is now significantly “softer” than the HD fare from CBS, NBC and Fox.
According to Dave Converse, VP of engineering for the ABC station group, the way the NetVX encoders are now set across the group gives the primary ABC channel an average of 11 Mbps, peaking as high as 14, while Live Well HD averages around 6.5 Mbps. The Local AccuWeather Channel is delivered at an average bitrate of 1.5 Mbps; it is hard-coded at a constant bitrate at some stations and statistically multiplexed with the two HDs at others. Converse believes that setup delivers good picture quality for the primary feed and still gives enough bits to Live Well, whose “talking head”-type material isn't as demanding.
“We have a set of rules, and No. 1 is 'Do no harm to the D-1 channel,'” Converse says. “We think we've accomplished that.”
While the ABC-owned stations are the first big-market outlets to multiplex two HDs, a number of small-market stations have already taken the leap. Usually, this involves a Big Four affiliate picking up a secondary affiliation, such as a CBS affiliate adding a CW or MyNetwork TV affiliation that didn't previously exist in the market. These stations have generally multiplexed two 720p streams because doing 1080i streams isn't really feasible, even with the latest MPEG-2 encoders.
Vendors say there are a few examples of a station transmitting both a 1080i and a 720p. One is KXII, Gray Television's station in Sherman, Texas, which has been using Harmonic encoders since 2006 to simultaneously broadcast CBS programming in 1080i, Fox in 720p and MyNetwork TV as a 480i SD stream. The three streams are stat-muxed together, with the 1080i CBS stream typically getting the priority when it comes to bits.
“Sometimes when there's a lot of activity going on, such as sports against sports, you may see a little bit of fighting for bits,” says Dennis Kite, KXII co-chief engineer. “But overall, it works out real well during normal programming.”
KXII probably doesn't have any room left to transmit mobile DTV programming. But parent Gray, like many other large station groups, is a big believer in mobile DTV's potential. And vendors say they are already fielding inquiries from customers about how they might incorporate ATSC-M/H streams into their existing multiplexes. In that vein, Harmonic's new Electra 8000 encoder already supports ATSC-M/H encoding alongside HD and SD encoding, all within the same box.
How stations will balance HD picture quality versus existing SD subchannels and new services like ATSC-M/H remains to be seen. The answer will likely be decided by whatever attracts the largest number of viewers, many of whom frankly aren't as discerning over picture quality as the HD buffs weighing in on the AV Science Forum.
According to John Mailhot, technology architect for Harris' infrastructure and networking business unit, “The fundamental challenge for a TV broadcaster is, 'I've got a 6 MHz channel—now how do I find a business model that works for me to make money with that channel?'”
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