(Broadcast Engineering via NewsEdge) As we move further into the world of digital television, the spinmeisters are hard at work playing up HDTV as a major drawing card for terrestrial broadcasters, cable system operators and direct broadcast satellite services. It's true that there is more and more HD content available with each passing month. Major networks ABC, CBS, FOX, NBC, PBS and the soon-to-be-joined WB and UPN all offer film-style and live programs in the 1080i and 720p formats. On the cable and DBS side, you can choose from Discovery HD, ESPN HD, NBA TV, HBO, HDNet, Showtime, Starz, TNT and many other services.
All this is well and good, except there's a small matter of the digital pipeline from the source of the programming (network) and the viewer. That pipe has a fixed, measurable capacity that can't be exceeded. Thanks to MPEG-2 digital compression technology, it's possible to send HD programming down the pipe with excellent picture quality.
But the glass is half full here. One can also say that, thanks to MPEG-2 digital compression technology, it's possible to send HD programming down the pipe with mediocre image quality.
Some HDTV programming being offered since 1999 has been outstanding, such as CBS' 1080i telecast of the 2003 Grammys, NBC's 2006 Winter Olympics footage, and ABC and ESPN's coverage of the 2006 World Cup.
But also left much to be desired, such as ABC's early attempts to simulcast Monday Night Football in 720p and 480i.
If you are engaged in the production of HD program content, or will be, then you ought to pay close attention to just how many ways your artistic vision can and will be compromised along the way.
The digital shoebox
The concept of signal compression and multicasting, sending multiple program streams in one "channel," is always an attention-grabbing topic.
The size of the channel varies from one content provider to another, as well as the signal modulation method. Terrestrial broadcasters transmit DTV programs in a 6MHz channel using 8VSB, while cable companies employ two flavors of quadrature amplitude modulation (QAM) in that same space.
DIRECTV and Dish Network use much larger channels -- 24MHz and 36MHz - in combination with yet another modulation system, quadrature phase-shift keying (QPSK). In the terrestrial broadcast system, the maximum data rate is 19.39Mbps, though in the real world, the ceiling is closer to 18Mbps. At this data rate, a 1920x1080 HD program encoded in a 4:2:0 color space has been packed down by a factor of 55:1, while a 1280x720 HD show is delivered with 49:1 compression. These are certainly practical compression factors with good-quality MPEG-2 coding.
But what happens if the broadcaster decides not to fill the channel with a single HD program‾ What if instead the decision is made to send out two or more programs in a multicast, as many TV stations do around the country‾
Take the case of WPVI-DT in Philadelphia. The local ABC O&O TV station bit stream consists of three programs: 6-1 is the HD program stream; 6-2 is standard-definition version "talking heads" programming; and 6-3 carries a 24/7 weather service.
To jam all of this into the 18MHz terrestrial shoebox means something has got to give.
That something is the bit rate for the HD programs. Typically, that means that 6-1 may now be dropped to 14Mbps, while 6-2 chugs along at about 2.8Mbps, and whatever table scraps are left over go to the 6-3 weather channel, or around 1.3Mbps.
How is picture quality affected‾ For starters, any 720p HD programming on 6-1 has now been packed down by 68:1. The programs on 6-2 shouldn't suffer as much as 3Mbps is probably the lower limit of what an SD program can withstand - or close to the average bit rate of a DVD. As for the weather channel, it's mostly static graphics with a quarter-resolution video window.
Pack and ship
The cable and DBS worlds aren't immune to this issue either. By using 64-QAM and 256-QAM digital modulation, cable system operators can choose from maximum data rates of 27.7Mbps or 38.8Mbps, respectively. With some judicious bit-rate compression, it is possible to put 10 standard-definition HD programs, each with 2.7Mbps data rates, in a single 64-QAM channel. Two off-air HD broadcasts and a pair of SDTV programs can be packed into a 38.8Mbps 256-QAM payload.
Satellite services have a bigger problem in that their transponders are expensive to lease or own. So they also have an incentive to cut the bit rate and keep costs down by offering more program streams in their 27Mbps channels.
For instance, HBO transmits its 1080i HD movies and live programs at 15Mbps, a 27% reduction from the optimum terrestrial bit rate of 18Mbps, making it easier for satellite and cable systems to add other channels. And there is evidence that data rate may dip as low as 12Mbps by the time it gets to your living room. 1080i HD is compressed 83:1 at 12Mbps, while 720p is packed down by 74:1.
One possible way to get around the problem is to use a technique called statistical multiplexing, or variable bit-rate encoding. It requires the MPEG encoder to constantly look at all of its program streams, borrowing bits from one program and giving them to another as needed. The programs that, at any given instant, don't have much motion in them lose their bits to the programs that do.
In the WPVI example, the weather service on 6-3 will probably get whacked the most, though with a base data rate of less than 2Mbps, it doesn't have much to spare. So the tug-of-war takes place between the HD and SD programs. Statistical multiplexing is not an easy trick to pull off. An HD program, with spinning graphics, camera zooms and pans, plenty of motion, and fast picture refresh rates, needs lots of bits. If another program is simulcasting the same action and camera moves, who wins and who loses‾
A way out‾
As the transition to digital TV continues, cable companies and satellite operators can and will groom the bit rate of their signals to conserve bandwidth. While two 18Mbps HD programs can fit into one 256-QAM carrier, so can three HD programs at 12.5Mbps. Two such programs can also dovetail nicely into a satellite 27Mbps transponder.
Will the resulting HD images look as good‾ No. Will many viewers notice‾ Probably not, if they are watching on a smaller HDTV set with screen sizes under 34in.
Instead, most of the howls will come from viewers who have invested in large flat-panel and rear-projection TVs, all of which have a higher native resolution than picture tubes.
One possible solution to the issue is to use a more advanced compression system, such as MPEG-4. This format allows for much lower bit rates, but it has longer groups of pictures and is not part of the terrestrial digital TV broadcast standard.
Another way to solve the problem is to preserve bandwidth for the HD programs and not multicast during certain times of the day. None of CBS' O&O stations are multicasting, and its 1080i HD shows go out nominally at 18Mbps. Some FOX O&O stations carry one 720p program with bit rates from 12Mbps to 14Mbps.
Even PBS stations are experimenting with minimum practical bit rates. Philadelphia PBS affiliate WHYY-TV currently transmits one digital minor channel with 1080i content at 14Mbps, allocating 4Mbps to a second SD channel. Earlier experiments with lower bit rates showed diminished image quality with some programming.
More than meets the eye
The quality of MPEG encoding and the chosen bit rate make the difference between "That's quite a bit better than SDTV" and "Holy Cow!" Both the new HD-DVD and Blu-ray formats will face the exact same problems of balancing bit rate, bandwidth (disc capacity) and cost.
The increasing popularity of large-screen TVs using flat-panel and microdisplay technology will only magnify any encoding flaws. Once viewers see for themselves just how good HDTV can look, they won't settle for less. Will program providers and content distributors rise to the challenge of quality or choose the expedient route of cost cutting in an attempt to add more channels and sell more advertising‾ Only time will tell.
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