HDTV data multiplexing

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. “Everyone wants HDTV, and we've got [fill in the blank] channels of it! Compelling movies! Exciting sports action! Special programming! Sign up now!”

It goes on and on. It's certainly true that there is more and more HD content available with each passing month. Of the 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 could also say that — thanks to MPEG-2 digital compression technology — it's possible to send HD programming down the pipe with mediocre image quality.

I've watched a lot of HDTV programming since 1999 on a variety of TVs and monitors. Some of that programming 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.

Unfortunately, some of it has 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

I have taught a course on digital television at every InfoComm show since 1998, and I try to cover a gamut of issues in the two-hour time slot. The topic that always grabs everyone's attention is the concept of signal compression and multicasting — sending multiple program streams in one “channel.”

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). A comparison of each modulation system is beyond the scope of this article. But a comparison of the bit rates each can carry is worth a look.

In the terrestrial broadcast system, the maximum data rate is 19.39Mb/s, though in the real world, the ceiling is closer to 18Mb/s. At this data rate, a 1920 × 1080 HD program encoded in a 4:2:0 color space has been packed down by a factor of 55:1, while a 1280 × 720 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 (or, digital shoebox, as I like to call it) 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?

Here's a real-world example. WPVI-DT in my market (Philadelphia) is a local ABC O&O TV station. The WPVI 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's gotta give. That something is the bit rate for the HD programs. Typically, that means that 6-1 may now be dropped to 14Mb/s, while 6-2 chugs along at about 2.8Mb/s, and whatever table scraps are left over go to the 6-3 weather channel (around 1.3Mb/s).

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 3Mb/s is probably the lower limit of what an SD program can withstand (close to the average bit rate of a DVD). As for the weather channel, it's mostly static graphics with a quarter-resolution (360 × 240) 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 27.7Mb/s or 38.8Mb/s, respectively. With some judicious bit-rate compression, it is possible to put 10 standard-definition HD programs, each with 2.7Mb/s data rates, in a single 64-QAM channel. Pull out the ol' calculator, and you'll see that two off-air HD broadcasts and a pair of SDTV programs can be packed into a 38.8Mb/s 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 27Mb/s channels. For instance, HBO transmits its 1080i HD movies and live programs at 15Mb/s — a 27 percent reduction from the optimum terrestrial bit rate of 18Mb/s — making it easier for satellite and cable systems to add other channels. And there is evidence that data rate may dip as low as 12Mb/s by the time it gets to your living room. 1080i HD is compressed 83:1 at 12Mb/s, while 720p is packed down by 74:1.

So, what makes more sense: filling channels with high bit-rate HD programming or adding more channels to the mix and keeping advertisers and subscribers happy? Put yourself in the position of a cable company executive looking at potential revenue streams or a satellite operator facing a stack of bills for transponder space, and you can probably guess the answer to that one.

It's a balancing act

One possible way to get around the problem is to use a technique called statistical multiplexing. That's a $10 word for variable bit-rate encoding. And it requires the MPEG encoder to constantly look at all of its program streams, begging and borrowing (or outright stealing) 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 less than 2Mb/s, 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 (such as Monday Night Football), 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 18Mb/s HD programs can fit into one 256-QAM carrier, so can three HD programs at 12.5Mb/s. Two such programs could also dovetail nicely into a satellite 27Mb/s transponder.

Would the resulting HD images look as good? Nope. Would many viewers notice? Probably not, if they are watching on a smaller HDTV set (screen sizes under 34in). Instead, most of the howls would 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 (GOPs) 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 18Mb/s. Some FOX O&O stations carry one 720p program with bit rates from 12Mb/s to 14Mb/s.

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 14Mb/s, allocating 4Mb/s to a second SD channel. Earlier experiments with lower bit rates showed diminished image quality with some programming.

As a means of comparison, you can see just how good 1080i HD really looks if you have access to Toshiba's new HD-A1 HD DVD player and a demo disc. The live HD content, shot in the HDCAM format, is encoded at a constant bit rate of 36Mb/s — twice that of terrestrial HDTV, and you can see an improvement in picture quality.

Joe Kane Production's “Digital Video Essentials” also has some amazingly detailed 720p and 1080i sequences mastered from both HD tape and 35mm film formats. All of it was edited and encoded at the D-Theater data rate, and the tapes are a good way to work out your HD-ready monitor or integrated HDTV.

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.

Peter Putman is president of ROAM Consulting.