What Happens to that Big, Fat, HD Signal When It's Telecast?
It’s certainly true that there is more and more HD content available with each passing month. Of the major networks, CBS, NBC, ABC, PBS, WB, and UPN all offer several filmed prime-time programs in the 1080i and 720p formats as well as live sports and special events programming. On the cable and DBS side, you can choose from Discovery HD, HDNet, HBO, Showtime, ESPN HD, NBA TV, and many other services.
All 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 (like CBS’ 1080i telecast of the 2003 Grammys and HBO’s 2001 Bruce Springsteen concert). Unfortunately, some of it has left much to be desired, such as a 720p HD telecast of Monday Night Football I watched at RCA’s corporate headquarters during Cedia Expo this past September.
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 be compromised along the way. Read on, and find out!
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. One 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 does the signal modulation method. Terrestrial broadcasters are sending out DTV programs in a 6 MHz channel using eight-level vestigial sideband (8VSB), while cable companies employ two flavors of quadrature amplitude modulation (QAM) in that same space. DirecTV and Dish use much larger channels (24 and 36 MHz) 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 19.39 megabits per second (Mb/s), although for practical purposes, the ceiling is 18 Mb/s.
At this data rate, a 1920à—1080 HD program encoded in a 4:2:0 color space has been packed down by a factor of 55:1, while a 1280à—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-owned-and-operated TV station. The WPVI bitstream consists of three programs: 6-1 is the HD program stream, 6-2 is a standard-definition version of 6-1, and 6-3 carries 24/7 Doppler radar.
To jam all of this into the 18 MHz shoebox means something’s gotta give, and that’s the bit rate for the HD programs. Typically, that means that 6-1 may now be dropped to 13.5 Mb/s, while 6-2 chugs along at 3.5 Mb/s and whatever table scraps are left over go to the 6-3 weather channel (just over 1 Mb/s).
What has that done to image quality? For starters, any 720p HD programming on 6-1 has now been packed down by at least 65:1. The programs on 6-2 shouldn’t suffer as much, because 3.5 Mb/s- close to the average bit rate of a DVD- is probably the lower limit of what an SD program can withstand. As for the weather channel, how much data do you need for a fixed graphic with a rotating line?
The added compression on the HD channel means the MPEG encoder must work harder to preserve image quality. And that’s not always a given, as several members of the press saw back in September at RCA’s press dinner.
The local ABC affiliate, WRTV, was testing different MPEG encoder configurations, and the quality of the Redskins/Jets game was simply awful- plenty of pixel-blocking artifacts were seen, even with static images. You can imagine how bad it looked on RCA’s new 61-inch DLP rear-projection TV set (1280à—720 native resolution), as no MPEG encoding flaw escaped our eyes.
Pack and Ship
The cable and DBS worlds aren’t immune to this sort of thing, either. Thanks to the use of 64-QAM and 256-QAM digital modulation, cable system operators can choose from data rates of 27.7 Mb/s and 38.8 Mb/s, respectively.
With some judicious bit-rate compression, it is possible to put 10 standard-definition programs in a single 256-QAM channel. Pull out the ol’ calculator, and you’ll see that two off-air HD broadcasts and a pair of SDTV programs can also be packed into that 38.8 Mb/s payload.
Satellite services have a bigger problem in that their "channels" (transponders) are quite 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 27 Mb/s shoeboxes.
One popular network, HBO, transmits its HD movies and live programs at 15 Mb/s- a 27 percent reduction from the optimum terrestrial bit rate of 18 Mb/s- making it easier for satellite and cable systems to add other channels. And there is evidence that data rate may further dip as low as 12 Mb/s by the time it gets to your living room. (1080i HD is compressed 83:1 at 12 Mb/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 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.)
A Juggling Act
One possible way to get around the problem is to use a technique called statistical multiplexing. That’s a ten-dollar 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 Doppler radar program on 6-3 will probably get whacked the most, although with a base data rate less than 2 Mb/s second, 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 (like Monday Night Football) needs lots of bits. If the competing program is the standard-definition version of MNF that is simulcasting the same action and camera moves, who wins- and who loses?
The analogy is like an adult and a child fighting over a kid’s meal at a fast food restaurant. Each will get something out of the bag, but it’s a sure bet neither will be satisfied with the bits and pieces they managed to grab, and both will still be hungry afterwards.
Less is More?
Multicasting in itself is not a bad idea. Another digital station in my market (WLVT) uses its 6 MHz channel to simultaneously transmit four different PBS standard-definition programs 24/7. HDTV may be in the picture down the road, but right now they can address more viewers with everything from kid’s shows to political and community affairs programs.
In this case, each SDTV stream on WLVT gets about 4.5 Mb/s, which makes for a nice 480i picture. Another local DTV station, WFMZ, sends out two SD program streams, with 69-1 made up of their usual local news, infomercials, and reruns of older shows, and 69-2 carrying a local (and extremely useful) AccuWeather forecasting service.
The local Fox digital station (WTXF) sets aside 8 Mb/s of its available data stream to send out 480p/30 SDTV programming. That’s a nice speed for this format (think of progressive-scan DVDs at a constant bit rate), although 10 Mb/s would be even better. While it allows for other SD programs to be simulcast down the road, Fox has indicated it will move to the 720p HD format toward the end of 2004- and the data rate will have to go up!
Here’s an even more unusual example. Religious broadcaster WFMZ in West Orange, NJ, transmits 10 minor channels as part of its digital broadcast. The first is SDTV video. The second (last time I checked) was a graphic that asked any viewers who were picking up the digital signal to call the station and let them know. As for minor channels 3 through 10, they were (and continue to be) FM stations from around the USA with no video.
No Way Out?
As the transition to digital TV continues, cable companies and satellite operators can and will groom the bit rate of any signals to conserve bandwidth. While two 18 Mb/s HD programs can fit into one 256 QAM carrier, three HD programs at 12.5 Mb/s can also fit into this slot. Two such programs could also dovetail nicely into a satellite 27 Mb/s transponder.
Would the resulting HD images look as good? Nope. Would many viewers notice? Probably not on a smaller HDTV set (screen sizes under 34 inches). Instead, most of the howls would come from viewers who have invested in large flat-panel and rear-projection TVs, all of which have higher native resolution than picture tubes.
One possible solution to the digital shoebox paradox is to use a more advanced compression system like MPEG-4. This format allows for much lower bit rates, but is quite a bit more complicated to encode and is not part of the terrestrial digital TV broadcast standard.
Nevertheless, one new satellite service- VOOM- claims to use MPEG-4 encoding and decoding for 39 channels of HDTV (I’ll believe that when I see it!). For a 1080i program, MPEG-4 can get by with about 9 Mb/s. As we saw earlier, a typical transponder channel has a maximum bit rate of 27 Mb/s. So, VOOM would (in theory) need only 13 transponder channels to send out those 39 HD programs.
Another way to solve the problem is to preserve bandwidth for the HD programs and not multicast during certain times of the day. No CBS-owned-and-operated stations are multicasting, and their 1080i shows go out at near 18 Mb/s. The same can be said (for now) with NBC-owned-and-operated stations.
Even some PBS stations are looking more at preserving bit rates. The Philadelphia PBS affiliate (WHYY) currently sends out one minor channel with 1080i content, having given up on an earlier experiment where a second minor channel was in use to send out classical music.
As a means of comparison, you can see just how good 1080i HD really looks if you have access to JVC’s D-Theater playback system and a few tapes. This digital VHS format has a working data rate of about 25 Mb/s, and there have been several feature films released in this format. That’s 40 percent higher than terrestrial HDTV, and it shows when watching films with fast action sequences.
The new edition of Digital Video Essentials, developed by Joe Kane Productions, 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
Like everything else in life, HDTV should come with a disclaimer. The quality of MPEG encoding and the chosen bit rate are what make the difference between "That’s quite a bit better than SDTV" and "Holy Cow!" All of the new HD-DVD formats being proposed will face the exact same problems- bit rate vs. bandwidth (disc capacity) vs. cost.
The increasing popularity of large-screen TVs using flat-panel and microdisplay technology will only serve to magnify any encoding flaws in HDTV programs. Trust me, 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? Only time will tell.