(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.