At the National Show in Chicago last month, the big cable television convention, the major subject was HDTV. The minor subjects were HDTV, HDTV, HDTV, and HDTV.
In programming, there were announcements of new HDTV channels and even HDTV video on demand (VOD) to complement the many existing and previously announced HDTV services. There were new HDTV set-top boxes, some of them incorporating TiVo-like HDTV functions. There was HDTV headend equipment and there were HDTV business plans. And then there was Pace’s new Digital Cable Adapter (DCA).
Of all the media that can be used to deliver HDTV to homes, cable seems the most appropriate. U.S. markets are limited to somewhere between one and a couple dozen broadcast digital television channels each. Those channels are limited to about 19 million bits per second (Mbps) each, and MPEG-2 encoding must be used (for the primary video service) for transmission. MPEG-2 seems to need a minimum of about 12Mbps for HDTV (some would say that’s too low), so broadcasters can’t offer more than one HDTV program at a time, even without taking reception problems into account.
Satellite can offer much more HDTV programming than can terrestrial broadcast (in any one market), but it will have a problem if it ends up eventually having to carry more than 1,700 local digital broadcasters, each consuming 19Mbps. Spot-beam technology can help, but it’s not fine enough to distinguish between such tightly packed, broadcaster-filled northeast markets as Boston, Hartford-New Haven, New York, Philadelphia, Baltimore, and Washington, DC.
Cable need never deliver non-local digital television broadcast programming, gets roughly 39Mbps into the same size channel limited to 19Mbps for broadcast, isn’t restricted to MPEG-2 compression technology, and has considerable transmission capacity. That is, it has considerable transmission capacity—but also many demands on it.
HDTV might require 4-6 times as much capacity as standard definition programming, but that pales in capacity to other bandwidth hogs—VOD, cable modems, and cable telephony—to name three. Of course, those are easily juggled as part of a business plan. If HDTV brings in more money, devote more capacity to HDTV. If it’s cable modems, send capacity that way.
The biggest cable-capacity hog, unfortunately, is none of the above. It’s plain analog TV.
Any cable system can offer at least two HDTV programs in a single 6-MHz channel. Ten or 12 standard definition programs can fit into that same channel; 24 have been demonstrated. Or, for analog TV delivered by cable, viewers can get just one program from a 6-MHz channel.
The analog service might not look as good as the digital, and it offers many fewer channels, but it has a big advantage for many subscribers. Outside of certain systems that have always required set-box boxes, viewers can plug the analog cable directly into their TV sets and VCRs. If a cable company delivers 80 analog channels, that’s 480MHz of bandwidth—equivalent to about 260 simultaneous HDTV programs at 12Mbps each, not counting advantages of non-MPEG-2 coding and statistical multiplexing. Even at a raw 18Mbps per HDTV program, 80 analog TV channels could carry 172 simultaneous HDTV programs, with room to spare. In every other aspect of television, HDTV consumes more capacity than analog; in cable, it’s less.
That’s what made Pace’s DCA so exciting. For a reported cost of just $70, it’ll connect a consumer’s ordinary analog TV and VCR (or two TVs) to digital cable. It’s so simple to install that subscribers are expected to do it themselves. It’s got a bypass mode that will pass analog signals through until the transition date. It’ll work with existing TV and VCR remote controls, and it’ll even deliver stereo to a home audio system.
It won’t deliver HDTV—at least not directly. But, if by eliminating analog transmission, it allows 80 existing analog channels to be squeezed into even as many as eight (a relatively trivial digital cable feat), then it will free up enough space for roughly 150-230 new HDTV programming services.
That’s quite a few more than have been announced to date.
Mark Schubin is an engineering consultant with a diverse range of clients, from the Metropolitan Opera to Sesame Workshop.
