Current Online
Multicasting—the practical engine that’s driving public TV’s digital transition
Adapted from Current, April 22, 2002
By Steve Behrens

For the media cognoscenti who can't wait for the next paradigm to shift, multicasting is an unexciting option for digital TV, not nearly as cool as the interactive enhancements now in R&D and certainly lacking the wow factor of high-definition pictures.

It's just more TV—a chance to air two, four or more digital channels at once in spectrum that that could previously accommodate only a single analog channel.

Yet multicasting has become the workhorse that's pulling the public TV's transition to digital transmission. It's the best selling point for viewers, who don't have HD sets yet; for legislators and foundations, which see its practical potential and dismiss HD as "pretty pictures"; and for cable companies, which want to stock their digital tiers with channels of diverse appeal.

While few public TV stations have decided exactly how to use their multicast channels, they are developing schools of thought. Many are pledging to use at least one channel to aid formal education—in Texas, a partial channel that would bring limited Internet-like services to rural areas. Others are planning local and regional services, including public affairs coverage. A few anticipate specialized channels for the arts or for Spanish speakers, in the model of cable TV's narrowcast channels. And more than a few think the best use of the spectrum would be repeating the best material from public TV's main channel, so that more people will see it. See the links to additional articles at right.

Multicasting has been part of public TV's digital dream for years, with system leaders predicting they will divide the 19.4 Megabits-per-second (Mbps) bitstream into four standard-definition (SD) channels during the daytime and switching to a single HD channel at night.

Allocating DTV bitstream

Long-discussed model:
Daytime: 4 standard-def channels
Primetime: 1 HD

Alternative planned by some stations:
Daytime: 4 SD channels
Primetime: 1 SD + 1 HD

It may be premature to count on HD, some station executives believe, however. Few Americans have displays capable of handling HD, while multicasting is becoming a real service, with cable operators in various cities adding multicast public TV channels to their digital-tier menus. Though some stations are keeping HD largely for the wow factor, others are delaying an operational HD service.

At night, if they switch to HD at all, stations in Dallas, Seattle and elsewhere are limiting its bit rate so they can squeeze in an SD channel at the same time.

How many channels?

It's not just technical and esthetic considerations that will determine how many program streams will go out on a DTV channel. The FCC and cable companies will have their effects.

The FCC's simulcast rule, for one thing, will tend to knock down the number of distinct streams a digital station can transmit by requiring the station to simulcast its old analog channel on its digital channel. As of April 1, 2003, a DTV station will have to air half of the programming from its analog channel on its digital channel. In April 2004, the share rises to 75 percent, and in April 2005 to 100 percent.

That would mean one less distinct multicast stream if the rule takes effect. Broadcasters' lobbyists may argue to postpone or eliminate it.

APTS, for instance, will look at the value of the simulcast rule, "seeing whether or not it makes sense, given stations' current service plans," says Marilyn Mohrman-Gillis, v.p.

Cable and DBS companies will be a greater factor in determining whether Americans ever see multicast services, because three-quarters of households now get their TV via cable or DBS. Since the FCC has declined to give must-carry protection to DTV multicasting channels, cablers can reject all but one of the program streams from a public DTV station. Conversely, they could choose to carry more channels than are broadcast over the air if those channels entice subscribers to upgrade.

By dealing directly with cable operators, stations can avoid the duplication encouraged by the FCC simulcast rule. A station that feeds its programming directly to cable operators by wire can deliver a different selection, says Lewis Zager, v.p. of technology at WETA. It can make sure, for instance, that none of its multicast channels is a redundant simulcast of the analog channel already carried by the cable system. The cable operator not only would appreciate the differentiation but might also require it.

Def jam

Of course, it's advances in the obscure art of digital compression that will decide just how much capacity a DTV channel has.

Those advances have fallen short for some broadcasters who expected to be cramming five or six SD channels into a broadcast channel by now. So far, you can't get more than four decent-looking broadcast programs into a channel, says Bruce Jacobs, chief technology officer at Twin Cities PTV. If you try five, the result is "scummy," he says. "It looks like there's grunge around the edges of things."

Others stretch capacity further. Louisiana Public Broadcasting squeezes three SD channels and an 11 Mbps HD program into one channel, says network engineering chief Frank Kleinpeter.

Further improvements in compression are inevitable, according to Zager at WETA. How quickly they come depends on the demand for better compression and how much money manufacturers spend on R&D.

In a demonsration videotape made by Oregon Public Broadcasting to show differences in compression quality, a viewer can't easily distinguish a full-bitstream signal of 19.4 Mbps from a signal compressed to 8 Mbps.

Squeezing the program further to 4 Mbps yields acceptable pictures if the subject moves very little, but a scene of birds flying breaks up into a pixelated frenzy and even the edges of a color chart begin to blur. At 2 Mbps, a talking head is still of passable quality, but the image of flying birds disintegrates. At 1 Mbps, even a still color chart and a talking head are spoiled by jittery artifacts.

In practice, digital broadcasters can squeeze in much more quality by constantly reallocating bitstream to the images as required—giving 6 Mbps to a fast-moving picture and just 2 Mbps to a static one, says Don McKay, an Oregon Public Broadcasting engineering exec. The process, called statistical multiplexing or "statmux," allows DBS operators to cram in TV channels at an average of perhaps 3 Mbps each.

Within quality constraints, broadcasters have great leeway in allocating their bitstreams. They may want to air three SD channels for reception by TV sets and 10 microchannels of low quality that would be picked up by computers instead, says tech consultant Steven Vedro. In instructional programs, the video image might occupy only a fraction of the screen and PowerPoint graphics could fill the rest.

If pubcasters are developing a new consensus on the matter, it may be that high-def doesn't need the full 19.4 Mbps bitstream. Stations in Dallas, Seattle and Orange County, Calif., are airing an SD program along with their HD programs in the evening.

In Seattle, KCTS will always simulcast its analog channel even when airing an HD program, says Cliff Anderson, director of broadcast operations and engineering. The station gives 3.5 Mbps to SD and a little more than 13 to HD (slightly compressed from the PBS feed, which weighs in at 18 Mbps).

KERA in Dallas likewise has decided it can have its cake and eat it, too, says Rick Owen, chief technology officer. The bitrate allocations in Dallas are similar to Seattle's.

In Orange County, Calif., KOCE President Mel Rogers highlighted the importance of its planned local multicast channel, Real Orange TV, by announcing last month that it will operate 24 hours a day, even during HD broadcasts.

What to put on the channels?

When pubcasters first heard they might soon have four multicast channels to fill, some were stunned by the challenge of delivering so much programming. But now they have more channel ideas than channels, and some of those notions are not only doable but done.

Since 1999, PBS has provided a PBS Kids feed and the adult-ed channel PBS You, both initially packaged for DBS but also offered for multicasting. Annenberg/CPB also supplies a full-time educational channel. At least a couple stations are already packaging specialty channels for local cable operators that could be built into national feeds for multicasting. And numerous stations have ambitious plans to originate local and regional channels.

Not far over the horizon is the day multicast airtime will be so fully packed that programmers will gaze upon a multidimensional common-carriage schedule as complex as a Rubik's Cube and struggle to squeeze in another play of To the Manor Born.

What's all this about multicasting?

Overview: What is it, and how many channels does it add to public TV?

Repeats: The rationalists' preference for additional channels

Education: An offer that fits the mission and public policy

Online access: How Texas stations aim to bring new media to the outback

Local and regional programs: Hopes for coverage that's often squeezed out of public TV today

Specialized genres: Replicating the model that works for cable

To Current's home page
Current Briefing on DTV.
Earlier article: APTS analyzes stations' multicasting plans, 2001.
Outside link: What KAET in Phoenix tells viewers about multicasting.

Web page posted Aug. 6, 2002
The newspaper about public television and radio
in the United States
A service of Current Publishing Committee, Takoma Park, Md.
Copyright 2002