Early last week many papers, including USA Today, carried an Associated Press article describing the battle between broadcasters and cable companies over carriage of broadcast DTV channels. The article, New Digital TV Rules Draw Fight from Cable Guys, does a good job describing the situation, but like many of the discussions on broadcast digital must-carry it misses some technical issues that have a critical impact on both sides of the argument.
When looking at the digital must-carry debate, few discussions mention that whether a DTV station is broadcasting a single high-definition channel or multiple, lower-resolution channels, they are limited to a maximum data rate of 19.39 Mbps and the broadcaster determines how much of that data rate is dedicated for a single channel. Most broadcasters will want to use the majority of their 19.3 Mbps data bandwidth for critical HDTV programming such as the Super Bowl or Olympics. The TV engineers reading this report already know this, but for some reason many non-technical reports on the debate miss this point.
Current FCC digital must-carry rules state that cable companies required to carry a DTV channel are only required to carry the "primary" program service if a DTV station is multicasting multiple channels. The rules also state the cable company cannot degrade the quality of the signal. If a broadcaster chooses to run one channel of HDTV programming (or upconverted SDTV programming) using its entire 19 Mbps data stream, the cable company will need to dedicate close to 19 Mbps of data bandwidth to that station, depending on how you define degradation.
Because digital signals carried over cable do not have to deal with the interference and multipath present with over-the-air reception, they can use higher order (more complex) modulation to carry this 19 Mbps of data using only fraction of the channel space required for one analog TV channel or they can combine multiple 19 Mbps DTV signals into the space occupied by one analog channel. It doesn't matter if the DTV station being carried is broadcasting one 19 Mbps HDTV signal (video and audio) or five standard-definition (SDTV) signals of 3.8 Mbps (video and audio) each, the same amount of data and channel bandwidth to is required to carry all the station's program streams.
Of course, if the broadcasters insisted those five SDTV signals were carried as analog signals, it would have a severe impact on the number of other analog channels a cable system could provide. Alternatively, since the cable company is required to carry only one of the multicast signals in the example above, it could free up 15.2 Mbps of data bandwidth, which it could use for additional digital cable channels. However, if the multicasting DTV station switched to HDTV (perhaps for prime-time programming) using all its data capacity, the cable company would then have to return the 15.2 Mbps of data capacity it had "borrowed" and drop the extra cable channels. Therefore, while the argument that carrying all programs in a DTV stations' multicast service could displace cable channels is correct if the multicast channels are carried in analog format on individual analog channels, the cable company could not count on keeping any digital cable channels gained by carrying only one of the multicast programs.
Even if the technical argument for only carrying one channel of several that a DTV broadcaster may decide to transmit is flawed, the non-technical issues raise real concerns for cable operators. It is possible that some of the programs on the multicast stream duplicate those already on the cable system. Should the cable company be required to carry the same program on two channels, even if one is included in the same data bandwidth a broadcaster might later use for HDTV? It would be hard to argue for cable carriage of encrypted multicast streams that are designed to directly complete with cable companies, like the cable program services being offered on USDTV. (See the January 19 RF Report)
Broadcasters have the opportunity to use multicasting to offer additional local programming - traffic reports, expanded local news, local weather - that would benefit viewers whether they watch TV off-air or over cable. Multicasting is a way to offer alternative programming in different languages or for different ethnic groups.
If the cable industry, broadcasters and the FCC could agree on a system that gives all viewers access to these services, while not requiring cable companies to carry duplicative programming, it would be a win-win situation for everyone involved. Broadcasters, knowing they would have cable carriage and a better chance for a decent return on their investment, would be more likely to launch new multicast programming services. Since these services would not be available in analog format, they would help encourage the DTV transition, an FCC goal, and provide cable viewers an incentive to upgrade from analog to digital cable service, providing more revenue opportunities for cable companies from digital services such as pay-per-view.
