During the NAB Show, FCC Chairman Genachowski announced that the FCC would convene an "Engineers Forum" to address technical issues raised by the National Broadband Plan (NBP) and "develop the next path forward." Until yesterday, I hadn't seen any public announcements concerning the Forum or the topics to be covered.
On Wednesday the FCC announced [PDF] details about the forum and the topics to be discussed. The list will be familiar to anyone who has seen the recommendations in the NBP, and they are clearly focused on ways in which the FCC can achieve its goal of taking what amounts to almost half of the usable TV broadcast spectrum remaining after the DTV transition a year ago. (This excludes land-mobile, channel 37 and channels 2 through 6.)
One topic is "Cellularization of Broadcast Architecture," which is has been proposed as a way to allow channels to be reused without the normal spacing normally required to avoid interference between high-power single site transmitters. While this technology has some promise, it wouldn't solve the problem of the insufficient number of channels available in larger markets, and would require a huge build-out of towers, microwave or fiber links, and transmitters, along with extremely complex engineering to avoid interference to existing receivers, particularly in markets with terrain issues such as Los Angeles and New York.
Another group will look at "Methodologies for Repacking the TV Band."
As difficult as it was to move TV stations from channels 52-69 to new slots last June, most stations operating at cutover time then had been authorized two channels. In many cases if they couldn't stay on their pre-transition DTV channel, they could use an existing analog antenna and perhaps convert an existing analog transmitter for DTV broadcasting on their old analog channel.
However, if stations were forced to transition again, things would be more difficult for many of them, and it would be a lot more difficult for stations in the most populated parts of the county.
Stations moving from UHF to VHF would need new antennas, new transmitters, and maybe new towers, although in some markets if celluarization can be made to work, a single large antenna, transmitter and tower could be replaced with 20 or more (perhaps many more, depending on the area and power) small to medium power transmitters, antennas and towers.
All of the channels that would be reallocated to wireless carriers are UHF, which means VHF channels, including low-band VHF, may be the only option for broadcasters not willing to give up their own off-air broadcasting. It isn't surprising then that one forum group is devoted to "Improvements in VHF Reception." However, the disadvantages of VHF channels, even high-band channels for DTV, became clear after the analog shutdown. As I've been reporting, advances in metamaterial antennas could improve receive antenna efficiency and reduce some of the VHF disadvantage on high-band channels, but it won't help if noise from switching power supplies, TV sets, motors and power lines overwhelms the DTV signal. Increasing transmitter power could raise other interference issues due to the enhanced propagation that can take place at lower VHF frequencies.
Another way to minimize the impact of taking away such a large block of broadcast TV spectrum is to use the remaining spectrum more efficiently. Stations that are not interested in multicast, mobile DTV or high quality, fast action HDTV might be willing to cash in their channel assignment and share a 19.39 Mbps channel with another TV station. Moving U.S. broadcasting from MPEG-2 to MPEG-4 (AVC) compression technology would free up data bandwidth, but would obsolete every existing DTV set and DTV converter box.
And what if stations want to hold onto that "found" data bandwidth for their mobile DTV operations?
Stations currently broadcasting an HD channel and one or two SD channels may be interested in partnering with a station that isn't using as much data bandwidth in order to add more mobile DTV channels and services. The question remains: Would Mobile DTV be able to compete with Verizon, AT&T and the other wireless carriers for this spectrum?
The engineers participating in these groups won't have an easy time finding a way to eliminate almost half the currently usable TV broadcast spectrum.
The FCC is not allowing public access to the working groups, but on Friday, June 25, the reports from the groups will be available online. The reports are limited to 30 minutes per group. Before the reports, there will be a short welcome and overview at 3 p.m. EDT.
Doug Lung is one of America's foremost authorities on broadcast RF technology. He has been with NBC since 1985 and is currently vice president of broadcast technology for NBC/Telemundo stations.
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