With increasing frequency, we are required to become familiar with a new term, and sometimes this requires unlearning an older meaning of that term, or one that is similar. For example, those of us in the television industry used to use the term “white areas,” or something similar, to describe those few small areas in the United States that are not served by any broadcast television signal. Those areas probably still exist, but we hear few references to them in recent years.
We have recently heard many references to “white spaces,” but in this case, white spaces refer to the segments of the broadcast television spectrum in a given geographic area that are not occupied by television signals: the vacant channels.
IT’S TABOO
The reasons for the unoccupied television channels have been with us as long as television itself. As television engineers, most of us are well familiar with the reasons that, for example, there is no NTSC television signal on Channel 4 in Philadelphia or in Hartford, Conn. The reason is these markets are too close to New York, Boston and Washington to avoid interfering with Channel 4 signals in those cities. For the same reason, New York, Boston, and Washington do not have signals on Channel 3, but Philadelphia and Hartford do.
In the UHF spectrum, particularly the higher UHF channels, signals do not travel as far as VHF signals, but there were also the NTSC “taboo channels” in a geographic area. These channels were vacant of NTSC signals because the intermodulation between signals on two other occupied UHF channels generates products that fall on top of the “victim” or “taboo” channel. This taboo channel problem does not appear to be as serious when we are considering DTV-to-DTV interference as it is in NTSC-to-NTSC interference.
Indeed, when we look at the occupied television channels in large markets such as New York or Los Angeles—the two largest markets today—we note that so many channels are now occupied with both analog and digital signals that there are not many white spaces to be found in large swaths of the UHF spectrum, where many adjacent channels are occupied. This is, of course, not true in the VHF spectrum. Additionally, in television broadcast, there is licensed use of unoccupied UHF channels in various markets for wireless microphone signals.
There is a coalition of companies in various computer and online businesses that wish the FCC to legalize the use of unlicensed devices that are intended to operate in the white space spectrum, to be used for wireless Internet connectivity and other such applications. The argument is that these devices, using current technologies, are capable of detecting the absence of television signals on unoccupied channels and only operating on such vacant channels; and that they therefore will not cause interference to DTV signals.
Members of the coalition—which includes Google, Microsoft and Intel, among others—recently provided prototypes of such devices to the FCC for testing, and the FCC reported that their test results show that the devices at least some of the time failed to detect active television signals, and had thereby caused interference. At the request of the coalition and some public interest groups, new prototypes of white spaces devices will in the near future be provided for retesting.
PROVE IT
This is an important issue for terrestrial broadcasters, who are largely against the legalization of such devices absent incontrovertible proof that they will not cause interference to DTV signals. One of their fears is that if a number of such unlicensed devices, particularly portable ones, get into the field, they can never be recalled, even if they prove harmful to DTV reception. Nor are broadcasters alone. The cable industry is likewise concerned about ingress of these signals into cables, and they have pointed out that cable has no white spaces: Every available channel is used.
If these devices do cause interference to DTV signals, the result of releasing them into the marketplace will in many ways be more problematic to DTV signals than would such interference to analog signals, as noisy analog signals are often considered viewable, particularly when the interfering noise appears to be random, as digital signals probably would be. The effects of interference on DTV signals, however, range from pixilation and freezing of the picture and/or interference to the sound signals, all the way to the complete loss of the video and/or audio signals altogether.
Broadcasters are particularly concerned about the effect of the possible loss of television reception white spaces devices might precipitate once the off-air television viewer is restricted solely to digital signals. They are urging the FCC to exercise great caution as they consider whether to permit such uses of the television broadcast spectrum.
