CAPITAL CURRENTS: DTV receiver standards on the way?
DTV receivers are susceptible to IF interference
In April, the FCC released a study of the performance of DTV receivers that were subject to interference from signals on nearby TV channels. While there have never been any FCC performance standards for TV receivers, the ATSC has a Recommended Practice (ATSC A/74) that specifies how receivers should perform. The major conclusion from the FCC study is that most DTV receivers don’t meet those performance objectives.
The FCC has a proceeding underway to allow low-power unlicensed transmitters to be used on unused broadcast channels for Internet access and other communications networks. The FCC has to decide what frequencies can be used in which locations, and what power levels will be permitted, in order to prevent interference to DTV receivers in viewers’ homes. This was the motivation for the interference study.
A few years ago, the FCC adopted a channel allotment plan for assigning DTV channels to existing analog broadcasters. The FCC made fairly tight assumptions about the ability of DTV receivers to reject interference in deciding how far apart the DTV stations would have to be.
In simple terms, the question in this recent study was what happens to TV reception on channel N when there is an interfering signal on the adjacent channel (N±1), or the second adjacent channel (N±2), or the third, and so on, up to N±15. These are the channels that were protected by the analog UHF “taboos” that were designed to protect inexpensive analog TV tuners against interference. The analog UHF taboos are actually spacing requirements; for example, the minimum spacing between analog channels 14 and 21, or any two channels separated by 7, is 95 kilometers. The FCC adopted DTV-to-DTV spacing requirements only for co-channel and first adjacent channel assignment, but decided none were needed for other channel separations. That decision now seems wrong.
The FCC receiver performance assumptions were tighter than the industry guidelines in ATSC A/74. Now, the FCC study says that none of the eight receivers tested even met the loose A/74 guidelines, let alone the tighter FCC assumptions. One possible reason for poor interference rejection is the use of single-conversion tuners. The original Grand Alliance prototype receiver used a double conversion tuner. Cable set-top boxes use double conversion tuners. But all the receivers that the FCC was able to acquire for testing had single conversion tuners.
The analog N+7 separation is intended to protect against interference into the 44 MHz Intermediate Frequency of analog tuners. The measurements of DTV receivers showed that they are susceptible to IF interference, just like analog tuners. It was expected that DTV receivers would be more resistant to interference from a signal on the second adjacent channel (6 to 12 MHz away) than a signal on the first adjacent channel. The measurements showed just the opposite. DTV receivers are more susceptible to interference from signals on the second adjacent channel than the first adjacent channel.
The industry’s A/74 specs are voluntary guidelines for DTV receivers. However, the federal government plans to subsidize off-air set-top converter boxes so that households with analog TV receivers can continue to use them after the DTV transition is completed in 2009. The agency that is administering the distribution of discount coupons for these set-top converter boxes recently released the technical specs that these boxes need to meet in order to qualify for the subsidy; they need to satisfy the A/74 specs. A/74 may be voluntary for DTV receivers, but it is mandatory for the off-air set-top converter boxes.
The FCC has never before adopted receiver performance specifications or interference rejection requirements. It has the authority to do since Senator Goldwater introduced a bill that became Public Law 97-259 in 1982, which added the italicized language below to the Communications Act:
The Commission may, consistent with the public interest, convenience, and necessity, make reasonable regulations establishing minimum performance standards for home electronic equipment and systems to reduce their susceptibility to interference from radiofrequency energy.
That law applies to DTV receivers.
I believe the FCC will impose interference rejection requirements on DTV receivers. These study results suggest that unlicensed communications devices on broadcast frequencies will cause real interference to DTV receivers, at least, those receivers on the market today. The FCC’s original expectations were that only the co-channel frequencies needed protection; that is, communications devices need be barred from only the channels actually used by broadcast stations, but could operate on any other channels. This study shows that, in addition, at least the first adjacent channels, the second adjacent channels and the N+7 channel must be protected. That means that for every DTV station in a town, at least six TV channels are taboo. That doesn’t leave much for unlicensed devices – unless, of course, DTV receivers are improved to the point where they are no longer susceptible to interference from transmitters on those channels. And that’s where the receiver standards come into firstname.lastname@example.org