How Hard is it To Receive DTV? Part II

Last month, I looked at the technical arguments in the debate over how to determine whether a household can receive a local network affiliate's DTV signal and thus is not allowed to receive a distant network DTV signal via satellite. This month, I'll finish the discussion with more details on the debate and reply comments from Meintel, Sgrignoli and Wallace (MSW) on behalf of NAB.
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Last month, I looked at the technical arguments in the debate over how to determine whether a household can receive a local network affiliate's DTV signal and thus is not allowed to receive a distant network DTV signal via satellite. This month, I'll finish the discussion with more details on the debate and reply comments from Meintel, Sgrignoli and Wallace (MSW) on behalf of NAB.

Much of last month's column was devoted to the technical arguments made by Hammett & Edison (H&E) supporting EchoStar's assertion that the FCC DTV planning factors and methodology used to determine whether a household received local analog TV service, are not appropriate for determining DTV service.

In its filing, H&E argued that reduced signal due to antenna pointing must be considered, and that much stronger signal levels are required for reception using indoor antennas.

MSW responded that without the use of directional outdoor antennas, there would have been "no way to give all NTSC stations an extra 6 MHz channel without causing excessive interference to each qualifying television station during the transition." MSW noted an inconsistency in EchoStar's argument, that a correctly pointed outdoor antenna should not be required for a household to be considered as one receiving service from a local DTV TV station.

ANTENNA ARGUMENTS

"As pointed out in our initial Engineering Statement, outdoor directional antennas that are properly and carefully pointed provide better and more reliable performance for both terrestrial DTV and DBS services," MSW told the FCC. "Since DBS antennas must be properly mounted outside, it is reasonable to assume that antennas for terrestrial DTV reception will be (at least in some areas of weaker signal strength) likewise need to be mounted outside."

MSW took issue with H&E's measurements of consumer DTV set sensitivity. H&E tested four consumer DTV receivers and one professional receiver and determined that their sensitivity did not meet the FCC planning factors of -81.2 dBm for VHF and -84.2 dBm for UHF. MSW argued that one reason the H&E tests did not yield the same sensitivity as the planning factors was that H&E measured the sensitivity using over-the-air signals, instead of traditional test methodology in a well-controlled laboratory environment. Laboratory tests use a well-defined 8-VSB RF source with SNR values greater than 30 dB, an absolutely constant DTV level and minimal splatter, no multipath or antenna-like signal spectrum tilts, and no adjacent channel signals before carefully attenuating the signal to the threshold of errors. MSW said that H&E did not cite the level of multipath or interference present in the over-the-air sensitivity tests. Three of the receivers used in H&E's tests are likely fourth-generation receivers, missing the planning factors by only 2.3 dB at VHF, and a maximum of 3.8 dB at UHF.

"How much of this problem is due to any existing short delay multipath or interference cannot be determined from their test data," MSW said.

MSW explained that a number of design issues, such as high tuner noise, lack of enough IF gain, equalizer noise enhancement due to a poor algorithm, and internally generated beats within the RF and IF band ccould cause variations in receiver sensitivity. Receivers that handle multipath and interference well may have a noise figure of 10 dB rather than 7 dB. As MSW pointed out in its original comments, these variations are small enough that a mast-mounted preamplifier can mitigate them.

MSW argued that the 90 percent time variability parameter used in FCC DTV coverage studies is valid and that there is no need to change to 99 percent time variability for determining household eligibility to receive distant DTV signals by satellite. MSW noted that the entire DTV allocation process was based on "certain key assumptions" and 90 percent time variability was one of them.

"According to H&E, the 99 percent time variability requirement would require a 17.5 dB UHF correction factor, based on the six tests they elected to report," MSW said. "Since stations cannot deliver these additional dB without violating the commission's limits on ERP, it is difficult to see how stations can fairly be penalized (through loss of viewers) for not meeting this brand-new standard."

MSW explained that 90 percent time-variability does not mean there will be a DTV signal failure 10 percent of the time (or 36.5 days per year), since the assumed loss of service is only at the outermost limit of the service area and is not a "typical" figure across the station's entire service area, and that the potential time or duration of these outages is unknown. MSW said many outages are likely to occur during parts of the day when no one is watching or be so short there is only a momentary service interruption. Finally, MSW reiterates that the number of outages can be substantially reduced through the use of a mast-mounted preamplifier.

Late in August, EchoStar filed additional data from H&E to rebut MSW's comments and reply comments. H&E did not respond to MSW's comments on receiver sensitivity, nor did H&E strengthen its justification for a 99 percent time variability. H&E did note, however, that an argument by MSW that signal strength is typically worse during the day when signal measurements are likely to be made than at night when most viewing occurs did not match its findings, which showed three stations it measured were typically stronger during the day while one was typically stronger late at night. The typical hourly variation for all stations monitored over a 24-hour period was less than +/- 1 dB.

BOTH SIDES NOW

If you took time to read the comments on both sides, you probably noticed that H&E and MSW used different studies or in the the case of 8-VSB field tests, a different subset of the results to justify the EchoStar and NAB positions. While the two engineering firms argue over details, they agree on key technical points. Both agree antenna pointing makes a difference. H&E says antennas are not pointed correctly and this has to be accounted for when determining service. MSW says that FCC allocation rules assume antennas are aligned correctly, just as satellite antennas have to be aligned correctly.

To support EchoStar's position, H&E focused on how people typically receive analog TV--whether in a strong- or weak-signal environment--and used this data to justify a higher standard for determining DTV service at households with satellite TV. MSW, backing up NAB's argument that the planning factors are fine and only minor adjustments to the analog methods are needed, showed that with some effort, field tests show DTV reception can be achieved at locations predicted to receive an adequate DTV signal based on existing FCC planning factors.

The main difference is EchoStar and H&E would require stations to deliver a strong enough DTV signal to a household so that little or no effort is required to receive it, perhaps even using indoor antennas, before distant signals are not allowed. NAB and MSW argue that FCC rules require viewers to make some effort to receive the DTV signal. This could include rotating the antenna (using the antenna positioning output available on newer ATI VSB decoder chips, for example), installing a mast-mounted preamplifier or perhaps even replacing their DTV receiver with a later model.

I have not discussed the arguments on the procedures for measuring the strength of DTV signals and some other important topics. For the full story, visit the FCC "Search for Filed Comments" page at http://gullfoss2.fcc.gov/prod/ecfs/comsrch_v2.cgi. Enter "05-182" in the Proceeding box to retrieve a listing of all comments filed in this proceeding. You may also be interested in reading the comments submitted by Z Technology, the well-known manufacturer of 8-VSB field measurement equipment. View them at http://www.ztechnology.com/pdf/shvera.pdf.

Questions and comments are always welcome and appreciated. E-mail me at dlung@transmitter.com.