Charles W. Rhodes
In February, the National Association of Broadcasters told the FCC that it was currently opposed to any changes in OET- 69, which describes the calculations used to forecast the coverage areas and interference factors for full-power and Class A low-power TV stations. The association’s response came after the FCC released its TVStudy software which modified Bulletin OET-69.
In fact, the NAB believes that the FCC probably does not have the authority to change it without issuing a Notice of Proposed Rule Making. This to me is a game changer as the FCC has announced that the repacking of UHF TV spectrum will be done using OET-69. In the bulletin, Table 5A deals with DTV-DTV interference and the only such interference is adjacent channel interference, or ACI.
CRITICAL INTERFERENCE ISSUES
So, without any changes to OET-69, the commission could not take up any additional planning criteria such as interference between DTV signals having a channel relationship other that N±1 and N (ACI).
In my last column, in the Feb. 13 issue, I spelled out technical strategies for the mitigation of interference between DTV signals other than ACI. The FCC wouldn’t need to accept any of the test results my colleagues and I have obtained as the FCC Laboratory made such measurements of all 115 NTIA-approved converter boxes. Those results were published by Steve Martin, senior engineer of the FCC Lab in the December 2010 issue of “IEEE Transactions on Broadcasting.” The tables of greatest interest are No. VIII and X on p. 449.
The NPRM route is, as the NAB suggests, appropriate considering that the fate of over-the-air broadcasting may be decided by how the FCC repacks the UHF spectrum, and what the broadcasters tell the FCC in their comments.
If Table 5A of OET-69 as it now stands is the basis for dealing with interference, future over-the-air DTV receivers would either have to be designed to be much more robust at rejecting interference or they might simply not be available. Manufacturers might decide to concentrate on PCs to provide DTV display for CATV and DBS-delivered signals and signals from an ATSC set top decoder via an HDMI connector which may already be on some PC offerings.
The problem is one of economics, not technology. Suppose a really robust tuner adds $10 to the bill of materials. That adds $25 to the retail price. This price increase only provides added value for the 10 percent of buyers who depend on OTA broadcasting. From this sort of reasoning, the only way robust receivers could be made would involve the Congress mandating such robustness in all receivers just as it did with the “All Channel Act” 50 years ago. That was before any alternative delivery means were available. Is this likely now that these alternatives are available for receiving TV?
INTERFERENCE PROTECTION
So, as I see it, broadcasters should plead for adequate interference protection in the way the UHF band is repacked. In reading the 323 (or so) comments filed mostly by broadcasters concerning the FCC plan to update OET-69, interference issues were only vaguely and indirectly mentioned.
Fig. 1: Spectrum plot received off-air from a transmitter about 14 air miles from the author’shome in Vancouver, Wash., over a line-of-sight path.
(Click To Enlarge) I believe that the FCC should use the data its own lab produced in Tables VIII and X of this IEEE paper, except that the data concerning ACI is not useful in predicting coverage. That data resulted from tests in which the U ATSC signal was generated by a very high-quality laboratory signal generator, not by a DTV transmitter. ACI is due to the sideband splatter radiated by DTV transmitters. This is the only instance where the test results my colleagues have produced are at significant variance with the test results produced at the FCC Lab. Absent ACI, the noise floor in each adjacent channel is about –99 dBm which is the receiver-generated noise. For ATSC, the minimum SNR is 15.2 dB, so the minimum usable ATSC signal, absent interference, is –84 dBm. If the U signal is received at, say –45 dBm, the noise in the desired (D) channel would be –90 dBm. The combined sideband splatter and receiver generated noise totals –89 dBm.
With this noise floor, the receiver is desensitized by 10 dB. The D signal would have to exceed –74 dBm. Note that the U signal power is –45 dBm and the minimum D power is –74 dBm, so the threshold D/U power is –29 dB. Notice how close this comes to the transmitter’s D/U ratio (–28 dBm where the U signal is on channel N+1 and –26 dB when the U signal is on N–1).
It should be noted that if there are U signals on both N+1 and N-1, reception would fail. The FCC must not allocate both N-1 and N+1 where there is, or will be, a transmitter on channel N. Remember that the transmitter D/U is measured at the transmitter when at rated power and driving a dummy load. Even when co-sited with the D transmitter, the sideband splatter into the D channel from the U transmitter may not be as expected for several reasons, and when the two transmitters feed different antennas, all bets are off. Fig. 1 shows the spectrum plot received off-air from a transmitter about 14 air miles from my home in Vancouver, Wash., over a line-of-sight path.
The “shoulders” in the adjacent channels are the sideband splatter actually radiated by the station on Channel 30 (569 MHz center frequency). It is the noise power within these “shoulders” that is noise in the D channel when the receiver is tuned either to Channel 29 or Channel 31. A signal on either of these channels must be more that 15.2 dB above this noise to be decoded. This is the actual mechanism underlying ACI.
A very low-noise preamplifier was needed to overcome the high noise figure of the analyzer. The ATSC signal flat top is 37.4 dB above the “shoulders” in both adjacent channels.
The FCC does not require that transmitters on channel N and the adjacent channel be co-sited, although many are. I believe these should be co-sited and that the FCC should mandate co-siting of transmitters on adjacent channels before re-packing.
Stay tuned.
Charles Rhodes is a consultant in the field of television broadcast technologies and planning. He can be reached via email atcwr@bootit.com.
