In my last column, I dealt with a triplet of three contiguous channels, such as channels 35, 36 and 37, as shown in Fig. 1 of the July 23, 2008 issue of TV Technology. It was shown that if all three signals are received at the same power (D/U about zero dB), interference between these signals should not result. It was then proven that if the D/U were near the FCC limits, the center channel (36 in the example cited) would suffer interference unless the receiver was perfectly linear with two signals at –25 dBm each on first adjacent channels. Allotments of three contiguous channels are found in the latest table of channel allotments for a significant number of communities.
It can be argued that in many of these cases all three transmitters are co-located (or nearly so) so that large D/U ratios will not exist within the coverage area in common to the three stations of the triplet. However, consider a triplet of three contiguous channels where for example, channels 35 (N–1) and 37 (N+1) are in one community while the channel 36 (N) signal is broadcast from another community. Well inside the coverage area of the stations on channels N–1 and N+1 stations, their signals will be strong, while the signal from the other community may be significantly weaker; hence we have a high D/U ratio. Human nature being what it is, some people will want to receive the unreceivable signal on channel N.
RUNDOWN OF ALLOTMENTS
This month, I looked for channel pairs of the form N–1 and N+1 allocated to the same community in the latest FCC Table of Channel Allotments. Every state has some community or communities with such allotments. My list of such channel pairs and the community to which they are allotted appears in the accompanying sidebar (p. 40). This list should be of interest to station management and engineers. Let's look at three examples of such allotments and what they might mean to broadcasters after the sunset of analog TV broadcasting a few months from now.
Take Phoenix, which has two such channel pairs 15, 17 and 24, 26. If there is a station near Phoenix on channel 16 it would be subject to interference from the signal pair 15, 17. The same applies to a nearby station on channel 25.
A much more complex situation exists for Los Angeles. That community has allocated to it, channels 7, 9, 11 and 13, so any signal on channels 8, 10, or 12 may suffer interference, which is in part due to the sideband splatter radiated in the adjacent channels and to third-order distortion products (IM3) generated by the strong undesired signals (on channels 7, 9, 11 and 13) within the receiver suffering such jamming. In addition, channel pair 34, 36 is also allocated to Los Angeles, so any signal on channel 35 may suffer the same fate if anyone tries to receive it in Los Angeles. Before we leave this example, consider the signals on channel pair 34, 36. They can generate IM3. The spectrum of IM3 of DTV signals resembles beehives. They are centered on channels 32 and 38 and these IM3 products have a spectrum three channels wide with significant power in channels 31 and 33, and in 37 and 39, too. So signals on these channels may suffer interference in receivers, which are nonlinear in the presence of strong undesired signals on the channel pair 34 and 36.
TAKE A LOOK AT LOCAL
Some consecutive triplets are found in the table of allotments. For example, Topeka, Kan., has 11, 12 and 13. In addition to the channel problem discussed last month, IM3, which would be generated by signals on 11 and 12, falls into channel 10. IM3 generated by signals on channels 13 and 11 falls in channel 9 and so forth.
Readers may want to analyze the situation for their local stations (after Feb. 17, 2009) to prepare as best you can for complaints. Perhaps your DTV signal cannot be received by some viewers who did receive your analog signal before Feb. 18, 2009. Not all cases of reception failure will be due to interference, so let's talk about the bigger picture for a moment. For example, Portland, Ore., has a PBS station transmitting DTV signals on channel 27, while their NTSC signal is on channel 10.
That will change with their signal on channel 10 being DTV after Feb. 17, 2009, and channel 27 being vacant. So there could be antenna problems or antenna orientation problems then. Now the viewer's DTV receiver or downconverter must be tuned to channel 10. If the station changes its PSIP data correctly, this will happen automatically. However, an error in updating the PSIP data could leave many viewers in the dark. Then there are the millions out there who get their TV signals from TV translators. Translators convert the received signal to another channel before they radiate it. Very few translators will be able to change the PSIP of signals they rebroadcast, so viewers will have to get their DTV receiver or downconverter to scan the spectrum and look for signals.
In some cases there may be a need to re-orient antennas. That we won't know until the transition takes place. Quite recently, I took down a rooftop antenna that had been in place since 1973. It worked fine for FM radio reception and it would still fetch low VHF signals, but the high VHF and UHF signals eluded this antique and worn-out antenna. Once on the ground, it was obvious that corrosion had more or less insulated the antenna. I'm not the only one who will find that 2009 will be a great year for new antennas, once broadcasters settle into their new and permanent channel.
In the accompanying sidebar is my list of channel pairs of the class N–1 and N+1 allocated to the same community. Where these are both received with strong signals such as above –25 dBm and a weaker desired signal, which is on channel N, may be jammed by interference as explained above.
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