Now that we pretty much know the permanent channel allotments from the 7th Report and Order and Notice of Proposed Rule Making dated October 2006, I asked my friend Louis R. du Triel Jr. to revise the data in Table D.1 of the ATSC Recommended Practice: Receiver Performance Guidelines document A/74. I was not acting for the ATSC; I was asking for my own interest.
These guidelines give the calculated received power from all stations in the Miami area. Bob Jr. did the compilation and now I can share with you the received DTV signal power of UHF stations in the greater Miami area. This makes it possible to evaluate potential interference to DTV reception in a major market after the sun sets on NTSC in February 2009. I also used the FCC Report entitled “Interference Rejection Thresholds of Consumer Digital Television Receivers Available in 2005 and 2006,” which provides actual performance data of receivers.
(click thumbnail)Table 1: DTV Received Power Analysis; Location: Hollywood, Fla.DTV RECEIVED POWER
From the revised table, I computed the total received DTV signal power within the UHF band at a receiver at the site specified in the ATSC Table D.1, in Hollywood, Fla. This power is –2.7 dBm, which I believe would overload any receiver being fed by the rooftop direction antenna specified in OET Bulletin No. 69, and aimed towards the same station as was assumed by the authors of ATSC Table D.1. The revised table is shown in Table 1. Many thanks to Bob for his kind assistance.
I have drawn some very interesting conclusions to share. First, I will compare the threshold of interference D/U ratios given in the ATSC guidelines document A/74 with the measured data from the FCC Laboratory Table A-5 as both are in D/U for D = –53 dBm.
Table 2 gives the measured threshold of interference for the median receiver at D = –53 dBm. This means that 50 percent of the receivers tested failed at the stated U power. Only a small percentage of your station’s coverage area enjoys received signal power above –53 dBm. You certainly don’t want to lose 50 percent of your potential audience where D = –53 dBm or less.
The FCC also provided the mean D/U threshold power, which is quite close to the median D/U, and the corresponding value of the Standard Deviation (SD). Sixty-eight percent of the area under the Gaussian bell shaped curve is within one SD of the mean U power. That means that 32 percent of the area is outside and we are only concerned with the part where the receivers fail. That is the 16 percent. So, if the mean value were increased by one SD, the number of lost viewers would be reduced from 50 percent to 16 percent. If the mean receiver threshold U power were somehow increased by two SD, 95 percent of the area under the Gaussian curve would be OK, and only 2.5 percent would fail statistically.
(click thumbnail)Table 2: Comparison ATSC A/74 Guidelines D/U vs. FCC Measured D/U for D = –53 dBm.Perhaps the statistics for this small number of receivers is a bit shaky, but I think that you would like to see the mean U thresholds of DTV receivers improved. That is not the direction that the ATSC guidelines would take us. Look at the threshold D/U for N+5 (–42 dB), U max = –11 dBm. The FCC measured –49 dB D/U. That means the U max for the real world median receiver is actually –4 dBm. I believe the ATSC should either revise or retract A/74. The strongest DTV signal power likely to be present at a receiver input is –5 dBm according to the ATSC guidelines document A/74. This is supported by the revised table by du Triel. Now, let’s look at the updated Table 1 data.
TOO HOT TO HANDLE
In this updated table the viewer’s directional rooftop antenna is still aimed towards the tower of WSFL-HD, Channel 19, which is 6.1 km distant and at a compass bearing of 204 degrees. Under these conditions, the received DTV signal power in UHF band is –2.7 dBm, (my calculation). I do not believe that any receiver can handle all this power without attenuation.
Suppose this viewer wishes to watch Channel 28. That signal is received at –41 dBm. We lack any data for D = –41 dBm so we must improvise, finding a way to use the nearest data to D = –41 dBm. The solution is to attenuate the antenna signal by 12 dB and we will then have the –41 dBm desired signal at –53 dBm. This will reduce the total undesired DTV signal power from –2.7 dBm to –14.7 dBm. Perhaps the receiver can handle this.
From the updated table there are two channel pairs of interest relative to reception of Channel 28; N–5 and N–10 and N+2 and N+4. Table 3 shows the effective attenuation on the received signal.
The best case would disregard all other signals. The power sum of these four U signals is –20.5 dBm. We see that the threshold U power for the first U channel pair is –19.5 dBm, which is significantly more than the total power of the first channel pair so that pair of U signals will not block reception of the desired signal on Channel 28, if we have the 12 dB signal attenuator at the receiver’s input port. Without the attenuator, the first U channel pair alone would block reception, as both U signals exceed the U power of this channel pair at which reception is blocked –19.4 dBm for the best 50 percent of receivers.
(click thumbnail)Table 3: The effect of attenuation on the received signal for the two channel pairs of interest.The second U channel pair, N+2 and N+4, would also block reception without the attenuator. However reception would be possible with 12 dB of signal attenuation. Note that the desired signal is –53 dBm with the 12 dB attenuator so there is still a large signal level margin above the noise.
In Table A-10 of the FCC Laboratory report, for the N–5, N–10 U channel pair, at D = –53 dBm, the threshold U power per signal was –19.4 dBm. At D = –53 dBm, the maximum IM3 would be – 53 dBm – S/N+I (15.2 dB for the ATSC Signal) = –68.2 dBm. For U = –19.4 dBm and IM3 = –68.2 dBm, the third order Intercept Power (IP3), a figure of merit concerning linearity, 2*IP3 = 3*(–19.4 dBm) – (–68.2 dBm) IP3 = +5 dBm. The median receiver tested had this figure of merit.
For the best 50 percent of receivers tested, IP3 exceeded +5 dBm. The best receiver tested had a threshold U power per channel (N–5, N–10) of –14.1 dBm. Using that number for U, 2*IP3 = 3(–14.1 dBm) – (–68.2 dBm) = +12.95 dBm. Dr.Oded Bendov and I have both suggested that an IP3 of +16 dBm would be highly desirable for consumer DTV receivers. I am pleased to see that some receivers can closely approach our recommendation of +16 dBm. Re-arranging the equation for IP3, we can get an equation for IM3 for a given IP3 and a given U. IM3 for the Bendov/Rhodes IP3 of +16 dBm is:
IM3 = 3*(U) – 2*IP3
[IM3 = –42.3 dBm – 32 dBm = –74.3 dBm a 6 dB improvement.] I suggest that this justifies receiver designs for IP3 = +16 dBm.
This 6 dB improvement over the best receiver tested by the FCC would eliminate interference from all channel pairs tested by the FCC. It would also eliminate interference from a single undesired signal DTV or unlicensed device in many cases. It would also provide some safety margin against general interference from unlicensed devices. This industry needs to improve receiver IP3, and here is a specific proposal for what that goal should be: +16 dBm. This goal is, I believe, possible with today’s technology, which we have shown here, has produced an IP3 up to +13 dBm.
IN MEMORIAM
Before I close my column, I’d like to pay tribute to a pioneer in this business, and someone who was also a close friend.
At my age, you get used to hearing that a friend or former co-worker has passed away. Recently, I lost one of my dearest and closest of these, a broadcaster, or perhaps I should say “The Pioneer Broadcaster of Alaska.” Augie Hiebert passed away in September at the age of 91. He built—and I mean that he constructed—the first radio transmitter in Alaska for KFAR in 1939. There were more to follow in Anchorage, Fairbanks, and places you may never have heard of.
Augie, an avid ham radio operator, fired up his short wave receiver one Sunday morning in 1941 and learned the Japanese were bombing Pearl Harbor. He knew exactly who to call—nearby Ladd Army Air Force Base. That is how the Commanding General Army Headquarters for the Alaska Defense Command learned that World War II had begun. After the war, Augie fought for, and got, the FCC to allow KFAR to operate on 660 kHz, despite the fact that this frequency was “owned” by a Clear Channel station in New York.
KFAR was about all there was in Alaska in the way of radio then. Augie fixed that by building more radio stations. One of these he had to build twice, as a flood wiped out the first try. It is no exaggeration to say that Augie was involved in many instances where radio was the only means of communication in Alaska, and he understood the significance of wireless communications in time of disaster.
After the war, Augie brought more radio Alaska and then moved into television. This was an inevitable thing for Augie to do of course, but the conventional wisdom was that no community in Alaska could support a TV station. However, Augie thought otherwise and he built KTVA in Anchorage and it is there today. He also was involved with Fairbank’s first TV station, KTVF.
Another of his projects was in establishing Alaska’s first FM station in 1960. He also pioneered in the use of translators to extend the reach Anchorage and Fairbanks television transmitters. After that, Augie played an important role in bringing satellite communications to Alaska in 1969. For the first time, Americans in Alaska had real-time live TV from the Lower 48.
The modern telecommunications system in Alaska owes much to the memory of Augie Hiebert, and I believe Alaskans know it. I know that Alaska’s broadcasters know it. Augie always had a vision of the future and he made it come about for Alaska. If you would like to get to know this man, read his biography “Airwaves Over Alaska” with its forward by Walter Cronkite, ISBN 0-942381-09-2. Perhaps your children would be inspired by what Augie did for Alaska.
To me, Augie Hiebert showed how much one man with a purpose and a plan can still accomplish.
ERRATUM
In my last column, “How Unlicensed Devices Could Affect Your Future,” in the Oct. 17 issue, incorrect figures were listed in the far right column of Table 1. The correct figures can be found online in the updated chart at
www.tvtechnology.com
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Stay tuned.
