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Sorting Out Reception Obstacles

Last month, this column reviewed the paper Gary Sgrignoli of Zenith and I wrote for the International Conference on Consumer Electronics (ICCE), which closely followed the Consumer Electronics Show (CES) in Las Vegas. The CES had about 130,000 attendees. ICCE had some 400 engineers. At CES, you would have looked in vain for broadcast receivers with their 4:3 aspect-ratio CRT displays in the 19- to 35-inch screen-size range. Gone were traditional CRT displays and 4:3 screens; plasma panels as big as 102 inches and LCD and DLP displays have taken over!

In short, the future is digital, 16:9 and gigantic. And I believe the future is now.

The future has arrived at my home. I now have a HTDV set-top tuner/decoder. We view the output on a 42-inch HTDV plasma panel. You know, it looked pretty large when this panel display replaced our 27-inch NTSC (4:3) receiver. That lasted several weeks, now the 42-inch screen looks just about right, or maybe a little small.

I recall visiting New York City in June 1946. TV was taking the city by storm as night baseball was being televised "live" in glorious monochrome. Everywhere one looked, banners and signs identified those bars having TV. In fact, it was from watching baseball in bars that the public learned they needed TV at home. When I saw those giant widescreen displays at CES, I was sure CEA members were betting history repeats itself.

Readers will be wondering what DTV tuner/decoder I found to work at my terrain-challenged site 16 miles from the Washington towers, and 26 miles from the Baltimore towers. The bearing angle from my home to the DC towers and those in Baltimore is about 80 degrees. So I have always had my rooftop antenna on a rotator.


The good news is that I can receive and decode the HDTV signals, but I had to calibrate my antenna rotator very carefully. My rotator swings the beam too fast for DTV. The line-of-sight to D.C. is blocked by a hill. It would not be appropriate to identify the brand or model number in this column, but we are now enjoying HDTV, and we find that we are watching more TV. But when "so-called" HDTV (actually NTSC upconverted and re-branded as HDTV) is broadcast, we channel-surf or slip a disc into the DVD player.

My antenna rotator swings past the best angle so quickly that the receiver has trouble locking up on-the-fly. What a proper DTV antenna rotator would do is to slew the antenna to, say, within 10 degrees from where it is set to stop, and then creep up to the stopping point. Now that would claim to be an HDTV antenna rotator.

A more serious problem with the rotator is that despite my efforts to calibrate it, when the weather changes, the rotator needs a slightly different angle, so I can't just rely upon my calibration. But I can and do make it work. When spring springs and the trees leaf out, UHF signals are going to be attenuated here. I will probably replace my VHF-UHF antenna with a top-of-the-line UHF log periodic, but I am in a wait-and-see mode for now.


Last month, this column featured some spectrum plots I used in the ICCE presentation. One of those (slightly modified) is reproduced here as Fig. 1. It shows two undesired DTV signals on Channels 33 and 35 that are identified as n-4 and n-2. This pair of channels can produce third-order intermodulation (IM3), which fall in channel n-6 and channel n. Either might be your channel, as this column has noted several times now.

Take a closer look at the noise level of the other channels in this figure. Channels n-6, n-5, n-3, n-1 and channel n all have about the same noise level, which is due to IM3 being generated in the overloaded amplifier under test. You can see that the noise drops off in channels n-7 and n+1. Fig. 2 shows the effect of reducing the signal power in channel n-2. The IM3 in the other channels is greatly attenuated--proving that it is due to IM3 generated between the signals on this pair of channels.

If receivers are designed to avoid being overloaded by multiple strong DTV signals (up to -5 dBm average power, 0 dBm maximum symbol power per channel), this potential problem would not materialize. The current FCC DTV channel-allotment plan tacitly assumes no receiver overloading. How could manufacturers half-way around the world suspect that our channel plan is based on this assumption?

The bad news is that the n-4, n-2 and the n+2, n+4 channel pairs may be real threats to the reception of your signal on any of a whole host of channels near either of them. This is due to the spectrum-spreading aspect of these particular channel pairs. Spectrum spreading is even greater where both channel pairs--n-4, n-2 and n+2, n+4--are in the same community.

The odds against you have just gone up with this spectrum plot, and they may go even higher as more stations maximize their DTV facilities. But the good news is that if just one of either channel pair is not used near the other, this problem is solved, as Fig. 2 illustrates.

In the case of n-2, n-4 channel pair, n-2 should be banned because that gets rid of two of the most infamous channel pairs--n-4, n-2 and n-2, n-1. In the case of the n+2, n+4 channel pair, there may be better reasons to ban the n+4 member of that infernal n+2, n+4 pair. That is under investigation.

Broadcasters could bring this to the attention of the FCC before the final DTV channel allotment plan is developed. Stay tuned.

Charlie Rhodes is a consultant in television broadcast technologies and planning. He can be reached via e-mail at charleswrhodes@worldnet.

Editor's Note: In the Jan. 19 edition of TV Technology, the illustrations labeled Fig. 1 and Fig. 3 in this column were inadvertently transposed.