I recently attended and participated in the annual meeting of the National Translator Association, which is composed of TV translator operators. You might be surprised at their number. I believe about 130 attendees came this year.
I was curious about how many were readers of TV Technology, so I asked. I was pleased to see a show of hands of about 75 percent or more.
Then I asked when the first HDTV transmitter went into operation. Some of my readers will recall that the BBC began regularly scheduled television service on its 405-line, 25 fps all-electronic TV system in 1936.
An engineer with a decided British accent raised his hand, and with his first words, I knew he knew--and he did. For those of you who suspected this was a trick question, you were right. The 405-line TV system remained in operation in the United Kingdom from 1936 to 1987. In 1936, it was indeed HDTV with 405 lines per frame, as the only other system under development in England was a mechanically scanned 240-line system developed by John Logie Baird.
TV translators go back to about 1950. Some say they first appeared without the blessings of Washington, D.C., in eastern Washington State, Idaho and Montana. Some were the first on-channel boosters, carrying KXLY, Channel 4, Spokane, Wash. into sparsely populated valleys.
In any event, TV translators sprung into existence as fast as the word spread that they could bring television to shadowed valleys. They became so popular that the FCC created order out of the chaos, establishing Part 74 of its telecommunication rules. TV translator systems now provide millions of viewers with a free over-the-air television service in the United States and Canada. Keith Larson, now chief engineer of the FCC Media Bureau, wrote Part 74.
Nearly all these systems receive TV signals at mountaintop sites where there is a direct line-of-sight path from the broadcast transmitter to the translator's receiving antenna. The received signals are translated to another channel and retransmitted to the viewers. Many of these are owned and operated by county governments.
The hot topic at the NTA meeting this year was clearly the transition to digital television. In my book, it all began when Paul Burkholder, communications director of the Humboldt County, Nev., translator system, presented his "Report on HDTV Field Tests, Phase One, November 1999--May 2000," before the NTA in Medford, Ore., in May 2000. Paul built a DTV translator system and tried it out in 1999. I regard his report as a milestone in the history of DTV. He carefully documented that it worked!
At that same meeting, Sam Zboroski reported on his field-strength measurements in the hilly terrain near Pittsburgh, Pa. Initially, Sam thought an on-channel repeater would not work, but he reported that his measurements suggested such a device would work. That was a great relief to me, as my own paper was a tutorial on why on-channel repeaters could be used with 8-VSB modulated DTV signals to provide coverage in terrain-shielded areas. I was reporting the results of an experiment carried out by the Advanced Television Technical Center, which built and tested an on-channel DTV repeater in West Virginia. That too worked.
(click thumbnail)Fig. 1 Two-channel DTV translatorusing off-air received signals This year, Keith Larson and Bruce Franca of the FCC briefed NTA members on both the technical and administrative aspects of the new DTV translator rules. Applications will soon be accepted for DTV translators.
My presentation this year concerned IM3 (third-order intermodulation) as the primary interference mechanism behind DTV-into-DTV interference, for adjacent channel and taboo channel interference. Briefly, I noted that it takes two (or more) undesired signals on certain pairs of channels to generate IM3, and that some of this will fall into a third channel, which may be the desired DTV channel.
Fig. 1 shows a DTV translator receiving off-air DTV signals from Channels 22 and 31. These signals may be about -50 dBm at a mountaintop site as far as 100 miles from the broadcast transmitter. They will suffer attenuation in the downlead of 7 dB and another 4 dB of loss in the two-way signal splitter. The DTV signals will be at -63 dBm at the input to the two DTV receivers. At that signal level, the RF amplifier will be at maximum gain to give the best possible SNR (signal-to-noise ratio) power at the mixer, where most receiver-generated noise is produced.
Fig. 1 also shows that these incoming signals are being translated to Channels 25 and 28. A small part of the signals being retransmitted will find their way back into the receive antenna. These signals may be at -28 dBm at the input to the receivers. They will be amplified by the RF amplifier and be much stronger at the mixer input, perhaps -14 dBm, which will overload the mixer with the result that IM3 will be produced by the channel pair 25 and 28. Some of these IM3 fall into the desired Channels 22 and 31. Now this IM3 increases the noise under the desired DTV signal. As we all know, these receivers cannot decode the 8-VSB signal unless the SNR is greater than 15.2 dB in theory--and much higher in practice--to deal with signal fading and phase noise.
So Fig. 1 demonstrates how a poor selection of retransmit channels can threaten translator failure.
The rule of thumb is that where the desired signal is on Channel n, pairs of retransmit channels of the form:
n-2K, n-K and n+K, n+2K should not be chosen.
K is a small integer. Thus:
2(n-1) - (n-2) = n = 2(n+1) - (n+2)
2(n-2) - (n-4) = n = 2(n+2) - (n+4)
2(n-3) - (n-6) = n = 2 (n+3) - (n+6)
2(n-K) - (n-2K) = n - 2(n+K) - (n+2K)
As a practical matter, K may be as high as 4 and cause IM3 to be generated in the receiver. Above K = 4, in my opinion, a well-designed receiver should reject one or both undesired signals, thus preventing the production of IM3 in the receiver front-end.
There is a special case where both n channels are being received off-air at the translator site. Channels n-1 and n+1 should not be used for retransmission.
In this case, experiments have shown that strong IM3 will be produced in Channel n. So if the translator is to receive on Channel n, neither n+1 nor n-1 should be used for retransmission. If the incoming signal is on Channel n, it should not be retransmitted on either n+1 or n-1 if the translator also uses the other adjacent channel as a retransmit channel for any purpose. Many translators use a group of odd- or even-numbered channels such as 20, 22, 24, 26 and 28. That would be a bad practice with DTV.
OK, I've reviewed what I presented to the translator engineers and their consultants. How does this relate to broadcasters? As you may know, the FCC has placed the ATSC proposal for Distributed Transmission (multiple DTV transmitters per broadcaster) on its fast track for adoption. I believe that many broadcasters interested in using multiple DTV transmitters expect to feed them from a primary transmitter over-the-air. That would be cheaper and faster than using fiber-optic or point-to-multipoint microwave relays. Look again at Fig. 1. The undesired signals (in red) being picked up by the receive antenna on Channels 25 and 28 may overload the receiver.
Of course, the solution is to put a bandpass filter ahead of the off-air receiver. This bandpass filter must be flat over the entire desired channel (n) within 0.5 dB, and it must not produce group envelope delay distortion of any frequency within Channel n. That means it cannot provide much attenuation of signals on Channels n+1 or n-1.
I think a well-designed filter can provide 25 dB of attenuation for n+/- 2, 3, 4 and above. Broadcasters will have no control over the channels in use in their communities, so they must engineer around them. Of course, this may be impossible in the case of first adjacent channels, because those undesired signals cannot be attenuated adequately without seriously degrading the desired signal on Channel n by group envelope delay distortion in the bandpass filter.
It appears that before May 2006, a number of DTV translators will be in operation in the United States. The first step--receiving a DTV signal at the translator site and converting it into an NTSC signal for retransmission on another channel--has been achieved by a number of translator operators. They report that the public is thrilled with the resulting benefits. This is probably most evident where multiple translators were previously employed, as was often the case in the western United States and Canada.
I've heard of five repeaters in cascade. I can imagine that the video quality over multiple analog translators would leave something to be desired. With DTV, the signal can be error-corrected at the translator so what viewers get is of extremely high quality, even if it has to be home-delivered in analog form a while longer.
Oh yes, another hot topic at the NTA was the date by which all NTSC terrestrial broadcasting will end. I don't know, and no one else knows either. Time will tell.
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