Perhaps the biggest surprise of the DTV transition has been the poor performance of many VHF stations as digital-only stations. On the whole, their signals don't seem to reach as far or penetrate buildings as well as those of their UHF counterparts.
Since the June 12 analog cut-off, broadcast engineers at VHF stations have been trying to sort out the problems of digital Vs. And in this effort, they have had the support of consulting engineers, experts on the peculiar behavior of radio frequencies.
Among those deep into DTV is Richard Mertz, a principal withCavell, Mertz & Associates.
In this interview withTVNewsCheck, Mertz says high-band VHF (channels 7-13) for fixed and mobile reception can be remedied with more transmission power, something that he hopes the FCC will consider granting. As for low-band VHF, his message is simple: Abandon all hope.
An edited transcript:
What's the problem with reception of digital VHF signals?
A lot of people don't want to install an outside antenna and, if you recall, the planning factors for DTV are based on a properly oriented, high-gain antenna mounted 30 feet in the air outside. Rabbit ears inside don't take into account the attenuation that the building materials in a home provides and or the reflective surfaces in a home that cause multipath.
I remember as a kid fighting with the rabbit ears and trying to tune in the station and you got an acceptable picture. Now with digital, you can't do that. Kind of getting the signal is not acceptable anymore. You won't get a picture with a ghost. You'll get nothing. I know from my own experience: A lot of people don't want to get up and move the rabbit ears around.
Another problem is the receivers. We have no receiver standards. We have decoding standards, but we don't have performance standards for the receiver. So depending on which receiver you have, you have different sensitivities, you have different abilities. Some of the decoder boxes can handle a severe amount of multipath; others cannot.
In some cases, in trying to get a good signal, people overdo it. I've been to several homes where people are complaining they can't get high-band VHF stations and the problem is they put an antenna amplifier between the antenna and the converter box. A lot of these amplifiers are noisy or they're overloading the front end of the converter boxes.
You mentioned high-band VHF. What about low-band VHF. Is that a particular problem?
We don't recommend using those channels. I know that several people have gone back to ch. 6, but I don't think that's a wise idea. There have been different studies done and papers published that talk about man-made noise, celestial noise in those frequencies.
So, if you're in the low V band, you should be thinking about getting out of there.
I don't think they should have gone there in the first place. It's spectrum the commission may recover eventually. I don't think it works very well. Propagation-wise, studies years ago suggested that high-band VHF was the best place to be and we tried to move people there as we saw fit.
What is the inherent problem with high-band VHF that makes it harder to receive?
This is a multifaceted problem, but it's basically power limitation, especially in the Northeast. You need more power to overcome building attenuation and get to indoor antennas.
The maximum power you're allowed in high-band VHF is 30 kilowatts at 305 meters of average terrain. The reason they did that was because they were working towards replication of the analog grades. Some of the planning factors may have been very conservative and didn't really take into effect how people actually watch TV over the air.
And an amplifier in the antenna line won't help?
Typically, you try not to have the amplifier. Let me give you an aside. We had a UHF LPTV client that was being picked up and carried by a local cable system, but the signal would drop out periodically. They had a dedicated antenna for this one low-power television station. I talked to them on the phone and discovered that the antenna was pointed in the wrong direction. Then, I asked if the signal passed through any amplifiers. Yes. Can you unplug the amplifier and bypass it? Can you plug the antenna directly into the receiver? We can. Do you have enough signal now? Oh, more than enough. They haven't had a drop-out since.
What is it that the amplifier does that causes the signal problems?
It can either overload the initial stages in the receiver or it can create noise that the receiver thinks is data because of the nature of the ATSC signal. So by taking the amplifier out |More …
Copyright 2009 TV Newsday, Inc. All rights reserved.
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