How to keep DTV from going astray Reclaim the Media
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Over the past 10 years, he has shared his wisdom with broadcast engineers through more than 85 all-day seminars on 8-VSB. In this interview with TVNewsday, Sgrignoli talks about what broadcasters -- and viewers -- need to do to make sure viewers can find the DTV signals and keep the pictures sharp and steady. An edited transcript: I have been hearing lately that the DTV signal is not everything that it should be, that the reception is poor even in areas where the field strength meters are saying it should be strong. What's going on?
I have not heard a great deal of clamoring regarding this since a third of the TV stations turned off analog by Feb 17. Now we've quietly heard that there are certain areas where they're questioning whether high-VHF signals are being received properly.
My concern is that there may not be enough radiated transmitter power in some cases, and that people are using rabbit ears that are not very good in other cases. Also, getting the signal indoors sometimes is a problem with high-VHF, if the only decent path to the inside is through the windows, which typically are not large enough for these high-VHF signals to pass through easily.
Of course, it's worse for low-VHF signals, but there are only roughly 40 stations in the whole country that are going to end up in that band. There are plenty of areas where people are pleased with the coverage. Unfortunately, you don't hear much from them because when people are happy, they don't bother to call in and write in and so forth. So, obviously, I believe it's slighted towards the negative side.
That said, there are three areas where problems can creep in -- the transmit side, the propagation side, and then the receive side. The broadcasters are working hard on the transmit system. They're learning, they're gaining more test equipment, and they're learning how to use it and so forth, and so we've seen some really good things there.
The propagation side: Hey, you can't change the terrain or the buildings that are there. Hopefully, the broadcasters have optimized where they've placed their transmit antennas and the height of the antennas and their patterns and all that. That leaves the receive side, which I believe is the biggest concern. That's where the industry is trying to catch up now.
They have to educate the viewers on what kind of antennas they need and how to use them. One of the problems is with "managed expectations." Viewers were told to just get a DTV set or a converter box, hook up some kind of antenna either indoors or outdoors, and assured that it would work. While that can work in some cases, it does not work all of the time.
That is a problem that can only be solved with more viewer education. Let's talk about the broadcast side first. What's going on or not going on at the transmitter site that may be causing problems? And how do you rectify it? The broadcasters are pretty much sending out a decent VSB signal in terms of signal quality. They've learned to do linear and nonlinear pre-correction, which means to make the signal as pristine as possible before it's sent up the transmission line to the antenna.
They've worked out some of the issues with average power measurement, FCC emission mask compliance, phase noise and things like that, but there may still be some issues with clock jitter. Clock jitter? In the early days, we saw that sometimes the transport clock going into the exciter was jittering enough that it caused the symbol clock of the 8-VSB to also jitter to the extent that it affected the receivers out in the field.
This jitter can come in anywhere -- from the studio to the transmitter. So that's just one thing to be careful about. The other thing is the actual symbol clock frequency. In the early days, the symbol clock frequency was sometimes not within the recommended 2.8 parts per million tolerance. Test equipment now is in place to at least inform them about the frequency tolerance of the symbol clock. Lip syncing has apparently been a chronic problem. What's being done to correct that ?
It's something that they're working on within the broadcast industry. It has nothing to do with transmission, of course. It's a product of the video and audio processing. Because the video requires far more processing than the audio, it delays it longer than the audio.
What they do is try to delay the sound to match it, but every time it goes through a different set of processers, they've got to be careful to do that again, and that's what they're working on now. I believe that they will solve this problem in due time. Is the transmitter less forgiving in the digital world than it is in the analog world? In other words, do you have to take better care in calibrating and maintaining the transmitter than you did in analog?
I guess that's fair to say. But in some ways things are a little bit easier now because digital and modern technology have caught up with or advanced to the point where it's not so hard to maintain this. On the other hand, the transmission system for 8-VSB is far more efficient than the old analog system.
You're sending five times the amount of video information in digital and using less radiated power to do so. Since you're using the channel much more efficiently, you have to be more careful about the creation of the 8-VSB signal and its transmission. But the broadcasters, I think, have that down fairly well. The equipment is there to do what is necessary to create this pristine signal. That assumes the broadcasters are paying attention and doing their job. Right.
That's not so much a problem. As a general rule, they've come a long way. Some of them have been doing this for ten years. The reason it's being focused on now is because we've had a third of the stations turn off their analog signal and station management realizes that digital is all they've got now. There's nothing else to fall back on.
So, obviously, they want viewers to have a good experience with their free, over-the-air signal. Has the test and measurement equipment caught up with the technology yet? Yeah, essentially it's out there. However, I can't say with certainty that every station has all the equipment that they really should have because it's a money thing.
A lot of times test equipment is on the lower end of the list of things to buy, especially in a bad economy like we have right now. What kind of investment do you need to make sure you have the proper test and measurement gear? That's a hard one since it can vary greatly. It depends on the accuracy of the equipment. Of course, commercial grade test equipment is also available for less money, but with less accuracy.
Then you need a power meter, but you probably have that from the analog days. It is also a good idea to buy an MPEG transport stream analyzer so that you can analyze the actual data packets. So it depends on how far back in the chain you want to go. Let's talk environment issues -- buildings, terrain and other forms of interference.
What are you learning about how they affect the signal? What we're finding out is that interference from multiple different signals is still an unknown at this point. There had been the assumption in the laboratory and in the FCC planning that there would be one primary interferer.
We think we need to go further and start looking at multiple interferers. Obviously, the focus is going to be on digital-into-digital interference as analog turns off. We've also recently had some problems with FM radio stations.
The second harmonic of those stations fall into the high-VHF television band and are causing problems and interference with broadband amplifiers. The broadband amplifiers could be inside the antenna or they could be external to the antenna on the mast or they could be distribution amplifiers in the home.
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