[opendtv] Interview: ARE MANY TRANSMITTERS BETTER THAN ONE?
- From: Craig Birkmaier <craig@xxxxxxxxx>
- To: OpenDTV Mail List <opendtv@xxxxxxxxxxxxx>
- Date: Sat, 15 Sep 2007 08:55:35 -0400
http://www.tvnewsday.com/articles/2007/09/13/daily.4/
TECH ONE OF ONE WITH MERRILL WEISS
SFN: ARE MANY TRANSMITTERS BETTER THAN ONE?
TVNEWSDAY, SEP. 13, 9:04 AM ET
There is no doubt in the mind of the technical consultant who helped
develop the technology. He believes that a single frequency network
of transmitters operating on the same channel can blanket a market
with a stronger, more uniform signal than the conventional big-stick
approach, improving indoor and planned mobile reception.
To get the greatest coverage, you put an antenna at the highest
possible point on the tallest tower you can find and you plug it into
the most powerful transmitter the FCC will let you have.
But, despite all those superlatives, simple might not always be best.
In the new world of digital broadcasting, it might be smarter in some
markets to deploy a network of complementary transmitters operating
with less power and relatively low antennas.
The so-called single frequency networks (SFNs), according to their
proponents, blanket markets with a more uniform and pervasive signals
than the conventional big-stick approach, improving indoor and
planned mobile reception.
SFNs also may prove a handy alternative when, for whatever reasons,
the big stick just isn't practical.
Indeed, that's the case in New York City. There, two separate groups
of broadcasters are experimenting with SFNs, having concluded that
the their common "tower"-the Empire State Building-just won't
accommodate all their digital signals come the analog cut-off date of
Feb. 17, 2009.
Representing the 10 largest stations in New York, the Metropolitan
Television Alliance is looking at SFN technology as an interim
solution, a way to bridge the gap between the analog cut off and the
day in 2011 when the all the stations are supposed to migrate to a
new mast atop the 1,776-foot Freedom Tower, the tallest of five
skyscrapers being built where the Twin Towers fell on 9/11.
MTVA President Paul Bissonette told TVNEWSDAY in an interview posted
on Tuesday that testing will soon begin of a prototype SFN system,
including four low-power transmitters in Brooklyn.
Meanwhile, Ion Media and Richland Towers have reported positive
results from their preliminary testing of a separate SFN system
comprising one transmitter in West Orange, N.J., and another in Times
Square.
The technology is still in its infancy. Only a handful of systems
have been deployed in the U.S. with its 8-VSB digital broadcasting
standard. Because of that, Bissonette offers no guarantee in New
York. "Nothing like this has ever been attempted before in an urban
environment," he says.
For a better understanding of SFN technology-its variations and
potential, TVNEWSDAY turned to Merrill Weiss, a technical consultant
based in Metuchen, N.J., who may be the foremost authority on the
technology. He holds a key patent and has been involved in most of
the applications, including those in New York.
The recipient of the NAB's 2006 Television Engineering Achievement
Award, Weiss explains the technology and how he expects a full-blown
SFN system that he is working on in Philadelphia to soon settle any
question about SFN feasibility once and for all.
An edited transcript follows:
TVN: Give me the short course on single frequency networks.
Weiss: A single frequency network is a system in which multiple
transmitters are used to deliver signals to an area that a station is
trying to cover, and the signals from those multiple transmitters
overlap one another at least in some areas.
We talk about small-cell systems and large-cell systems. In the
large-cell systems, you're basically spreading the transmitters far
apart and you try to minimize the amount of overlap. In a small-cell
system, you have transmitters close together and you actually
intentionally overlap the signals from adjacent transmitters, but you
try to prevent overlap two cells away.
Now, when you talk about single frequency networks, there are three
types of transmitters-digital on-channel repeaters, distributed
transmitters and distributed translators.
An on-channel repeater basically receives the main signal over the
air and simply retransmits it. The issue there is that there is some
time delay that causes an echo. So the trick is to minimize the time
delay.
Distributed transmitters all share the same channels, but they are
simultaneously fed with a signal and synchronized in such a way that
they do not interfere with one another.
The third approach is distributed translators. You actually need two
channels. You put the main signal on one and then you can have
multiple translators on another channel that are synchronized just as
the distributed transmitters are.
TVN: Basically, you're talking about multiple transmitters operating
at lower powers rather than one big stick.
Weiss: Maybe. You could put up a large facility just as you would
have built in the past, but find that it doesn't reach all of the
service area because of some terrain blockage. You then could put in
an additional transmitter to fill in the part of the service area
that you could not reach from your single transmitter.
TVN: In the distributed transmitter and distributed translator
approaches, how do you prevent the transmitters operating on the same
frequency from interfering with each other?
Weiss: In reality, they do interfere with each other. The trick is
that we take advantage of some technology that's built into each one
of the digital receivers that is designed to deal with echoes that
occur in a natural environment. That technology is the adaptive
equalizer. There's one in every digital receiver and they've been
getting better over time.
The big reason that the early digital receivers didn't work too well
and people were concerned was because the adaptive equalizers were
not really quite good enough.
But over time they've gotten a lot better and as a consequence, the
VSB transmission system is working a lot better. And so we can take
advantage of that in the systems that use multiple transmitters by
making the signals from those transmitters look like echoes that
occur in the natural environment due to signals bouncing off of
buildings, mountains and things of that sort.
TVN: When you say echoes, you mean ghosts.
Weiss: Yes, except that ghosts are an analog phenomenon. They create
ghost images in the picture. In the digital world, you get exactly
the same phenomenon, but instead of showing up as ghosts in the
picture, what it can do is destroy the receiver's ability to extract
the data.
TVN: What was your particular contribution to this technology?
Weiss: I developed a way to synchronize the transmitters so that the
signals that they emit would be seen as echoes by receivers.
TVN: How does that work?
Weiss: When you do distributed transmission, you need a
studio-to-transmitter link to each of the sites and what you get for
that is the ability to adjust the relative timing of all the
transmitters. You have complete control over them.
We insert a little extra data in the signal to each of the
transmitters. That little extra data tells the transmitters to
produce outputs that are appropriately timed with one another.
TVN: How many of SFN installations are you aware in the U.S. right now?
Weiss: Well there are five distributed transmitter systems either on
the air or in under construction that I'm aware of. There was a sixth
that was put up for test and experimentation purposes.
TVN: Where are they?
Weiss: One is in State College, Pa. Penn State put that on the air in
2003. There's one down in Virginia, operated by Shenandoah Valley
Public Television. There's one going on the air in Philadelphia-an
eight transmitter network that soon will be on the air. They are
testing two systems in New York City. And Tribune had one in
Indianapolis that was for test purposes.
TVN: I know that you were involved in the State College project. What
does that look like?
Weiss: That is designed for four transmitters, one of which is a
megawatt-class transmitter. It's 810 kilowatts, but it's high enough
that it's equivalent to a megawatt, and then there are three gap
fillers, one for each of the three major population centers-State
College, Altoona and Johnstown.
TVN: So they have one big stick and three little ones.
Weiss: Yes, but only one of those smaller ones is on the air at the
moment. They initially only had the money to do those two
transmitters. Also, they were doing it on an experimental license
basis and so they have to complete the field testing and get data to
the FCC. Once they do that, they'll go ahead with the other two
transmitters. That field testing is almost complete.
TVN: What's the verdict there?
Weiss: Oh, it's great. It works. I mean it basically shows that, in a
preponderance of locations in the State College area where people
could not have received digital television from the main transmitter,
they now can get it, and, in many cases, they can get signals that
are strong enough for reception indoors on a set-top antenna.
Now, just to be complete, there are some locations where there's some
amount of interference between the two signals and it actually makes
it a little worse.
You have to be aware of that when you build one of these networks,
what you're doing is balancing the gains versus the losses. You try
to gain a lot and lose only a few. And you try and do it in a way
that the few can still receive a signal, if they use a directional
antenna.
TVN: You've got a big project right now-WTVE Reading in the
Philadelphia market. Tell me about it. [Editor's note: WTVE is just
coming out of Chapter 11 bankruptcy. Trustee George Miller is hoping
improved coverage and $2.6 million in annual revenue will attract a
buyer at his asking price of $12 million. The station, which operates
on analog channel 51, is now filled with infomercials, paid religion
and paid sports.]
Weiss: Reading is a large-cell, distributed transmission system with
eight transmitters operating on digital channel 25. Some of the
transmitters are 80 to 100 miles apart.
They had a construction permit that allowed them to have a megawatt
class transmitter that was about 50 kilometers or about 30 miles
outside Philadelphia. What we did instead was put a relatively high
power transmitter in at the Philadelphia antenna farm and then fill
out the service area with the other distributed transmitters.
It's got one additional issue that we had to deal with and that is
that there is an adjacent channel station [channel 26] in the market.
Part of the management of interference between adjacent channel
stations is to co-locate the transmitters whenever possible. Because
of the fact that there was an adjacent channel in the market, what we
had to do was put a higher power transmitter in the neighborhood of
that adjacent channel station, but at a lower power and close enough
to it and close enough in power that [the adjacent channel] signal
didn't overwhelm the signal from the distributed transmission system
that we're trying to put in.
So, to make all this work, we were pretty much forced to put a higher
power transmitter in at the Philadelphia antenna farm. So that in the
regions where that other station, which is a megawatt class station,
is very strong, we had a strong enough signal that we could overcome
the interference from it.
As we move away from the adjacent channel station's transmitter to
where its signal gets much weaker, we use lower power transmitters.
We are making the signal from the distributed transmission system
stronger than the adjacent channel, but not so strong as to cause any
interference with it. We also designed a special antenna pattern that
prevents what I call hot spots around the base of each of the
transmitters.
TVN: So you put a big transmitter at the antenna farm with everybody
else and seven other transmitters scattered throughout the market.
What type of towers are you using outside the antenna farm?
Weiss: Six of them are cell towers and one of them is a broadcast
tower. Some of them have the antenna only a hundred feet off the
ground even though the towers are sometimes much taller than that.
And some of them are higher than a hundred feet depending on what
we're trying to do.
TVN: When will the system be up and running?
Weiss: Well, we're turning it up now. Part of it's on the air already
and we're turning on additional transmitters day by day.
TVN: And at the end of the day you're going to have much better
coverage within your permissible coverage areas than if you had put
one big installation in at the antenna farm?
Weiss: That's correct. The market includes outlying communities like
Reading, Lancaster and Allentown. They're modest sized cities. The
signal will be about 20 dB stronger in those places than we could get
from the antenna farm.
And using the antenna farm alone was not really an option because we
couldn't cover Reading, the city of license, from there. There was an
obstruction in the way.
TVN: Is this going to more expensive than the conventional approach?
Weiss: It turns out that it will be in this case because of some
things we had to do to meet some FCC limitations, but it didn't
inherently have to be.
TVN: I'm told that the distributed transmission approach is much
better for mobile broadcasting. Would you agree?
Weiss: Yes. When you are using a mobile receiver, you can imagine
going around a corner or just going down the street and going past a
building that blocks the signal from a single transmitter. But, if
there are multiple transmitters, then you have signals coming from
multiple directions and it's much more probable that if you lose the
signal from one, you'll still have the signal from another.
Of course, this depends on how the network is designed, but, if the
network is designed for mobile applications, then you get what's
called transmitter diversity that allows you to continue to receive a
signal even though major obstructions may appear and disappear in the
path to a single transmitter. That's just one factor. Another is that
having transmitters closer to receivers provides a more reliable
service. You can see that in the cell-phone system.
TVN: ATSC is working on an in-band mobile broadcasting standard. I
suppose it will have the same kind of adaptive equalizing technology
that the DTV receivers do.
Weiss: Right. One of the big challenges is to make the equalizer work
better. Or, you could eliminate the need for the equalizer in certain
ways by making the signal less susceptible to echoes.
TVN: What kind of interest are you getting from broadcasters in
single frequency networks?
Weiss:Well, let's put it this way: I know that a couple of years ago
one group broadcaster that took a look at its stations figured that
it could use distributed transmission in half of its markets.
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