The Clicker: Single Frequency Networks and OTA HDTV
posted May 5th 2005 4:20PM
One look at the eye-popping picture and you were hooked. The window-like effect dazzled you. You all but drooled
over the rock-solid picture. There's no snow. There's no ghosting. The resolution is to die for. You saw the light and
its name was High-Definition. There's only one problem: you can't seem to pull in a reliable OTA (over-the-air)
signal.
Oh sure, cable is an alternative for many. However, over the years, you've been spoiled; you're hooked on your PVR.
And, as a connoisseur of PVRs, you understand that the cable companies offer anemic light-weight versions. You demand
more. You're not happy unless you've got your TiVo or your MCE (Microsoft Media Center Edition). You've even been known
to utter the phrase, "They'll have to rip my TiVo out of my cold dead hands." Worse yet, you meant it – LITERALLY.
Fear not – all hope is not lost. Despite previous attempts to scale your house with antenna in hand only to be
denied a clean signal, there are two reasons to still have hope: a) Single-Frequency Networks and b) 5th generation
receivers.
Since the beginning of broadcasting nearly all stations have followed the same steps when it comes to transmitting
their signals: a) do your best to locate the tower in the middle b) build the tower high and c) add power. The problem
is that, in some regions, this method just doesn’t cut it. Natural terrain presents issues as do man-made buildings.
Signals bounce around or, worse yet, fly right over your head. If State College, Pennsylvania’s WPSX is a harbinger of
things to come, the answer to your woes just might be Single-Frequency networks (SFNs).
More commonly discussed within the context of Europe’s COFDM, SFNs are beginning to show signs that they might just
work with America’s 8-VSB modulation scheme also.
So what are SFNs?
Simply stated, single-frequency networks are when a broadcaster uses multiple transmitters to send the same signal
over the same frequency. The idea is that in certain geographically-difficult areas broadcasters will have much better
success if they can fill coverage voids by utilizing smaller, usually-less-powerful satellite towers in addition to
their main tower.
It sounds like a simple idea, but, as always, the devil is in the details. Unlike the repeater towers that are
sometimes used in western states (AKA translators), SFNs aren’t simply receiving the signal, error-correcting, and
retransmitting on a different frequency. That is an extremely inefficient use of the spectrum.
So what are they doing?
There are two main methods of creating a single-frequency network. The first is through the use of on channel
boosters. On channel boosters quickly receive the OTA signal from the main tower and retransmit the signal on the same
frequency. The problem is that there is no error correction. So, any errors in reception are simply retransmitted along
with the slight echo caused by the booster itself. On channel boosters are also limited in their placement as they need
reliable reception from the main tower.
So, when WPSX took the plunge into SFNs, it opted for option two, distributed transmission. Working with the New
Jersey-based Merrill Weiss Group, WPSX has created one of nation’s most technologically-advanced broadcasting
systems.
With distributed transmission, the signal is delivered to each of its transmitters via fixed channels (land-based
delivery). Then through the use of GPS-based reference clocks at each tower (for both time and frequency), the signals
are synced so each can emit a perfect copy of both signal and symbol data. The result is a group of towers working in
harmony.
So why isn’t this done more often (at all)?
No matter how synchronized the output from the towers may be, there will always be some amount of multi-path to deal
with, and until recently receiver hardware wasn’t equipped to handle anything but very minor levels of multi-path.
Multi-path? Yes, multi-path is the result of your receiver seeing the same signal more than once. This can be caused by
a variety of things. For instance, in a crowded downtown area the signal can bounce off buildings many times before it
hits your antenna. The problem occurs when some copies of the data arrive more quickly than others. This, in effect,
can jam your receiver.
The same effect occurs within SFNs. Because waves from different towers have the potential to reach your antenna at
different times, a receiver’s ability to handle multi-path is paramount to its success.
With that said, LG has continued to make great strides in their ability to handle multi-path issues. Through the use
of techniques such as adaptive equalizing they have even begun to turn a negative into a positive. First generation
tuners had a multi-path window of -3 to +10 us. Fifth generation tuners have increased that window to -50 to +50. As
always bigger numbers are way better.
So, if you’re like me, unable to receive a signal despite near-heroic efforts. Have hope; there’s technology out there
that might just help one day.
But for now – we wait.
If you have any comments or suggestions feel free to drop a line at
theclicker@theevilempire.com.
Until next week, save my seat!
Kinda depressing article. I was hoping there would be an offered solution other than "technology will fix this sometime in the future". That's like a bad Star Trek episode where all problems are solved by a tachyon burst from the main deflector.
Anyone have any luck with OTA antennas? I've tried two to match my soon to be obsolete D* HD Tivo and neither could get me UPN reliably in NJ for my Veronica Mars fix. The Silver whatever one is what I'm trying now (looks like a series of boomerangs) but it doesn't work well.
Get a 10db booster, radioshack has them, i got one because i was just short of getting pbs reliably
I would love to see SFN implemented in San Francisco - Bay Area, California. With so many hills and mountain, the area is a prime candidate for SFN coverage.
I used the Silver Sensor for about four months with my HDTivo, but found an indoor antenna couldn't cut it no matter where I put it --- as high in possible in the attic or even outdoors on the roof.
I'm now using the ChannelMaster 4221. I'm about 20 miles from my tower; it's mounted on my roof on a six-foot mast, about 20 feet in the air. Pretty small and inconspicuous compared to traditional antennae; it's a highly-directional vertically-mounted grid that's a little smaller than 2x3 --- an oversized tennis racket, only square. I'm happy with the reception; I see one second of pixellation maybe once a day. People farther away from the tower use the 4228; it's basically the same but twice the width. The 4228 captures up to 60 miles, apparently.
These models are UHF only, but since UHF frequencies are better for DTV signals, they're standard even for stations that broadcast analog on the VHF band. Check antennaweb.org to make sure all your local digital channels are in UHF (including UPN), and get yourself a ChannelMaster, especially if you have no interest in receiving standard-def analog VHF channels. If UPN in NJ is broadcasting on VHF, your Silver Sensor won't pick it up, and neither will the ChannelMaster. Also, since the ChannelMaster is highly directional, it helps if you're getting all your signals from a single multi-station tower (as in SF) or the same basic direction; say, NYC.
For some people, the day is already here:
http://valueelectronics.com/DLP,%20LCD%20and%20CRT%20Direct%20View%20HDTVs.htm
I might get the 30" or its Slim CRT brother.
This would also go nicely in my HTPC when it arrives eventually:
http://www.digitalconnection.com/Products/Video/fusion5.asp
I'm sure there are plenty more products with 5th generation tuners on the way. All new LG HDTVs with digital tuners have 5th generation tuners in them. But strangely enough, no LG 5th generation set-top box yet.
The 5th generation tuners will solve all your problems.
you guys are lucky you even have ota hd. down here in arkansas there isn't any at all.
Why were PVRs mentioned? At all? Honestly now, you shouldn't write an article with a heading paragraph and bait people in to reading what might be some sort of magical upcoming solution to PVRing HD content without regard to where it comes from and such. But no, you never mention PVRs again after the intro. Quite disappointing.
Single frequency networks sound great... for local HD networks. Big freakin whoop. What about all the rest? You know, the stuff people actually watch that isn't censored to hell and back? How will that be delivered in an HD form that can actually be PVR'd without resorting to some locked-down restricted box provided by a company who listens to the faux death cries of the movie industry and produces DRM solutions that destroy the usability of their product? What's going on with the broadcast flag? Is the court ruling that said the FCC can't mandate that it be adhered to actually going to have effect? Or will the hardware makers bend over like the cable companies do by bundling HDCP into their cable boxes?
He's already written on some of those issues. If you go back and read the column on cable cards you can hear what's going on in the cable box arena.
5th generation chips are not the answer. LG 5th generation chips are not the answer. LG has however, using a 5th gen chip and other front end technology built a prototype receiver that does work minimally well. We tested it last summer. If they are putting that technology into their HDTV sets good but we have yet to see any set top box that has this chip that works as well as the prototype we say last summer.
We are testing two receivers this week that use LG 5th gen chips. So far one of these does not match the LG prototype. One we tested two months ago did not either though the one we have now is better.
So don't think that a receiver or HDTV set will work just because it had LG or any other companies 5th gen chip. There is more to it than that.
As to SFN's and 8-VSB. I will beleive it when I see it. COFDM works flawlessly with SFN's. 8-VSB AFAIK still needs hills to hide behind. Might as well reuse the same spectrum on the moon and call it an SFN.
The US should change to COFDM and MPEG4 while our DTV transition is still stagnate. France, China, Japan and other countries are using such advanced technology.
We started down the HDTV road in a big hurry to beat the Japanese. We are now rapidly falling behind not only the Japanese but virtually everyone else.
Congress has blinders on and is about to promote the spending of tax payers money to put 8-VSB receivers in millions of homes to get the transition off dead center. These receivers will not be 5th gen anything. They will be cynically produced cheap receivers made by the lowest bidder. They are being shown in DC right now to uncritical members of Congress who have no idea what a good receiver should be.
Manufacuters are putting these junk receivers into TV sets to satisfy the FCC mandate. They are putting in the cheapest stuff because they know that 85% will never be attached to an antenna and used. Since there are no standards competition dictates that they put the worst junk at the lowest price in these TV sets.
This is the time to stop and take another look at the disaster that we have in our digital transition not the time to blindly charge ahead.
I do OTA HD, exclusively. That's right, I don't subscribe to cable or satellite. Got me the Zenith 32V37 with built in tuner and the biggest honkinest antenna money can buy at the friendly home depot. Mounted the monstrosity on a tripod and a 6' boom on my room, cabled it with quad sheilded coax, and never looked back. I just love the $45 a month that I don't pay to Comcast anymore.
I'm 28 miles from the Sear Tower, outside Chicago (60103 represent), and I get ALL the stations wonderfully, even WBBM. That bitch broadcasts on channel 3 at like 60 watts, it is notoriously difficult to receive.
I look forward to the day that the FCC forces the TV stations to give back their analog frequencies. Hopefully the low VHF frequencies are put out to pasture and I only have to deal with UHF. Then I could get a much smaller antenna, and I would be 100% happy.
The much hyped and totally unrealistic DTV transistion of 2006 is a "solution to a non-existent problem"...What do I mean? Well, first off I use 2 OTA Samsung tuners at home and they are wonderful when they get a signal.. even on aging 480i sets. The bottom-line is that the vast majority of the general public really doesn't care about "rushing" out to buy a new TV. There may be some 270 to 300+ million analog sets still out there and that's just in the US ! When "Joe" consumer goes out to "whatever -mart" he/she wants to buy based on price and low price usually ends up in "joes" living room. On a side note, it amazes me that most shoppers still buy curved tubed sets when the better and not THAT much more expensive sets are nearby on the shelves!! There are certainly a lot (millions) of consumers who want the "good" stuff, but it's just a drop in the bucket by comparison...That may change somewhat as DTV chipsets continue to be more integrated and price drops, but it will be many years. As far as current OTA goes, I live in a major market and reception is "fair" when all stations are considered. This is interesting since the transmitters are in a cluster in one section of the city. It's really annoying to notice that people walking around in a room causes "pixel city" and dropouts and total loss of signal....does not say much for the current OTA system !!!!
Follow-up to previous post! I meant consumers choose the cheaper and inferior curved tube sets when the FLAT tube sets are just a little more money but are better...well worth the extra cost.
In response to Mike in Philly.
Limit that to the current OTA system in the US. The rest of the world is light years better than what the US is stuck with.
We talk about corruption in other countries but all of them except the US, Canada, S. Korea and Mexico were able to resist the money and lobbying that stuck us with our current completly asinine modulation.
Thank Congress for putting their war chest above any public interest once again. Live with it or vote the b******s out.
Right now at the top of my list of those who sold you and the rest of us out is Congressman Dingell of Michigan. He personally intimidated broadcasters to vote against their own and your self interest and for the junk that you will have to buy in your next TV set. He also intimidated the Department of Defence when they brought up the issue of Homeland Security in 2000.
The other digital modulation being used in the world are far far far better for Homeland Security but Dingell silenced them on the subject.
Then you have the current group in the House, Markey, Upton, Barton and others who with Dingell on the House Commerce Committee are working to FORCE the transition down our thoats with NO receive on the market that should be.
We need receiver standards for the current horrible modulation we are stuck with or a new modulation.
In response to Mike in Philly.
Limit that to the current OTA system in the US. The rest of the world is light years better than what the US is stuck with.
We talk about corruption in other countries but all of them except the US, Canada, S. Korea and Mexico were able to resist the money and lobbying that stuck us with our current completly asinine modulation.
Thank Congress for putting their war chest above any public interest once again. Live with it or vote the b******s out.
Right now at the top of my list of those who sold you and the rest of us out is Congressman Dingell of Michigan. He personally intimidated broadcasters to vote against their own and your self interest and for the junk that you will have to buy in your next TV set. He also intimidated the Department of Defence when they brought up the issue of Homeland Security in 2000.
The other digital modulation being used in the world are far far far better for Homeland Security but Dingell silenced them on the subject.
Then you have the current group in the House, Markey, Upton, Barton and others who with Dingell on the House Commerce Committee are working to FORCE the transition down our thoats with NO receive on the market that should be.
We need receiver standards for the current horrible modulation we are stuck with or a new modulation.
it's a highly-directional vertically-mounted grid that's a little smaller than 2x3 --- an oversized tennis racket, only square. I'm happy with the reception; I see one second of pixellation maybe once a day. People farther away from the tower use the 4228; it's basically the same but twice the width. The 4228 captures up to 60 miles, apparently.
These models are UHF only, but since UHF frequencies are better for DTV signals, they're standard even for stations that broadcast analog on the VHF band.