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Digital Television Transmission Standards
I wanted to know what Slashdotters thought about the COFDM vs. 8VSB digital television transmission standard issue. However, I was suprised to find nothing related to this topic in a Slashdot search. This is an important issue, and it's a big one: almost half of the broadcast television stations in the US support the movement, lead by Sinclair Broadcast Group, to change the standard.
Here's the issue (another NY Times article talks about this -- search for DTV). Digital television, or DTV, is projected to replace America's current NTSC transmission system sometime in the earle 21st century. Stations have already begun to move to the new trasmission format. It promises better picture quality, no ghosting, and (here's the big one for "nerds") 19.2 megabits per second of raw binary data. One of the major forseen applications is delivering data like the data on the PointCast Network to mobile devices. Of course, however, the main application is television. Broadcasters have a choice: either they can transmit one channel of amazingly high-resolution, stunningly detailed high-quality video (HDTV), or several channels (4, I think) of standard-quality video, which is better than the video of the current system because there is no ghosting and fading up until a point where it doesn't come in at all. The issue centers over that point.
The current system is known as 8VSB, and it passed advanced laboratory testing and even some basic field testing almost ten years ago. However, last year, when Sinclair did actual, in-home, average-viewer's-setup testing of this system, it didn't work as well as the NTSC system. They could not receive HDTV signals from a station near an NTSC station whose picture came in clear. Then Sinclair did more, and more detailed, tests at home in Baltimore. This time they brought for comparison a sample modulator for the European transmission system called COFDM, and a demodulator / decoder box to receive it. They tried it in streets with tall buildings, parking garages, and apartments, all places where multipath, which causes ghosting, is prevalent. Each time they tested the two systems--European COFDM and American 8VSB, the COFDM receiver picked out its signal without fail--"It was hard to find a place where it didn't work," says my dad, who was part of the testing--but it was hard to find a spot where 8VSB would
work. Continued testing convinced Sinclair officials that the current system would not be able to work in the real world, and are pushing for a change to COFDM, a system that has been proven in Europe. They wrote up a petition to the FCC, and almost half of the television stations of the US have signed it.
Finally, somewhere where the EU is ahead of the USA.
Don't forget GSM phones. They work everywhere but the US.
--GnrcMan--
Basically, here's the gist: Broadcast companies could care less about broadcasting HDTV. For all their talk about drastically improving the quality of television, their eyes are on the really big asset they're sitting on: their spectrum.
First, a little history. (Sorry for the slight tangent, but bear with me :-) Unbeknownst to most people, network TV stations are the only companies in the country that get free transmission spectrum. This was done in 1932 (or sometime around then) when there were few other uses for the bandwidth and the government wanted to encourage broadcasting because they felt it would be in the public good to have universal access to this new communications medium. Since then, of course, that spectrum has become incredibly valuable, but the broadcasters continue to get it for free.
Enter HDTV. Using modern compression standards, broadcasters can fit the entire datastream of an HDTV picture into the same 6MHz T.V. channel currently used for NTSC. But broadcast companies started looking at it the other way around. Using modern compression standards, they could fit 6 NTSC channels into one spectrum slice. Or... they could fit 1 NTSC channel into 1/6 the slice, and use the other 5/6 slice for other services e.g. data transmission, cell phones, etc. After all, they're getting a full 6Mhz for free; if they can continue their current broadcasts (thereby continuing their current revenue) and add other profitable services without having to pay for the spectrum, why not?
Look at it this way: they could either use the 6Mhz to a) transmit 1 HDTV channel b) transmit 6 NTSC channels c) transmit 1 NTSC channel and a bunch of other services. It's clear that options b & c would be far more profitable than option a. This is why there is no one HDTV standard, but a whole spectrum of standards. Note how NTSC defines one picture standard, but HDTV defines 18 (all of which must be supported by a TV in order for it to be sold as an "HDTV")! One of those happens to include compressed, digitized NTSC...
Grease the palms of our honorable legislators enough, and it's not hard to get a sweet deal. And the networks are sitting on an incredibly sweet deal. First of all, they can decide which picture standard to use (ranging in quality from crappy NTSC to fullblown HDTV) assured that consumers have paid for the expensive decoder chips to watch whichever standard they choose to broadcast. Secondly, they can decide which mix of channels/services/etc. is the most profitable for them with no regulation whatsoever that forces them to use their spectrum for actually broadcasting HDTV. And they can do it all on free spectrum that otherwise would have cost them $70 billion (according to estimates of how much that spectrum would have fetched the government if it was auctioned)!
Are you feeling sick? Do you want to lead a consumer revolt by not buying HDTV sets? Don't worry; they have that covered too. In 10-15 years, by law, all NTSC broadcasts will be halted and everyone will be forced to switch over to HDTV. Unless you want to quit watching TV of any kind, you *must* purchase an HDTV set. Note how if you have a B&W T.V. from the 40's, you can still watch T.V. today, but 10 years from now, your NTSC set will be useless; why do you think they couldn't come up with a way to maintain backward compatibility when they were defining the HDTV standard? Or at least allow the market to determine the rate of HDTV acceptance as it saw fit? Perhaps because broadcasters knew that once people began to see that they essentially bought expensive new sets in order to watch the same crappy TV just so that the network companies could make more money off their spectrum, no one would buy HDTV sets and networks may have to continue broadcasting NTSC and miss out on all their extra profits...
So to segue back on-topic, broadcasters could care less about the quality of TV transmission and the details about penetration rates, signal quality, etc. etc. Because no matter how bad the transmission quality is, in 10 years, everyone will be forced to adopt the new standard anyway. And why should they care if half the people in their station area can't receive their TV signal and are thus not watching their advertising? They'll be making far more from all those extra services they'll be selling on their newfound $70 billion bandwidth horde...
I am a professional television broadcaster. I run engineering for one of the major channels on almost all cable systems. HDTV is almost surely doomed to failure.
First and foremost, the majority of Americans watch TV via cable or direct-to-home satellite. There is currently no way for consumers to get HDTV programming this way. There is no benefit to cable companies to send HDTV to cable customers. HDTV uses up much bandwidth and there is no way for cable headeds to insert their own commercials. The VSB vs. CODFM debate is small potatoes compared to the fact that no HDTV is available via cable system.
Second, there are *no* standards for broadcasting HDTV besides transsmission. Broadcasters are free to choose any systems that fits into the VSB transmission system. There is not even agreement on making the system progressive (what computers use) or interlaced (what analog TVs use). This has led to a bad situation for the television set makers. They have to make a choice on the 'native display format' for they televisions. That means that if Sony make a 720 Progressive tube set, all broadcasts not in 720P must be deinterlaced or upconverted to that format. That conversion leads to even more picture degredation
Remember, the base HDTV signal used in post-production is 1.5 GHz and that is squeezed down to 19.4 Mbit/s for transmission. Compression can only go so far.
Lastly, while the VSB vs. CODFM indeed does rage, that is nothing compared to the simple fact that there is no business reason to broadcast HDTV. The only reason broadcasters are doing it is because the FCC is forcing them to do it. No one has been able to demonstrate any revenue plan for HDTV yet.
BTW, what people have in Europe is digital television, it is not High Definition TV. All of the direct-to-home satellite services in America are basically the same as DTV in Europe. I think that Dish Network even uses the DVB (European) format for transmission. Currently, Europe has no HDTV.
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It really depends how bad the picture is that you see on analogue TV at the moment. If the major problem is 'ghosting', ie spurious extra images caused by multiple reflection paths off the mountains, COFDM is particularly good at sorting these out to achieve constructive interference between the different paths.
If the major problem is 'snow', a signal amplifier box can help, just like for analogue TV. But if there's too much snow, this will destroy the digital signal.
One other trick with COFDM is that the TV company can add additional transmitters on the same frequency to sort out reception blackspots. With analogue signals this would just cause particularly bad destructive interference, and would be a Bad Idea. But because COFMM can make the two signals interfere constructively, this opens the door to networks of transmitters all on the same frequency.
This article by Craig Birkmaier is very good on COFDM vs 8-VSB generally, and covers the possibility of Single Frequency Networks in a bit more detail.
I hang out at Digital Theater to keep up with Digital TV happenings. They've had many discussions on this topic, and recommend reading the FCC report, DTV REPORT ON COFDM AND 8-VSB PERFORMANCE. The summary of which is
8-VSB was choosen over COFDM during the 9 year period in which the DTV standards were created. One of the major benefits of 8-VSB is it covers a much larger area which is a very important benefit in the US. It also carries more data than COFDM, which means a better picture can be received. In Europe, where everything is much closer together and multipath is a bigger problem, COFDM was choosen as their broadcast standard.
The 8-VSB reception problems shown by Sincliar were exhibited in 1st generation HDTV sets. The manufacturers of the sets have already come up with better ways of cancelling out the multi-path interference that plays havoc with HDTV reception. The newest sets work much better than the 1st generation, and it is expected this trend will continue.
The general consensus at Digital Theater is that Sinclair is not pursuing this for the benefit of us, but for their own benefit as they stand to gain a lot financially if the US changes to COFDM.
In 6 years, I imagine most of the televisions in our house will still be in service. TVs last a long time, and I'm not concerned with the "latest and greatest" stuff as long as I can watch the occasional show or Yankee game.
What I want to know is, what happens in 6 years when stations quit broadcasting in their current format. Will my non-cable-box-connected, normal, cable-ready televisions still be fine? Or will I have to toss them and buy new ones, something that'll probably end up costing a couple thousand dollars if I have to replace 5 or more sets. If the latter turns out to be the case, I can assure you that I and *millions* of other Americans will be incredibly pissed off.
I'm not sure this whole thing was planned out right, except maybe with the question, "How can the electronics and television industries make a shitload of money in one fell swoop?" being the only objective. What of the millions of people who can't *afford* new HDTVs or the little box to convert down to "normal" broadcast signals of today? Do they get told, "Sorry, you're fucked"? There are plenty of people out there that simply will not stand for this, if the options are either buy a new television or give up TV.
Do I have my facts messed up? Is this really what's going to happen in a scant 6 years' time? If so, it's going to backfire bigtime. Expect to see common analog signals being broadcast far into the future alongside HDTV signals, until the marketshare of people with older, "inferior" TVs is such that ignoring them and turning off the analog broadcast towers for good is more cosf-effective than not.
If I'm wrong, someone please enlighten me.
- A.P.
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