Why VHF is bad for DTV

[Carmine5]

Here is an excellent article courtesy of TV Newscheck explaining why DTV is problematic on the VHF band. One factor often ignored is its high susceptibility to impulse noise from other electronic equipment. Even an antenna amplifier can hinder digital VHF.

The article also points out tactics broadcasters can use and what those who receive OTA DTV can do to improve VHF reception.

http://www.tvnewscheck.com/articles/2010/03/25/daily.1/

c5

 

[landtuna]

This article seems to be full of omissions and commissions. A few I noticed:

"Even though only 10% of the average station's audience is watching off the air,"

Here we go again with yet another baseless estimate of OTA viewing. Just having questioned another poster and received a qualified 30% number. Without a verifiable number how are broadcasters going to qualify their decision(s) to upgrade OTA viewers?

"After all, the FCC assigned digital channels to stations based on the assumption that viewers would be tuning in with antennas 30 feet above the ground."

WHAT? Do we really have a collection of imbeciles at the FCC who think everyone lives in a two-story house and has another 10' of antenna post roof-mounted?

 

[tripinva]

Yes, the FCC standard is 30 feet. The Longley-Rice coverage maps I have on RabbitEars assume a height of 13 feet instead.

- Trip

 

[Markxxx]

Is VHF really that bad or does building and geography also deal with it? I mean I noticed a lot of Chicago people have trouble with the VHF but those in the suburbs do not have trouble with digital VHF. Is it the buildings? I get no DTV on VHF or UHF and I live in Chicago and was told it was the buildings and you need an outside antenna.

Does VHF need more power? One thing I found odd was WTTW in Chicago on Channel 11 was fine in analog. Then when a TV station in Lafayette Indiana started broadcasting digitally on channel 11 it messed up WTTW's analog signal.

 

[w9wi]

Is VHF really that bad or does building and geography also deal with it? I mean I noticed a lot of Chicago people have trouble with the VHF but those in the suburbs do not have trouble with digital VHF. Is it the buildings? I get no DTV on VHF or UHF and I live in Chicago and was told it was the buildings and you need an outside antenna.

I think there are two fundamental problems with VHF DTV:

- Interference. Everyone who could get a 7-13 channel for DTV took it, and at times the FCC packed them in a bit too tight. It's looking as if DTV-into-DTV interference is worse than analog-into-DTV.
- Antenna hucksters. Stores are selling UHF-only antennas in markets where major stations are on VHF. I got another call like that just this afternoon: store sold the guy a UHF-only panel antenna, leaving him wondering why he can't get NBC or PBS. Those high-VHF stations that have had to move back to UHF should be billing Wal-Mart.

Does VHF need more power?

There is some thought to that effect. I'm really not so sure it'll do much good.

To the extent that interference or multipath are the problem, more power will have no effect. While the desired signals will get stronger, so will the undesired ones.

_________________________________________________

Trip is right: the FCC propagation curves assume an antenna height of 9 meters (30').

I've run a few Longley-Rice maps using 3 meters, though even that figure is probably optimistic in urban areas. (Trip's 4m figure is probably about right for most outside-antenna viewers, and obstructions/attenuation are so severe indoors that I'm not sure there's much point to running an analysis for indoor-antenna viewers!)

 

[landtuna]

Yes, the FCC standard is 30 feet. The Longley-Rice coverage maps I have on RabbitEars assume a height of 13 feet instead.

- Trip

I'm not disputing your numbers but am definitely questioning the engineers(?) at the FCC.

I really doubt the average height of an outside antenna is 9 meters AGL or that average rabbit ears are 13 feet.

Most residences are 10 feet per story. Add another couple feet for the antenna post and you have an average well under 30 feet for one-story homes and still under for two-story.

My two-story rabbit ears are 4-5 feet AGL on the ground level and 15-20 feet on the second story.

I don't know how much difference a few feet makes in either case but it doesn't seem like the averages are even close.

 

[dave73]

For full power DTV stations, I haven't had any major problems getting the stations (once the bugs were worked out), including when WBBM-TV was broadcasting their digital on 3. Low power DTV stations are ones I have yet to get, despite having a big antenna. The FCC website along with tvfool.com show WWME-LD having coverage over my area, but I can't get the signal. WOCK-CD is another station that tvfool.com shows as having some coverage over my area. That one I have yet to get, but saw a post on avsforums.com where someone from South Bend getting WOCK-CD. My antenna is only about 15 ft. off the ground. I am planning to get a new antenna or 2 (either Hi-VHF/UHF combo or a separate Hi-VHF & separate UHF) for Chicago stations & a small UHF only antenna for WYIN. I only had minimal interference with WBBM-TV on 3 (onlywhen there was a lot of lightning, but didn't make the picture unwatchable), but no interference of any type on 7 (WLS-TV) or 12 (WBBM-TV since June 12th).

I don't expect those living closest to Downtown Chicago to get any stations due to the skyscrapers being so high along with so many close together (whether VHF or UHF).

 

[w9wi]

I'm not disputing your numbers but am definitely questioning the engineers(?) at the FCC.

I really doubt the average height of an outside antenna is 9 meters AGL or that average rabbit ears are 13 feet.

In the analog world, they worked from a set of propagation curves developed in the early days of TV. At the time it was probably in fact reasonable to presume anyone in areas far enough from the transmitter to worry about signal strength would put up an antenna 9m high. (to this day you can still see some of the towers in some places...) Changing the receiver antenna height, while analog was still in operation, would have invalidated most of the allocations rules & led to a heck of a mess.

That said, when things went digital they switched to a new methodology for estimating coverage. It was a golden opportunity to change the reference receiver antenna height. Why they didn't, I don't know.

 

[landtuna]

That said, when things went digital they switched to a new methodology for estimating coverage. It was a golden opportunity to change the reference receiver antenna height. Why they didn't, I don't know.

And that "new methodology" suggested, in everything I read, that indoor rabbit ears were the preferred way of receiving DTV signals. Only when the flood of reception complaints occurred did the outdoor antenna receive publicity but it is clear that is only a partial resolution as my situation can attest.

From my personal perspective (as a non-broadcast person) it appears the FCC dropped the ball initially and over and over again on the analog-to-digital switch. Their selection of Wilmington, NC as a test site for DTV was an exceptionally poor choice and has proved it wasn't much of a test. Perhaps this site was selected for political and not technical reasons? Are there any engineers still working at the FCC or are they all lawyers now?

I'd strongly suggest they get back in the business of overseeing the technical aspects of our broadcast industries and worry less about controlling the Internet and expending limited resources on things like station logs.

 

[Markxxx]

I agree they should've transitioned over a period, start with NYC and work their way down a market or two at a time. Not moving till the bugs in one market were worked out.

Of course this would've hurt advertisers so that was out.

With my laptop and a USB Antenna, in Chicago, if I go outside away from the buildings to say a park I can usually pick up all the UHF stations. I still can't get VHF WBBM on Channel 12, with my laptop, but I can get all the other stations away from my flat.

So it definately has to be the buildings effecting it. My neighborhood is full of dense buildings.

 

[landtuna]

So it definately has to be the buildings effecting it. My neighborhood is full of dense buildings.

And I would agree with you on the buildings as they were a problem in the analog days as well.

But I live in a suburb of Phoenix which is relatively flat, no obstructions between me and the towers (about 8 miles in a clear shot). I get many types of signal loss on both indoor and outdoor antennas. One of the most puzzling is the 1800 MST pixelation for 30-40 minutes as the sun sets behind the mountain on which the towers reside. (The towers are almost due west from my location.)

Cloudy, clear, windy, calm, sunny, nighttime - nothing seems to matter. Never had one single issue with analog. VHF or UHF. In fact, 10 years ago I used to live 30 miles from the towers and got excellent reception with a big antenna in my attic (with foil insulation even!). The guy who lives in my old house went cable because no matter what he tried he couldn't get reliable OTA.

 

[Markxxx]

I never had any problems with analog TV, even the low power stations came in very clear. WWME low power channel 23 was the best analog channel it was crystal clear like cable

How's Los Angeles doing with VHF? All the high VHF analog stations returned to those channels for digital. Did they have the issues?

 

[tripinva]

Are there any engineers still working at the FCC or are they all lawyers now?

Yes, there are engineers there, just that the politicians ignore them when they say inconvenient things. For example, during the White Space Device hearings, the engineers were locked out of the room. Only politicians were allowed.

- Trip

 

[Carmine5]

Does VHF need more power? One thing I found odd was WTTW in Chicago on Channel 11 was fine in analog. Then when a TV station in Lafayette Indiana started broadcasting digitally on channel 11 it messed up WTTW's analog signal.

Supposedly, 8VSB requires less power to cover an area comparable to that of the earlier NTSC system. However, I noticed that our own KTTV Ch. 11 in Los Angeles applied for an ERP increase from about 11Kw to 115Kw in order to replicate their former analog coverage.

Of course, there may be other factors such as the terrain or typical housing of the area which may necessitate this increase. As I understand it 8VSB is more susceptible to dynamic multipath and when used on the VHF band shows a much weaker penetration in single story homes surrounded by trees, which is characteristic of a great deal of L.A. County housing.

c5

 

[landtuna]

Supposedly, 8VSB requires less power to cover an area comparable to that of the earlier NTSC system. However, I noticed that our own KTTV Ch. 11 in Los Angeles applied for an ERP increase from about 11Kw to 115Kw in order to replicate their former analog coverage.

11 to 115KW is a HUGE increase!

Of course, there may be other factors such as the terrain or typical housing of the area which may necessitate this increase.

Well maybe...but my guess is that neither the terrain nor the typical housing has changed much since the digital migration. Therefore the issues must reside with DTV itself.

As I understand it 8VSB is more susceptible to dynamic multipath and when used on the VHF band shows a much weaker penetration in single story homes surrounded by trees, which is characteristic of a great deal of L.A. County housing.

Typical of Phoenix area older neighborhoods too although all of my issues with DTV are on the UHF RF's.

 

[dave73]

I can't get most signals in my home. In order to get FM & TV reception, I must have an outdoor antenna. TV barely came in on rabbit ears for analog & non-existant for digital. The only signals I get in my home are from cellphone services Verizon Wireless & Nextel on the 800 band network plus Verizon Wireless's PCS network for 3G (since in my immediate area, 3G is PCS only while voice is 800 band only). I'm lucky that I can have an outdoor antenna where I live. I would be screwed if I lived somewhere that made it impossible to have an outdoor antenna. I also live well away from Chicago and all full power stations on both VHF & UHF are no problem to get. Low power stations are another matter. I don't get those yet (including WWME-LD on 39).

 

[kenglish]

It's too bad that there are really very few techs out there who can properly specify and install antennas.
And, it was kinda depressing to note that we could not even take delivery on a reasonably priced "installer"-type ATSC analyzer until Thanksgiving weekend 2008.

Do most cities and towns even HAVE any qualified installers. or just dish technicians?

 

[vchimpanzee]

And that "new methodology" suggested, in everything I read, that indoor rabbit ears were the preferred way of receiving DTV signals. Only when the flood of reception complaints occurred did the outdoor antenna receive publicity but it is clear that is only a partial resolution as my situation can attest.

When I tried to buy an outdoor antenna at Sears, I was told their indoor amplified antenna would be better. They didn't sell outdoor antennas. A man who lived in my area had bought the indoor antenna at Sears and picked up the stations I wanted. So did I, until I had to get a new TV and moved the converter box for the first time. Things haven't been quite ideal since, and even with UHF bad weather is a serious problem.

VHF? I was fortunate the one station to do this gave up trying.

I mostly watch cable. It's almost a necessity for certain stations in my area, and it eliminates those annoying glitches. And for VCRs and definitely for TiVo, it's the only way to go.

 

[vchimpanzee]

"After all, the FCC assigned digital channels to stations based on the assumption that viewers would be tuning in with antennas 30 feet above the ground."

WHAT? Do we really have a collection of imbeciles at the FCC who think everyone lives in a two-story house and has another 10' of antenna post roof-mounted?

This wasn't in the Wikipedia article about the DTV transition until I put it there several weeks after the transition was complete. I just happened to see it in one of the many Broadcasting & Cable articles about the VHF problem.

 

[kenglish]

"No TV will work at all with a 29-foot antenna : ."

This argument sounds like the old Charlie ("Why do EAS? If they want to know if it's raining, go look out the window.") Ergen tirade about Distant Networks.

The FCC planning factors, like anything scientific, require some form of quantization. The thirty-foot height is just a marker for making measurements, nothing more. Some people have interpreted it as REQUIRING a 30-foot mast, even if you're well within the station's coverage area.