How does digital TV broadcast prevent ghosting effects?

[Jan Panteltje]

Good idea. And yes, I think much of the multipath comes back
polarization-twisted.

Joerg, I have been thinking about your case, where you have one direct path
and one airplane reflected path.
With 2 path destructive interference can cause zero signal.
Then I had this funny idea:
Why not use a wider beam antenna then your log periodic?
With some more refections the sum is not so likely to become zero...
But this will only work with sufficient signal strenth,

I turned my antenne 90 degrees to vertical, it is a bow tie, it has a wide angle.
I can now get some other stations, likely (but not 100% sure) one 120
miles away (40kW high tower), but some are now on the same frequency, so
hard to say if I get the one or the other...
There must be a station ID in the DTV tables, but I would have to look up the Etsi
documents, and write the soft to grab it perhaps.

 

[Jim Thompson]

Good idea. And yes, I think much of the multipath comes back
polarization-twisted.

I used corrugated fiberglass roofing rolled into an 8' long tube, then
used 1/4" copper tubing as the helix ;-)

...Jim Thompson

 

[Joerg]

I used corrugated fiberglass roofing rolled into an 8' long tube, then
used 1/4" copper tubing as the helix ;-)

That stuff doesn't always last long out here. The repeated heat/freeze
must take a toll. One day it goes phhhst ... crunch and just breaks
open, fibers sticking out like fuzz. Seen it again during our dog walk
yesterday.

 

[Mark]

That's "frequency selective fading" and tends to be difficult to fix
unless you have a very very very narrow front beamwidth with no side
lobes and no ground bounce. If you have the possibility of
reflections from behind, also good f/b ratio. If these are difficult,
you might want to consider dual diversity antennas (about 10
wavelengths apart), with dual tuners and a diversity switch. If the
signal level to one antenna falls due to frequency selective fading,
the odds are good that the other antenna will still have a usable
signal.

Freq Selective fading is casued by multipath, they are two views of
the the same thing.. one view in the time domain the other the freq
domain. If you have deep nulls across the channel the two rays are
almost equal in power. If you have one null then the delay is about
1/6MHz, two nulls 1/3MHz etc.

8VSB needs a good equalizer in the receiver to overcome multipath.
COFDM is more naturally immue to multipath but is more suseptable to
impulse noise. Each impulse wipes out all the carriers and a lot of
symbols. With 8VSB, each impulse wipes out only a few symbols.

Mark

 

[Joerg]

Maybe you should try turning it then.

Did that :-(

Perhaps it has so many damned element that you are contributing to your
own perceived problem.

It certainly ain't perceived. Yesterday was the worst: Channel 13 news.
Watched it on digital channel 13.1. Clouds rolled in, picture became
blocky, audio cut out at times. Image got more blocky, then froze, audio
gone as well. Switched back to 13 analog -> H-sync a wee bit tipsy but a
clear image. The comb filter didn't get all reflections, could see about
half a dozen weak ghosts. But the fact of the matter is we could watch
the news on the analog channel while on their digital channel we could not.

So unfortunately I have to stand by what I suspected all along: DTV is
not as reliable as analog. Not even close. The local politicians might
want to stock up on anti-depressants come early 2009 ...

Mine worked fine from indoors set top, and I was over 40 miles East
from the transmitters, and down in a valley from them.

Funny thing was, from there, I only needed to point in one direction to
get all the stations. From just 12 miles both North and East from them,
I had to point in several directions dependent on what direction whatever
particular station was in from where I was. Now, I am 40 miles North of
them at sea level, and still have several vectors needed. Most all are
Southerly, however.

They are in San Diego. I was in Santee for years with no problem and
great signal. In Clairemont (an SD suburb), and now in Oceanside, I have
to move the antenna around a lot more. Perhaps what HD tuners need is
true diversity, where there are like four tuners inside, and four
antennas, and the signal is garnered from them all to compile a complete
data stream. It sounds like overkill, but diversity really does work.
That's what they use at all the NASCAR race tracks, and at Indy.

Diversity receivers.

http://en.wikipedia.org/wiki/Diversity_scheme

I know diversity well. We even have it at church for the wireless mikes.
However, terrestrial broadcast is low on the pecking order because the
majority is in the hands of the big cable and sat companies. So I doubt
the industry will invest much there. Certainly not a 2nd tuner, probably
not even a 2nd F-connection.

 

[Joerg]

Bingo. Are you *SURE* it's really a log periodic antenna or some
kludge that only looks like one? Duz it look like one of these?
<http://en.wikipedia.org/wiki/Log-periodic_antenna>

It does, just a whole lot bigger.

I modeled a few of the Radio Shock TV antennas with 4NEC2 to see what
they were doing. The results were awful. On some channels, it had
more gain out the back of the antenna than in the forward direction.
Impedance matching was awful and varied wildly across the frequency
range. Side lobes were all over the place. There were nulls in the
forward direction on some channels.

Under ideal conditions, a real log periodic has a forward gain of only
5 to 7dBi. Howver, that's about 6dBi across the entire frequency
range which is the point of such a broadband antenna. Having such a
low gain, it also has a rather wide forward beamwidth. My guess is
30-50 degrees at -3dB points horizontally. That's NOT what you want
if you want to avoid multipath.

It is much sharper. I spend hours on the ladder turning it. You can
completely lose a station into the noise at those 30-50 degrees.

I live in the Santa Cruz mountains. Before I gave up and went with
DirecTV satellite TV, I used a mast, rotator, and tangle of antennas
on the roof. Multipath from the nearby mountains was a serious
problem with Radio Shack antennas. Even the one station that was line
of sight had reflection problems.

So, I started to play with antennas. My first solution was a simple
bow tie antenna with a flat barbeque grill reflector. That worked
very well for UHF, but was too small for the VHF channels. It's main
advantage was that it was very broadband, few side lobes, and
excellent f/b. However, it didn't have much gain (about 4dBi).

Looking for something better, I decided that single channel yagi
antennas were the only way. The commerical versions are:
<http://www.blondertongue.com/media/pdfs/catalog_classes/reception/bty.pdf>
However, I built my own. I had 4 different dipoles, pointed at the 4
transmitter locations. Gain was about 10dBi for VHF and 12dBi for
UHF, a substantial improvement over broadband antennas. Each antenna
had a really ugly homemade Dual Gate MOS FET RF amplifier at roof
level, with a combiner feeding the TV. I removed the rotator as it
was un-necessary with one antenna per transmitter/channel. Fine
aiming required a 20ft fiber glass pole to bang on the boom in one
direction or other.

The results were a dramatic improvement in reception signal strength
and quality. There were still a few ghosts visible but nothing as
horrible as with previous antennas. I was happy for about a year,
when a large fir tree branch managed to mangle 3 out of 4 antennas. As
I was recovering from surgery at the time, and was in no condition to
do anything major, I ordered DirecTV and gave up.

Well, I don't wnat to go to that length ;-)

TV isn't too important and nowadays monitors have PC connectors. So
eventually people might be able to cut it all loose and obtain their
teevee from the web. Some day.

That's "frequency selective fading" and tends to be difficult to fix
unless you have a very very very narrow front beamwidth with no side
lobes and no ground bounce. If you have the possibility of
reflections from behind, also good f/b ratio. If these are difficult,
you might want to consider dual diversity antennas (about 10
wavelengths apart), with dual tuners and a diversity switch. If the
signal level to one antenna falls due to frequency selective fading,
the odds are good that the other antenna will still have a usable
signal.


Nope. Politics is the major problem. If the FCC would have approved
European COFDM modulation instead of crappy 8VSB, we would not have as
many multipath problem.
<http://en.wikipedia.org/wiki/8VSB#8VSB_vs_COFDM>
The FCC has several chances to listen to sane technical comparisons,
but elected to follow the path paved by the US patent owners.

Well, I guess it is too late now. Whatever happens, when this leads to
unpleasant consumer feedback in 2009 someone will dig into the matter of
"who dunnit".

That's going to be very difficult to reduce multipath if both the
incident and reflected signals are coming from so close a direction.
Diversity reception might be the only workable band-aid. However, in
my limited experience with such multipath, that doesn't happen as much
as one would think. The reflected path ghost delay is usually so
small, that it's only visible as a slight smear to the right. The
reflections that generate the really visible ghosts, usually come from
the back, and sometimes from the sides. A good antenna pattern will
eliminate those.

Out here there are echoes with very long delays. Tens of microseconds.

 

[Joel Koltner]

So unfortunately I have to stand by what I suspected all along: DTV is not
as reliable as analog. Not even close. The local politicians might want to
stock up on anti-depressants come early 2009 ...

You can bet that the cable and satellite TV companies will use the cessation
of over-the-air NTSC as part of advertising campaigns to get people to start
paying them every month for the "privilege" of watching local TV.

I'm sorry to hear that even a new ATSC receiver isn't working very well for
you, Joerg.

---Joel

 

[Jim Thompson]

You can bet that the cable and satellite TV companies will use the cessation
of over-the-air NTSC as part of advertising campaigns to get people to start
paying them every month for the "privilege" of watching local TV.

I'm sorry to hear that even a new ATSC receiver isn't working very well for
you, Joerg.

---Joel

I think the whole object is to dispose of over-the-air transmission.
For 37 years I've had some mountain or other in the way, so for the
last 25 years or so I've had cable... I'd never go back.

...Jim Thompson

 

[Joerg]

Well, after looking at the standards I had a hunch that this would
happen. They probably never did real field tests, at least I haven't
read about many.

I think the whole object is to dispose of over-the-air transmission.
For 37 years I've had some mountain or other in the way, so for the
last 25 years or so I've had cable... I'd never go back.

It just irks me that a company can foist $40/mo plus on us just for the
"privilege" of watching the news and some old movies because they were
handed a quasi-monopoly. It's not about being able to afford that, it's
about the principle. Every year they dip into the pot deeper, claiming
"programming costs have gone up" and yada, yada, yada.

Also, I am afraid other people in this area will react more belligerant
once they realize they've been had over the barrel. Possibly after
investing some four-digit sum into a "new and improved TV experience"
that then fails to materialize. Soon the blame game will start and
shortly thereafter it will be mined for political gains. And that won't
be pretty at all.

 

[Jeff Liebermann]

It does, just a whole lot bigger.

Which one of these? I believe you mentioned it was a Channel Master:
<http://www.pctinternational.com/channelmaster/0612/antennas_outdoor.html>
If it's one of the 36xx series, they're similar to the Radio Shock
monstrocity that I modeled. (I'm still looking for where I buried the
NEC2 files).

It is much sharper. I spend hours on the ladder turning it. You can
completely lose a station into the noise at those 30-50 degrees.

Ladder? I thought you had a tower or pole. If you're that close to
the ground or roof line, you're going to get a quite a few local
reflections.

Well, I don't wnat to go to that length ;-)

It wasn't really all that bad. What I should have done was contact
the local CATV company and ask them if they had any unused single
channel antennas at the local headend. I know they have these because
they're moving away from over the air feeds, and switching to fiber
and satellite distribution. However, I built my own. The first
prototype was a piece of PVC pipe, with welding rod for elements. Dual
T-match and balun on the driven element. Once I got one channel
working, and determined that the others would probably work just as
well, I built 3 more antennas and amps. Figure on one weekend per
antenna.

TV isn't too important and nowadays monitors have PC connectors. So
eventually people might be able to cut it all loose and obtain their
teevee from the web. Some day.

Nope. The future is "personal TV", where you wear a 3D headset and
are literally immersed in the story. Anything less than terrestrial
fiber doesn't have the bandwidth. You read it here first.

Well, I guess it is too late now. Whatever happens, when this leads to
unpleasant consumer feedback in 2009 someone will dig into the matter of
"who dunnit".

Notice how the deadline on Feb 17, 2009 is AFTER the election in Nov
2008. Congress and the FCC know a problem, especially when they
create it. If there's every going to be a "popular uprising", it will
be over something like the loss of analog TV.

Out here there are echoes with very long delays. Tens of microseconds.

Then, they are NOT coming from bouncing around the indicent path.
They're coming from the sides, or more likely, from the back. Since
your antenna is apparently within easy reach (on a ladder), you get to
try a really dumb experiment. Find a 20ft fiber glass window washer
or painting telescoping pole. Mount the biggest wire mesh you can
find to the top to block the signal. Move it around the sides and
back of the antenna until the reflections (ghosts) disapper. If you
have multiple reflections, this has the potential of making things
worse, but at least you'll get a clue as to from which direction the
reflections are coming from. My guess(tm) is from the back. If the
wire mesh makes it worse, try again with something that absorbs RF
(wet towel works nicely) and doesn't reflect RF.

If you don't want to have the neighborhood suspect your sanity by
running around your roof with a giant fly swatter, find a bow tie
antenna (with a flat reflector) and try it on one of the UHF channels.
This won't solve the VHF problem, but for UHF, it has a highly
desireable high f/b ratio. If the reflection is from the back, as I
suspect, then it should eliminate it.

Also, please try not to fall off the roof. You have but one life to
sacrifice to the HDTV gods.

There's also an antenna selector web site at:
<http://antennaweb.org>
It just declared that there are no TV stations that I can see at my
location. Oh well.

 

[Joerg]

Which one of these? I believe you mentioned it was a Channel Master:
<http://www.pctinternational.com/channelmaster/0612/antennas_outdoor.html>
If it's one of the 36xx series, they're similar to the Radio Shock
monstrocity that I modeled. (I'm still looking for where I buried the
NEC2 files).

None of those, AFAIK it's a commercial grade version for CATV heads.
Very old and not a spot of wear (other than the occasional bird
droppings), pretty impressive.

Ladder? I thought you had a tower or pole. If you're that close to
the ground or roof line, you're going to get a quite a few local
reflections.

The mast is bolted to the side of the house and you have to get on a
ladder to untighten it, turn the whole mast, then retighten.

It wasn't really all that bad. What I should have done was contact
the local CATV company and ask them if they had any unused single
channel antennas at the local headend. I know they have these because
they're moving away from over the air feeds, and switching to fiber
and satellite distribution. However, I built my own. The first
prototype was a piece of PVC pipe, with welding rod for elements. Dual
T-match and balun on the driven element. Once I got one channel
working, and determined that the others would probably work just as
well, I built 3 more antennas and amps. Figure on one weekend per
antenna.


Nope. The future is "personal TV", where you wear a 3D headset and
are literally immersed in the story. Anything less than terrestrial
fiber doesn't have the bandwidth. You read it here first.

So all pedestrians are now schlepping fibers behind them?

Notice how the deadline on Feb 17, 2009 is AFTER the election in Nov
2008. Congress and the FCC know a problem, especially when they
create it. If there's every going to be a "popular uprising", it will
be over something like the loss of analog TV.

And that one could be huge. There is stuff that voters forget, and then
there is stuff they don't forget. And you can be sure that this will be
mined in the election after that one, along the lines of "these are the
guys who dunnit".

Then, they are NOT coming from bouncing around the indicent path.
They're coming from the sides, or more likely, from the back. Since
your antenna is apparently within easy reach (on a ladder), you get to
try a really dumb experiment. Find a 20ft fiber glass window washer
or painting telescoping pole. Mount the biggest wire mesh you can
find to the top to block the signal. Move it around the sides and
back of the antenna until the reflections (ghosts) disapper. If you
have multiple reflections, this has the potential of making things
worse, but at least you'll get a clue as to from which direction the
reflections are coming from. My guess(tm) is from the back. If the
wire mesh makes it worse, try again with something that absorbs RF
(wet towel works nicely) and doesn't reflect RF.

I've tried those tricks. Thing is, reflections (bounces) seem to happen
multiple times. Meaning a back reflection comes bouncing back from the
front again. That's why it gets really bad when a 747 freighter glides
in, probably because it's a nearly perfect reflector.

If you don't want to have the neighborhood suspect your sanity by
running around your roof with a giant fly swatter, find a bow tie
antenna (with a flat reflector) and try it on one of the UHF channels.
This won't solve the VHF problem, but for UHF, it has a highly
desireable high f/b ratio. If the reflection is from the back, as I
suspect, then it should eliminate it.

Also, please try not to fall off the roof. You have but one life to
sacrifice to the HDTV gods.

There's also an antenna selector web site at:
<http://antennaweb.org>
It just declared that there are no TV stations that I can see at my
location. Oh well.

Quote "These large, multi-element rooftop antennas are used in weak
signal areas for maximum possible TV reception. These antennas can be
used in ANY LOCATION, but require an amplifier and roof mounting to
receive blue and purple channels. Amplifiers are not recommended for
yellow channels."

Hmm, not all that helpful ...

 

[Mark Zenier]

OK, if you say so.
But I also clearly remember the discussions in the related newsgroups at
that time,
about the decision, and 8VSB versus COFDM, and an attempt by the BROADCASTERS
to force COFDM and reject 8VSB, some had alreay installed the COFDM transmitters

The stuff I remember (vaguely) was greatly refreshed by reading
<http://www.atsc.org/history.html> and the related pages on the
"Grand Alliance" and "FCC adopts...". They started with this
in the late 1980's, came up with several systems around 1991 and
combined the results into the standard by 1995.

They lost (against politics).

I'm pretty sure that nobody had put up anything but a test transmitter
by then. I think some groups thought that they could lobby their way
around it as COFDM got better developed after 1995.

You also have to remember that in the decentralized US broadcast
industry, signal quality is a competitive tool. (Or used to be,
now that most everybody uses cable or satellite).

Mark Zenier [email protected]
Googleproofaddress(account:mzenier provider:eskimo domain:com)

 

[krw]

One thing you have to do with the Vizio is adjust the settings. At least
the color. It's over-saturated but that's easy to correct. I didn't even
have to get off the couch for that

I played with the Vizio for a while. I couldn't get the picture
"right", so decided to go a bit up-scale for my own. I was
considering Olivia (?) and Westinghouse too, but decided I'd rather
spend a little more money now than be disappointed for years. An
HDTV is too obvious to get wrong. ;-)

 

[Michael A. Terrell]

So unfortunately I have to stand by what I suspected all along: DTV is
not as reliable as analog. Not even close. The local politicians might
want to stock up on anti-depressants come early 2009 ...

Just wait till they discover that a lot of people won't be able to
see their annoying campaign commercials.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[ChairmanOfTheBored]

Quote "These large, multi-element rooftop antennas are used in weak
signal areas for maximum possible TV reception. These antennas can be
used in ANY LOCATION, but require an amplifier and roof mounting to
receive blue and purple channels. Amplifiers are not recommended for
yellow channels."

Hmm, not all that helpful ...

Have you tried an MATV amplifier?

 

[Joerg]

Have you tried an MATV amplifier?

It's all built with the good stuff. None of this plastic stuff. No
shortcuts. Antenna -> mast amplifier -> quad shield -> big distribution
amp in the comms closet -> some more quad shield -> TV. All runs are
dual quad shield so content can be fed back to the head amp. After
installation I did some tests such as feeding a gen signal all the way
back to the mast head and then checking it coming all the way back. Was
perfect.

 

[Joerg]

I played with the Vizio for a while. I couldn't get the picture
"right", so decided to go a bit up-scale for my own. I was
considering Olivia (?) and Westinghouse too, but decided I'd rather
spend a little more money now than be disappointed for years. An
HDTV is too obvious to get wrong. ;-)

It's fine. The only minor difference is that its black level isn't quite
as black as that of >$1500 units. But that difference wasn't worth $750
to me. It's way too bright anyhow so I'll probably reduce the backlight
juice a bit (has a menu setting for that). Reviews were favorable as
well except that many rated its audio as being a bit measly. Strange
thing, I think the audio is great.

However, since DTV is of inferior reliability it might be that only the
VGA function will be useful here starting in 2009.

 

[David DiGiacomo]

However, since DTV is of inferior reliability it might be that only the
VGA function will be useful here starting in 2009.

Joerg, ATSC works fine, the problem is with your setup.

Here are some ways you can fix it:

1. Exchange your TV for a model with a more robust ATSC tuner.

2. Re-aim your antenna for best ATSC reception.

3. Replace your antenna with something with better directionality and F/B
ratio.

4. Accept that living in the boonies has costs and benefits, and one of
the costs is paying for cable or satellite TV.

5. Download your TV shows from the internet using BitTorrent.

Of course if you're not willing to make any changes, you're not going to
solve the problem.

 

[David DiGiacomo]

It's all built with the good stuff. None of this plastic stuff. No
shortcuts. Antenna -> mast amplifier -> quad shield -> big distribution
amp in the comms closet -> some more quad shield -> TV. All runs are
dual quad shield so content can be fed back to the head amp. After
installation I did some tests such as feeding a gen signal all the way
back to the mast head and then checking it coming all the way back. Was
perfect.

Despite everything being perfect, you should try disconnecting everything
from the antenna and running a simple 100' cable to the TV. If you can
drag the TV out to the antenna, that's even better.

 

[Joerg]

Despite everything being perfect, you should try disconnecting everything
from the antenna and running a simple 100' cable to the TV. If you can
drag the TV out to the antenna, that's even better.

Believe it or not but that's what I did to initially find the best
position for the antenna. TV out on the deck, about 40ft of quad shield,
me on the ladder with binoculars in the belt. There was no difference in
image quality after bringing it back inside where it was fed via the
distribution network.