How does digital TV broadcast prevent ghosting effects?

[Jan Panteltje]

On a sunny day (Thu, 22 Nov 2007 11:31:27 GMT) it happened Jan Panteltje

Correction:
'Frames' are in the millisecond range (one every 40mS for example for
25 fps), at the speed of light the refecting object would
have to be about 300 000 000 * .04 / 2 meters away = 6 000 000 meter, or
6 thousand kilometers.

 

[StickThatInYourPipeAndSmokeIt]

With analog video signals, ghosting is caused by mutipath distortion,
where a 'second copy' of the signal arrives at the receiver delayed by a
small amount of time. That second copy is displayed as a second image,
offset on the screen by a distance equal to the path delay times the
horizontal scanning rate of the CRT.

With digital video, the broadcast is sent in several types of frames.
Each frame is labeled with a sequence number indicating where in time it
should be displayed. While mutipath distortion can still interfere with
digital signal reception, it does not manifest itself as a second image
offset on the screen spatially from the original. The only way to
produce a ghost in a digital stream would be to spoof the packet timing
data.

Not even then. The packet data, as you call it, is bottled up in the
transport stream, and the "frames" you mention are inside that encrypted
(usually) stream. so the main broadcast signal packets differ from the
packaged up transport stream, which is where any ordering sequence takes
place. The FEC is applied to any lost data segments, in the broadcast
stream, *then* any missing parts of the transport stream get cast aside,
and skipped, as in NOT PROCESSED, therefore it would be impossible to
have such an event as a double image as properly decoded frames get
applied to the final picture data and cast aside frames do not get any
processing as they got shitcanned. So even "spoofed" data would not get
processed as the FEC which gets sent would consider it as a bad frame to
begin with and cast it out. Rarely, some data of in individual frame
gets jumbled and the viewer might see some bit blts that are garbage, but
not often. Usually a high bit error rate will cause frames to drop
completely, and many "tuners" will not even display the channel at all if
said bit error rate is too high. Somewhere around 10% BER.

 

[ChairmanOfTheBored]

Multipath is a large problem in digital transmission though it doesn't
manifest itself as trailing edge smears in the picture. It shows up as
lumps in the RF response (comb filtering) that make it difficult to
impossible to extract the data. There are active equalizers that can
correct some pretty bad reflections. The newest chipsets (#6 coming
soon) can correct a broader range of errors and do it faster - almost
to the point of working in a moving car.

http://www.hdtvprimer.com/ISSUES/what_is_ATSC.html

Excellent link!

Maybe that dope that thinks the only HDTV mode is 1080p will get a
clue!

 

[ChairmanOfTheBored]

Nah, what they need is sentient computers so they can kick the striking
screen writers.... at least right now.

I'd imagine robotic actors would sell better in the personal porn industry,
at least first. You know, when personality doesn't matter.

You would know.

 

[ChairmanOfTheBored]

Correct so far, but not delays are in the microsecond rang.
Always there is a HORIZONATL shift (ghost), never a vertical one.
For exaple in a NTSC 64 uS line 1 uS is about 1/60 of the screen width.


'Frames' are in the millisecond range (one every 40mS for example for
25 fps), at the speed of light the refecting object would
have to be about 300 000 000 000 * .04 / 2 meters away = 6 000 000 000 or
6 million kilometers.
Not likely, and the signal strength of that reflected signal would be very very low...



No, even if you changed the frame time stamp, the frame would be displayed
at the same position.
But the decoder may well abort.

Digital will simply take the strongest signal in the simplest case.

No. Digital MAKES the correct packet or it doesn't and it uses whatever
it receives to do so. It doesn't pick a signal and cast the others out.

That is called diversity, and I don't think that modern tuners are
doing diversity yet. It ain't cheap.

True diversity would use multiple antennas, and THEN the best signal
gets chosen.

 

[Joerg]

At 37" your minimum requisite should have at least been 1080p.

1368x768 looks pretty grainy.

You essentially bought three year old technology at a closeout price.

That is aside from the fact that Vizio doesn't use the best panel
vendors around, so the end product ain't too hot. Hell, you should have
researched a bit. You can get a Viewsonic at that size and spec for the
same price, since you are buying the old, marked down stuff.

I've looked at Samsung and other brands side by side with it, all the
way up to $1300. The difference wasn't that great. Very slightly in
motion artefacts maybe but definitely nothing to write home about.

 

[Joerg]

Sharp Aquos Full HD for me.

So, what's usually playing in HD these days? Tried it out yesterday, it
gets some HD channels, others drop out a lot, and yet others it can't
decipher on account of multipath. But all the DTV stuff was standard
def. Some of it wide screen though.

The picture is nice and crisp BTW. No complaints here.

 

[ChairmanOfTheBored]

I didn't post this. Learn to quote properly.

So, what's usually playing in HD these days?

HD offerings grow continually.

Tried it out yesterday, it
gets some HD channels, others drop out a lot, and yet others it can't
decipher on account of multipath. But all the DTV stuff was standard
def. Some of it wide screen though.

If it was in wide format over the air, it was HD.

HD starts at 1280 x 720

The picture is nice and crisp BTW. No complaints here.

There is a link in this thread that declares what is and what is not HD.
You should give it a read.

I know, however, that over cable signals get compressed, and even
though many are wide aspect, few are true HD streams. The dirty bastards!

Sad too, since they fit onto a std 6MHz wide slot, and the
cable/satellite twits didn't need to re-compress a goddamned thing, yet
the idiots do it anyway.

 

[Joerg]

I didn't post this. Learn to quote properly.

Huh? Read again: Sez " [email protected] wrote:". AFAIK that
ain't you.

HD offerings grow continually.


If it was in wide format over the air, it was HD.

But it read 480SD in the OSD. When I turned the set to "normal" the
format on the digital channel switched a lot depending on what played.
News in standard with the black strips left and right, unless I switched
to "wide" but then everyone appears overweight.

HD starts at 1280 x 720

There is a link in this thread that declares what is and what is not HD.
You should give it a read.

I know, however, that over cable signals get compressed, and even
though many are wide aspect, few are true HD streams. The dirty bastards!

Sad too, since they fit onto a std 6MHz wide slot, and the
cable/satellite twits didn't need to re-compress a goddamned thing, yet
the idiots do it anyway.

Well, we have neither cable nor satellite. It's only what's available
OTA. I am not a ballgame fan but I'll check this afternoon just to see
how HD might look like on this set. Our vicar said the Packers play the
Lions (he roots for Detroit) today so hopefully that will be in HD.

BTW, someone mentioned the Sharp Aquos: Costco has the 42" for around
$1200 after rebates. Says the ad.

 

[Joerg]

Yes, although it's also dependent on the intended transmission path -- getting
video to a cell phone in, e.g., someone's moving car is a worse environment
than just getting it to a stationary TV antenna with decenet gain.

Where I work the way we choose the amount of error correction is to take a
prototype with no error correction, transmit known test patterns, collect
error statistics while operating the receiver in the intended environment
(e.g., at someone's home, driving around in a car, etc.), and then play around
with the amount of error correction to try to balance data rates with
robustness.

If you're looking at "well known" over-the-air standards, it's probably a safe
assumption that someone came up with a model of the environment and did plenty
of simulations before committing anything to silicon -- for high data rate or
highi volume devices, that's where the error correction is implemented
(whereas, at least to date, everything we've done has been slow enough to do
it in software).


Well, it's supposed to be, but I don't personally have enough familiarity with
them to say.

We live in heavy multipath. All I can say so far (after one day) is that
we'll seem to be losing some stations in 2009. What happens is one of these:

a. TV doesn't recognize the DTV channel, data probably too messed up.
b. TV says something like "Receiving data" but that's it.
c. Picture gets blocky or stops at times. IOW not very useful.

Some work well and there I have to say it's nice. There are clearly
disadvantages when picture content changes rapidly but heck, there is no
free lunch. Shannon said it more scientifically though. Most of the time
the DTV picture is really nice (for channels where it works).

 

[Jan Panteltje]

We live in heavy multipath.

I have heard that US did chose 8VSB because Europe had an other system,
and 8VSB would help US industry, as Europe then had the same hurdles to
market as the US.
That apart, I have terrestial digital too (no more analog here),
and its uses 64 QAM, with a FEC of 1/2, so a lot of correction bits,
the channels take up 8MHz.
Not all of Europe does this, but the Netherlands does.
I had a weak signal, and reflections, in analog,
perfect reception now in digital, even from stations far away.

I have never understood the 8VSB idea other then political.
http://www.digitalradiotech.co.uk/cofdm.htm
http://en.wikipedia.org/wiki/DVB-T

I have not seen any HDTV over DVB-T here yet, not sure any is planned.
But we have plenty over satellite.

 

[Joerg]

I have heard that US did chose 8VSB because Europe had an other system,
and 8VSB would help US industry, as Europe then had the same hurdles to
market as the US.

Don't know but I'd think it's more because the sports fans here want
their football games in hi-def.

That apart, I have terrestial digital too (no more analog here),
and its uses 64 QAM, with a FEC of 1/2, so a lot of correction bits,
the channels take up 8MHz.
Not all of Europe does this, but the Netherlands does.
I had a weak signal, and reflections, in analog,
perfect reception now in digital, even from stations far away.

I have never understood the 8VSB idea other then political.
http://www.digitalradiotech.co.uk/cofdm.htm
http://en.wikipedia.org/wiki/DVB-T

I have not seen any HDTV over DVB-T here yet, not sure any is planned.

Well, that's where Europe seems to be lagging. I just saw an HD program
over the air (not satellite). A dog show, not very interesting, but the
picture quality was quite stunning. I just don't see the need for it
since standard-def TV is good enough for me.

But we have plenty over satellite.

That's expensive over here since we don't have Astra.

 

[Joerg]

Multipath is a large problem in digital transmission though it doesn't
manifest itself as trailing edge smears in the picture. It shows up as
lumps in the RF response (comb filtering) that make it difficult to
impossible to extract the data. There are active equalizers that can
correct some pretty bad reflections. The newest chipsets (#6 coming
soon) can correct a broader range of errors and do it faster - almost
to the point of working in a moving car.

http://www.hdtvprimer.com/ISSUES/what_is_ATSC.html

The Vizio set I just bought does a fairly good job here. It did decode a
few streams where I was almost sure it wouldn't. But not all because
multipath is really bad out here.

Question: How do you find out what's on those digital channels? Ok, I
found some via the local station web sites, such as 13.1 or 13.2. But
then others are scattered. For example 54.1, 54.2 and 54.4 are listed
under KTEH whereas 54.3 is on the KQED web site. The latter is confusing
because it is listed as channel 9 out of San Francisco. Our TV guide
here turns out not to be very useful because it only lists the analog
channels for OTA stations, the rest is all cable and satellite.

 

[ChairmanOfTheBored]

Don't know but I'd think it's more because the sports fans here want
their football games in hi-def.

Broadcast HDTV was QAM until recently.

 

[Jan Panteltje]

Don't know but I'd think it's more because the sports fans here want
their football games in hi-def.

DVB-T does not exclude HD,
It is even possible to have SD and HDTV on the same carrier, and have the receiver
switch to SD if signal becomes really bad, see:
http://en.wikipedia.org/wiki/DVB-T
They are also working on DVB-T2 (2009).

Well, that's where Europe seems to be lagging. I just saw an HD program
over the air (not satellite). A dog show, not very interesting, but the
picture quality was quite stunning. I just don't see the need for it
since standard-def TV is good enough for me.

Well Europe is not lagging, we did see already long time ago that satellite
is a much more clever way to bring content, only one small transmitter needed up there ;-)
The new standards is DVB-S2 (look it up, within 0.7 dB of the Shannon limit, great system:
http://en.wikipedia.org/wiki/DVB-S
and HDTV is now broadcast both on the old DVB-S and DVB-S2 system.

Lagging is US.
It was sort of first with color (NTSC), then Europe got PAL, PAL was better.
Then Europe went digital.
US also wanted digital, and did chose a lesser system with 8VSB, putting it
more in the third world category, as you can also see from the dollar, your
current dictator, and your true elections replaced by pre-programmed Republican
voting machines ;-) Now that is funny.

That's expensive over here since we don't have Astra.

Well, you cannot see Astra, it is below the horizon, but if you had not moved so far west you could have.
There is not only Astra here though, Hotbirds, many more, the sky is full of sats here,
that is why I have a motorised dish.

On the other bright side, in the cities, cable companies can easily pick up
the satellite signal, I have heard Netherlands is something like 80% cable.

But seriously, or more seriously, I fail to see the advantages of 8VSB especially in the cities.

There is an other issue too, with digital processing in LCD receivers the picture needs de-interlacing
if not transmitted progressive, and that basically reduces the resolution by half.
Further losses happen if the TV does not have the same vertical pixels as the transmitted signal, as
rescaling causes further loss of detail.
I would like to see all content transmitted in 720 or 1080 progressive, there is a EBU recommendation for progressive:
http://www.ebu.ch/en/technical/trev/trev_301-editorial.html
I dunno is US transmitting *any* progressive? If interlaced, then you have only half the resolution,
and that would be less quality then old NTSC with a good signal
http://en.wikipedia.org/wiki/Deinterlacing

 

[Joerg]

Broadcast HDTV was QAM until recently.

To be honest I don't know much about all the tech details because TV has
such low importance for me. But HDTV sure looks nice. Just for fun I
watched a documentary in HD about the Rhein/Mosel area in Germany. The
images of the castles and strongholds there were stunning (and I know
those buildings from close-up). It was almost like looking out the
window of a restaurant in Trier. Heck, you could even spot which street
lamps they forgot to polish before the camera crew arrived.

The sobering awakening came right afterwards. Clouds rolled in and then
multipath gets worse. All but one digital channel collapsed. That's
pretty much what I thought all along would happen :-(

 

[Joerg]

DVB-T does not exclude HD,
It is even possible to have SD and HDTV on the same carrier, and have the receiver
switch to SD if signal becomes really bad, see:
http://en.wikipedia.org/wiki/DVB-T
They are also working on DVB-T2 (2009).

So, then, why don't they use HDTV?

Our system seems not to be adaptive to poor signal quality. DTV is truly
digital, either you see stunningly video or nothing at all. In that
respect analog is certainly better.

Well Europe is not lagging, we did see already long time ago that satellite
is a much more clever way to bring content, only one small transmitter needed up there ;-)

Same here. However, it's in the hands of companies like Dish Network.
IOW not free. OTOH we don't have to pay a TV tax.

The new standards is DVB-S2 (look it up, within 0.7 dB of the Shannon limit, great system:
http://en.wikipedia.org/wiki/DVB-S
and HDTV is now broadcast both on the old DVB-S and DVB-S2 system.

But in your previous post you said HDTV is not seen over there. Is it,
or is it not?

Lagging is US.
It was sort of first with color (NTSC), then Europe got PAL, PAL was better.

Sure, it's usually easier to improve upon a technology than inventing it
from scratch. Later NTSC wasn't inferior anymore because they fixed the
phase drift problem. The challenge was to do it in a way that the older
sets would keep working and that excluded alternating the phase.

Then Europe went digital.
US also wanted digital, and did chose a lesser system with 8VSB, putting it
more in the third world category, as you can also see from the dollar, your
current dictator, and your true elections replaced by pre-programmed Republican
voting machines ;-) Now that is funny.

Ahm, I guess now you are drifting into propaganda (and I disagree with
all points).

Well, you cannot see Astra, it is below the horizon, but if you had not moved so far west you could have.
There is not only Astra here though, Hotbirds, many more, the sky is full of sats here,
that is why I have a motorised dish.

On the other bright side, in the cities, cable companies can easily pick up
the satellite signal, I have heard Netherlands is something like 80% cable.

But seriously, or more seriously, I fail to see the advantages of 8VSB especially in the cities.

Probably to have the capability of extreme resolution but I don't know
for sure. In the US football and baseball fans are the ones who drive TV
and subscription sales.

There is an other issue too, with digital processing in LCD receivers the picture needs de-interlacing
if not transmitted progressive, and that basically reduces the resolution by half.
Further losses happen if the TV does not have the same vertical pixels as the transmitted signal, as
rescaling causes further loss of detail.
I would like to see all content transmitted in 720 or 1080 progressive, there is a EBU recommendation for progressive:
http://www.ebu.ch/en/technical/trev/trev_301-editorial.html
I dunno is US transmitting *any* progressive? If interlaced, then you have only half the resolution,
and that would be less quality then old NTSC with a good signal
http://en.wikipedia.org/wiki/Deinterlacing

I believe our 1280*720 format is progressive but not sure. Anyhow, I can
only judge it as a viewer: The images are absolutely stunning. Until
multipath strikes, that is ...

 

[Jan Panteltje]

The sobering awakening came right afterwards. Clouds rolled in and then
multipath gets worse. All but one digital channel collapsed.

How far away is that transmitter? Curious.

 

[ChairmanOfTheBored]

Lagging is US.
It was sort of first with color (NTSC), then Europe got PAL, PAL was better.
Then Europe went digital.
US also wanted digital, and did chose a lesser system with 8VSB, putting it
more in the third world category, as you can also see from the dollar, your
current dictator, and your true elections replaced by pre-programmed Republican
voting machines ;-) Now that is funny.

You're a goddamned retard.

 

[ChairmanOfTheBored]

I dunno is US transmitting *any* progressive? If interlaced, then you have only half the resolution,
and that would be less quality then old NTSC with a good signal

You're an idiot.