1. Home
  2. Electronics Repair
  3. ghosts on digital OTA TV

ghosts on digital OTA TV

Before the advent of mandatory digital over-the-air TV, it was said that the picture would be near perfect, and that there woudl be no ghosts. Tonight I saw one, for a full half hour if the scene had the right kind of lines, including vertical lines.

From the beginning I had doubts about the claim there would be no ghosts. I gleaned or understood that, aiui, if a ghost signal was less than, say, 50% as strong as the direct signal, and the direct signal were 0, the electronics would "round down" and output a zero. If the direct signal were 1 and the ghost was less than 0.5 and more than zero, or more than -0.5 and less than zero, the sum might be 1.5, but the electronics would interpret that as a 1.

But that didn't account for every situation. What if the ghost signal was 0.6 the strength of the direct signal and the direct signal was a zero? Now, although I've never seen it discussed (and I do read a bit) I presume there are 3 signals making up the OTA video signal, and I'm ignoring that for now.

Anyhow, for the first time in 9 months, with no changes to the tv wiring of my house, tonight I saw a ghost on an OTA tv show. On channel 7.3 in Wash. DC, and I live about 30 or 40 miles north of the transmitter. A black and white show, The Rifleman, with Chuck Connors, from the 60's I think, but maybe the 50's.

I don't know from where the reflection could be coming. There is a 7 story building about 100 yards behind me and to the left (wrt channel

7) but that's not far enough away, is it, to account for 1/8" or 3/16" ghost displacement on a 19" tv? Woudln't any ghost from there be a small fraction of a millimeter from the main image?

Could the ghost be in the coaxial connection between my TV tuner (a DVDR) and the tv. There is 15 feet of co=ax, an RF amp/splitter, 8 more feet of co-ax, a splitter, another RF amp, another splitter, 30 feet of co-ax, then another splitter, then 10 more feet of co-ax. If that were the source, it wouldn't be attributable to a digital signal, but I don't think those lengths are sufficient to account for a ghost

1/8" from the main image, are they?

They also didnt' mention, said otherwise though I forget the words they used, that there would be checkerboard interference. I get that a lot on several stations, especially the ones that are 30 or 40 miles away.

How much tv did these people watch before they made their false claims? Were they only a mile from the transmitter in rural Kansas?

Does anyone know their names or email addresses so i could tell them how foolish they look?

I get two new stations I like with digital, so I'm not angry. (I wouldn't be anyhow.) I just think they make themselves look stupid when they make false claims that they should have known better than to make, and to an extent, the impression this leaves spills over on to other "experts" and "scientists", in other fields.

Reply to
mm
Loading thread data ...

It doesn't matter what the relationship is between the main signal and any reflected signals, they still won't cause ghosting in digital TV. They may mess the signal up to the point where the television doesn't produce a picture, but you won't get ghosting - digital TV simply doesn't work that way.

Sylvia.

Reply to
Sylvia Else

But I did get ghosting.

Who am I going to believe, your theory-based answer, or my own eyes?

Do you watch OTA tv these days?

Maybe it matters that the station is 30 or 40 miles away. Do you watch stations like that?

I also saw what was probably ghosting during a commercial right after that show. The commercial had big thick red letters against a white background, and the red continued about an eighth or three sixteenths of an inch to the right, lighter than the main red image.

Reply to
mm

They are not necessarily inconsistent. You may have seen something, but it wasn't ghosting, in the sense that it wasn't caused by your antenna receiving a main signal and some delayed reflections.

If you're using a set-top-box and feeding a composite video signal into a conventional TV, you may just be seeing an effect where abrupt changes in luminance feed into the colour signal.

Sylvia.

Reply to
Sylvia Else

No. I'm using a DVDR with a hard drive and a digital tuner and sending an RF signal on channel 3 to an analog tv.

Isn't that what people with digital cable and satellite do if they don't have composite or component inputs on their tvs?

Assuming for the sake of this paragraph that this makes a difference, those many people who promoted the high quality of digital tv never said anthing about this making a difference.

Reply to
mm

the

1.5,

signal

and

So what you were seeing were fringes introduced in overdriven RF stages or harmonics/sub-harmonics/beats within the IFs or combinations between

I would rather have ghosts instead of "no signal" blank screen and sound in bad reception environment but unfortunately not an option these days

Reply to
N_Cook

Okay. But then I used the same system with analog tv transmission. For 23 years I used a VCR and later for a year I used the DVDR using its analog tuner, in both cases with RF output to this tv and other tvs, all set on channel 3. Maybe the ghosts I saw then were also fringes introduced in overdriven RF stages as you describe. ??

Me too. In fact picture quality has never mattered to me much (despite this thread of mine about ghosts.) Much of the time I'm reading the newspaper, cooking, or looking at the computer screen, and even when I'm looking at the tv I don't care much. But even analog tvs in the last several years have blanked out the screen and the sound if the signal wasn't strong enough. It should have been an option. It should still be an option.

>
Reply to
mm

Please confirm that you do not drive a motor vehicle... If in fact you do drive, please indicate what areas we should be on the watchout for you.

Reply to
PeterD

I missed the original post, but the appearance of ghosts can be caused by a limited bandwidth of the video circuit.

Ghosts can also be caused by improper termination of the feed line from the antenna to the TV.

Reply to
root

On second thought, the digital processing should eliminate any such effects on the RF side.

Reply to
root

I, too, am confused. A sufficiently strong ghost should garble the data to the point where the image is corrupted -- not duplicated.

My guess is that the "ghosting" has some other source. It might even be in the signal itself. If the low-pass video filter is not properly phase-equalized, there can be severe ringing on the trailing edge of objects, which would appear as "ghosts" of vertical lines.

Reply to
William Sommerwerck

Hi!

Ghosts in a digital over the air TV signal won't be caused for the same reasons that analog over the air TV signals had them.

What's going out over the air in a digital TV transmission is a data stream, complete with some degree of error correction. Any such "ghosting" would show up in the signal as a duplicated datastream, which is probably ignored or considered errant by the video decompressor in the unit. If you were to get enough of these, you might notice some glitching ("checkerboard pattern") in the video.

When you receive digital TV broadcasts, they will be an all or nothing affair. Either you are receiving enough of the datastream to receive a picture and sound, or you are not.

The flaw could have been in the material being broadcast. No broadcasting method can make up for flaws in the original material. Older material is more likely to suffer from problems.

It sounds like you have a significant length of cable between your converter box and TV set, going to a significant number of places and through some amplifiers. If you are looking for interference, you are looking there. But I don't think you'll find any--or at least not much. In other words, this isn't the ideal situation, but I doubt it is the problem.

Everything after the converter box's RF output is an analog TV signal and therefore subject to the same things that can interfere with a much more powerful analog TV broadcast signal.

The only major thing I don't like about digital TV is the unwatchability of weak signals. Watching weak analog TV signals was never much fun, but at least it was doable if you had to.

William

Reply to
William R. Walsh

Is the trailing edge on the left side or the right side?

Reply to
mm

You may be seeing spikes before and after due to filtering. It might be more or only noticable viewing on an analog TV. I was seeing filtering and emphasis effects on my 36 in CRT Toshiba on analog signals which i assumed to be due to digital processing.

greg

Reply to
GregS

Cabling signal crosstalk between multiple tuners at the baseband.

--
Adrian C
Reply to
Adrian C

"William Sommerwerck" wrote in news:hj73hr$di4$ snipped-for-privacy@news.eternal-september.org:

it could even be a coax cabling problem;some mismatch or cable defect,or too long a cable from settop to TV.

OR the TV itself....

--
Jim Yanik
jyanik
at
localnet
dot com
Reply to
Jim Yanik

I've not had ghosting, but in the beginning of the transition, I used to get something that looked like the reds were shifted, like bad convergence. The shift changed, so that the red picture would sort of float around it's target. It might have been a combination of the broadcasters working out the bugs, and me working out the antenna, because my picture has been flawless for about 6 months. I did cut way back on my signal level by removing a distribution amp and padding the antenna output.

Reply to
bg

a

get

he

the

s

If it was a black and white picture, it could be there were some artifacts caused by the film to video conversion. Did you check any other sets or neighbors' tvs to see if they saw the same thing.

Reply to
hrhofmann

If it was a black and white picture, it could be there were some artifacts caused by the film to video conversion. Did you check any other sets or neighbors' tvs to see if they saw the same thing.

I had the same effect with two different brands of convertor boxes and on different TV's. My antenna was common to both. I don't know if too much signal could have caused it, or if it was broadcast with a problem. Most of the local broadcasters did seem to need a month or two to get things right, such as the sync between the video and audio, going off air etc ... and I was tweaking the antenna quite often too, wondering if it was me or them.

Reply to
bg

No. I figure I'll have a chance to do this later. To check this tv on a different episode (Heck, I watched the show last night but forgot to look.)

I've seen lack of sync but only once, I think, and not at the start.

I just got an antenna, but have to crawl into the attic to set it up.

Reply to
mm

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.