Black and white bars in digital broadcasts

Bob_Harris

On the Irish DTT channels you can see alternating black and white bars across the very top right hand half of the screen.

Sometimes on ITV you can see a black and white bars moving, like a small spinning black and white striped ball, in the top left hand corner.

Does anyone know the technical purpose of these bars?

See attached image for example.

Sponge Bob

ad break warning ??

Owenw

Cue Dots! (I've been wondering for years as well)

Bob_Harris

So that explains the ITVs black and white bars. I've never watched ITV long enough to draw the correlation between an oncoming ad and the bars.

I wonder what's the purpose of the ones on Irish DTT. They are there constantly.

copacetic

They aren't cue dots on RTÉ, it's VBI data. Normally not seen on TVs as there is always some overscan. Generally you would only see it on PCs or special monitors, you can likely adjust the settings on your PC.

Read more about overscan here:
http://scanline.ca/overscan/

maxg

Right it is teletext and the widescreen flag in the first half of line 23.
Only visible at a lcd if overscan is switched off for sd.

Sam Russell

Bob_Harris wrote: »

Sometimes on ITV you can see a black and white bars moving, like a small spinning black and white striped ball, in the top left hand corner.

Does anyone know the technical purpose of these bars?

FWIW, ITV was set up as a regionalised network with local input and distant input. They had local ads, local programmes, and network programmes. This was all setup in the 1950s, before transistors were generally in use, and long before computers were possible for these tasks. TVs were prety crude as well. People did nearly everything. The sync of the TV signals was vital to prevent picture jumping, so the local studio synched to the distant signal. When the changeover was coming up a sync difference signal was shown so the the two signals would come into sync. Presumably this was done manually originally. Why it was visible, I do not know.

Teletext was introduced in the 1970s by ITN (Oracle) and BBC (Ceefax) so that data could be transmitted in the unseen part of the picture. On some TVs it was not that invisible.

fat-tony

copacetic wrote: »

They aren't cue dots on RTÉ, it's VBI data. Normally not seen on TVs as there is always some overscan. Generally you would only see it on PCs or special monitors, you can likely adjust the settings on your PC.

Read more about overscan here:
http://scanline.ca/overscan/

The VBI data is visible only on RTE 1 and 2 (not TV3 or TG4). I can see it on my Mvision combo displayed via SCART on a 4:3 TV and on DVBviewer and on Windows Media Centre on two different PCs. It's probably not visible on a widescreen TV if you have overscan on, but it is obvious on a 4:3 TV or on a regular PC monitor with black bars on the top and bottom. I don't think it's a setting problem at the receiving end - seems to me to be an issue when the signal is being encoded.

copacetic

fat-tony wrote: »

The VBI data is visible only on RTE 1 and 2 (not TV3 or TG4). I can see it on my Mvision combo displayed via SCART on a 4:3 TV and on DVBviewer and on Windows Media Centre on two different PCs. It's probably not visible on a widescreen TV if you have overscan on, but it is obvious on a 4:3 TV or on a regular PC monitor with black bars on the top and bottom. I don't think it's a setting problem at the receiving end - seems to me to be an issue when the signal is being encoded.

TG4 and TV3 generally have less data in the vertical blanking so it may not be as noticeable. I'm not sure how it could be an 'encoding' issue, as all the channels are broadcast the same way.

Even with overscan off on your TV, most receivers will still overscan slightly to hide the datacasts. Your examples would all show it alright as the 4:3 TV isn't ideal and the PC apps generally don't have any overscan on by default.

I don't think it's a setting 'problem', but if you don't want to see it then you need to overscan by a line at least imo.

fat-tony

I appreciate what you're saying, but it's peculiar that the same issue does not occur with the UK satellite channels - BBC, ITV etc which also carry some limited teletext data etc. in the VBI. Are RTE using more of the lines to carry teletext? I would imagine that the Mvision supresses the normal number of lines when it's generating the black bars to present the 16:9 image on a 4:3 TV but it has no user control over the overscan, nor does Windows Media Centre or DVBviewer, so it's an annoying artefact - but only on RTE 1 and 2!

Bob_Harris

I'd assume carrying teletext and other data in the overscan region is analogue specific, because the DVB standard allows for these things to be transported "in the background".

copacetic

fat-tony wrote: »

I appreciate what you're saying, but it's peculiar that the same issue does not occur with the UK satellite channels - BBC, ITV etc which also carry some limited teletext data etc. in the VBI. Are RTE using more of the lines to carry teletext? I would imagine that the Mvision supresses the normal number of lines when it's generating the black bars to present the 16:9 image on a 4:3 TV but it has no user control over the overscan, nor does Windows Media Centre or DVBviewer, so it's an annoying artefact - but only on RTE 1 and 2!

interesting, not sure on satellite issue. Are you viewing RTE channels off same satellite?

I'd suspect the issue is that the sat platform does the overscan on BBC and ITV and you are looking at RTE off DTT? However if you are seeing the issue on RTE and no others off same platform then it would appear to be an RTE issue all right.

I don't see the issue on sky platform no matter what settings I use, but I know I'm losing at least a few lines top and bottom on sky.

fat-tony

copacetic wrote: »

interesting, not sure on satellite issue. Are you viewing RTE channels off same satellite?

I'd suspect the issue is that the sat platform does the overscan on BBC and ITV and you are looking at RTE off DTT? However if you are seeing the issue on RTE and no others off same platform then it would appear to be an RTE issue all right.

I don't see the issue on sky platform no matter what settings I use, but I know I'm losing at least a few lines top and bottom on sky.

I've checked the sat version of RTE1/2 - same Mvision box as terrestrial. There are no VBI lines visible. The VBI lines are only visible off DTT and on all methods available to me - STB and two different PC platforms. The reason I mentioned encoding at all was that I presumed RTE (RTENL?) might have some control over what lines were made "visible" in the MPEG data stream. Out of the 625 lines which are in the PAL standard, usually only 576 are present in DVB-T or DVB-S standard definition. I'm not au fait with the actual conversion/encoding process, but I assume there is some level of control available to the broadcaster as to how much of the actual raw picture signal is presented to the encoding process. It's presumably a non-issue if you're viewing on a 16:9 display, unless you have the overscan turned off completely on your TV.

copacetic

fat-tony wrote: »

I've checked the sat version of RTE1/2 - same Mvision box as terrestrial. There are no VBI lines visible. The VBI lines are only visible off DTT and on all methods available to me - STB and two different PC platforms. The reason I mentioned encoding at all was that I presumed RTE (RTENL?) might have some control over what lines were made "visible" in the MPEG data stream. Out of the 625 lines which are in the PAL standard, usually only 576 are present in DVB-T or DVB-S standard definition. I'm not au fait with the actual conversion/encoding process, but I assume there is some level of control available to the broadcaster as to how much of the actual raw picture signal is presented to the encoding process. It's presumably a non-issue if you're viewing on a 16:9 display, unless you have the overscan turned off completely on your TV.

Well that makes a bit of sense to me then. I agree NL have control over coding, but they code TG4 and TV3 also so I'm pretty sure they are setup the same way as RTE1/2.

So my conclusion (without anything really to go go!) is that at the moment during DTT testing they are putting out the full provided picture for all channels and RTE1 and 2 show the line of data more clearly as they have a lot more data there than TV3 or TG4.

fat-tony

Funnily enough, this morning I could see VBI data on some shots on TV3 on DTT during the morning, studio-based programme:rolleyes:. I also realised that the aspect ratio on TV3 on DTT seems to be fixed at 16:9, even though the studio-based stuff is not widescreen. TV3 on satellite, however, seems to switch properly between 4:3 on studio stuff and 16:9 during the advertisements or on programmes originated in widescreen. RTE seem to switch properly between 16:9 and 4:3 on DTT and satellite depending on the programme material.

I believe that Sky boxes don't support (or use) line 23 widescreen switching which is why you won't see this on a Sky system. You most likely would see it on UK DTT. I've also seen it on a Lidl FTA satellite system, More4 +1 always showed line 23 on screen while the regular More4 didn't.

There are bitmaps on the net of the WSS data, apparently if you load it on a PC when connected to a TV it will switch aspect ratio.

PyeContinental

I've noticed the same phenomenon on RTE1 and RTE2 DTT channels in (Win7) Media Center. In the attached pic you can see them from the top left, across to about the area above the parting in Franc's hair.
I assumed they were something to do with overscan alright, definitely not ad break warning like you might see on ITV for example.

I only see these lines when I bring up the menu or EPG overlay. It doesn't happen with TV3. What I find more annoying is that the display goes very briefly blank, as though the display is having to switch resolution. I've a feeling that's something to do with what's going on.

scath

Guess you all probably saw this...just though I'd attach...continue convo, don't mind me lol.