67 replies to this topic

[bob_the_builder]

Hi,

I ended up buying a 46PZ800A Panasonic Plasma TV - it's so amazing and fits the room so well - I was so happy I went back to the salesman to tell him all about it... I turned off IFC and sports (AFL) and even normal DVD looks fantastic. Mind you my previous TV was a 4:3 52cm CRT Teac so I guess even an oil painting would have been an improvement!

Anyway, I noticed that on some occasions when viewing FTA digital TV on the bottom horizontal edge of the screen i see a line or half line of weird pixels. Sort of like black and white blocks. However, if I turn on the Overscan function then it dissapears. I figure it can't be an issue with the TV because it only happens on some channels. As far as I can tell Overscan is like a very small zoom in function, and it makes the rough/weird looking edge issue dissapear. This is the only issue I have with the screen. I was wondering if anyone had this issue, or used the overscan feature?

Thank you.

[Junkyard]

yeah some stuff i am watching seems to have lines at the bottom or top if overscan is off , so i leave it on

[pneu]

Overscan is present on all TV's to hide any unsightly patterns that are deliberately added to the edges of the frame, usually for technical reasons.  Panasonic has provided an option for turning overscan on/off in the new models.  The main use of the feature would typically be for PC input to prevent edges of the screen getting cut off, such as the start menu.  You may also like to turn overscan off to improve picture quality slightly when you know the edges of the video are "clean" , eg. blu-ray.

[dalemissen]

pneu, on Jul 30 2008, 01:22 AM, said:

Overscan is present on all TV's to hide any unsightly patterns that are deliberately added to the edges of the frame, usually for technical reasons.  Panasonic has provided an option for turning overscan on/off in the new models.  The main use of the feature would typically be for PC input to prevent edges of the screen getting cut off, such as the start menu.  You may also like to turn overscan off to improve picture quality slightly when you know the edges of the video are "clean" , eg. blu-ray.

i so can't wait to buy one of these puppies!!!!!

[tonymy01]

Ten SD uses the bottom line in the 576i image to do some signalling to their remote transmitters etc, so if you don't have overscan you may see this (some STBs will now blank out the edges of the images to stop the dozens of "why does my STB have junk on the sides" complaints).   You also see this junk when they upscale something from SD to go to HD (e.g. the MotoGP broadcast when shoved in the earlier timeslot instead of Dexter on HD had this on the HD channel).
It seems when Nein upscales their SD, some junk ends up on the top line or so of the 1080i image also.
Also anything coming from analogue world has the potential for junk on the top mainly and often even the sides can be bad.    Overscan is always accounted for when framing anything (even film!), so it isn't a huge loss losing it, some TVs lose over 10% of the image!!
The Samsung LCD series has a "Just Scan" feature that maps the 1080i/p digital broadcast (internally, or via HDMI) to the 1080p pixels on the screen, so you will see this junk.    But I prefer to leave it in Just Scan mode and "tune out" to the bits you can see on the edges, as I reckon that partially zooming a 1080 image to a screen that is native 1920x1080 means stuff will just not line up to the pixels properly any more.
Regards

[Owen]

tonymy01, on Jul 30 2008, 02:35 AM, said:

But I prefer to leave it in Just Scan mode and "tune out" to the bits you can see on the edges, as I reckon that partially zooming a 1080 image to a screen that is native 1920x1080 means stuff will just not line up to the pixels properly any more.

It does not really matter, video from a camera (the real world digitized) is very different to PC generated graphics, there is no resolution at 1920x1080 in video only 1920x1080 pixels which is not the same thing at all.
If the scaler is any good there should be no loss of resolution with overscan, but since the image will be bigger it may look a little softer, which can be compensated for with the sharpness control.

Edited by Owen, 30 July 2008 - 03:52 PM.

[smokenz]

just scan or 1:1 pixel mode is important for HD I believe

any bluray, 1080p game etc will fill the edge perfectly with just scan, and if you have your tv set to just scan oppose to 16:9 it makes a huge difference i think.   i became aware of this feature hardout when i used a bluray test image from thoese hidden eggs ages ago with the casino royale bluray and a few other sony blurays

maybe turn off with HDTV if you get this weirdness, but leave on with a PS3, Bluray player or any HDMI device outputting 1080i or 1080p

[Owen]

Believe what you like but it’s a myth, 1:1 mapping is not important for video.
The visible resolution of video is ALWAYS lower then the pixel count would indicate.
For anything shot with a 1080 digital camera or film transferred to 1080 digital video you cannot ever have 1920x1080 output resolution, its an impossibility due to digital sampling limitations and required low pass filtering.

[smokenz]

run a bluray 1080p calibration pattern, it shows a lot of measurements of where the true 16:9 or 1.77:1 image should be touching the border of the screen.

if you don't run 1:1 or just scan it will be over scanned by 3-5%, loosing that much of the picture, also blowing out the true aspect of the video because when the image is over scanned it stretches all boundaries, which means if the movie is made 2.35:1 aspect ratio, it won't be shown in that aspect ratio, will be slightly tall.

get a bluray, put it in and try it.  if you select between overscan and not you can tell the image is not scaled correctly.  

i know a 16:9 TV is a different aspect ratio to movies, but with just scan the black bars are put in the correct way at top and bottom to simulate the correct aspect ratio with vertical pixels staying 1920 but horizontal having a different range that would calculate out to the correct aspect ratio and fitting within a 16:9 TV.

irrelevant of the pixels the aspect ratio is displayed correctly with justscan or 1:1 and it uses the full panel to do this, from edge to edge, without overscanning anywhere.

i can put some pictures up if you want as well.

[Owen]

Overscan is supposed to cut of the edge of the picture that’s its purpose, but it should do so equally horizontally and vertically, therefore aspect ratio is unaffected.
If your display is altering the aspect ratio with overscan enabled it is defective.
Other then a slightly larger image with the edges cropped off there should be no loss of picture quality, if there is there is a problem.

BluRay movies normally don’t need overscan so the use of “Just Scan” is appropriate, for TV broadcasts it is generally not.

I have been running 1080 from my HTPC to my 70” 1080 display for 18 months and to my previous 57” 1080 display for 2 years before that, both with and without overscan.
I can control under-overscan in 1% or 1 pixel increments at the touch of a key in real time, so I know exactly what overscan is about.

[bobbyf]

Owen, on Jul 30 2008, 03:20 PM, said:

It does not really matter, video from a camera (the real world digitized) is very different to PC generated graphics, there is no resolution at 1920x1080 in video only 1920x1080 pixels which is not the same thing at all.
If the scaler is any good there should be no loss of resolution with overscan, but since the image will be bigger it may look a little softer, which can be compensated for with the sharpness control.

Hi Owen
Many of us of here, who have read your posts over the years know that you know your stuff, especially about overscan, but there are many who won't necessarily understand about why video resolution cannot be 1920*1080. (i'm one of them actually-makes my brain hurt)
We see video cams in JB and the like every week with 1920*1080 stamped all over them, so could you explain what is actually happening when you record on these cams and what is the actual resolution?
I'm sure you've posted this before somewhere so maybe you could cut and paste into here too.
Thanks
Bob

Edited by bobbyf, 30 July 2008 - 09:12 PM.

[smokenz]

If you overscan an anamorph video, typical hollywood blockbuster bluray, you loose more visual left and right data than top data because anamorphed movies have black bars above and below them, that is effectively empty space.

if you overscan that movie it will have a different aspect ratio because the tv can't display the left and right data.  i know it's not changed because behind the TV bezel is where the 2% overscan of each side would be hiding if you imagine it.  but because the side data is not shown, im sure the aspect has changed and then movies look slightly stretched.

its like a small version of playing a 16:9 movie on a 4:3 tv and you completely cut off the left and right.  sures it still a 16:9 movie, but you loose alot of the left and right data which alters how you see it on the TV.

[bobbyf]

smokenz, on Jul 30 2008, 08:51 PM, said:

if you overscan that movie it will have a different aspect ratio because the tv can't display the left and right data.  i know it's not changed because behind the TV bezel is where the 2% overscan of each side would be hiding if you imagine it.  but because the side data is not shown, im sure the aspect has changed and then movies look slightly stretched.

I think you're mistaken here smokenz.
The aspect ratio will remain the same, you just see an extra 2% of the vertical image, because the lines you lose are not part of the actual image. The fact that you can see 2% less of the horizontal image does not mean that the ratio has changed.
Hope that makes sense

Bob

Edited by bobbyf, 30 July 2008 - 09:45 PM.

[smokenz]

yeah i realise now I was using the wrong term.

the best I can explain what I was meaning was watching a movie overscanned is similar to watching a 16:9 broadcast movie on a 4:3 TV, but of course not as extreme.

the reason movies on 4:3 tv's were cut was to make it preserve the aspect ratio to the TV's aspect or format ratio.

so if a bluray movie is designed to be output on a 16:9 TV, without overscanning, then overscanning is changing the output ratio when it doesn't need to be,  then making the movie look stretched.

so back to the original post i put, it is better to have just scan or 1:1 mode enabled when watching a movie or PS3 game etc.

i dont see the point of having overscanning or any image errors if broadcasts are digital, tvs are digital.

Edited by smokenz, 30 July 2008 - 11:41 PM.

[pneu]

smokenz, on Jul 30 2008, 08:51 PM, said:

If you overscan an anamorph video, typical hollywood blockbuster bluray, you loose more visual left and right data than top data because anamorphed movies have black bars above and below them, that is effectively empty space.

Blu-rays are not encoded in anamorphic format - there is no need.

As bobbyf has mentioned, overscan does not cause incorrect aspect.  What will cause incorrect aspect on the Panasonics is using the "Just" ratio which preserves the centre part of the image and stretches the extreme edges out - is this what you are seeing?

[smokenz]

sorry i don't have a panasonic tv, i just saw the title about overscanning.

i have played with sony, pioneer, samsung, philips, lg, sharp etc, all which were 1080p panels, and had 1:1 or Just Scan options.

I don't think you guys get what I'm talking about.

1920x1080 is a 1.777778:1 aspect ratio, or 16:9  so is 1280x720. 1366x768 is around 1.778:1.

When a bluray movie is made, it is made to output on a 16:9 screen and has the aspect ratio encoded in it.  This movie will only display 100% correct as the producer wanted if you have it output at 1920x1080 1:1 pixel.  Or if your TV has Just Scan, it will scale the image to fit perfectly in a 16:9 screen, and the player will output the black bars to make the aspect ratio correct.

If you put a bluray disk in a PC, or an HD movie rip and make it full screen on a laptop that has a 16:10 screen.  You will get dark black bars at top and bottom to crop it to 16:9.  Then the software, Media Classic, PowerDVD etc, will output the aspect ratio within that 16:9 window.

TV's only started to recently allow a 1:1 mode or Just Scan.  Obvious it's to prevent overscanning.  Obviously overscanning is bad for some reason.

put in a sony bluray disk.  at the main menu press 7669 the enter.  go to the second hidden calibration screen that shows all the little triangles touching the outside of the panel.  it should say 16:9 on the top right and left. 1.78:1 bottom right and left.  with just scan off, you won't be able to read 16:9, and only see the 1 from 1.78:1

you will then see what i mean about Just Scan or 1:1 being the preferred method as it will not corrupt the output anyway.

EDIT::

i thought I would host some images to show people that don't have a bluray player what i'm talking about

this image here has no triangle markers on a calibration, which is set to 16:9 on this plasma, or standard over scanning of the plasma

[img=http://img142.imageshack.us/img142/5715/16x9jw4.th.jpg]

this image here is using the Just Scan feature.  Which if this tv was 1080p it would be doing a 1:1 pixel match.  but Just Scanning on a non 1080p set produces the same result as the panels are all 16:9

[img=http://img517.imageshack.us/img517/2615/justscanua7.th.jpg]

You can see the difference that overscanning causes on a bluray and translates through to watching the movie.

Edited by smokenz, 30 July 2008 - 11:10 PM.

[tonymy01]

I don't know what you are smoken :-)
Overscan is there for a reason that has nothing to do with maintaining aspect.    If you overscan an image by 2% on all 4 sides, the aspect ratio is still maintained.   If the image was letterboxed a bit (e.g 2.35:1 movie), then by overscanning, you lose a little of the "black bits" on the top and bottom, and you lose a little bit of the image on the left and right, but the aspect ratio remains identical.   In fact, your screen is slightly more filled with image than black when overscanned, but the aspect remains identical.
Overscan is a phenominon that dates back to the invention of the television, it is there to give the sweeping electron beam time to get back to the beginning of the line, and often you will get irregularities on the edge both from the beam settling down again, and because the very edges of the beam are the most difficult to focus etc, so everything made for TV is framed with overscan in mind.   Even making movies is the same, as movie theatres can't 100% make the projection system exactly match the projection screen, so a little overscan is even in movies!
Now, saying this, if a movie has been digitally made (Nemo/etc) and you have a digital display, then there is no reason not to have the movie 1:1 pixel matched as there will not be any junk top/bottom/left/right on that image.   Even most digitally filmed stuff these days will be the same.   Shame that Ten has the signalling garbage in the actual 576lines of image rather than using a digital signalling system... but they don't do this on HD at least (unless they have upscaled an SD show).

You edited your message.
See, in both cases, your circles are still circle, thus the aspect ratio is not changed when overscanning (why should it, as you are "zooming" an identical amount on all 4 sides).    The thing is, even though it looks like "oh no, I am losing stuff"... you aren't really losing much, as everything is filmed assuming at least 5% of the image will be lost to overscan.   In fact, if the editor is using a monitor that is overscanned to do the editing, they may not notice that a boom microphone or some kid making silly faces is off on the edge of the screen, and you may end up with this junk in the final product.    But, as has been suggested, with Blueray I guess they author it with the assumption of "it is a digital HD source going to a digital display, there is no need to worry about overscan anymore", and so turning 1:1/JustScan on is worth it in the end, if you can just "tune your brain" out of seeing any occassional junk that makes its way into the edges of the frame for other sources like TV.
Regards

[smokenz]

if you look at my edited post you wil see the image showing what i mean.

all blurays have no image problems and are designed for 1:1, no overscanning.

the fact you overscan a bluray takes away that ultra wide look they recorded a movie in.

i originally said 1:1 or Just Scan is the way for HD movies on stuff like a bluray player or PS3.  and use overscan for HDTV broadcast etc.

ask any AV enthusiast.  they will not want you to overscan a bluray, that's why almost all new TV's are now coming out with this feature.

[tonymy01]

I don't think you are reading what everyone is writing here.   The aspect ratio is not causing squishing or any distortion of the image when overscanning vs 1:1.    But you are correct, you lose bits off the side (and top/bottom), that is what the term overscan means!
Regards

[smokenz]

sorry bud, i see what happened here.

i was focusing more on the overscanning.  i know overscanning is like a zoom.  the reason i guess im going hard about it is i had this mean debate with this philips technician quite a few years back when I got an LCD about an overscanning 720p set i had.  In the end he and philips couldn't solve it because back then there was no just scan options on tvs.  and he put a comment through to philips head design team to look into it.

i questioned him why playing a 720p image, 1280x720, on a 1366x768 panel needs to be overscanned.  you can see my train of thought with that, it's already stretching a 720p format up to 1366x768, but then it overscans it.  this was causing distortion of text on TVs, which was why I complained about it.  this was like 5 years ago or so before 1080p was out in TV's.

since then, most digital tvs now have just scan or 1:1 modes, and you find the early reviews of 1080p sets that started to include it, it got mention and usually had a paragraph about how good this was because you could see the movie in full in it's aspect without loosing any picture.

agree it's a pity broadcasters are still overscanning with HD broadcasts.  It doesn't need to be used now with digital i believe, bluray is already showing that.

[pneu]

I think some people just have a personality type that disagrees with the idea of overscan.  Personally I don't give a stuff if you chop 2-5% off my picture given the benefit of doing so.  I doubt anyone could pick the difference in a video source that had lost 2-5% of its resolution in a double blind test.

[smokenz]

there is no benefit in overscanning with a digital video type.  if you had a monitor that was perfect 8mp resolution for photos.  would you then view all your photos at 105% instead of 100%, causing you to loose the outer edges?  i don't see any logical reason to do that, and you are loosing a 1:1 pixel match.  

if you watch 2.35:1 aspect ratio movies regularly at 1:1 or just scan, you will notice that it gives a more wider feel to it and you can tell the difference when overscanning is applied.  talk to a bluray producer, im sure they will recommend 1:1 or just scan on a 1080p panel.  the movies are made to fit 1080p perfectly and the player controls how it should look.  once your tv overscans that signal it's not output how it was intended.

Edited by smokenz, 31 July 2008 - 09:42 AM.

[Owen]

bobbyf, on Jul 30 2008, 09:12 PM, said:

Hi Owen
Many of us of here, who have read your posts over the years know that you know your stuff, especially about overscan, but there are many who won't necessarily understand about why video resolution cannot be 1920*1080. (i'm one of them actually-makes my brain hurt)
We see video cams in JB and the like every week with 1920*1080 stamped all over them, so could you explain what is actually happening when you record on these cams and what is the actual resolution?
I'm sure you've posted this before somewhere so maybe you could cut and paste into here too.
Thanks
Bob

I’ll save some time by quoting a post I made to a camera forum.

“Take a camera with a 1920 pixel wide image sensor and focus it on a test card with alternating black and white vertical lines, which buy chance happen to be spaced to line up exactly with the pixels on the sensor. Also assume the impossible that we have a perfect lens and no pre filter-defuser in front of the sensor so that the lines are perfectly focused onto the sensor pixels. In this case we can easily resolve 1920 lines (960 line pairs) with great accuracy.

We now adjust the image or camera so that the test lines are moved across half a pixel. Now we have a big problem as every pixel on the sensor sees half a black line and half a white line and will therefore output a uniform 50% grey, we will not be able to resolve anything at all.
Obviously in the real world we cannot force single pixel size details to fall exactly on image sensor pixels, some will most wont.”

If we don’t filter or role of the cameras response to high spatial frequencies we will get aliasing, moiré and shimmer as fine detail move into and out of alignment with the image senor pixels.
For this reason ALL digital images including those from film scanners are filtered or blurred, first by the camera/scanner lens, then by a diffuser in front of the image sensor which makes sure that that single pixel sized details are spread over several pixels, then after capture to the digital domain by digital low pass filters in the camera/scanner.
After all that we then have video compression which blurs/filters fine detail (high spatial frequencies) even further.

Every step in the acquisition process introduced MTF loss and what we see on screen is the sum of all those combined losses, therefore its completely impossible for 1920x1080 video to have 1920x1080 “resolution”, its can only have 1920x1080 pixels which is not the same thing at all.

A good but technical explanation of MTF can be viewed here:

http://www.normankor...orials/MTF.html

An interesting and informative article on the practical performance of digital and film cameras and how that translates to image sharpness can be read here:

http://bssc.sel.sony...calSeminar2.pdf

Unfortunately most people think that a 1920x1080 camera will give use pixel perfect 1920x1080 “resolution” images when displayed 1:1 on a 1920x1080 monitor, however nothing could be further from the truth, 1:1 pixel mapping is therefore NOT important.
The only single pixel detail you will get in video sourced from a camera is the sharp edges of compression artifacts, and the last thing we want to do is faithfully reproduce them as they are nothing but a distortion and not in the original image.

I hope this generates some understanding and dispels the common misconceptions surrounding “resolution” as apposed to pixel count.

[Owen]

smokenz, on Jul 31 2008, 09:42 AM, said:

there is no benefit in overscanning with a digital video type.  if you had a monitor that was perfect 8mp resolution for photos.  would you then view all your photos at 105% instead of 100%, causing you to loose the outer edges?  i don't see any logical reason to do that, and you are loosing a 1:1 pixel match.  

if you watch 2.35:1 aspect ratio movies regularly at 1:1 or just scan, you will notice that it gives a more wider feel to it and you can tell the difference when overscanning is applied.  talk to a bluray producer, im sure they will recommend 1:1 or just scan on a 1080p panel.  the movies are made to fit 1080p perfectly and the player controls how it should look.  once your tv overscans that signal it's not output how it was intended.

If all you ever view is the PC desktop or BluRay movies you can get away with zero overscan, however for everything else it’s desirable to have overscan, the big question is how much. In my experience 2% (1% on every side) is adequate most of the time, but manufacturers play it safe and normally go for 2.0-2.5% on each side to be safe.
ALL video produced for TV has overscan taken into consideration and it is assumed the display WILL overscan.

Overscan DOES NOT affect aspect ratio, if it does on your display it is faulty. This remains true no matter what the pixel aspect ratio of the display, the image is scaled proportionally to fit the physical display area and aspect ratio.

Edited by Owen, 31 July 2008 - 01:36 PM.

[tonymy01]

I get the point smoken is trying to make, but he is making it the wrong way.   When you slice 2% of the top/bottom/left/right of a 2.35:1 movie, the over-all aspect ratio remains identical, but because of the letterboxing, the only bits lost are some black bits (who cares) and some real image on the left/right.   So if you divide onscreen image width by onscreen image height, then the aspect ratio of the visible image on screen changes.   But the loss is so small, who cares really, and NO distortion has been created by this process.

Your point about filtering is spot on.
Regards