Another Underscan/Overscan Question

[yukon33]

From my research it seems that modern TVs still operate with an overscanned picture to accomodate older transmissions [and ensure accurate framing of those] rather than to deal with image drift, which has become much more rare in LCD and CRT displays.

Some basic info here:-

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

My question to those well versed in overscan/underscan usage: Is it true that 'modern TVs' will not vary much - if at all - between each other is respect of picture sizes, and there will be almost no image drift between different viewers?

Reason for asking, is that I can make great use of the valuable 7% or so of picture frame size I can gain by reducing the image in a current Avid Liquid editing project - but this means leaving the underscanned area as a black frame with little or no room for TVs displaying 'shrinkage' [for if they do, my film will appear with a black frame with no 'extra picture' available].

I suppose I can leave a few pixels width or so in the underscanned area just in case, but it seems pointless to 'fill it' if modern TVs seldom need all that area for any [unlikely] image drift.

Grateful for any thoughts/advice.

[jman98]

Many HDTVs still overscan. It varies from model to model. I've heard of a few where it could be turned off, but many do not have this option. For example, my particular Samsung HDTV overscans. There's a patch from Samsung that removes overscanning only on HD sources, but if you apply it it causes the TV to overscan even more on standard definition sources. That makes it a no go for me as I watch a lot of standard def video on my TV and I don't want it to be even worse.

There is still garbage sometimes at the outer boundaries of high def TV signals and if consumers could see it because their TV didn't hide it by overscanning, they would flip out and say that either the TV itself or their TV source was broken. Nobody wants to deal with this, so that's why TVs still overscan. I wish it could be turned off for those of us who do know what we are doing, but that's rarely an option.

So if you use that overscan area, you do run the very real risk that it won't be seen on many modern TVs.

[yukon33]

Thanks for your reply jman.

I have established that most new televisions still overscan, but my real question is to what extent does image drift/picture squash occur as a natural symptom in modern televisions.

If it occurs little or never, I may choose to shrink my project picture by 7% to bring the underscanned area into the overscanned 'TV' area visible in my editor. This will give me more flexibility with re-framing during editing, and applying zoom effects.

I appreciate it is sometimes possible for home viewers to access the underscanned area of a picture - for example with a DVD player and television not both set at the correct aspect ratio. And perhaps some HD TV broadcasts, as you mentioned. Also I imagine having vertical and horizontal hold incorrectly set on the TV may reveal the underscanned area.

However there are no artifacts visible at the edges of my current 16:9 footage - my 'underscanned' area will be a neat black border if I shrink the picture down so the full captured frame fits into overscan mode.

So in the event that a modern television does still image drift a little, this black border perimeter should be all that is capable of 'creeping in' when viewing my DVD.

[jagabo]

I have established that most new televisions still overscan, but my real question is to what extent does image drift/picture squash occur as a natural symptom in modern televisions.

None at all with direct view fixed pixel displays like LCD and plasma.

[yukon33]

Thanks, jagabo.

This essentially leaves only user error capable of exposing the underscanned area on LCD/Plasma TVs [unless underscan is a switchable option].

Concerning modern CRT TVs, I believe image drift is unlikely, and if occuring, minimal. It seems acceptable therefore to leave only a small percentage of video picture in the underscan area just in case.

This from http://www.repairfaq.org/sam/crtfaq.htm:-


"Why do TVs Overscan?

(The following includes material from: Jeroen Stessen (Jeroen.Stessen@philips.com).)
TVs are always set up to generate a picture which is 10-15 percent large than the visible face of the CRT. Why?

In the early days of TV, this was probably done to make the design easier. Component tolerances and power line voltage fluctuations would be masked even if they caused changes in picture size.

There certainly is almost no reason today to have any more than a couple of percent overscan. Most modern TVs have very well regulated power supplies and component values do not really drift much.

Computer monitors, for example, are usually set up for no overscan at all so that the entire image is visible.

We are constantly reminded of that, now that we are building TV's with VGA inputs (PD5029C1 in the USA, US$ 2000). This mixed application has overscan in TV mode and underscan in VGA mode. Geometry adjustment is quite critical if you see border-on-border.

Unfortunately, TV's may be good but VCR's certainly are not. If you have too little overscan and then put the VCR in any feature mode (like picture search) then one (black) picture edge may become visible. Bad form. Viewers do not like this.

While design considerations may have been the original reason for overscan, now it has become accepted as a de facto standard, and broadcasters are counting on the overscan being a certain percentage. One wonders whether it will ever change or whether this really matters.

I suppose when we have true flat panel digitally addressed displays, we might have 0% overscan.

At the Japan Electronics Show all the signs are pointed toward flat panel displays so maybe I will not have to hold your breath for much longer.

Physically, as with an LCD display on a laptop computer, there will be 0% overscan (no need to build the extra pixels) but that doesn't mean that all 480 lines will be visible"

[edDV]

Several issues:

CRT TV sets:

These aren't dead. Many will still buy the $99-150 K-Mart DTV specials that include ATSC tuners and scale to a 640x480i frame buffer, then read out using normal analog RGB CRT scan technology. The CRT TV sets in the stores still show normal overscan.

Here is the problem. As CRT's age,the phosphors lose brightness. To compensate, the user or service technician increases brightness. This stresses the high voltage supply which in turn causes image blooming or picture size instability.


PC/Game port:

Many plasma and LCD-TV sets default to no overscan on the "PC/Game" or VGA port so that computer and console games can be viewed to the picture edge. Some offer overscan off (Samsung calls this "Just Scan") on the HDMI and analog component ports. Tuner or composite/S-Video inputs usually maintain overscan.


Production standards:

Each potential client for your program will have acceptability standards for "safe/action", title/graphics placement and templates to allow 16:9 and/or 4:3 viewing. Programs or ads intended for broadcast will have strict standards that optimize for 16:9, 4:3 or heavily overscanned old CRT TV sets.

Usually a black frame is unwelcome. The magazine publication industry uses the concept or "page bleed" to beyond the page edge because they can't predict the accuracy of the printer's edge trim. TV is similar. They like a frame or background bleed that matches the material in the center.

[edDV]

Graphics for TV news are placed for overscan rules in 16:9 or 4:3 like this.

16:9


4:3


Overscan will take 5-10% off the edges.

[edDV]

This frame shows the graphic safe limits for CNN-HD. This is typical for broadcast.

It works for 16:9 or 4:3. The "CNN HD" overlay extends outside the 4:3 safe area.



This is how it looks on CNN-SD