Why is there an overscan area?

[Michelle Cox]

I read an explanation once on why the overscan area exists, but I can't find it and can't remember it. Someone asked me what the purpose is since you can't see it on a TV and it's just junk anyway. That got me curious to remember why, but my searching isn't turning up anything. Anyone know?

Thanks,

Michelle

[Dr Mordrid]

It's not junk. It's there for a very specific purpose: to have extra coverage for TV's that are out of adjustment, and there are a LOT of those.

Now....if/when we go to fully flat panel digital displays there won't be a need for it. Until then.....

Dr. Mordrid

[Michelle Cox]

I guess I don't understand... If your TV is out of adjustment so that you see the overscan, wouldn't seeing the flickering noise be worse than blackness?

I apologize if this is a silly question. I don't know much about TVs and only learned there was such a thing as overscan when I started capping.

Thanks,

Michelle

[Dr Mordrid]

The full D1 video frame is either 720x480 or 704x480, depending on what standard you're quoting.

The "TITLE SAFE AREA" is what normally shows on a properly adjusted TV set.

The "OVERSCAN AREA" is the extra video beyond the Title Safe Area that provides an extension of the viewable signal for TV's that are out of adjustment.

By that I mean a TV whose picture is horizontally or vertically compressed so that the Overscan Area is drawn into the viewable portion of the screen. This often happens with older sets when their transformers or voltage control circuits deteriorate.

Without the Overscan Area an out of adjustment TV would display black bars instead. This is what would happen if you just captured or cropped to the size of the Title Safe Area.

Here's how it looks if you draw it out;



Dr. Mordrid

[LvR]

Because it is simply not possible to be 100% spot-on with the adjustments on every set leaving the production line, manufacturing processes runs between some minimum and maximum realistic/achievable tolerances.

These figures are chosen so that just about all the sets leaving the production line will be viewable by its owner without noticable deficiencies (that is unless he views his set right next to an optimally adjusted identical set).

Also - there are some very clever people out there in the industry thinking up useful applications for those overscan areas like Teletext etc, and the not so useful and great PITA (If you are running a Matrox card and doing editing) Macrovision.

The idea is to have the "out-of-adjustment" of the production line fall in a section off the viewable screen, and thereby hiding the problem to a large extent

edit - OK I see Doc beat me to it, but I will leave it in anyway.

[wkulecz]

Not to start an arguement, but I believe the overscan area goes to the root of TV design in the pre-semicondutor era -- with tube circuts that could be sold economically it was impossible to make the sweep generators be linear enough to give a good picture over the whole screen so the overscan area represents the central area where linearity of the sweeps was "good enough".

If you still have an old set with "pots" to adjust H&V size and position you can usually make the overscan area visible to see how distorted this area is.

On modern sets no overscan would be needed if designed like your computer monitor is, but its cost and tradition now.

-wally.

[Greebe]

I pick...

D) All the above


...is correct. The overscan area was implemented originally due to linearity problems with tube circiuts of yesteryear, but as an extra, this also helped manufactures out with alignment problems.

[LvR]

Wally

We are stuck with overscan on a PC monitor as well - jack up you monitor brightness and you will find an area just appearing that is wider and higher than your picture content (the raster).

Even on PC monitors where direct drive to the sync and RGB stages is used, manufacturing tolerances (production and time/temp other influences) still dictates that "a little more than what is needed" should be scanned - the owner is given the monitor adjustments to satisfy the optimum setting for the use of the monitor in his particular environment - he can even move the picture content from the display card around within the "raster" area of the monitor and thereby effect a non-symetrical overscan - just as with a normal TV.

On PC monitors, most people I know for some reason lives with a black border around the picture and never bother to stretch the picture to cover the full viewable screen - often loosing a good 20mm or so on the diagonal

[Dr Mordrid]

Like I said; until we're all using solid state digital displays we're stuck with overscan.

Dr. Mordrid

[wkulecz]

Don't confuse overscan, where visible information is cropped off with retrace blanking where no visible information is allowed.

Its the H lines during the V retrace that is used for teltext, closed captioning, etc. Since they are not visible, one can do pretty much anything with these 45 "lines" in NTSC (the missing part of 480 displayed of 525 total).

A properly adjusted monitor (easy on modern monitors) will show all NxM pixels without overscan (cropping). I agree manufacturing tolerences generally mean you have to leave about 1mm of the blanking interval visible in the viewable rectangle.

Anybody have one of those LCD displays with built in tuner & analog A/V inputs? I'm curious if they "overscan" to show what people are used to seeing when a VCR tape or TV broadcast is displayed.

--wally.