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Discussion Starter · #1 ·
What exactly is overscan and why is it used? What is the PJ doing when you increase the overscan percentage?


I have a HT1000 and the default seems to be 5% overscan. Why?


I see what changing the overscan does to the picture - it zooms in on the center of the frame but you loose edge info, i.e. part of the image is cropped. Do you loose resolution by increasing the overscan rate?
 

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Quite a lot of DVDs don't fill the width of the image, and leave unsightly black sidebars. 5% overscan will usualy hide this, and is probably less then the overscan built into your tv.


Gary.
 

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Discussion Starter · #3 ·
Couldn't you correct this problem just by zooming out a little, i.e. what if you have a 16:9 screen and set the zoom so that a widescreen DVD just fits the width of the screen - conversely, why couldn't you set the zoom to fit a 4:3 movies into the height of your screen, wouldn' t the width then be set correctly?
 

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Paul,


Most displays are direct view or rear projection. These display device rarely have perfect geometry. But having a little bit of overscan hides the geometry flaws (pin-cushion, barrell distortion, etc.)


With that in mind, a lot of source material created with that in mind.


The most obvious problem comes from 720p. 720p (like 1080i) isn't guaranteed to have 1280 X 720 pixels of actual program material. This is dictated by the ATSC. Quite often there is junk around the top and sides when 720p is shown pixel-for-pixel on a 1280 X 720 display. Most of the before mentioned displays have 5% or so overscan so the junk is hidden but digital display often show the hole picture with 0% overscan.


The HT1000 lets you select how much overscan you have. Unless you are doing computer desktops, I would recommend the 5% setting if you start to see junk around the borders.


Good luck,


-Mr. Wigggles
 

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Discussion Starter · #5 ·
Thanks Mr. Wiggles. I don't see "junk" around the edges by occationally I see a thin bright line along the top and bottom edge of the image. The line seems to be independent of overscan percentage. It looks like the image edge "wraps" on itself. Any idea what this might be from?
 

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The bright line that most people see on the top of the image is time code data from some broadcasts. 5% overscan should remove it.


-Mr. Wigggles
 

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Paul,


Using 5% overscan has the same effect as using a bit of digital zoom and it does make the picture softer. I found the best setting on my HT1000 with PSO lens is 0% overscan along with a variable screen masking system to crop off any unwanted row(s) of pixels. This is probably the best method of getting 100% of the image to the screen outside using a HTPC for sizing/cropping.


Thumper
 

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Overscan is a relic from the tube TV days - circa 1960. Old TV's had all tube circuitry. Tubes age and drift so the deflection systems would change in size daily- in fact hourly. Until about 1975 even solid state TV's did not have regulated power supplies except for the high voltage supply which was always mandatory in color TVs. These factors all contributed to wandering H&V size. So overscanning hides these errors.


SMPTE even adopted a SAFE TITLE and SAFE ACTION specification so titles and important action could be insured to be seen even on highly overscanned TVs. Even today broadcast television and most home video transfers adhere to that specification.


With today's stable and high tolerance analog electronics, overscan could be dispensed with. And for digital displays, it simply is not needed.


But old standards die very slowly. Expect SAFE TITLE and ACTION limiting to be with us at least until NTSC dies.
 
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