Cheezmo
Senior Member
Registered: Apr 2004
Location: Plano, TX
Posts: 229 
Everyone should be aware of a few things with regard to overscan.
On RPTV's especially, it is typical to have more horizontal overscan than vertical. Almost everyone I calibrate, involves decreasing the width to correct this.
Every TV screen is not exactly 16:9! Measure yours and see.
Now, my overscan was calibrated with an Accupel HDG3000 test pattern generator and an overlay grid. I am convinced that my geometry and linearity are very accurate.
So, what does the HDNet pattern show me?
The circle in the middle is exactly 47cm high and 47cm wide. Good.
Anywhere I measure 3 of the squares on the grid pattern, they are 20cm high (or across). Good.
Overscan is just over 4% on the top, 6.5% on the bottom. OK, slightly offset but my grid may have been slightly offset that way, and vertical positioning often varies a bit for different DVD players/STBs.
Overscan on the left is 10%, overscan on the right is just barely under 10%. That can't be right, can it?
This confuses me. The circles is round. All the squares are square. Why is the geometry off. I measured my screen and it is 16/9.04. That shoudn't make it different by more than 1%.
So, where is the problem? Counting the grid they use, it is 27 squares by 15 squares. Huh? That is 16.2x9. This is getting interesting.
They appear to be marking the "numbers" as 1/8 of a square. Um, but look at the bottom ones? They go 14, 12, 10, then a line, then 8, 6 , 4... Whoops, they skipped one! Sure enough, the last one before the edge is 4, not 2. So, subtract 2 from all your bottom overscan numbers, please. OK, now I'm just over 4% top and 4.5% bottom. That's better.
Well, we normally consider overscan in terms of percentage of 100% of the picture. 1% cuts off 1%. This pattern does not represent that!
Horizontally, there are 27 squares. So their numbers actually represent 1/216th's not percentages. So, multiply the left and right numbers by 100/216 to get an actuall percentage. My 10% is now 4.6%.
Vertically there are 15 squares. So their numbers actually represent 1/120ths, not percentages. My 4% and 4.5% are now 3.3 and 3.75%.
OK folks, here is the way to get actuall overscan numbers from the numbers on the HDNet grid (probably worth a seperate post!).
Top: HDNet# * .83
Bottom: (HDNet#  2) * .83
Left and Right: HDNet# * .46
Wow, I really never would have thought that they would have botched that pattern so badly. How many people are using it to make the top/bottom and left/right numbers the same. Doing so is a bad thing!
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Steve Martin
ISF Calibrator
Smart Calibration, LLC
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