: THX Mode dealt with overscanning on my HTPC


hugh
2011-04-01, 08:46 AM
Okay so I hooked up my new HTPC to my Panasonic Plasma.

The result was overscan. Now I know you can make adjustments with overscan in the video settings to correct the output from the computer but I wondered if I could make a simple adjustment on my TV.

So I looked at Picture modes on Panasonic and first thing I did was switch it to THX mode and voila everything was perfect.

Can anyone explain to me what just happened and why its all perfect now?

Would there be any advantage to playing withe video card setting vs. just setting the TV to THX mode.

Signed,
Curious in Toronto

jvincent
2011-04-01, 08:58 AM
I assume you are using an HDMI input and the HTPC resolution is set to 1920x1080.

If so, I'm surprised that changing the mode on the TV changed the scaling. Generally a digital input will be 1:1 pixel mapped if you have it set to the display's native resolution.

I have seen funny things where the HTPC is sending YCbCr vs RGB and that changes how the display behaves.

hugh
2011-04-01, 09:00 AM
I assume you are using an HDMI input and the HTPC resolution is set to 1920x1080.

That is correct.

57
2011-04-01, 11:12 AM
I'm assuming this is on your Panasonic. In the Panasonics, the overscan option is usually remembered separately for each input. It's not usually related to the picture mode although perhaps when you switch to THX the HD size defaults to Size 2 instead of Size 1. On any mode that has "Advanced Picture Settings", you need to go into the Advanced Picture settings and choose Size 2 under "HD Size" for no overscan. I recommend people use Size 1 with about 2-3% overscan for TV viewing - here's the FAQ on the topic:

http://www.digitalhome.ca/forum/showthread.php?p=793236#post793236

If you download the operating manual and search for "size" it should come up as one of the results. It will say that Size 1 is 5% overscan, however, that represents 2-3% top/bottom and 2-3% on the sides. If you put up an overscan test pattern it will usually indicate about 2-3% all around.

As indicated in the FAQ, different TV makes/models will have this option stored in different locations in the user menu- sometimes with more than two size options, but usually only with HDMI inputs. Sometimes it's simply under the various stretch modes . On some TVs the setting is universal, while others it's saved by input. Of course if you're going through an AVR using only 1 TV input, you'll need to change the size if you want different sizes for different devices.

PPL4GOLF
2011-04-04, 01:21 PM
As Vincent said, as long as the HTPC is putting out 1920x1080, you're good.

Video cards have overscan compensating resolutions (like 1176x688, etc) and those are last resort if there is no 1:1 mode. Those resolution should not be used otherwise. I have no problem using overscan/underscan scaling compensation as there is no degradation in quality.

It sounded like the THX mode forces 1:1 which should be the default mode anyways.

I view exclusively OTA and always use ScreenFit (1:1 mode) on my Samsungs. The channels or programs that have noisy borders and not common. For the 1080i channels, the picture is noticeably softer when it is on the overscanning 16:9 mode. Both Samsungs of mine overscan 720p signals. I wish there is a ScreenFit mode for 720p channels like ABC and Fox.

ScaryBob
2011-04-04, 02:45 PM
Would there be any advantage to playing withe video card setting vs. just setting the TV to THX mode.
I would say no. Scaling always has the potential to create artifacts. The ideal solution is a 1:1 mapping end to end. That is, no scaling on the HTPC and no overscan on the TV.

audacity
2011-04-04, 02:54 PM
Scaling always has the potential to create artifacts.

... which is especially visible when viewing text on a HDTV.

Ultimately, you want your HDTV to behave as much like a computer monitor as possible.