Friday 30 August 2013

TV Calibration Challenges

It would have been easier if I had an expensive spectrum analyser or independent TV calibration hardware to make sure the TV is set up correctly in the first instance but oh well...

I have the following sources, all via HDMI directly into the Panasonic P65VT65:

1. SkyHD
2. XBMC on Acer Revo on XBMCBuntu
4. Blu-Ray player (in future this will share the HDMI TV input with the CCTV via a HDMI switcher as the TV has only 3 HDMI inputs)

CCTV, I'm not going to worry about at all.  
Blu-Ray player I'll do when I get it (though it could help me calibrate the TV as it could act as the reference source).

The SkyHD box is a problem as I can't load test images on it.  Sky used to send out a test card (replicating the old BBC test card but updated with Myleene Klass):

They do not send it out any longer it seems.

BBC themselves used to broadcast a specific HD test card:

They too have stopped.  

So the only thing I can do is calibrate Sky by eye, not the most perfect or elegant solution.  I've set this to THX Cinema mode (very happy with this mode in general) and tweaked the contrast and brightness a little.

XBMC on Acer
There are a few things that complicate this.  As I'm running XBMC on a Linux PC (XBMCbuntu) I have to take in to consideration the graphics card (Nvidia) and drivers and software on top i.e. Linux and XBMC.  Each of these could have an effect.  The largest impact by far is by Xorg/X11 (providing the GUI) and the Nvidia setup. 

The default output for X is a monitor.  The values used for RGB is 0-255 (0 = perfect black, 255 - perfect white).  The issue comes when you use a TV as a display with source which is not created with 0-255 in mind...i.e. movies...they use 16-235 (16 = perfect black, 235 = perfect white).  Why don't they use the full range?   The shooting and digitisation process is not perfect so some head room and foot room is given, maybe to accommodate a filter overshoot for example.

So I made sure that my XBMC setup was outputting 16-235.  After a lot of research the solution was a simple addition to my xorg.conf:
Option "ColorRange""Limited"

In fact the XBMC devs have recognised this:
Video calibration...
See #Video calibration...
  Future Gotham feature: Use limited color range (16-235)  
Use limited color range instead of full color range (0-255). Limited range should be used if your display is a regular HDMI TV, while full color range should be used if your display is a PC monitor.

So now that my basic XBMC setup was done I went through the calibration steps from AVSForum guys: As I have the THX blue filter glasses, I was able to get the skin tones done too.

I recognise that this is not all there is to calibration and different sources will still produce different issues but this is enough for me at least.

So now I have a calibrated picture for both Sky HD and my XBMC setup...problem is when I go to other peoples homes, I am instantly distracted by the tendency of their TV's to oversaturate and use too much brightness!

Future Possibilities
I have a Canon 5D Mark III....I'm wondering whether I can use the capabilities of the camera somehow as an accurate spectrum analyser i.e. I take a picture of an RGB 128,128,128 (gray), output it on to the TV, measure with camera again, tweak until the picture is close to RGB 128,128, that with multiple colours and grays etc.

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