Author Topic: HDTV Calibration Explained  (Read 16652 times)

Offline KenMasters

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HDTV Calibration Explained
« on: May 17, 2012, 08:56:06 AM »
Here's a quick rundown on TV calibration and why it's needed, without a whole bunch of technical jargon:

What is calibration all about?

A comment I often hear from calibration nay sayers is: I don't want my TV to look like real life, I set it to look like I like it. This is a notion based on a simple misconception. Calibration is not about making images look true to life, they are about making images look true to what the person who created them wanted you to see.

Film, TV or video games can look any way their creators want them to look. They can be bright and colourful, true to life, dark and moody, or entirely composed of pink and blue! The point of calibration is to remove your TV's influence from the equation, so that it adds or subtracts nothing from the original image.

Now how does that work you might wonder? How does the calibrator know how the TV should be set to view the content correctly? How can he be sure that his settings are good for all forms of content? Well that's easy, all content created for HDTVs should adhere to a set of parameters defined by a standard called Rec. 709.

Rec. 709 defines exactly how every aspect of an image should be displayed on screen. When professionals create the content we view on our TVs, they film their material and work on monitors that adhere to Rec. 709, judging their visuals accordingly. So basically, by calibrating, we can see our movies, TV series and games as they were intended to be seen.

How does Rec. 709 define HDTV images?

In this section I will attempt to explain exactly what a calibrator attempts to do in order to bring an HDTV in line with the HDTV spec in easily understood terms. Rather than being technically correct, my explanations are meant first and foremost to be simple to comprehend.

Greyscale / Colour Temp:

Think of all images on your TV as being black and white, with colour only being overlayed afterwards. This is the greyscale and it is the canvas of your TV. The greyscale should consist of red, green and blue mixed together in equal parts to create a neutral grey (neutral being defined as D65 on the CIE diagram [colour temp 6500K]) from the darkest grey (black) through to the brightest grey (white).

When this balance is uneven, it affects all colours on the screen once they're overlayed. Too much blue and everything gets a blue tint, too much red and everything gets a red tint. By calibrating we seek to even the balance to ensure a neutral greyscale throughout.

Black Level:

There are 256 levels of red, green and blue in our video system. Black is defined as being at level 16, all levels below that should not be visible. We therefore need to adjust our sets so that everything from sixteen and below is solid black in order to maximise our TV's potential. Set the level below sixteen and your blacks will become washed out and grey, set it higher and detail in the darker areas of the image are lost.

White Level:

Level 235 of the RGB scale should come across as white on a TV, but not a bright white, it should be at a comfortable brightness level for your viewing environment. It should be like the white of a white T shirt or table cloth when viewed under normal lighting conditions.

Unlike black however, the levels above 235 should remain visible. These are our highlights, those really bright areas created by something like the sparkling highlights off of a Harely's chrome or the sun glistening on the waters of a lake.

Gamma:

Gamma is related to the greyscale in that it changes the slope of greyscale. It alters how quickly white rises out of black. If your gamma is too low, then the 50% grey point is moved down the scale and your blacks end up being too closely packed together, crushing black detail. Move it too far up, while darker shades become more distinct, you lose punch and detail at the higher levels.

It's worth noting that there is no one correct gamma setting, it depends on your viewing environment as it is easier to perceive close shades of black in a darkened room than it is in a brighter room. So there's a trade off. In a darker room you could set your gamma to 2.4, which would bring blacks closer together adding more depth to your image and richer colour (as the colour is now more saturated). Or in a bright room you could set it to 2.0, in which case shades of black would be further apart and easier to distinguish, but the overall image would lack punch. 2.2 is considered a good figure for the average lighting conditions in a home.



Colour:

HDTV colour is plotted on the CIE chart (Commission Internationale de l'Eclairage), a chart that covers the visual spectrum. R, G & B as meant to be seen on an HDTV are plotted on this chart by way of a triangle, the points of the triangle representing each colour at 100%:



Calibration seeks to bring your TV's colour in line with these coordinates. Notice the D65 in the centre of the triangle? That is where all three colours meet in equal amounts, that is the colour coordinates of our greyscale. So you see, if our greyscale isn't set correctly, on a correctly functioning set we would not be able to balance our colours correctly either.

What about everything else?

Okay, now what about all those other settings you find on your TV? All those enhancement options? Basically, they are only there for one of three reasons.

One is that they were designed to compensate for problems inherent in the TV technology itself. To judge the value of an enhancement, you need to consider if it's helping to provide as accurate a rendering of the content you're feeding your set as possible. If the enhancement is obstructing this, it needs to go, if it is working towards this, then it can stay. Generally speaking, enhancements that genuinely benefit the image are few and far between, which leads us to the second type.

The second kind are enhancements manufacturers make up for more devious reasons. Their sole purpose is to differentiate their product from the products of others, to make them stand out on the showroom floor. Many of these settings only exist to compensate for the errors caused by the boosted picture settings and far too cool colour temp of the showroom picture preset.

The last sort of settings are legacy settings and are totally pointless in modern day systems. Sharpness comes first and foremost to mind. Sharpness is useful in compensating for low quality video sources such as VCRs and poor analogue reception. It works by creating white oulines around contrasting edges and contrary to it's name, actually obscures fine detail.
« Last Edit: May 17, 2012, 12:27:49 PM by audiomuze »

Offline KenMasters

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Re: HDTV Calibration Explained
« Reply #1 on: May 17, 2012, 09:28:28 AM »
Basic steps for setting up a TV correctly:

Make sure your TV is set to a more accurate mode, like Movie/Cinema.

Then go through your TV's menu and switch off any enhancement type options (dynamic contrast/black, edge enhancement, noise reduction etc. [should find that these are switched off in Movie/Cinema mode by default])

Use this pattern to see which screen ratio option displays the full pattern on screen: http://i174.photobucket.com/albums/w116/KenMasters_photo/Overscan-2.jpg. This is the ratio you would normally want to use when playing games and watching DVDs and BDs (1:1 pixel matching), with TV you should use a bit of overscan, see which option crops the pattern closest to the 2.5% mark.

Okay, now with 1.1 pixel matching selected we can set the basic settings. Using this pattern, turn your TV's Sharpness setting up until you can easily see the white outlines developing around the black lines. Then turn it down until they disappear (although not always the case, don't be surprised if the correct setting is 0).

Now move on to adjusting Brightness, for that use this pattern: http://i174.photobucket.com/albums/w116/KenMasters_photo/BlackLevelNoise.jpg You must adjust Brightness to the point where all the dark strips from 16 and below disappear, but all the strips above that are visible.

Then for Contrast, use this pattern: http://i174.photobucket.com/albums/w116/KenMasters_photo/Contrast.jpg and turn Contrast up until the white strips towards the right of the scale begin to blend, then turn it back down until you can differentiate each shade of white. If you find any colour tint creeping into the white at this level, turn it further down until it disappears. At this point you might also want to adjust your Backlight Brightness if you find it too bright for comfortable viewing, or not bright enough for your room.

Give the Brightness pattern another look and make sure the Brightness adjustment is still valid, give it a quick tweak if not.

If your TV signal comes through an RF or composite cable, you might want to switch the Noise Reduction feature back on for TV.
« Last Edit: May 17, 2012, 12:28:26 PM by audiomuze »