Professional TV & Projector Calibration: A Walkthrough
Part Two of our picture calibration series, by calibration expert Tony O'Brien. A walkthrough of the actual calibration process itself.
What’s actually involved in having your display (TV/Projector) professionally calibrated is more detailed and involved than most people expect. Those with some understanding of display calibration, may just think it’s using a test disc to adjust the basic picture controls, such as brightness, contrast and colour etc. While properly setting these controls is essential in achieving a great looking picture, bringing out all of that detail and colour accuracy from a bluray disc means we need to go much further.
If you have ever looked at the advanced picture controls on your display, you may have seen controls for white balance, gamma and the Colour Management System. These controls require specialised measurement equipment and software to calibrate.
Your calibrator will usually start by calibrating the display itself (other equipment, such as bluray players, will be looked at later in the process), using test patterns from a signal generator (connected to a HDMI input) and blurays/ DVD’s. I usually start by setting the aspect ratio, sharpness and motion controls. This ensures that none of the picture is being cropped, no artefacts are being introduced into the image (due to incorrect sharpness settings) and movement doesn’t look too jerky, overly smooth or unnatural.
With this completed, I maximise the dynamic range of the display, by setting the brightness control, (sets the level at which your display produces black) and the contrast control, (sets the level for peak white). This allows your display to produce the darkest blacks and brightest whites, without clipping picture information and will provide more pop to the calibrated picture.
Setting the picture controls up until now has only required the right test patterns, but calibrating the advanced controls requires some specialised measurement equipment and software. I use both a spectroradiometer and colorimeter (referred to as a meters) for this purpose.
The meter is connected to a laptop and in the case of a plasma or LED/LCD TV rests flat against the screen. If your display is a projector the meter is tripod mounted and aimed at the projector screen. Amongst other things, the meter will read colour accuracy and light output, and the results are displayed on a laptop.
1 pro 2 spectroradiometer calibrating plasma TV
d3 colorimeter calibrating projector
Moving onto the advanced controls, it’s best to start by calibrating the greyscale (also referred to as white balance). Simply put, greyscale refers to the black and white picture information, which is reproduced using a mixture of red, green and blue (RGB). To accurately reproduce colour, the display needs to have neutral greyscale tracking (most have a bias towards blue).
In the absence of being able to show you the impact of correct / incorrect greyscale tracking on an image, I will talk you through the measurements as shown on the Pre-Calibration Greyscale diagram. The right hand axis shows the RGB balance of the greyscale and the bottom axis shows the steps ranging from 0% black to 100% white. You will notice that while green is close to 100% (our target), red is too low and blue is off the chart. While the software shows a 0% reading (black), please ignore it, as meters simply can’t measure this low accurately and it’s presented for completeness.
To achieve a neutral greyscale, the calibrator will be taking readings from your display and using it’s white balance/greyscale controls to balance red, green and blue (RGB) from 10% to 100%. This is a reiterative process, calibrating one step can put another step out of balance.
The calibrator will use a specific set of coordinates to measure each greyscale step between 10-100% but for the purpose of this illustration, it will suffice to say that red, green and blue should sit as close as possible to 100%. You will notice the Post-Calibration Greyscale diagram shows the greyscale is tracking much closer to our red, green and blue target point of 100% across all steps.
Closely related to greyscale, is gamma, which controls how quickly your display comes out of black. If the display comes out of black to slowly, the picture will look too dark. Conversely, if the display comes out black too fast the picture will lose much of its depth and look washed out, or flat.
The Pre-Calibration Gamma diagram illustrates both the target gamma (represented by the curved yellow line) and the actual display gamma (represented by the curved blue line). You will notice that at 0-30% the gamma is close to our target gamma curve, but then rapidly rises from 30-90%. With actual viewing material, the picture lacked a sense of punch.
Correctly setting display gamma requires a meter and software. This is also a reiterative process, properly calibrating any one step between 10%-100% will most likely affect another step. Furthermore, hitting our target gamma curve is going to affect the greyscale balance, so the calibrator will be going back and forth between the greyscale and gamma controls, until everything is just right.
With the basic picture controls set and the greyscale and gamma calibrated, the picture will have a nice sense of depth, and detail and colours will be more realistic. If your display also has a colour management system (CMS), your calibrator will be able to make colour more accurate, by calibrating the primary and secondary colours to the Rec. 709 colour standard (with the aid of a meter and calibration software).
The Colour Gamut Before / After diagrams shows how colour was tracking on this display both pre and post calibration. Most of the colour error was reduced with correct greyscale tracking, but using the CMS increased accuracy and all colours were reduced below the threshold of visible error.
Now that the display itself is calibrated, it’s time to make sure that all of the equipment within the video chain (bluray player, receiver etc.) is also calibrated. The calibrator will use a bluray disc with test patterns to make sure that everything is working how it should be. During this stage, you can expect the calibrator to revisit all of the previous steps and make further adjustments to both the display and any other equipment within the video chain.
Up until now, there has been a lot of measurements and adjustments made to your display and video equipment and technically, it should be looking fantastic. But… it’s important to make sure your display is looking fantastic with actual viewing material. This involves putting your display through its paces with a variety of different scenes to make sure everything’s looking fantastic. It’s also not uncommon for some fine-tuning of the picture settings to take place, to get that last little bit of performance from your display. If you haven’t already, now is an appropriate time to say ooh and aah! ;-)
With the requisite praise for your calibrator completed, it’s time to kick them out of your house (we generally prefer to be paid first ;)) and settle in for some serious movie watching!
Calibrated Image, taken from my mobile phone, the actual TV picture looked much better!
Test pattern images and the picture from the Calibrated Image were taken from the Spears & Munsil High Definition Benchmark 2.0. Calibration figures are from Chroma Pure Professional Calibration Software.
Read on for Part Three: Frequently Asked Questions
About the Author
Tony O'Brien is the owner of Adelaide based 'Clarity Audio & Video Calibration', providing calibration services to home theatre owners and video production companies for film and TV. He is an ISF Certified calibrator.
For more information visit http://www.claritycalibration.com.au
As the owner of Adelaide based 'Clarity Audio & Video Calibration', Tony is a certified ISF Calibrator. Tony is an accomplished Audio-Visual reviewer specialising in theatre and visual products.
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