What Adjustments Should Be Included in Video Calibration?

Written by the ISF Forum Library Community
Thursday, 17 May 2007

[ Q ] What are the adjustments that I should expect to be performed by my ISF calibrator?

[ A ] This will vary, depending on the type of display and on the specific services offered by your calibrator. However, the following six key objectives should be addressed as part of any professional video calibration:

1. Evaluate the Viewing Environment -- Anything short of a completely darkened room will present some degree of a challenge for accurate imaging, and it's important for the calibrator to recognize those issues and minimize them as much as possible via the calibration process.

For example, uncontrolled ambient lighting in the viewing environment or highly reflective walls, ceiling and floors will have a direct impact on how a viewer will perceive black level. In environments where "critical viewing" is done during both the daytime and in the evening, it may be necessary to calibrate your HDTV separately for these different viewing conditions.

2. Ensure Correct Configuration of the Display and All Video Sources -- Before making any adjustments to your TV, your calibrator should check the configuration of the display and the various video sources it is receiving. The calibrator should ensure that high-bandwidth interconnects (component video, RGB, DVI, or HDMI) are being used for high-bandwidth sources, and that disc players and cable/satellite/teleco receivers are set for the correct screen shape and optimal image resolution.

Between untrained cable and satellite installers and consumers who are overwhelmed by system set-up complexity, it's not uncommon for a calibrator to find multiple system configuration issues during this initial inspection phase. These configuration errors can impose a serious penalty on the viewing experience, if left unaddressed.

3. Optimize Contrast Ratio -- It's important to understand the difference between "optimized" and "maximized." Published contrast ratio data for many displays are actually based on extreme contrast and brightness settings that would not be tolerable for any period of sustained viewing.

The calibrator's goal is to discover the optimal combination for black level and peak light output. This means achieving the "blackest" possible black level, without going so low that near-black image areas (shadow detail) is obscured, and then setting peak light output as high as possible, without risking the loss of fine picture detail that can occur if light output is overdone. Excessive light levels can also make it impossible to achieve accurate grayscale tracking, so this is another factor that will determine optimal contrast ratio.

4. Calibrate Grayscale to the D6500 Standard -- Grayscale may seem irrelevant when you think about a color image, but its importance can be illustrated by lowering your TV's color control to the minimum level. What remains when the color signal is muted is grayscale, an image that contains virtually all of the picture detail of the original image but with no apparent color.

From this exercise, you can see for yourself how grayscale forms the foundation upon which a full-color image is created. If your HDTV's grayscale foundation is skewed toward blue, or red or green (or some combination of those colors), then the color error in the grayscale image will carry through when the color signal is overlaid onto that grayscale foundation.

The standard for the color of white has been determined by the Society of Motion Picture and Television Engineers (SMPTE) and that standard is referred to as D6500. All films and television programming produced around the world adhere to the D6500 standard throughout the production process, and your display needs to be calibrated to same standard in order to produce accurate color images.

5. Correct Color Balance & Color Decoding -- Once grayscale has been calibrated, the color control should be set for the proper level of saturation of the red, green and blue primaries, with the tint control used to fine-tune the cyan, magenta and yellow secondary colors.

If your HDTV's color decoder is aligned correctly, then the user-style color and tint controls should be sufficient to produce accurate color balance. However, many HDTVs have "tweaked" color decoders that favor product differentiation over accuracy. In these situations, accessible color decoder controls allow the calibrator to adjust each of the primary and secondary colors independently, and this is the best approach to ensuring correct color reproduction.

6. Reduce Sharpness & Edge Enhancement -- Sharpness, edge enhancement and detail enhancement are "features" that manipulate areas of an image where dark-to-bright transitions exist. Highlighting or "edging" is added to these transitions in order to make the overall picture appear "sharper."

And, in fact, these electronic additives may be effective in creating the perception of a sharper picture, but this is accomplished by adding "noise." This noise doesn't exist in the original signal, and its presence actually ends up obscuring picture detail that should be viewable. The net result of excessive sharpness or edge enhancement is an actual sacrifice in picture resolution.

Your calibrator should minimize, if not eliminate, all extraneous enhancements of this type, particularly in calibrating the display for high-resolution sources that do not normally require sharpening.

Other adjustments that are sometimes included, or provided optionally: maximizing picture viewing area through reduction of overscan; ensuring 1:1 pixel mapping for sources that match the native resolution of the display; eliminating timing errors between the color and black and white components of the signal; adjusting focus; reigning in electronic or optical solutions that dynamically adjust black level or peak light output; and cleaning of lenses or other optics, where present.

Also, for CRT-based rear-projection or front-projection installations, calibration will typically include: adjustment of picture geometry; and, convergence of the independent red, green and blue images.

Contributions to this Calibration FAQ answer were provided by the following ISF Forum calibrators:
Clearly Resolved, Perfect Home Theater

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