How Playback Conducts Video Tests

Posted by: David Birch-Jones at 3:03 pm, April 27th, 2009

(This blog is based on an article that is currently running in Playback issue 19; you can access and read the issue online from this site, at no charge).

OVERVIEW

Almost without exception, the typical TV’s factory default picture settings aren’t designed to produce the most accurate picture quality but rather to impress shoppers in a brightly lit store environment. Just about all of the settings are amped up to the max, which in a residential environment invariably produces a harsh, overly bright and garishly over-colorful image. A recent trend among some manufacturers is to provide a choice of “Retail” or “Demo” mode along with a “Home” mode–which, if the “Home” mode is chosen, provides picture settings that are dialed down somewhat, closer to where they should be.

To thoroughly optimize a set’s picture characteristics, a professional calibration is always recommended (and with a front projection setup, it’s all but mandatory); however, we recognize that very few TV buyers are ever offered that opportunity. Our goal here at Playback is to find the optimum combination of user-adjustable picture settings that bring a set as close as possible to that of a calibrated studio-quality HD monitor, and only then do we judge its picture quality.

Testing begins with a thorough analysis of a display’s basic picture quality parameters, including colorimetry (accuracy of primary and secondary color points), color temperature (or color of white), gray scale linearity (the ability, or not, of a display to hold color temperature consistent at all luminance levels) and gamma (electro-optical power conversion factor). Most sets provide a number of preset picture modes, and more often than not, one of them will prove to be closer than the others to the technical ideal.

TEST EQUIPMENT

To measure these parameters, we use a pro-grade color analyzer (Konica-Minolta’s CS-200, a NIST-traceable calibrated instrument—see illustration photo), along with an SD/HD test pattern generator (Sencore’s VP403C) and color analysis software (ColorFacts Pro and Cal-Man). On hand are a variety of test DVDs and Blu-ray discs, including Joe Kane’s Digital Video Essentials series—a must-have for any HDTV buyer, since they have the test patterns you’ll need to tweak your set for maximum performance, and reference-grade demo clips for further confirmation of proper adjustment.

A Tale Of Two Sets

The factory default out-of-the-box picture mode for most HDTVs is often jokingly referred to as “Torch” mode–again, to pump out the brightest, most vivid image to compete against harshly bright store lighting. Examples are so-called “Vivid” or “Dynamic” modes, where everything’s cranked up to “11.” Within these modes, the white balance (color of white) is often decidedly bluish (which gives the impression of extra brightness), the contrast settings are so far over-the-top that they can often cause eye strain when viewed in moderately-lit home environments, and various so-called “picture enhancements” are activated—despite the fact they usually degrade overall picture quality noticeably with clean HD source material. 

Often, a set is equipped with not one but two colorimetry choices–one of which provides color palette characteristics that greatly exceed the boundaries specified by the HDTV standard, which usually results in excessive color emphasis that often can’t be tamed. We look for the setting that provides the closest match to the standard, as that is always the better choice to ensure rich but not overly garish color intensity (especially with respect to skin tones).

The first CIE color chart below (Fig. 1)presents a typical scenario, with the triangle representing the boundaries of the HDTV standard, and the six + points representing the target points of the three primary (red, green and blue) and three secondary (cyan, magenta and yellow) colors. This LCD flat panel HDTV’s colorimetry shows that only the red, yellow and blue are close to where they should be (the white dots representing the actual measured values). Note too the white dot along the curved dark line in the center of the triangle–that’s the black body curve—and the white dot shows that this TV’s white characteristics will have a decidedly blue tinge (the D65 spot at the center of the dashed crosshairs represents the technical ideal).