How Playback Conducts Video Tests

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

The second CIE chart (Fig. 2) shows what a properly adjusted set’s colorimetry and white point measurements look like. In fact, the result shown in (Fig. 2) is about as good as you’ll get with most sets after a full calibration. What makes this chart noteworthy, however, is that the measurements were taken from an uncalibrated Pioneer Kuro Elite plasma, using the set’s “Pure” picture mode, “Low” color temperature choice, and one of the two available color palette choices. Pioneer designed the Pure mode to mimic the characteristics of a true broadcast/studio HDTV monitor, and the above is about as good a score card as you’ll ever see–with a sumptuously rich, natural color quality that really sets the standard for HDTV flat panel picture quality.

The next chart (Fig.3)shows a typical gray scale linearity result, this time from an LCD flat panel.The result is reasonably flat over most of the range, though it is quite a bit above the dashed line that represents the D65 ideal, and goes off the chart at the darkest grays–very typical of LCD sets. Unlike most other sets, this one didn’t offer a choice of color temperatures however.

Here again is the same chart type, this time from the Pioneer Kuro Elite set (Fig.4). Folks, it doesn’t get any more accurate than that (again, measurements were taken using the set’s “Pure” mode and “Low” color temperature option). This is a spectacular result.

Another critical item we look at is a set’s gamma curve, which is the electro-optical power conversion factor, and is essentially supposed to be the inverse of the studio camera’s optical-electrical power conversion factor. Ideally, it should be a smooth arc, without bumps, which of course, the Pioneer Kuro Elite provides in this example (Fig.5).

Note that the curve sits, to an astonishingly accurate degree, right on top of the dashed target line (veering just the tiniest wee bit near the top right). The measured ratio of 2.15 is very close to the ideal, preserving shadow detail along with clean un-clipped whites.

The next example (Fig.6)  shows a more typical result, one representing a set whose gamma curve probably can’t be fixed even with a proper calibration. The example shows the gamma curve is below where it should be in the blacks and dark grays, and above where it should be in the light grays and whites—a result often referred to as an “S-curve.” This kind of gamma curve produces a punchier-looking picture, but it can often be accompanied by visible picture defects such as crushed blacks and clipped whites. The 1.82 value is also low, which will produce a washed-out look, compared to a set with accurate gamma.

TESTING FRONT PROJECTORS

For a front projector, many of the same measurements (along with numerous others) are entailed, but since the projector maker can’t possibly know which one of the various screen brands and types a buyer might choose, we strongly recommend that you have your setup calibrated by a pro. For the slight additional dollars that you’ll invest, a calibration will ensure that the projector is correctly matched to your screen type and size, and many projectors provide a full suite of calibration controls that will allow the calibrator to dial everything in just about perfectly. The reference screen we use is a Stewart CineCurve, which is a deluxe motorized variable aspect ratio type, with a fixed-frame Stewart GrayHawk RS 16:9 screen on hand as an occasional alternate.