Display Calibration & Film-like Video Quality
Dear Gary: I’m a fan of Widescreen Review and I thought I’d turn to you for a neophyte technical question that I can’t seem to get answered by Sony’s “Technical Support” people. I’d like to get my TV (Sony XBR450) to display as accurate an image as possible according to NTSC standards, without resorting to an expensive ISF calibration. There is a control for color temperature with three settings: “Cool” (whites with a bluish tint), “Neutral” (whites with a neutral tint), and “Warm” (whites with a reddish tint). Intuitively, I would think that “Neutral” would be the “correct” setting, but the manual says that “Warm” is the NTSC standard. Is this correct? Should whites have a reddish tint? It seems that whites won’t look white (at least to what I understand white to be or am used to). I’ve tried it and black and white movies have a sepia look to them. Is this correct? I know 6500 Kelvin is the correct temperature, and I’m trying to decide which of these settings would be closest to correct. Do you have any advice? Plus, I purchased the AVIA Guide To Home Theater DVD for its test patterns to help set up my TV correctly. It has test patterns for Contrast (white level), Brightness (black level), Sharpness, Color (color saturation), and Tint or Hue. Setting Brightness, Color, and Tint is relatively straightforward using the patterns, but I found setting Contrast and Sharpness to be difficult to set and highly subjective. For example, AVIA shows what level not to go above for Contrast—the point of blooming. Yet it doesn’t really show where to set Contrast at below this point. The sharpness problem is similar. I’m always reading about DVDs with great video and its “film-like quality,” which I think means a soft picture. Elsewhere I’m reading about pictures praised for their sharp images. Assuming that TVs offer similar ranges for Contrast and Sharpness (and this assumption may very well be wrong), could you tell me roughly at what point these two settings should be set? Half-way point? A third? My controls range from 1 to 64.
Edward Liu
Video Technical Editor Greg Rogers Comments:
These are good questions and I would like to answer them in some detail since they should be of interest to other readers. Color Temperature It is likely that the Warm color temperature is closest to 6500K (degrees Kelvin), which is the correlated color temperature of D65, the standard reference white for NTSC. This is consistent with the instructions in your manual and my experience with Sony XBR TVs. The Warm, Neutral, and Cool color temperatures are usually several thousand degrees Kelvin apart, so the Warm temperature will appear relatively more reddish than the other settings. Adjusting color temperature to achieve accurate black and white and color pictures is more complicated than it may seem. The correct reference white color must be maintained over the entire brightness range between black and peak-white. This is referred to as gray scale tracking. Most manufacturers don’t individually calibrate the gray scale tracking of each TV they produce, so the color temperature may vary significantly over the gray scale range. Our TV reviews show a plot of color temperature versus brightness for products as we received them from the manufacturer and another plot after our calibration. You can get a pretty good idea of gray scale tracking for your TV by looking at the horizontal and vertical gray scale linearity patterns on AVIA. These patterns have gray horizontal or vertical stripes with brightness levels in 10 IRE steps. The gray stripes should all appear to have the same hue. If some stripes appear a bit more reddish, bluish, or greenish than others, it indicates that the gray scale tracking is incorrect, which affects color accuracy and the hue of black and white pictures. Color temperature is actually inadequate to accurately specify the standard reference white. For more information about standard reference white and correlated color temperatures versus CIE x,y coordinates, refer to my reply on this subject in last issue’s letters column. If you want the best color accuracy your TV can produce, then my advice is to obtain a gray scale tracking and color decoder calibration from a professional technician that uses an electronic color analyzer for both adjustments. Contrast Control Adjusting the Contrast control is somewhat subjective, but you should avoid a setting that creates spot blooming, which could accelerate CRT aging or even damage a CRT. In general, as Contrast is increased, resolution decreases, and other effects may also degrade picture quality before blooming becomes apparent. Each TV has a different Contrast control range, so there’s no one numerical setting that would be correct. But you can use test patterns to select a Contrast setting that produces a pleasingly bright picture in your normal viewing conditions without unacceptable picture degradation. As the Contrast control is increased, the electron beam current must increase to produce additional light output when the electrons strike the CRT’s phosphor screen. A higher beam current means there are more electrons in the beam to repel each other, which spreads the beam apart, increasing the spot size. The larger spot size obscures fine image details, which reduces picture resolution. If the Contrast control is increased too far, the spot size may suddenly become so large that it is said to “bloom,” and the edges of bright objects will appear to expand outward. Use a gray scale step pattern with vertically stacked rectangles of increasing brightness to look for spot blooming. The top rectangle will appear to expand if the contrast is set high enough to create blooming. The Contrast control should always be set below the point of any visible blooming to protect the CRT from possible damage or premature aging. Another effect that can occur in CRT-based displays is geometric distortion if the high-voltage supply dips when the beam current is increased for high brightness parts of the picture. A drop in high voltage expands the picture until the power supply recovers to its original voltage. Use a needle pulse pattern to check for geometric distortion from high-voltage power supply limitations. This is a straight vertical line that is white against a black background in the upper half of the picture, and black against a white background in the bottom of the picture. When the background abruptly changes to high brightness at the middle of the screen, a sudden increase in beam current is required. This may momentarily pull down the high-voltage power supply, causing the lower half of the picture to expand, which makes the needle bend outward. As the high-voltage power supply recovers, it re-establishes its original voltage and the needle will return to its original vertical path farther down the screen. The amount of needle bend, and the length of the bend, is an indicator of the power supply's reserve and recovery time. AVIA provides a calibration pattern that combines two needle pulses (one at each side of the screen) with a five-step gray scale brightness pattern. The Contrast control should be set low enough that the needle pulse shows minimal bending, and always below the level where visible spot blooming may occur. In many consumer TVs, the Contrast control may have to be set much lower than expected to avoid these problems. Picture resolution will be dramatically reduced if spot blooming is noticeable on the rectangular brightness patterns. But the maximum display resolution gradually degrades as the Contrast is increased before the point of blooming is reached. You can make a final adjustment of the Contrast setting to trade off picture brightness versus resolution, by observing the horizontal and vertical resolution wedges on the AVIA resolution patterns. Fixed-pixel front and rear projectors (LCD, DLP™, D-ILA®) don’t have to worry about spot blooming or geometric distortion from high-voltage power supply limitations, but they must avoid another problem called white-level clipping. If the Contrast control is set too high, a fixed-pixel projector will achieve its maximum brightness at a level below the standard peak-white signal level. Any brightness details above that level will be clipped at the maximum brightness and lost. AVIA includes test patterns with moving stripes that are just below the 100-IRE peak-white level of the background. If those stripes disappear into the background, the Contrast control is set too high. Sharpness Control When Sharpness controls are used to “increase” sharpness, there are usually adverse consequences. When set too high (anywhere above minimum on some controls), outlining artifacts appear adjacent to lines or edges of objects. Outlining usually appears as white ghost lines that are particularly visible against gray backgrounds. Increasing Sharpness settings can also increase the visibility of noise and MPEG artifacts in the picture. So the usual and safest recommendation is to simply set the Sharpness control where it will be “off” or effectively out of the signal path. Some controls are “off,” or out of the signal path, when adjusted to their minimum settings. These are controls that only “increase” sharpness. Others are able to “increase” or “decrease” sharpness and usually have a center position where they are effectively out of the signal path. But there are many types of analog and digital Sharpness controls, and many types of consumer displays. Some displays actually produce a more uniform frequency response with a slight boost of the Sharpness control. Some video processors also include sophisticated detail enhancement circuits that can produce a significant improvement in sharpness without adding noticeable edge outlining. I stress that these circuits must be used very judiciously to avoid artifacts, but they can be especially effective with standard-definition video sources on large screen displays. So I prefer to set Sharpness or Detail enhancement controls with test patterns while evaluating these effects. Most Sharpness controls only affect horizontal video bandwidth, which may create outlining on the sides of vertical and diagonal edges. Some displays, DVD players, and video processors also include Vertical Sharpness (or Vertical Detail) controls. Those circuits can produce outlining artifacts above and below horizontal lines and edges. Sharpness and Detail controls may also produce frequency response aberrations that can selectively over-emphasize and brighten picture details within a particular size range. This can make a picture look overly harsh. Frequency response aberrations appear as regions of non-uniform brightness in a frequency sweep or multiburst pattern. The AVIA resolution pattern has a dedicated sharpness adjustment pattern that can be used to look for all of these effects while also observing the sharpness of alphanumeric characters. There are horizontal and vertical black lines of varying widths against a gray background to observe edge outlining, and horizontal and vertical frequency sweeps with a multiburst pattern to look for frequency response aberrations. Set the Sharpness control to produce the most even brightness over the sweep and multiburst patterns, and minimal edge outlining on the black lines. FilmLike DVD Quality I would also like to address your comments about soft pictures and film-like video quality. I don’t equate DVDs with great video and “film-like qualities” to soft pictures. To the contrary, the best DVD transfers have the sharpest pictures and look the most like film. HDTV has higher resolution and much more sharpness than DVD, and the pictures are even more “film-like.” DVDs that look the least like film have edge-outlining artifacts created by excessive edge enhancement processing. Displays and processors that have Sharpness controls that can reduce sharpness can sometimes be used to reduce the awful edge outlining included on some DVDs. In that limited sense, a softer picture may be more film-like and hopefully more tolerable, but this also reduces picture detail and sharpness, creating a picture that is decidedly unlike film. Occasionally we get a DVD with poor quality video compression that has too many MPEG artifacts. In that case, reducing sharpness may blur out some of those artifacts. Since film has no MPEG artifacts, I suppose the picture could be considered more film-like, but it’s now a blurrier picture than the film. The only acceptable solution to these problems is better DVD quality, not to cover them up by reducing picture sharpness.
You can E-mail Widescreen Review @ mailto:editorgary@widescreenreview.com