ISF Calibration of an HDTV Rear Projection Display
Andrew Marshall watches London Audio’s Rick Ho ISF calibrate
the AIG Reference Pioneer PRO-710 HD and sees the difference
Some of us thought that HDTV sets would come ready to make the best of all images, and I assumed this when I acquired my 64″ Pioneer Elite PRO-710HD rear-projection TV a year or more ago. It has been superseded by newer, less expensive models in the same size, as HD prices have rapidly come down. The buzz these days is around the larger plasma sets (up to 60″), which can be hung on a wall. I’ve reviewed numerous models of these, and I still come back to my 7″ CRT-based PRO-710 for its higher resolution and fewer video artifacts. The greatest virtue of the plasma technology is the vivid bright picture and vibrant colour.
A certain amount of massaging is required for the best plasma picture, and many new sets incorporate scalers and other picture processing. However, there is little on-site setup required for plasma and LCD sets, while 3-gun CRT RPTVs really do need fine tuning. There are several reasons for this. One is that movement slightly displaces the aim of the CRTs, and they have to be realigned. This process is called convergence, and can be a long and tricky process. A set’s convergence can also be affected by its orientation in the viewing environment relative to the earth’s magnetic field, as can colour values.
And speaking of the question of colour values, as Rick Ho notes, “In addition to geometric alignment, the red, green, and blue CRTs must also have their output levels adjusted to reproduce accurate black and white levels. This adjustment affects not only Contrast, the balance between light and dark scenes, but also overall colour balance as well. Television pictures are made by overlaying colour infomation over top of a black and white picture [Chroma over Luminance information], so achieving accurate blacks, whites, and all the steps in between are critical to [displaying] a great picture. Achieving the correct balance of the three CRTs is usually referred to as grey scale tracking.”
In short, these are the main elements of an Imaging Science Foundation calibration process. The technicians doing these are all trained and certified by the ISF, and use specialized equipment based around a laptop computer and a device that reads the light output from the set’s screen so it can be precisely set to the correct values for a perfectly balanced picture.
Though all sets have adjustments for picture contrast and brightness, as well as resolution (Usually called “Sharpness”) and other values, these cover a fairly limited range, and are seldom set for optimum values at the factory. Most often they are set for excessive contrast and brightness, what some call the “fluorescent effect”, that is, to show high brightness and contrast in a well lit dealer’s showroom.
As well, factory adjustments of greyscale will be somewhat unrefined, without a smooth, even transition from the darkest to the brightest parts of the picture. This severely compromises picture realism by obscuring fine detail and making it overly contrasty, and in worst cases, downright blotchy. Photographers will understand exactly what I mean.
As a result, one of the first stages of the ISF process is often opening up the TV to fine tune internal adjustments of greyscale and colour values. Sometimes TVs have secret service menus that allow adjustment using just the remote control. Such tools as the Video Essentials DVD are very handy during this and other stages. Computer generated colour, greyscale, and linearity test patterns are also used by Rick and other ISF certified technicians to tweak the image.
Before getting into more specifics on the calibration process, I should note some of the conditions for a really good picture, whether it’s high definition or good old NTSC (Never Twice Same Colour). Rick, again: “Image levels, or Brightness, are measured in lumens or Foot Candles, [while] the colour temperature is measured in Kelvin, and the reference level for an NTSC [display] is 6500. There are scorchingly bright displays that can accurately track a flat 6500K grey scale in a bright room. The issue is that most displays (regardless of format) when set up for the correct maximum output and accurate grey scale tracking simply do not put out much light. This has nothing to do with the colour temperature, per se, but cranking the contrast will affect correct grey scale tracking.”
Therefore, a perfect picture from your screen works best in a fairly dimly lit room with controlled indirect lighting, preferably above or behind the set, so there are no reflections on the screen, rather like a movie theatre, though with front projectors there is sufficient scattered light to not require additional lighting. In both cases, the room should not have highly light-reflective walls: white walls are not a good idea in a home theatre environment.
Brightness, Black, & Sharpness
Many ISF adjustments are made internally on the display device, or by using a special hidden factory remote control menu, so they cannot be altered by the end user to degrade picture quality beyond the range of the set’s normal controls, and the ISF settings can be immediately restored by reverting to default values.
Black level (usually called “Brightness” by manufacturers) is set to reproduce the absence of light correctly. This requires precisely matching the output of each colour gun, since the absence of light is, in fact, full output of each colour at once. This also ensures that white, the absence of colour, will not have a caste of its own. Over the years, certain brands have tended to have a leaning to blue in whites, or to green or brown, because of higher or lower relative output from colour guns, this especially true of tube CRT sets.
Picture level (”Contrast”) is also a part of this stage of calibration, and involves setting the maximum peak light output: if it’s too high the picture will bloom, wiping out detail and, at worst, causing geometric distortion. Again, there will be the option for the viewer to adjust this default setting for varying natural light conditions within the range of the set’s controls.
It’s interesting that the first step in dealing with Sharpness is to reduce it to avoid over accentuation, which can put white borders around objects on the screen and accentuate any video noise present. As well, the SVM (Scan Velocity Modulation) circuit (found in many high end TVs) must be turned off, because it tends to over-accentuate contrast and reduce picture resolution.
Colour Temperature & Tint
The next stage Rick got into involved special instruments. First is a device that is mounted close to the screen to measure light output, and allow precise setting of colour and greyscale values. These can be recorded on a chart with before and after values. Part of this process also involves a box that outputs very accurate colour, greyscale, and geometric test patterns.
Rick sums up the process and objectives: “To accurately reproduce a colour picture there are three issues to address: the correct adjustment of the grey scale, the setting of the colour decoding, and the adjustment of hue. Ween I look through the tinted gel at the colour bars, I am not actually adjusting the blue, red, or green CRTs, I am adjusting the colours decoder in the set. The colour and tint (or hue) are adjustments of the colour decoder. There are additional controls available in the service menu that affect how the colour decoder works.” Again, like black level and grey scale values, these are set to a default that can be immediately restored using the standard settings for the set.
Convergence, Convergence, Convergence
Central to a High Definition display is its true picture detail and sharpness. This can only be achieved if every pixel emanating from the three colour guns is precisely aimed at the proper spot on the screen. With the PRO-710HD, this was a long and arduous process, the red and blue guns being lined up with the green at, if I recall correctly, 72 positions on the screen from edge to edge vertically and horizontally using a grid (see photo above).
Because this is an HD set, there are several aspect ratio settings for each of four inputs and for the TV’s tuner. In other words, with this set, convergence had to be set individually for every aspect ratio, from Full (HD) to Normal (NTSC), with all the screen-stretching modes in between. This took Rick a couple of hours to do, but at least he could do it using the remote control, though the button presses had to be extremely precise to line up the lines, so to speak, both vertically and horizontally. I had gone through the process myself when I first got the TV, but I could not manage the level of precision that an experienced technician like Rick could.
Benefits, Benefits, Benefits
The end result, after 5 hours, was a much sharper and more naturally coloured picture, with a wealth of detail, especially with DVD and HD sources. A major improvement was image depth. Fine tuning the convergence obviously makes the picture sharper, but the effect of properly calibrating colour temperature and grey scale values results in the greater depth of image, and a reduced level of eye fatigue relative to a set with hopped up brightness and contrast.
Even people in the industry who’ve seen lots of HD pictures have been wowed by the extra excellence of my ISF-calibrated display. And the bigger the screen (especially if it’s a CRT RPTV), the greater the difference it makes.
I should note that the calibration process is somewhat simpler for fixed pixel displays, though the setting of colour and grey scale values is still an exacting and necessary route to the best picture possible (see our review of the Elite PRO-910HD plasma in the Winter/Spring 04′ issue). Thanks, Rick, for making my day over and over again in front of my HDTV.