Tru-line Video Technologies
Projector alignment information and procedure
First, lets talk about what a color projector is and what it is trying to accomplish. The projector (rear screen or front) is a 3 CRT display device that relies on the science of "Additive color mixing" as opposed to paint systems, which are subtractive. -I.e. if you take red paint, green paint and blue paint and mix them together you will get a color very close to black. This is because the blue paint is a surface color that simply "absorbs" all the other colors but blue from the white light (daylight) that falls upon it. When you mix The red green and blue paint together they will reflect almost nothing -i.e. black. If you take a red spotlight, green spotlight and a blue spotlight and focus the beams together you'll get a white light, this is the same principal that a color projector works on. There are 3 CRT's or "picture tubes" each with a different phosphor coating on the front, red, green and blue. These 3 images are converged onto the same point on the screen producing a complete "color" picture.
What is color? Colors are something we see with our eyes. This "something" is waves of electromagnetic energy with wavelengths between 380 and 780 nanometers. Sir Isaac Newton explains, "The rays of light are not colored. There is nothing in the rays but a certain disposition that gives a sensation of a certain color". In other words, your eyes have receptors in them that are sensitive to three different wavelengths of light that is interpreted by your brain as red, green and blue color. When you stimulate them together your brain sees this as white. This means that one person may see colors slightly differently than the next; Our job is to give your eyes the best possible help in perceiving these colors objectively.
Clearly the need for careful calibration and setup become essential for the projector to perform to its full potential. I will attempt to explain each adjustment, its importance, and why the manufacturer doesn't do it properly "out of the box". IM not here to blame the set manufacturer's for not "doing it right" it is there job to sell sets and they must do this under the most extreme circumstance's. -I.e. bright showroom floors, poor input signals and most importantly against other sets.
This is how "bright" the white portions of the picture are and most commonly controlled by the "contrast control". This is one area where the manufacturer's will push the capability of the set completely past its linear operating range to "compete" with the other sets on the showroom floor. There is a trend for people to be attracted to the "brightest" picture, weather it's past the capability of the set or not! Most 55 inch sets out of the box are adjusted to approximately 30 ftl (Foot-Lambert's) of brightness when its limit is more like 4 ftl! This is adjusted with a LOW A.P.L. (Average Picture Level) 100 IRE (IRE: a unit indicating the level of a video signal from 0 to 100 measuring from 0 to 714 millivolts) window pattern from the center of the screen using instrumentation to the limit of the set.
This is how "bright" the black portions of the picture are and most commonly controlled by the "brightness control". This controls how much detail is evident in the dark portions of the picture. It is adjusted by eye using a combination of PLUGE (Picture Line Up Generating Equipment) patterns containing both a LOW and HIGH APL picture content. This adjustment is dependent upon the ambient light in the room and the amount of light falling on the face of the set. The use of both LOW and High APL test patterns is conducive of how well the set can hold the black level at black independent of picture content. (many sets cannot hold the level of black independent of picture content)
This is the amount of "color" in the picture and is controlled by the "color control". This is adjusted using the SMPTE (Society of Motion Picture and Television Engineers) color bar signal utilizing a proper optical filter and/or a oscilloscope viewing only the blue channel. This is not a straightforward adjustment due to manufacturer's "altering" the proper operation of the Chroma decoder again for marketing reasons mainly. In most consumer sets the red component is pushed beyond the green and blues. The "color" or "saturation" level is adjusted to the blue component then compromised slightly depending upon the level of inconsistency in the other two channels. This push in the red channel is commonly done to counteract the practice of higher color temperature's and/or "Auto Color" circuits adjusted into the sets. (see N: Color temperature) Auto Color is designed to make all flesh tones look more pinkish.
This controls the phase of the reference oscillator in the unit and determines the "hue" of the picture and is adjusted by the "tint control". This is adjusted using the SMPTE color bars and the blue optical filter.
This adjustment determines the beam spot size on the face of each CRT and is adjusted inside the set utilizing a pincushion or other sharp pattern.
This is the Len's focus adjustment inside the set and is adjusted the same way you would focus a camera Len's. Unfortunately some manufactures will glue this adjustment down trusting the original setup to the factory?
This adjustment centers the picture horizontally and is partially dependent on the source supplying the video signal to the set and how accurate the SCH (SubCarrier Horizontal) phase of the machine is. (SCH describes the phase relationship between the Chroma SubCarrier and the horizontal sync pulse present in each video scan line) This adjustment is done using the customers source player (preferably a LaserPlayer) and Joe Kane's "Video Essentials" laser disc utilizing the crosshatch pattern with centering and Overscan markers.
This adjustment centers the picture vertically and is done the same way as the horizontal adjustment.
This adjustment controls how much picture overflows outside the confines of the screen and is measured in percent. Consumer sets are typically as high as 20% overscaning. This means you are losing 20% of available picture area. This adjustment is done with the crosshatch pattern with overscan markers in it.
This adjustment is done separately from the horizontal but accomplished in the same manner.
There are usually several controls relating to geometric distortions which basically control how straight the vertical and horizontal lines appear when a crosshatch line pattern is observed. - I.e. the edges of building's in a picture.
This term is used to describe how each of the three images intersects onto the screen. This "convergence" of image must take place at the same point on the screen for each color to perform its "additive" function. The static adjustment is carried out by utilizing a crosshatch pattern and centering magnets on the CRT's. (course adjustment)
This is the fine convergence adjustment and is carried out by either internal adjustment controls or in the case of the newer sets it is done digitally thru software built into the set.
This is the most important adjustment to be performed and usually the most overlooked. Television is a "additive" light system meaning when all 3 picture tubes are driven in equal proportions the resulting light should be "white". Well, as we all know there are many different colors of "white", from the very warm orange glow from a 25 watt soft white light bulb to a very cool looking bluish glow from a high intensity metal halide lamp. Color temperature is measured in degrees Kelvin, from the 25-watt bulb at about 2000 degrees Kelvin to the metal halide bulb at about 9000 degrees Kelvin. The higher the "color temperature" the bluer the color of "white" looks. (degrees Kelvin correlates to the color of light emitted from an ideal black object re: carbon and heated to specific temperatures.) This is where the "balance" of the three CRT's becomes ABSOLUTELY CRITICAL!, If any one of the three colors (RGB) is driven harder than the other two the entire picture will be TINTED THAT COLOR !!. This color of "white" is the "neutral" or starting point to which color is added, the display device has no way of knowing what the correct color of white should be, this is where adjustment comes in. The next question, Is there a reference "color temperature" or "white balance" used in the television industry ?. YES!, have you ever bought a suit or a dress in a store and then found out that the colors look completely different outside in daylight than they did inside in the fluorescent lighting ?. If so, then you know that daylight is the best reference for evaluating colors. The "C.I.E." or International Commission On Illumination has named this point as the 6500 degree Kelvin or more precisely the "CIE (1986) S001 illuminant D65". This "reference" color of white IS attainable in any television utilizing CRT's as the primary light source!. As human light perception and judgment is very adaptive in nature it is IMPOSSIBLE to adjust the color temperature "by eye". This is adjusted using a 30 IRE Low APL window test pattern utilizing an instrument called a Spectroradiometer or Color Analyzer. ANYONE NOT UTILIZING INSTRUMENTATION TO ADJUST COLOR TEMPERATURE DOES NOT KNOW WHAT THEY ARE DOING !!.
This defines how well the set will "track" the reference D65 color of white from just above black thru maximum white. The color of white should not change relative to brightness level. This is where the manufacturer's REALLY CHEAT! Some fabric softener companies put blue dye into there products to give your white clothes that "Whiter Than White" look when in essence all you are doing is dyeing your clothes blue. The set manufacturers will intentionally put more blue into the color of white in their products. I'm not talking about a small amount of blue either, in most sets I've seen "out of the box" the bright end of the luminance range is adjusted as high as 10,000 degrees Kelvin and the dark end of the scale as high as 16,000 degrees or more ! The ENTIRE picture is tinted blue. This holds true for all consumer sets I've seen, this trend started a long time ago and now is difficult to stop due to if you have a properly adjusted set it will actually look "dull" next to the other sets on the showroom floor. This "battle" on the showroom floors is being lost in the living rooms of consumers, when you get these sets home they LOOK TERRIBLE !. This is where consumer education pay's off.
This is a term that when applied to a video signal is frequency peaking, usually at one center frequency, to compensate for loss of detail in the picture. Aperture correction is applied in the video camera. The sharpness control in a television serves a similar frequency compensation function. The drawback to this form of equalization to the video signal is that it cause's ringing or overshoot in the video signal, this is seen in the picture as dark "edges" around all bright image transitions. This control by the manufacturer's is usually adjusted much to high and give's the impression of a sharper image when in fact all it is doing is putting "edges" on bright image transitions. This is adjusted bye eye using a 20 IRE window pattern and a raised pedestal or black level and observing the amount of overshoot in the video signal.
Where aperture correction is used on the luminance signal Chroma detail enhancement is applied to color portion of the composite video signal. This circuit generally enhances transitions between colors horizontally. The drawback again is false "edges" between color transitions. This is adjusted using a real time SMPTE color bar test pattern. This circuit is not employed in all display devices.
Scan Velocity Modulation varies the speed of the electron gun as it travels from the left to the right side of the screen dependent upon picture content. If the image on that particular video scan line has a high APL it will speed up the scan and vice versa for low. If you have a luminance transition on the same scan line the effect is similar to having to much aperture or sharpness correction applied to the signal -i.e. dark or bright edges. This circuit is built into the sets and is not adjustable, but in some cases can be disconnected.
This is not a procedure to the set itself, but I feel it is important to be included in this document. Have you ever taken photograph's both outside in daylight and inside in incandescent lighting, and then found that in the pictures taken inside everything looked reddish ?. If your answer is yes, then you know how adaptive your vision is to the color of white when the film is not. What happens is your eyes loose their color reference. Again to have the correct reference for evaluating colors we need "daylight". This holds true not only when we evaluate the colors of a dress, but also when we evaluate the colors on a television. Also, watching television in a completely black room is straining on your eyes because the television is only occupying approximately 20% of your field of vision. This cause's the iris to open fully. Thus when a bright image comes onto the screen the iris is forced to close down quickly and sends the optic nerve into shock. This is where the television surrounding come into play. Both the color reference and luminance level can be solved by mounting a light behind the set. The light supplies the reference that our eye's need. Clearly the choice of the light is not arbitrary, this light should be as close as possible to the D65 color standard. The lamp should be rated at 6500 Degrees Kelvin with as high a C.R.I. rating as possible. (Color Rendering Index refers to how well a light source will "render" an object familiar. The rating is scaled from 0 to 100 with outside "daylight" being 100) The lamp used should have a rating of at least 90 CRI.
This has been published for informative purposes only. Tru-line Video Technologies assumes no responsibility for the accuracy, completeness or use of this information.
Robert P. McJimsey