Guy Kuo
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- Mar 6, 1999
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This post pertains specifically to the NEC LT150 and NEC LT150Z DLP projectors, but much of the underlying information is applicable to other digital projectors.
Out of the box, the NEC LT150 and its successor the LT150z have been low cost favorites for HT enthusiasts wishing for a reasonably priced entry level front projector. I picked up an LT150Z and after some experimentation have found or verified some techniques for quite massively boosting the image quality. Some of my internal mods like white segment silvering, lens aperture modification, light dump improvement, and light box blackout are probably similar to the secret mods done to the projector by KBK over at the AVS Forum, those are beyond the range of most users. Ultimately, it is possible to more than double the contrast ratio of the machine over the stock configuration, but doing everything would void the warranty.
This does, however, leave an easy non-warranty voiding maneuver to take advantage of the full range of the DLP chip for each of the three primaries. It is possible to make an LT150 or LT150Z look more like a CRT in terms of black level, contrast and white balance using an appropriate color filter and adjustment of white balance.
Think of the digital projector as being a set red, green and blue light valves. The state in which your projector reaches maximal contrast ratio is when each light valve is adjusted such that black makes each one go to its fully off state and white makes each one go to its fully on state. Pretty easy to understand since all three fully off means minimal light and all three fully on means maximal light. Maximal:minimal is the highest contrast ratio possible.
The default or neutral setting of color temperature and white balance is not necessarily the setting that gets all three light valves adjusted for maximal dynamic range. You can use test pattern from AVIA to adjust each light valve so it is at maximal range (just short of clipping both black and white). So you do that to your projector first. Now you have the maximal contrast ratio possible on your projector. I will cover how that is done later.
The color of the grayscale is now probably quite badly tinted by the native color of the projector bulb and color separation system. The trick now is to fix that tint in the grayscale by using a color filter rather than using the digital controls. Remember, that you just set the digital controls to the settings that yield maximal contrast range. Any deviation from those settings decreases contrast range since that would necessarily deviate the light valve away form either/or full off and full on for black and white. The trick is to do as much of the color correction as possible with a high quality coated optical filter. By use of a filter to perform white balance you get to use the full modulation range of the light valves AND get a good white balance.
For NSH and UHP bulbs which have an excess of blue and green and paucity of red output, a filter which works well for correcting the excessive green and blue is a Hoya monocoated (or even better multi-coated) FLD filter (aka FL Day). One can be had from www.2filter.com for under $30. You need one at least 49 mm in diameter for the LT150(z). Yet larger is fine. Placed in front of the projection lens, the Hoya FLD filter corrects down the green and blue most of the way, so very little deviation from the ideal maximal dynamic range settings for the primaries is needed to achieve final white balance. You sacrifice a little of the contrast ratio slightly touching up the white balance electronically, but much less than doing white balance electronically. If you did it all electronically you cut the dynamic range of the green valve to almost half of its maximum capability.
The result is a greater contrast range than if you used a bigger amount of digital adjustment to achieve good white balance. The overall light output is lowered, and this makes black blacker. The projector has plenty of light output anyway so the overall darker but higher contrast picture is considerably superior to one without a filter AND appropriate adjustment of the controls. You may initially think the image is too dim because you are used to the DLP blasting the screen. Give yourself a day to once more learn that bright = good. You learned that the first time you calibrated your display with AVIA. Most people need to learn that lesson all over again when they get a digital projector.
As an aside, do not even consider a lower grade non-coated filter like the commonly available Tiffens. The Hoya and even higher grade B&W filters have much less light scatter and reflection. Also pick up a bottle of Formula MC lens cleaner. Nothing else I have tried leaves less residue when cleaning a coated lens or filter, but that is another story.
So how do you set the red, green, and blue light valves for maximal dynamic range? You take advantage of the moving black and moving white bars in AVIA or Home Theater Tune-up. Those are special features for looking for signal clipping found only on those two DVDs. They can be found in multiple patterns on those discs but you could just use the ones in the basic brightness pattern and the color bar pattern.
If you are connected via a RGB cable which you can disconnect the other two primary colors while adjusting the gain and bias for the remaining visible color. Recall that the moving black and white bars can be used to quite sensitively detect when clipping occurs. The darker black bar disappears when bottom end clips more than 1 IRE. The brighter white bar disappears if the top end clips more than 1 IRE. You adjust the controls for each primary so the darkest black bar and the brightest white bars are barely short of being clipped while viewing only that color.
If you cannot isolate the individual primaries you can intentionally force the other two colors into clipping and that would allow you to see the changes in the remaining, unclipped color. You would write down the settings for that color, then go on to find the settings for the other colors. Once all three colors are done you go back and enter in the values you found.
An alternative technique would be to use a 100 IRE full field pattern and a light meter. The meter needs to be absolutely immobile and could even be a solar cell and milliammeter. You would display the 100 IRE Field and then adjust each primary gain until you reach the point for each at which clicking upward on the digital control no long creates an increment in light output. The dark end you would have to do using the technique just preceding.
If you are using an LT150, you will find the white balance controls in the advanced menu mode. First set color temperature to middle, do an initial brightness and contrast adjustment with AVIA, then go into the advanced menu mode and find the white balance controls. Once the three are adjusted for maximal range without clipping, do not change the main brightness and contrast controls any more. Put the filter in front of the projector and enjoy. You might need some very slight tweaks to white balance to get the grayscale even, but it will probably already be pretty good.
One warning, do not just pop the filter in front of the projector without doing the readjustment of the three primary color ranges. You will be sorely disappointed and discouraged by the lazy approach since the primaries will not be at the correct levels to work with the filter.
When done with the filter and reworked white balance, the machine will seem completely different.
Out of the box, the NEC LT150 and its successor the LT150z have been low cost favorites for HT enthusiasts wishing for a reasonably priced entry level front projector. I picked up an LT150Z and after some experimentation have found or verified some techniques for quite massively boosting the image quality. Some of my internal mods like white segment silvering, lens aperture modification, light dump improvement, and light box blackout are probably similar to the secret mods done to the projector by KBK over at the AVS Forum, those are beyond the range of most users. Ultimately, it is possible to more than double the contrast ratio of the machine over the stock configuration, but doing everything would void the warranty.
This does, however, leave an easy non-warranty voiding maneuver to take advantage of the full range of the DLP chip for each of the three primaries. It is possible to make an LT150 or LT150Z look more like a CRT in terms of black level, contrast and white balance using an appropriate color filter and adjustment of white balance.
Think of the digital projector as being a set red, green and blue light valves. The state in which your projector reaches maximal contrast ratio is when each light valve is adjusted such that black makes each one go to its fully off state and white makes each one go to its fully on state. Pretty easy to understand since all three fully off means minimal light and all three fully on means maximal light. Maximal:minimal is the highest contrast ratio possible.
The default or neutral setting of color temperature and white balance is not necessarily the setting that gets all three light valves adjusted for maximal dynamic range. You can use test pattern from AVIA to adjust each light valve so it is at maximal range (just short of clipping both black and white). So you do that to your projector first. Now you have the maximal contrast ratio possible on your projector. I will cover how that is done later.
The color of the grayscale is now probably quite badly tinted by the native color of the projector bulb and color separation system. The trick now is to fix that tint in the grayscale by using a color filter rather than using the digital controls. Remember, that you just set the digital controls to the settings that yield maximal contrast range. Any deviation from those settings decreases contrast range since that would necessarily deviate the light valve away form either/or full off and full on for black and white. The trick is to do as much of the color correction as possible with a high quality coated optical filter. By use of a filter to perform white balance you get to use the full modulation range of the light valves AND get a good white balance.
For NSH and UHP bulbs which have an excess of blue and green and paucity of red output, a filter which works well for correcting the excessive green and blue is a Hoya monocoated (or even better multi-coated) FLD filter (aka FL Day). One can be had from www.2filter.com for under $30. You need one at least 49 mm in diameter for the LT150(z). Yet larger is fine. Placed in front of the projection lens, the Hoya FLD filter corrects down the green and blue most of the way, so very little deviation from the ideal maximal dynamic range settings for the primaries is needed to achieve final white balance. You sacrifice a little of the contrast ratio slightly touching up the white balance electronically, but much less than doing white balance electronically. If you did it all electronically you cut the dynamic range of the green valve to almost half of its maximum capability.
The result is a greater contrast range than if you used a bigger amount of digital adjustment to achieve good white balance. The overall light output is lowered, and this makes black blacker. The projector has plenty of light output anyway so the overall darker but higher contrast picture is considerably superior to one without a filter AND appropriate adjustment of the controls. You may initially think the image is too dim because you are used to the DLP blasting the screen. Give yourself a day to once more learn that bright = good. You learned that the first time you calibrated your display with AVIA. Most people need to learn that lesson all over again when they get a digital projector.
As an aside, do not even consider a lower grade non-coated filter like the commonly available Tiffens. The Hoya and even higher grade B&W filters have much less light scatter and reflection. Also pick up a bottle of Formula MC lens cleaner. Nothing else I have tried leaves less residue when cleaning a coated lens or filter, but that is another story.
So how do you set the red, green, and blue light valves for maximal dynamic range? You take advantage of the moving black and moving white bars in AVIA or Home Theater Tune-up. Those are special features for looking for signal clipping found only on those two DVDs. They can be found in multiple patterns on those discs but you could just use the ones in the basic brightness pattern and the color bar pattern.
If you are connected via a RGB cable which you can disconnect the other two primary colors while adjusting the gain and bias for the remaining visible color. Recall that the moving black and white bars can be used to quite sensitively detect when clipping occurs. The darker black bar disappears when bottom end clips more than 1 IRE. The brighter white bar disappears if the top end clips more than 1 IRE. You adjust the controls for each primary so the darkest black bar and the brightest white bars are barely short of being clipped while viewing only that color.
If you cannot isolate the individual primaries you can intentionally force the other two colors into clipping and that would allow you to see the changes in the remaining, unclipped color. You would write down the settings for that color, then go on to find the settings for the other colors. Once all three colors are done you go back and enter in the values you found.
An alternative technique would be to use a 100 IRE full field pattern and a light meter. The meter needs to be absolutely immobile and could even be a solar cell and milliammeter. You would display the 100 IRE Field and then adjust each primary gain until you reach the point for each at which clicking upward on the digital control no long creates an increment in light output. The dark end you would have to do using the technique just preceding.
If you are using an LT150, you will find the white balance controls in the advanced menu mode. First set color temperature to middle, do an initial brightness and contrast adjustment with AVIA, then go into the advanced menu mode and find the white balance controls. Once the three are adjusted for maximal range without clipping, do not change the main brightness and contrast controls any more. Put the filter in front of the projector and enjoy. You might need some very slight tweaks to white balance to get the grayscale even, but it will probably already be pretty good.
One warning, do not just pop the filter in front of the projector without doing the readjustment of the three primary color ranges. You will be sorely disappointed and discouraged by the lazy approach since the primaries will not be at the correct levels to work with the filter.
When done with the filter and reworked white balance, the machine will seem completely different.
