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Optoma H79 Single-Chip 16:9 DLP Digital Projector

Part II

November, 2005

Steve Smallcombe and Sumit Chawla

 

On the Bench

Calibrations in the 720p mode were performed using an Accupel HDG-3000 Component Video Calibration Generator, a device capable of generating video calibration test signals in a wide range of video formats. Calibrations were performed after 171 hours on the bulb. I used a Denon 1600 DVD player for DVD viewing. I also watched HD and standard definition materials from my DISH 921 HD PVR.

Measurements

When we evaluate a projector, we not only look at images, we measure the color balance of the projector at various light intensity levels and determine the quality of what is called Grayscale Tracking. The idea is that black, white, and all shades of gray, should have the correct ratio of the three primary colors used in video projection Red, Green, and Blue. You can read more about the testing method in my past projector reviews on Secrets, or at www.smartavtweaks.com.

Using DVI signals from the Accupel, the proper brightness level was seen to be -2, and at contrast setting above 14, IRE levels just below 100 were lost or compressed. The measurements below were made with these settings. The Color Temperature was set to 2. As can be seen in the Color Intensity chart above, the H79 had good color balance through the mid IRE levels, but the red levels were considerably compressed at the highest IRE levels.

The Color Balance Graph above shows the same data, but in this case, plotted as a ratio between the various colors. The Color Balance chart not only shows the fall off in red at high IRE levels but also shows the there is too much red at the lower IRE levels.

The contrast ratio with the projector at this point was 2227:1. The light intensity at the screen was 13.7 ftL with the Brightmode off setting and 18.6 ftL with the Brightmode on setting. ANSI lumens measured 434 and 588 respectively. As shown above, the excess red at low IRE levels can be easily reduced by lowering the Red Brightness in the Advanced Menu, by 7 units in this case. This slightly overcorrects IRE 10, but undercorrects IRE 0, as the controls do not have full effect on the red leakage. This improves the contrast ratio to 2576:1.

Unfortunately, the above measurements are still a bit deceptive; ideally the contrast setting would have to be be lowered to avoid blowing out the red level at the highest IRE levels. The ideal contrast setting where the red does not start to clip reduces the measured contrast to 2050:1 (1770:1 without the lower bias setting). If one allows red to blow out, as shown above, then the highlights in the image often take on a cyanish tinge that does not look quite right. The problem is that if you lower the contrast so that red can keep up, you lower the contrast ratio and the image brightness a proportional amount.

It is therefore a dilemma when adjusting the contrast setting of a projector where one color, e.g., red, blows out before the others. Do you lower the contrast setting to get good gamma tracking, or raise the contrast setting for the best contrast ratio and brightness.

For me, this was Déjà vu, as running out of red at high IRE levels is exactly what limited the performance of many LCD-based projectors for years. This problem is what led me (Steve Smallcombe) to develop the use of CC filters with digital projectors to maximize the contrast ratio and improve black levels.

Measurements with an Added CC20R Filter

By adding a CC20R filter, which preferentially passes the color red and attenuates green and blue, and lowering the Red Contrast setting in the Advanced menu to restore color balance at the mid IRE levels, the grayscale tracking was now excellent right up to the highest IRE levels, (as shown below), and the contrast ratio had improved to 2620:1. With the CC20R filter added, the light level from the projector dropped by 35 percent. To compensate for the light drop, I enabled the Brightmode setting; enabling this mode increased the light level by 35% thus restoring the rather ideal 13.8 ftL at the screen (for IRE 100).

Gamma Tracking

The other thing we measure is Gamma Tracking, or how the light output of the projector responds to the input signal. If the projector's Gamma Tracking is off, then details in the image will either be lost, or the image may look flat and have little contrast. The Gamma Tracking graph shows the combined light intensity at the various IRE levels relative to a theoretical level. If the projector is accurately producing the intended light intensity level as a function of input or IRE level, all values should be close to 1 in the Gamma Tracking graph.

In the Gamma Tracking graph above, we can see that the H79 has reasonably accurate tracking that is well described with an overall gamma of 2.4. This measurement was made using a gamma setting of 2. The graph above is representative of the projector before and after grayscale/CC filter tweaking. There are actually 15 different gamma modes that can be access by various combinations of parameters, and with time, I may well explore other modes.

Viewing and Comments

I was able to mount the H79 on my ceiling and have it purring in a matter of hours, as it had a similar throw ratio as my previous projector. The Optoma projector mount allows some flexibility in both horizontal and vertical placement, but I did my best to place the mount so that the projector lens would be centered on the screen horizontally.

I have used the H79 for several months of viewing now, and it has never disappointed me. I particularly like its very quiet operation. I can't hear it from my chair about 6 feet below the projector, and of course the excellent picture quality is always pleasing.

There was a time when DLP-based projectors seemed to suffer from less saturated colors relative to their LCD-based competitors, e.g., my then reference Sony 11HT. Many people, myself included, were bothered by the rainbows which originate from the use of a single DLP chip and a spinning color wheel to produce colors. The dithering artifacts at low IRE levels, i.e., just above black, were also bothersome. What the DLP technology had going for it, however, was a smooth image with minimal Screen Door Effect (SDE) and good contrast ratios.

The current crop of DLP projectors, and the H79 is no exception, have managed to improve in all these areas. The H79 has wonderful, well-saturated colors, absolutely minimal SDE, and a respectable contrast ratio with image brightness usable with a unity gain screen, such as my 102 inch diagonal Dalite DaMatte screen.

Considering that I started with a Sony 11HT that had a contrast ratio of 150:1, and I managed to essentially double that with CC filters and other SMART-based tweaks, who would have thought that I would still want/need to tweak a projector with contrast ratios in the region of 2000 to 3000:1. Yet at 3000:1 blacks are not completely black and you can still see the shadow of your hand.

The CC filter-tweaked projector with a contrast ratio of 2620:1 and better grayscale tracking at high IRE levels is the way I am using the projector today. This is really made possible by the very quiet operation of the projector in the High Brightness mode. The image is a noticeable improvement over the 1770 obtained without the filter, the default color setting and the conservative contrast setting that preserves grayscale tracking into the highest IRE levels. It should be noted that the tweaked contrast ratio of 2620:1 is limited by red leakage at IRE 0; the blue and green contrast ratios are above 3000:1. Since the eye is less sensitive to red at low light levels, and these are very low light levels we are talking about here, this leakage may not be as deleterious to the perceived black levels as the numbers would indicate.

With DLP-based projectors, the various levels of gray, as well as colors, are made by tilting mirrors to turn the light for a pixel on and off. The faster this is done, the better the image, especially if the image contains motion. With each new DLP chip generation, Texas Instruments has improved the pixel fill factor, for minimal SDE, increased mirror tilt and smoothness for better contrast ratios and brightness, and increased the rate with which image data can be written to the chip. This latter factor has become more critical as the number segments in the color wheel has increased, e.g., by adding one and now two dark green segments to the color wheel to minimize dithering artifacts. The H79 has an 8-segment color wheel with two dark green segments, and dithering artifacts are simply not observable.

The other thing that should be discussed is the subject of motion artifacts. I became particularly aware of the motion artifact issue when I received an Optoma H77 for review and potential purchase. At first, I was very pleased with the overall image quality, but then, when a car in a movie was supposed to be slowly moving across the screen, instead of moving slowly and smoothly, it moved in several obvious steps. Obviously the image was not being updated fast enough for the image to appear to be moving smoothly. Symptoms of this issue included apparent jerky motion, or obvious blurring or pixelization of the moving image. This didn't happen all the time, but when it did, it was pretty obvious. That sort of ruled the H77 out for me, which was really too bad, as otherwise the image was great. I returned the H77 when the H79 became available, and was very pleased to see that these motion artifacts were not an issue.

Is the H79 my ultimate projector? Probably not. I would really like 1080p resolution, but it may be some time before it is available well below $10,000. Horizontal lens shift would be nice, but not really necessary for my setup. Many DLP projectors use an adjustable, but fixed (not dynamic) iris to allow the user to trade off brightness for contrast ratio. The H79 however has found a very good compromise in a fixed iris that works well with low gains screens, such as the one I use. However, those consumers with high-gain screens might appreciate trading some of the H79s excellent brightness for a bit better contrast ratio. And, oh, yes, the H79 does not have swoop-de-do curved lines or fancy colors - it's a beige box, but a nice looking beige box at that.

On the very positive side, the H79 with its excellent optics, well-saturated colors, and good contrast ratio and black levels, is better than my 11HT in virtually every way. The decision to buy it was really quite simple.

One downside I found with the H79 was that the brighter smoother image quickly made me aware of my screen - it was dirty, but I had not really noticed it with my 11HT. It was time to wash the screen, but that is another somewhat long story.

On a similar note, it is really nice to not hear a projector as background noise during quiet scenes in the movie. The H79 is so quiet that I can now hear the fan in my audio system keeping my amplifier cool, but one can hardly blame the H79 for that issue.

Conclusions

The H79 is an excellent example of how far video projectors aimed at the home theater market have come in the last few years. It produces a bright, smooth image with accurate, well-saturated colors, and has a respectable contrast ratio, especially given its brightness. It is the quietest projector I have experienced, another important factor in my buying decision. I bought one. You might want to as well.


- Steve Smallcomb and Sumit Chawla -

© Copyright 2005 Secrets of Home Theater & High Fidelity

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