MTF Revealed – Part III : JVC RS35 Update – The Secret Ingredient?
December 28, 2009
In part II of this series, we explored sharpness differences between LCOS and DLP technologies. In that article we also speculated that there appeared to be some unique changes to the JVC RS35 that improved sharpness and perhaps reduced chromatic artifacts as well. In this article we take a closer examination of the apparent differences that we saw between the RS20 and the RS35.
If you will recall, we noted that the RS20 edge pixels were diminished both in light output and geometry, and we suggested that this may be due to field fringing or other panel effects rather than lens effects. The RS35 on the other hand showed significant improvements to these edge pixels, but with one caveat – a transition dependence where black to white edge pixels (when moving from left to right) seemed to show the improvements while the white to black edge pixels (also left to right) seemed unchanged from what we saw with the RS20. We also saw chromatic artifacts on the RS20 edge pixels that were not present in the RS35, and we speculated that perhaps this was also related to the diminished edge pixels rather than the conventionally accepted causes of convergence and chromatic aberration.
As we noted at the beginning of part II, there are always variances among units and so it is always best to take such findings with a grain of salt. What adds credence to any speculation however, is repeated confirmation of the same behavior with other units. In this thread at AVS Forum, some members have commented on the same behavior on their RS35s and one member, Randal Feemster (Cam Man) graciously forwarded this high quality photo closeup of another RS35 owned by Erik Todoroff (HogPilot).
In it we can see the same transition effects that we discussed earlier, black to white transitions (from left to right) are well defined, but white to black (also moving from left to right) transitions show a large loss of pixel area. We also see a lack of the color artifacts from left to right that we saw with the RS20. So this photo provides additional support of the behavior that we noted in part II. We also hope to perform detailed MTF measurements on another RS35 soon to help further reinforce or dispel the findings that we have developed so far.
This photo also shows transition effects happening as one goes from bottom to top, but with red color effects occurring on the top most edge pixels that are also diminished the most. This is similar to what we saw with vertical line pairs with the RS20 in part II and as we noted in part II, a quick look at the RS35 line scans with white horizontal line pairs didn’t seem to show the same improvements that we were seeing with vertical line pairs. Incidentally, the photo above also appears to show blue misconvergence (seen at the bottom of the text) which is not related to the edge color effects that we noted in Part II.
This article will focus on the left to right transition improvements that we’ve seen on the RS35. We will do a similar treatment of bottom to top transitions as we’ve done here and if anything noteworthy is found, we’ll post a follow up article.
First let’s take a look at the RS20. If you recall, the photo below is from a vertical, 5 pixel wide line pair:
As we can see from this color photo, the right pixel in the 5 pixel line pair is lower in intensity and thinner. Chromatic artifacts exist on both edge pixels. Conventional wisdom would say that this is probably due to convergence errors from a misaligment of the three panels or chromatic aberrations in the lens. Let’s take a more detailed look though and see if those issues hold or if there is something else going on.
By taking separate red, green, and blue images of this pattern with a stationary line camera, we can see the performance of each individual panel. This was done below with white (all three panels) at the top and the other three colors aligned vertically below it so that we can easily check for convergence errors. Convergence errors will show up as a misalignment between the group of “on” pixels. The on pixels in one line scan image will be shifted either to the left or right and will not line up with the other panels if misconvergence is present. Using this method we can determine the degree of misalignment very precisely and thereby get some clues about the chromatic effects that we were seeing with the RS20. We can also use this method to dig deeper into the diminished pixels that we saw in Part II with white. We can for example see if the diminished edge pixels are due to one specific panel or multiple panels. The 5 pixel line scans are shown below.
From the line scans above we can see some surprising results. The biggest surprise is that the symmetric diminishing of the edge pixels that we saw with white is not actually symmetric when we examine each color individually. The red panel shows a problem with dark to light transitions (left to right) but is okay with light to dark transitions (also from left to right) and the situation is reversed for the green panel. We can see nearly spot on convergence between red and green, although blue is slightly misconverged. We can also see that the blue panel is generally not very well delineated, perhaps due to less than ideal blue focus (note: in Table I of Part II we showed that Blue MTF on the RS20 was reduced about 20% compared to the Red and Green panels).
Next lets examine the line scan images from a color perspective. The center points of each pixel seem to be roughly 175 camera pixels apart which would put the center of the left most edge pixel for both red and green at about line 1475 (based off of the camera pixel location). Looking vertically along this location at each line scan, it’s clear that the left edge pixel is diminished more in red than any other color. If we look at the color photo we see a cyan coloration. With a video RGB color system, any time that we reduce the amount of red from a white color mix, we will end up with green and blue which is the secondary color Cyan. Which just happens to match the same color of the chromatic artifact we see from the RS20 color photo. The lack of misconvergence or bleeding of other colors due to chromatic aberration is a clear indication that the color artifact that we are seeing in the photo above is primarily caused by the diminution of the red edge pixel.
Next lets take a look at the right edge pixel from the photo. This edge pixel follows the bright to dark transition (again left to right). Looking at the line scans we see that red is comparatively well maintained but green and blue are diminished now. Not surprisingly we will get a lack of green and blue in the RGB mix which gives us red. Looking at the photo, that is exactly the color that we see with this pixel. Since Green is the dominant color in Rec. 709 and Rec. 601, it’s also not surprising that we see a loss of brightness as well.
What is unclear is why the left Red pixel is diminished while the right Red pixel seems to be less affected. Similarly, it’s also unclear why the left green pixel is not as affected as much as the right edge pixel. Unfortunately this lack of symmetry complicates the speculation that we had earlier that field fringing was the primary cause of the diminished pixels as it seems as though this would cause both pixels to be diminished equally.
In summary, we have seen good indications that the color effects that we are seeing on this particular, well converged, RS20 are directly related to the diminished edge pixels that we discussed in Part II rather than the commonly accepted notions of misconvergence or lens chromatic aberration. Unfortunately we also have new questions about what causes the diminished edge pixels as it doesn’t seem to be symmetric within a given color and the left to right transition is different between Red and Green panels. Something new to ponder…
For now let’s put our pondering about the RS20 on hold and examine the RS35 using the same techniques described above. The photo that we saw in Part II is included below. Note the lack of color artifacts, although as we noted the right edge pixel is diminished in both size and light intensity.
Line scans from each individual panel were captured as we did with the RS20 and the results are shown below:
Here we see that the Red and Green panel alignment is also excellent and no convergence errors can be seen. As we saw with the RS20 there is a slight amount of blue misconvergence, but not a significant problem. Next let’s take a look at the Red panel. With the RS20 we saw that the left Red pixel was diminished which caused the Cyan tint that we noted with the left pixel. In Part II, we openly speculated that the RS35 might use some sort of compensation on some transition pixels which mitigates the fringing effects that create the diminished edge pixels. Looking at the Red Panel we see something very interesting. The first pixel (the one circled) is boosted and is noticeably brighter than the other pixels in the group by roughly 10%. This despite the fact that it’s an edge pixel where we expect some at least some degree of diminished light intensity. This boosting of light intensity is something that we hadn’t seen before with any other line scans on the RS20. In fact all of the edge pixels on all of the RS20 showed some degree of diminished light intensity. So this seems to be a strong indication that some sort of boost/compensation is being applied in the panel as it’s highly unlikely that lens or optical effects could cause this sort of spike. The net affects from this red spike are two fold: 1) The pixel intensity and geometry are preserved and 2) There is no longer a shortage of Red in the color spectrum for this edge pixel so we no longer see chromatic problems. Looking closely at these line scans compared to the RS20 we see similar but more subtle boosting/compensation going on with the first (left most pixel in the line pair) pixel in the Green and Blue Panels as well.
Incidentally, we also see much better definition with the Blue panel which confirms the MTF measurements that we took in Part II where we noted in Table 1 that the Blue MTF of the RS35 was improved by 20% over the RS20.
Now lets examine the right edge pixels on the RS35. Looking vertically along the line scans at this pixel position we see that all of the edge pixels show diminished light output and geometry which explains why this pixel shows up as being roughly 1/2 the size of the other pixels. On the RS20 we noted that only the right-most Red pixel in the group was fully formed which led to a red tint with this pixel. Curiously, this red pixel is diminished on the RS35 and now all 3 panels are back in color balance so we no longer see the same color artifacts that we saw on the right edge of the RS20 even though the pixel is still not as bright and fully formed as it should be. Overall, the diminished right pixel seems to be a “win” because of the lack of color artifacting, so perhaps this was an intentional change implemented by the JVC engineers. Unfortunately there is no way to know the “whys” although we do have a clear indication of improvements to the RS35 panel over the RS20, improvements that are clearly transition dependent.
Putting things together we have more information and perhaps a few answers, although we have also raised a few more questions in the process. Our speculation that the RS35 is using some form of boost/compensation in order to deal with fringing effects seems to have been given a lot of credence. The fact that the RS35 Red left edge pixel, after a “off” to “on” transition, is not diminished as it was with the RS20 and in fact is noticeably brighter than the other pixels in the group seems to be a clear indication of that. We have also seen the other edge pixels in a off to on transition maintain their light intensity better than we saw with the RS20.
Curiously, the rightmost red pixel on the RS35 has dropped a notch in brightness and delineation, although, overall this seems like a win as it keeps the colors balanced and prevents the distracting color artifacts that we saw with the RS20. We’ve also shown that what looked like convergence or chromatic aberration errors on the R20 was in fact related to the edge pixel issues that we discussed in Part II. These issues are likely panel related rather than lens related and they seem to have been corrected with the RS35, at least in the vertical direction. On the downside, we have new questions about what causes the fringing effects on the edge pixels, as the RS20 red and green panels do not seem to be symmetric in this respect.
The improvements on the RS35 edge pixels that we have seen with the left edge pixels directly result in improved sharpness and a lessening of color artifacts and likely explain at least some of the 9% higher MTF results that we measured with the RS35 over the RS20 with vertical line pairs. As we have seen, these improvements only seem to apply to the left edge pixels that follow a off to on transition. It’s unknown why similar improvements have not been made to the right edge pixels, but it gives us something to look forward to with future products. As we saw in Part II, LCOS is capable of high inter-pixel contrast and if the edge effects that we’ve discussed here can be further improved, we should see continued sharpness improvements in LCOS displays.