Quote:
Originally Posted by JimP
...to get darker blacks in a dark scenes, the iris will close giving you a darker black...The problem is when the dynamic iris sees a dark scene, it will darken the picture, but if you're in a well lit room, you've just crushed your blacks and have made your overall picture darker...
Does anyone have any insights as to how the dynamic iris system will work?? |
It sounds like you understand the mechanics of how the D-iris works pretty well, except for its relationship to blacks. It is important to distinguish the two main issues surrounding black performance (and the differences between them) to understand this. Crushing blacks is something that happens for one of two reasons, either the signal is clipped somewhere in the aquisition/production/delivery chain, or the picture/brightness/contrast settings in the display are misadjusted, allowing the darkest blacks to dip below the absolute black performance of the display. IOW, performance exceeds the linear window of operation of the transfer curve of the video amplifying system.
Any modern display can be set to not crush blacks. It is typically a one-time setting that works for all content (except content like "Cold Case", where clipped blacks are a part of the production style).
The reason this becomes thorny in LCDs and DLPs is because light leak inside the RP cabinet makes the absolute black level higher than in a plasma or CRT or direct LCD, and it makes the other problem, non-linear response, more noticeable, and more critical to tweak for.
But D-iris should not contribute at all to crushing black levels. If you take any similar application, such as a projector or a camera, decreasing the total light throughput by decreasing the iris aperture drops the light exponentially more for bright areas than it does dark areas, and black detail, while physically harder to see because it is simply darker than it was before, does not clip or crush in the conventional sense, because there is no non-linear part of the operational window. In optics, for all practical purposes, it is infinitely linear, meaning blacks can't clip.
Quote:
Originally Posted by JimP
...This isn't an issue in bright scenes because the iris in your eyes close down giving you the impression that smaller black areas are dark. Also this problem of blacks appearing as dark gray only really shows up when viewed in a fairly dark room... |
The reason D-iris can be effective is because it can actually lower the absolute black level for darker scenes without crushing the blacks. You struck here on exactly why this will work. When there is a predominance of light areas in the picture, our eyes (and perception) automatically compensate for the higher-than-normal absolute black level. Under such conditions a higher absolute black level doesn't seem that noticeable or out of place. Only when there is a dark picture does it seem higher than it should be. If a D-iris system can simply take an RMS value of the total brightness in a picture and use that voltage signal to close down or regulate the iris in front of the projector lamp (and also compensate by turning up the video brightness) during DARKER scenes the actual light LEAK level for dark scenes INSIDE THE RP CABINET will drop dynamically from its "always on full brightness" state, and absolute black levels will drop correspondingly for dark scenes, exactly where and when we need them to.
Quote:
Originally Posted by JimP
...to get darker blacks in a dark scenes, the iris will close giving you a darker black...The problem is when the dynamic iris sees a dark scene, it will darken the picture... |
This looks like where the confusion lies. For a D-iris system to work, it will allow full light output for the brightest scenes, and lowest light output for the darkest scenes, and correspondingly more for mid-bright scenes, and not change the actual black levels themselves at all, just the ABSOLUTE black level. That will lower the absolute black only when needed, and allow full brightness during bright scenes when the absolute black level is less objectionable and less noticeable.
And it may not darken the "picture" at all. What changes is the amount of light that illuminates the inside of the cabinet. It is light leak from that lamp that is causing the problem of higher absolute black in the first place. Placing an iris in front of it will drop the light LEAK IN THE CABINET and lower the absolute black dynamically dependent on content at any given second.
True, this will also lower the light that is directed through the light engine, but if the S/N levels used there can support raising the video level correspondingly high enough to offset the lowered light input during the moments when the iris is closed down, the resultant output video level itself should not vary much, if at all. It's all about balancing the RATIO of light leak inside the cabinet with the actual output video level delivered directly through the light engine. You manipulate both by lowering both for dark scenes, and then change the ratio for dark scenes by dynamically raising the video level back up to compensate, sort of like the way analog audio compressors work. IOW, the control signal voltage that closes down the iris has a correspondingly opposite control signal voltage that raises the video level back to normal output levels, and that is after the fact and separate from the manipulation of absolute black, so does not raise it back up.
The only trade-off should be noise performance in the blacks (noise levels will increase as the D-iris darkens the light output while video levels are amplified to keep the actual output levels normalized). If they can keep that low, then it will work effectively.
