Not sure if anyone's mentioned JVC's so-called hybrid interlace/progressive technique. Essentially, because of the 'limited' matrix resolution of D-ILA panels, pixels that aren't displayed in the first half of an HDTV frame are displayed in the second.
Apple VP engineering Mark Foster outlined the D-ILA's hybrid interlace/progressive display technique here some time back. This copy of his description now appears to be missing from the archived digital projectors section.
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OK - I'll try to tackle this one again. The D-ILA uses a very unique hybrid progressive+interlaced scanning method to display 1080i signals. To understand this, it's important to understand the way that 1080i works, so a brief review is in order.
1080i HDTV signals are transmitted as 1920 pixels per line, with 1080 visible lines/frame. However, as an interlaced transmission format, all the lines on the screen aren't sent sequentially. Instead, the even scan lines are transmitted during the first 1/60th of a second, and the odd scan lines are transmitted during the next 1/60th of a second. Each of these two "fields" are collectively called a frame": together, they comprise the complete 1920x1080 image. Your eye responds slowly enough that you are able to perceive the two fields as a single picture.
This very high resolution does create an interesting challenge: very few display devices are capable of resolving all of the information present in a high-quality 1080i image. The D-ILA, as the highest resolution digital projector on the market, is no exception. Its native panel resolution of 1365x1024 would appear to show "only" about 51% of a 1080i image ((1365 * 768) / (1920 * 1080)), as compared to XGA projectors, which show 28% of a 1080i image, or SVGA projectors, which show 17%. However, the D-ILA uses a very cool trick to place even more pixels on the screen than its native panel resolution would suggest!
What the D-ILA does is to create its own type of interlaced display which is quite different than broadcast 1080i. The first step in the process is that the D-ILA will receive a complete 1080i frame, loading all 1920x1080 pixels into its internal scaler memory. Next, the D-ILA will display as much of the information as it can during the following 1/60th of a second, placing 1365x768 pixels from the memory onto the screen. If you think about this for a moment, this means that there are still (1920 - 1365 =) 555 pixels/scan line which haven't been displayed yet, in addition to (1080 - 768=) 312 scan lines which didn't make it onto the screen. What happens in the next 1/60th of a second is what really sets the D-ILA apart for viewing HDTV!
What the projector does is to selectively replace the pixels on the screen with the pixels that haven't been displayed yet. Of the 768 scan lines on the screen, 312 of them will be completely replaced with new information (the 312 scan line that weren't displayed in the first pass), and the remaining scan lines will have 555 pixels replaced with new data. In other words, about 2 out of five scan lines are replaced, and about 2 out of 5 pixels are replaced on the remaining scan lines. The D-ILA then displays this new field, leaving the remaining 60% of its pixels unchanged.
If you reflect on this for a second, you'll realize that not quite all of the pixels from the 1080i image made it onto the screen. Specifically, for the 312 scan lines that were completely replaced, you never got a chance to see the 555 pixels that would have been replaced along those scan lines, if they had been on a non-replaced scan line. In other words, (312*555 / 1920*1080 =) 8.35% of the information never made it onto the screen, meaning that the D-ILA managed to cram 91.65% of the pixels from a 1080i image on the screen!
Note that this process is quite different than the interlacing method used in a CRT. In a CRT, interlaced scan lines are physically offset from one another, while the D-ILA actually overwrites existing pixels. Nonetheless, the effect is amazing: 1080i HDTV signals on the D-ILA are simply stunning!
As an aside, I had an interesting debate with another AVS Forum member about the advantages and disadvantages of this approach a while back. He felt that the approach used by the D-ILA was "crude", noting that the overwriting process would cause a loss of perceived information. It is true that this technique will cause you to perceive less of the image than if you had used a true interpolating 1080i scaler (so far, only the Faroudja 5000 can perform this task). However, compared to other digital projectors, which don't even attempt to utilize the additional information in a 1080i signal, the method used by the D-ILA yields a dramatic improvement in display quality! When playing with the scaler, it is easy to turn this feature off: when you do, the resulting image is far less lifelike than the method that the D-ILA normally uses. As a result, I think of the technique used by the D-ILA as a very elegant solution to the limits of the state of the art. By utilizing a unique hybrid progressive+interlaced scanning system, the D-ILA is
able to deliver a dramatic mprovement in its 1080i display. However, it is also true that once affordable interpolating 1080i scalers become available, D-ILA owners will be able to see an even better HDTV image than they do now!!!
Cheers!
MarkF
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--John
[This message has been edited by John Mason (edited 09-17-2001).]