Rear Projection TVs

The Wobbly Bits
The 0.65" wobulation DMD used in the WD-57731 has a 960 x 1080 array of pixels instead of a full 1920 x 1080. To display a full 1920 x 1080 pixels on the screen in the 1/60th of a second allotted to each 1080 progressive frame, half of the horizontal source pixels are displayed in the first 1/120th of a second, and the remainder are interleaved with the first in the subsequent 1/120thof a second. The image retention of the eye combines the two to produce the full 1920 x 1080 visible pixels per frame on screen.

The group of pixels displayed in the subsequent 1/120th of a second must, of course, be horizontally relocated on the screen to properly fill in the gaps. But the DMD itself is not physically displaced to do this. Instead, a separate, larger, "wobulation" mirror moves to shift the reflections from the entire 960 x 1080 array into the new position every 1/120th of a second.

While wobulation may sound suspiciously like interlacing, the analogy is weak. True, the second group of pixels is interleaved with the first in much the same way as an interlaced image is formed. But there is never any motion in the image itself between the first group of pixels and the next, since each is derived from the same frame. And there is a considerable difference between the 1/30th of a second it takes to display each field in traditional interlacing and the 1/120th of a second allotted to flash each group of wobulated pixels.

Bob and Wob
A native 1080i image consists of two interlaced fields with a vertical pixel count of 540. To properly assemble these into a 1080p frame with a minimum of artifacts requires a complex procedure that takes into account the movement (if any) of the image between each field. A similar process is required to properly de-interlace 480i to 480p, but a 1080i to 1080p conversion requires many times the processing power.

While some 1080p displays use a simpler technique called "bobbing" to perform this conversion, according to Mitsubishi none of its new 2007 models take this easy road. Sets that "bob" 1080i sources to 1080p simply take each 540 vertical pixel field of a 1080i frame and line double it to produce a 1080p frame, using interpolation to generate the intermediate lines. Each of these interpolated progressive frames is then displayed sequentially. This is much easier to do than proper de-interlacing, but it limits the resolution of the displayed image to 540p, which is the vertical resolution of the fields from which it was derived. At that point it's no longer true high-definition.