Tuesday, July 26, 2005

An Eye on DLP, No. 3

In An Eye on DLP, No. 2, a follow-up to (ahem) An Eye on DLP, No. 1, I tried to give some idea how front projectors and RPTVs that use Texas Instruments' Digital Light Processing technology produce images with (reasonably) smooth gradients. A minimum of banding and false contouring can be had with even a consumer-level display using just one digital micromirror device and one color wheel.

Even though the "bit depth" of the "grayscale resolution" is nominally just a barely sufficient 8 bits, that depth can effectively be extended to 10 or 11 bits, using tricks. An early, crude way to do that bit-depth boost is "dithering." "Dark video enhancement," involving a redesigned color wheel, now replaces dithering as a more subtle and successful method of keeping false edges out of darker portions of images.

Now I'd like to talk about another subtle and successful trick which the DLP mavens at TI have dreamed up. It's SmoothPicture, a way to get a DMD with 1080 rows and 960 (not 1920!) columns of micromirrors/pixels to display a 1920 x 1080, maximally high-definition picture that is progressively scanned — i.e., 1080p!

"A DMD that supports SmoothPicture contains an array of pixels aligned on a 45-degree angle," writes Alen Koebel in the August 2005 issue of Widescreen Review magazine. "TI calls this an offset-diamond pixel layout." The revised layout contrasts with the ordinary pixel arrangement, in which all the micromirrors' edges line up with the frame of the DMD.

With this new alignment, from each micromirror can be derived not one but two pixels on the screen. How? Koebel writes, "The incoming image is split into two subframes — the first containing half of the image’s pixels in a checkerboard pattern (odd pixels on odd scanlines and even pixels on even scanlines), and the second containing the remaining half of the pixels."

"During the first half of a video frame," Koebel continues, "the first subframe is displayed on the full [micromirror] array, addressed as 1080 rows of 960 columns. Just before the start of the second half of the video frame, a mirror in the projection path is moved [slightly] to shift the image of the DMD on the screen by exactly half a pixel horizontally. The second subframe is then displayed on the DMD during the second half of the video frame."

The illustration to the right may help in visualizing what is going on. (Click on it to see a larger version.) At the bottom is a representation of part of a mirror. This, I believe, is a separate mirror, one that is "in the projection path," not one of the micromirrors. It swivels slightly such that it forms first one pixel on the screen, and then a second, slightly offset pixel next to it.

Accordingly, the pixels are diamond-shaped, nor rectangular. It's too bad Mitsubishi owns the name DiamondVision, no?

Cheating, you say? Not true 1080p? Two points. One, there is a DLP chip that produces full-fledged, honest-to-goodness 1080p — or, actually, it does even better. It produces 2K horizontal resolution, fully 2,048 pixels across by 1,080 up and down. This digital micromirror device is called the DC2K, for "digital cinema 2K," and it's used in groups of three in (you guessed it) expensive digital-cinema projectors.

The DC2K is way too pricey for consumer-level gear, even in single-chip configurations. This is because it's physically larger than other DMDs. The larger the DMD, the harder it is to produce in quantity without flaws. The manufacturing yield is low, making the price correspondingly high.

The second point in favor of SmoothPicture is that the "slight loss of resolution, mostly in the diagonal direction," due to the overlapping pixels on the screen, "has an intended beneficial effect for which the feature is named: it hides the pixel structure."

"This is not unlike the natural filtering that occurs in a CRT projector, due to the Gaussian nature of the CRT’s flying spot," Koebel goes on to say. (By that he means that the moving electron beam in a CRT lights up a tiny circular area of phosphors: a spot. The luminance of the spot rises and then falls again in a bell-shaped ("Gaussian") pattern, proceeding from edge to edge.

So, says Koebel, "TI’s SmoothPicture is probably the closest a pixel-based display has yet come to the smooth, film-like performance of a projection CRT display."

"Comparative measurements by TI show that a 1080p SmoothPicture rear-projection display exhibits higher contrast at almost all spatial frequencies (in other words, more detail) than two popular competing LCoS rear-projection displays," writes Koebel. I'm guessing that he's referring here to something experts in imaging science call the "modulation transfer function," or MTF.

My understanding of the MTF — which is admittedly quite limited — is that it quantifies an odd characteristic of human vision: an image appears to be much sharper when it has a higher modulation (that is, when it has a higher contrast ratio). At a lower modulation/contrast ratio, the image appears less sharp — even though, technically, it has the same resolution!

So, I assume, TI's SmoothPicture display gives better contrast ratios than its LCoS competitors, and that higher modulation results in subjectively sharper pictures with subjectively more detail.

Another topic I'd like to touch on briefly is also contrast ratio-related: DynamicBlack.

DynamicBlack is TI"s name for automatic iris control. In front projectors there is often a user-adjustable circular iris in the light path which may be widened or contracted much like the iris in the eye. When you buy a front projector and screen, you have many options to choose from as to how large the screen is, how far away from the projector it is, and what the "gain" of the screen is. (Gain has to do with how much of the light from the projector is transmitted to the audience's retinas. The higher the gain, the brighter the picture.)

With all these variables, it's easy to wind up with a picture that is too bright. One possible remedy: lower the contrast control on the projector. A better remedy: close down the projector's iris somewhat.

But then dark scenes may appear washed out, lacking in detail. You'd like to open the iris all the way just for those scenes.

Under other installation conditions, you might want to do just the opposite. You'd prefer to open the iris wide for bright scenes to get the maximum dazzle effect, then close the iris as much as it can be closed to make dark scenes really dark.

This kind of thing, in fact, is what TI's DynamicBlack is designed to do, automatically.

In so doing, it increases the on-off or scene-to-scene contrast ratio of a DLP display to, says Koebel, "well in excess of 5,000:1; some [displays] may reach 10,000:1."

Not that DynamicBlack actually lowers the display's black level, the way DarkChip technology does. It accordingly has no effect on the “ANSI” contrast ratio measurement for the display, which compares light and dark areas in a single scene. (It is this single-scene or "simultaneous" contrast ratio that the modulation transfer function is concerned with — see above.) DynamicBlack merely makes transitions from high-brightness scenes to low-brighntess scenes seem more dramatic.

And that, for now, ends my investigation of the state of the DLP art. I may, however, come back to the subject in future installments. Stay tuned!

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