Technology Corner: Randy Hoffner

Achieving Full-Res HD with the 2K Projector

One of the nuggets of accepted wisdom in the early days of HDTV was there were few, if any, devices available that could display the full resolution of a 1920 x 1080 (either I or P) HD source. This nugget was true then, but, like many truths of those days, it is not true today.

The 73-pound Texas Instruments DLP cinema projector head.

Several types of displays that may be purchased today have this capability, including CRTs and LCDs. It is ironic that until late 2003, the advanced displays used for the demanding digital cinema application did not have such resolution capability.

Until that time, the DLP (micromirror) digital cinema display had a pixel count of 1280 x 1020. Having said that, we do not recall any complaints about the resolution of digital cinema, even though the screens used to view it can get very large. It is also noteworthy that 1280 x 1020 represents an aspect ratio of about 1.26-far closer to 4:3 than to 16:9. This means that for most cinematic features made in recent decades, produced with the intention to be projected at an aspect ratio of 1.85-and for HDTV, with its 1.78 aspect ratio-some of the display's inherent vertical resolution capability was lost. When 'Scope features are projected, they use even less of the total vertical line count.

MICROMIRRORS

Reviewing, we know that DLP projectors use semiconductor devices whose pixels are micromirrors-tiny, hinged mirrors that may be dynamically toggled between two positions. In one of these positions, a mirror reflects light from an external source so that it passes through a lens assembly that collects it, focuses it and projects it onto a display screen.

In the other position, the mirror reflects the light into a light-absorbing medium. With respect to the display screen, the light from a mirror is either on or off, making such a projection device binary in nature. Because this binary mirror can only assume one of two positions, it is incapable of generating an analog grayscale, so a grayscale is digitally synthesized by vibrating the mirror on and off at a frequency many times higher than the frame rate of the projected video material.

In this way, the duty cycle, or the ontime of the projected light beam per unit of time, may be controlled, which is perceived by the human eye as a grayscale.

One of the side effects of this high-frequency switching mode is the threshold of perception of large-area flicker is greatly exceeded, so 24fps material may be projected at its native format, rather than each frame being double-shuttered, as must be done when 24 fps film is projected.

ENTER THE BLACK CHIP

Late last year, Texas Instruments, the developer and sole manufacturer of DLP micromirror display chips, introduced the 2K "black chip" for digital cinema projection, and several companies are licensed to manufacture projectors using them. The 2K chip, as its name implies, has 2,048 horizontal mirrors and 1,080 vertical rows or lines, which permits mapping 1920 x 1080 HD images directly onto the pixel array.

(We note that a few extra horizontal pixels in a line are not used.) This makes these chips ideal for displaying 1920 x 1080 x 24p video and, as previously mentioned, because the mirrors in DLP chips are strobed at a high frequency, such images may be projected at true 24 fps.

Because these devices are designed for professional use, they are used in a three-chip configuration-one each for red, green and blue light, rather than in a single-chip configuration with a spinningcolor wheel. The three-chip configuration, which reduces the possibility of crosstalk between colors, plus the "black chip" enhancements (not new with the 2K chip), improve the contrast ratio that may be achieved with these devices to a claimed 1500:1.

One of the inherent problems with DLP projection engines is a difficulty in achieving true blacks, a problem caused by stray light leakage within the semiconductor device. The black chips address the internal light leakage problem, and this, with the three-chip configuration, produces a much better black than may be achieved otherwise.

CLEANER PICTURES

DLP projectors, like all other discretepixel devices, must be driven with progressively scanned signals. This fits well with using them to project 24p video images. Also, the combination of high resolution and progressive scanning, along with the freedom from the image bounce-and-weave that is introduced by even the best film projection systems, makes such images stunning, even at very large screen sizes.

It is, of course, possible to use them to project 1080i material, but 1080i material must be de-interlaced before being used to drive the projection engine. Interlace artifacts are, of course, not reversible, so they will still be present after de-interlacing and they will not be subtle on a 25- or 30-foot-wide screen.

When viewing 24p images sourced either from film or 24p video, it is quite apparent that the 1920 x 1080 x 24p scanning format is well able to convey extremely high-quality images. We have reached the point at which full-resolution HDTV signals may indeed be displayed, and at a large size. At the outset of HDTV broadcasting in the United States, one HD pioneer said that the HDTV we are seeing now is the worst HDTV we will ever see. That observation has certainly been borne out.

Randy Hoffner is manager of technology and strategic planning at ABC, New York, N.Y. The views expressed in his column are his own and not necessarily those of ABC. Write to him c/o TV Technology.