Date Visited: October 9, 1991
Report Author: W.E. Glenn
HDTEC (High Definition Television Engineering Corp.) is a consortium of Japan Key Technology Center (which is sponsored by MITI and the Ministry of Posts and Telecommunications) and several companies--primarily NHK, Seiko-Epson and NEC.
While HDTEC has a charter that covers all aspects of HDTV, its main focus seems to be the development of a consumer HDT light-valve projector using AMLCD panels and a special back projection screen.
Mr. Hirashima was developing PDP displays at NHK before being assigned to work with HDTEC. The staff at HDTEC normally spend about half of their time at HDTEC and half at their parent company.
Mr. Hirashima feels that for large hang-on-the-wall panels, both LCD and PDP are promising. But PDP is under a handicap that only about 1/20 as many people are working on this technology as on LCD panels. Their major problem is electrical-to-optical inefficiency.
GTC is working on developing large direct-view LCD panels. Mr. Hirashima does not feel that this will be possible within this century. He feels that consumer displays must sell for less than 500,000 yen to establish a consumer market. He thinks that within 5-10 years, back-projection AMLCD light valves can meet this objective.
The light-modulating element in the high-definition projector is an AMLCD panel designed by HDTEC and fabricated by Seiko-Epson. A description of this device is provided in reference 3.
The driver uses poly-Si TFTs on a quartz substrate. Polysilicon has several advantages; it has high enough mobility to make it possible to fabricate the addressing drivers on the module. A self-alignment design reduces the problems of registration between successive mock exposures. The insensitivity of polysilicon TFTs to light exposure eliminates the need for a light shield over the transistor, and its high on current reduces the size of the transistor and thus improves the aperture size of the pixels.
The design uses two drivers per pixel with redundant row and column drive lines. This improves yield since drive line breaks or open transistors have a redundant back-up. In addition to redundancy, yield is improved by repairing defects. Shorts are opened with a laser. Opens are repaired by opening holes through to the poly and depositing metal from a metalorganic gas between the holes.
In the design, row lines are poly and column lines are aluminum. The column driver is a simple bidirectional switching transistor.
In order to reduce the clock speed requirements, six sectors of columns are addressed in parallel. These sectors do not match exactly in transfer characteristic. Also a defect in one of the redundant drives results in some nonuniformity. In order to produce a uniform field, each pixel has a correction for block level, gamma, and contrast. These corrections are stored in a ROM and applied to the input signal.
The panel is scanned with a progressive scan at 60 FPS. Interlaced scan signal inputs are converted to progressive scan using line interpolation and time-base correction to derive all of the active lines from the current field of information.
The optical design of the projector is described in reference 4. The light source is a 250- watt metal halide arc with line spectra at the primary colors. It unfortunately also has a 580-cm mercury line that tends to desaturate both red and green.
Dichroic mirrors are used to separate and recombine the primary colors. Since two dichroic mirrors are required between the panel and first element of the projection lens, a rather long back-focus is required for the lens.
The panel itself has 960 x 1439 pixels with an aperture of 30% and a diagonal of 4.55 inches. The contrast ratio is 70:1. The optics in the cabinet are folded to give a 55" diagonal image with a cabinet depth of 55 cm. The optical throw distance from lens to screen was 1.5 meters. The optical efficiency was somewhat less than one lumen per watt. The exact number was not known because of severe shading in the image. The corner intensity was about 30% of the intensity in the center.
The present design uses air-cooled panels. When asked what limited the light output if higher power light sources were used, the answer was not known. However, it seemed that increased leakage from panel heating would be the limit if significantly more light flux was used to illuminate the panels. It was felt that about 70 degrees centigrade would be the upper limit that could be tolerated using larger pixel storage capacitors.
The projector uses a special back-projection screen (described in reference 5) to provide a high-contrast image in the presence of ambient light. The screen uses a fresnel lens to provide illumination uniformity, lenticular lenses to give a wider horizontal viewing angle, and a diffusing plate. To minimize moire patterns between the pixel structure and the lenticular plate, a pitch is used with the lenticles that has 3.5 lenticles per projected pixel. The half-integer number was felt to be important to minimize moire.