| The Director's Chair November 2001
An
Interview with Ian McMurray of Texas Instruments (European Marketing Manager for the
DLP™ Division)
Home Theater & Sound: Can
you give us a layman’s description of DLP™ technology?
Ian McMurray: DLP™ technology is
based on a revolutionary semiconductor, the Digital Micromirror Device (DMD), invented by
Dr. Larry Hornbeck of Texas Instruments in 1987, which uses microscopic mirrors that tilt
back and forth to act as light switches. Electrodes under opposite corners of the mirror
cause the tilting action. The basic principle is illustrated in Figure 1. The DMD as
fabricated is much more sophisticated and complex (see Figure 2).
Figure 1: Schematic of DMD principle of operation |
Figure 2: Schematic of DMD as fabricated |
The hinges are hidden underneath the surface
of the mirrors. Each square aluminum mirror measures 14 microns across -- around 1/5 the
width of a human hair -- and is equivalent to one pixel in the projected image. It is
capable of tilting ten degrees in either direction in response to the state of the
electrodes beneath two of its corners. The electrodes are mounted on a memory device.
The incoming data or graphics signal is processed and
causes the electrodes under each mirror to turn on and off -- up to 50,000 times per
second. A mirror turned "on" -- toward the light from the lamp -- causes a white
pixel to be projected onto the screen. A mirror turned "off" -- away from the
lamp -- causes a black pixel to be projected (see Figure 3 below). Shades of gray are
created by varying the proportion of time each mirror is on or off.
In a simple DLP™ subsystem, color is created by
placing a rotating color filter wheel between the lamp and the mirrors (see Figure 4).
Red, green, and blue light shine in sequence on the mirrors. Varying the proportion of
time any individual mirror is switched on for each color can create up to 16 million
different shades. For example, a purple pixel is created by having the appropriate mirror
turned on half the time when red light is shining on it, and half the time when blue light
is shining on it. The rapid switching of the mirror is integrated by the human eye into a
single, continuous tone.
Figure 3: The outer mirrors are turned "on"
-- towards the light -- creating a white pixel in the projected image. The central mirror
is turned "off," causing a dark pixel to appear. |
Figure 4: The rotating color filter is placed between
the lamp and the mirrors. |
HT&S: What
are the main differences between the abilities of the big three-chip systems and the
home-based one-chip systems? Will there be any trickle down, and if so, where?
IM: The primary differences are in
brightness, color saturation, and color reproduction. Three-chip projectors are brighter
because they have 3x the reflective area of a one-chip projector. The fact that red,
green, and blue lights are always "on" -- as opposed to being "on"
sequentially as with a one-chip system -- means that not only is brightness increased, but
also saturation. Finally, the color processing that takes place in a three-chip projector
is more sophisticated, using up to 13 bits per color rather than the eight bits per color
used by one-chip projectors, allowing a broader range of shades to be accurately
reproduced.
In general, it can be said that in our commercial
entertainment business, as in our DLP Cinema™ business, we have learned a lot about
what it takes to make great images, and much of this knowledge has been incorporated into
single DMD projectors.
HT&S: Why do DVD-based
images look so much better now than in previous generation DLP™ chips, especially in
single-chip projectors?
IM: Unsurprisingly, DLP™ technology
has evolved and developed over the last several years. TI has always taken a holistic view
of the performance of DLP™ technology, working with a variety of vendors to develop
the infrastructure necessary to enable DLP™ technology to deliver the best images you
can get. As noted below, numerous improvements have been made to the DMD in terms of
contrast performance. The way in which the DMD is driven has also been enhanced. The
surrounding electronics have been further developed, and new algorithms have been designed
to improve performance in a range of areas. The color filter wheel’s effective speed
is now 5-6x the speed of the original color wheels, significantly mitigating the effect of
some artifacts, while a range of possible color filter wheel designs are now supported,
giving manufacturers an increased number of options.
 It should also be noted that while
DLP™ technology has continued to improve, so too has the expertise and skill of the
projector manufacturers with whom we work: it is a very synergistic relationship, and
those manufacturers should take their share of the credit for the outstanding images that
projectors based on DLP™ technology are currently delivering.
HT&S: What is the exact
mechanism that tilts the mirrors? Is it safe to assume that no physical motion is taking
place in the chip due to latencies, inertia, and the frequencies involved?
IM: See above for a description of how
DLP™ technology works. The mechanism that causes the mirror tilt is that each mirror
is hinged and suspended above the surface of the underlying memory cell. Electrodes on the
surface of the memory cell, which are driven by the incoming graphics or video signal, are
turned "on" and "off." There are two electrodes for each mirror, which
are under opposite corners of the mirror. When an electrode is "on," the corner
of the mirror is attracted towards it, causing the mirror to tilt. Switching the opposing
electrode "on" causes the mirror to switch in the opposite direction. Physical
motion does not take place in the chip, but rather on the chip.
HT&S: What is the physical
size of the imaging area of current TI DLP™ chips?
IM: This varies according to resolution.
For SVGA and XGA resolution, it is currently a 0.7" diagonal. For SXGA resolution,
it’s a 0.9" diagonal. There have also been 0.9" versions of the XGA DMD and
1.1" versions of the SXGA DMD.
HT&S: Aside from color
issues, are there any special requirements for the light source used to illuminate the
DLP™ chip?
IM: The requirements have very little
difference from those for the light source for an LCD projector -- the two types use
pretty much the same lamps with the same characteristics.
HT&S: Are projectors
required to use heat-absorbing glass and/or infrared filters to remove heat from the light
before it reaches the chip? If such filters are used, are there life expectancy
concerns with these filters due to the temperatures they are likely to operate at?
IM: Both infrared and ultraviolet light
are filtered from the visible light before the light from the lamp strikes the surface of
the DMD. The heat generated by this absorption is not unduly significant in projector
design, nor are there any life expectancy issues that we are aware of.
HT&S: What techniques are
employed to make blacks as black as possible? Are there things that can be done
optically or on the chip to make further black-level improvements as DLP™ technology
matures?
IM: The primary technique used is to
control what would otherwise be stray light within the projector -- light that would
otherwise tend to wash out blacks and lessen contrast. Managing stray light in a
single-chip subsystem, with its simpler optical system, is fundamentally easier to do than
in a three-panel projector with its more complex optics. Numerous developments have taken
place in the DMD architecture -- rotating of the via (the hollow post that connects the
mirror to the substructure), making the gaps between the mirrors smaller, and coating the
subsurface with a light absorbing material have all contributed to managing stray light to
the point where projectors based on DLP™ technology are now capable of contrast
ratios of better than 1000:1. We continue to improve performance in this area and already
have lab results, which indicate that contrast ratios of 2000:1 and higher are achievable.
HT&S: When will we get the
Archimedes spiral, will it be retrofittable, and what improvements will it create?
IM: Sequential Color Recapture technology
was first announced at SID in May this year. It will first appear in commercial projectors
around third quarter 2002 and will not be retrofittable to projectors already installed.
It will allow projectors to deliver up to 40% more brightness, or alternatively it will
allow projector manufacturers to increase color saturation -- or some compromise between
increased brightness and increased color saturation, depending on the manufacturer’s
requirements.
HT&S: How long do the
light bulbs last? How much do they cost? Can users try different brands as tweaks? Do they
slowly wear out or do they offer 100% performance until the day they burn-out?
IM: The characteristics of the lamps used
in projectors enabled by DLP™ technology are virtually identical to the
characteristics of lamps used by LCD projectors.
HT&S: What is the maximum
contrast ratio possible with one-chip DLP™?
IM: Currently (see above) it is 1300:1.
HT&S: Do you recommend
using a negative-contrast-value screen (like Night Hawk)?
IM: Different projectors from different
manufacturers all exhibit different performance characteristics, according to the numerous
design trade offs that can be made. As such, it is difficult to recommend any specific
screen type, as a) screens are probably ideally matched to a projector rather than a
technology, b) the conditions/environment in which projectors are used vary widely, and c)
consumers have different preferences in terms of what they look for in an image. |
