LCoS Display Technology Shoot-Out
Dr. Raymond M. Soneira
President, DisplayMate
Technologies Corp.
Copyright © 1990-2006 by DisplayMate
Technologies Corporation. All Rights Reserved.
This article, or any
part thereof, may not be copied, reproduced, mirrored, distributed or
incorporated
into any other work without the prior written permission of DisplayMate
Technologies Corporation
Part B – LCoS Color and Gray-Scale Accuracy
Article Links: Overview Part A Part B Part C Part D
LCoS HDTV
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Shoot-Out
Hardware and Software Sidebar
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Introduction
This is the
second article in a four-part series examining Liquid Crystal on Silicon, LCoS, a
relatively new and obscure display technology that is now making its grand
entrance into the HDTV marketplace. Here in Part B we’ll begin with a
discussion of How We Tested and then examine the photometry and colorimetry of
all of the LCoS units in detail, which provides a quantitative assessment of
their color and gray-scale accuracy.
If
you read Part A,
then you can skip this Introduction. Already, LCoS provides the highest
resolutions, the highest non-CRT Contrast Ratios, and the most artifact-free
images of any display technology. For people that are sensitive to flicker and
eye-fatigue, LCoS operates at the highest refresh rates (120 Hz) for the
smoothest most flicker-free images. This article series will be an in-depth
examination of LCoS technology and five LCoS HDTVs, all but one of them
prototypes, in order to get an early look into this unfolding technology.
In
Part A we started
off with a description of the LCoS HDTV units that we tested, followed by an
overview of LCoS technology. Here in Part B we’ll begin with a discussion of
How We Tested and then examine the photometry and colorimetry of the units in
detail, which provides a quantitative assessment of their color and gray-scale
accuracy. In Part C we'll continue
with a revealing Test Pattern analysis, followed by a description of the
extensive Jury Panel testing and then provide individual Assessments for each
of the units, including Jury evaluations and comments. In Part D we’ll start
with an Assessment of LCoS Technology, followed by detailed technical
performance comparisons between all of the major display technologies: CRT,
LCD, Plasma, DLP, and LCoS, and we’ll finish with a discussion of the most
exciting new developments in display technology that will be the subject of
future articles in this series.
Note
that this article is the latest in a series of Display Technology Shoot-Out
articles that have covered CRT, LCD, Plasma and DLP display technologies. The
topics for the original series are: Part I: The Primary
Specs, Part II:
Gray-Scale and Color Accuracy, Part III: Display
Artifacts and Image Quality, and Part IV: Display
Technology Assessments. Online versions of these earlier articles are available
on www.displaymate.com.
When
we started planning the Shoot-Out there were just two shipping LCoS HDTVs
available, so I decided to enlarge the sample by persuading several
manufacturers to loan me their precious laboratory prototypes for the article.
The goal was to include every LCoS manufacturer that could provide a working
unit. There were only five candidates: JVC, Sony, Brillian, eLCOS and
SpatiaLight. We included units from both of the standard HDTV resolutions:
1280×720, which is roughly 1 mega-pixel, and 1920×1080, which is roughly 2
mega-pixels. They will be referred to as 720 or 1080 units throughout the
article.
Brillian, a small startup company based
in Tempe, Arizona, provided a prototype
of their 65 inch 720 unit (model 6501m, which
is now shipping) and also a prototype of their 1080 unit (model 6580i, which
is now shipping). eLCOS is another small
startup company that worked with light engine manufacturer JDS Uniphase to
deliver a 56 inch 1080 laboratory demonstration unit. JVC sent two units: the Consumer division sent
their shipping 61 inch 720 HDTV (model HD-61Z886),
and the Professional Products division sent a prototype of their 48 inch 1080
Reference Monitor (model DLA-HRM1,
which is now shipping) designed for television and movie post-production
studios. Note that the LCoS technology and devices in the two JVC units are
significantly different: the Consumer unit has a digital backplane that
controls each pixel with Pulse Width Modulation, while the Professional unit
uses analog voltage to control each pixel. SpatiaLight
was unable to deliver a prototype in time for the Shoot-Out and Sony declined
to participate. See Part
A for an in-depth description of the units and their technologies.
Caption:
The
Shoot-Out with the lights turned on. From left to right: JVC Consumer 720,
Brillian 720, JVC Professional 1080, CRT Studio Monitor, eLCOS-JDSU 1080, and
Brillian 1080 units. Photograph by David Migliori.
How We Tested
All of the
HDTVs were carefully set up side-by-side for simultaneous comparative viewing
and testing along a 35 foot wall. In addition to the five LCoS HDTVs there was
also a CRT High Definition professional broadcast studio monitor that was used
as a reference. The room was absolutely pitch black for all of the diagnostic
testing and photometric measurements, but four Ideal-Lume D6500 backlights were
used during all of the Jury Panel viewing tests (see the Shoot-Out Hardware and
Software Sidebar for information on Ideal-Lume). We used only digital video
signal sources together with an extensive all digital switching and signal
distribution system that was provided by Gefen (see the Sidebar). The
signal sources included Denon’s flagship DVD player, the DVD-5910, which we set
to produce 480p (see the Sidebar), two
digital D-VHS Players (one was provided by JVC), a Samsung SIR-T165 ATSC High
Definition Digital Broadcast Tuner, and two Windows PCs: one for running
DisplayMate diagnostic test patterns, and a second that included a large set of 720p and 1080p movie and video clips encoded
in Windows Media 9 Series that was provided by the Microsoft Digital Media
Division (see the Sidebar).
A
very important goal was to minimize the impact of the differences in front-end
signal processing between all of the HDTVs, because that would confuse the LCoS
picture quality issues that we were evaluating. (Signal processing is very
important for product reviews, but this article is a technology review.) The
first stage was to distribute identical DVI RGB digital signals to all of the
HDTVs rather than distribute HDMI YCbCr signals, which would have had to be
independently decoded by each set. This method forced each signal source to
perform all of the necessary decoding into RGB, so it was identical for all of
the units. The second stage was to use very high-quality video processors to
perform all of the necessary deinterlacing and scaling of the video signals so
that the individual sets wouldn’t have to perform this function either. For
this we had two DVDO iScan HD+ processors and two Silicon Optix prototype Niobe
Reference Design video processors with 1080i motion adaptive deinterlacing,
which were absolutely awesome. As a result we had separate 720p and 1080p feeds
for all of the HDTVs (see the Sidebar).
In
order to perform very accurate photometry and colorimetry on the units we had
two high-end Spectroradiometers from the Konica Minolta Instrument Systems
Division, the CS-1000, which we used in Parts I-IV, and the newly
introduced CS-200, which is much faster, and drastically reduced the amount of
time needed to perform all of the measurements (see the Sidebar). Finally,
we used our own advanced DisplayMate Multimedia Edition with over 500 High
Definition test patterns to perform in-depth diagnostic tests and to search for
and evaluate subtle image artifacts that might otherwise have gone undiscovered
(see the Sidebar).
All of our tests were done at the native 1920×1080 and 1280×720 resolutions of
each unit. Note that testing based on DVDs will miss most of the High
Definition display and processing artifacts. For additional information on the
testing procedures and instrumentation see the Sidebar and Part I.
All
of the HDTVs were set up and adjusted by their manufacturers, except for the
JVC Consumer unit, which was a shipping product and was set up using the
standard On Screen Menus without access to any service modes or menus. The
final adjustments and tweaking for all of the HDTVs were done during the actual
testing using the DisplayMate Set Up test pattern suite. For example, setting
the black-level accurately to digital signal level 16 is very important. So we
used a special split-screen test pattern that compared any digital signal level
to level 0. The Brightness Control, which controls the black-level, was
adjusted so that level 16 was indistinguishable from level 0, but signal level
17 was detectably brighter than level 0. Further details on the level
adjustments and set up procedures are provided below.
Black-Level
Any
ambient light that falls on a display screen spoils the picture quality by
reducing both contrast and color saturation. As a result home cinemas (and any
other application that requires high picture quality) need to carefully control
ambient light. When this is the case the ability of a display to produce a very
dark black becomes one of the most important display parameters because the
background glow from the screen can be quite noticeable in dark images and
scenes, and that glow also causes a loss of contrast and color saturation. In
Table 1 we list the Black-Level Luminance, in cd/m2, candelas per square meter (a unit
that used to be called “nits”), measured for each of
the HDTVs with the Konica Minolta Spectroradiometers. This was done using a
DisplayMate full-screen test pattern set to digital intensity level zero.
To convert to another common luminance unit, foot-Lamberts, fL, divide by 3.43.
Table 1 : Photometric
Measurements
|
Brillian 720
|
JVC Consumer
|
Brillian 1080
|
eLCOS-JDSU
|
JVC Professional
|
Black-Level Luminance
|
0.24 cd/m2
|
0.59 cd/m2
|
0.12 cd/m2
|
0.20 cd/m2
|
0.11 cd/m2
|
Color Temperature
|
6793 K
|
7881 K
|
6545 K
|
7281 K
|
7733 K
|
6577 K
|
Screen Gain
|
4.7
|
NA
|
4.9
|
4.9
|
4.0
|
1.25
|
Peak White Luminance
|
449 cd/m2
|
552 cd/m2
|
457 cd/m2
|
509 cd/m2
|
885 cd/m2
|
234 cd/m2
|
Contrast Ratio
|
1870
|
936
|
3807
|
4239
|
4425
|
2132
|
The
JVC Consumer unit had the highest black-level, but it was actually very easy to
visually see the differences in black-levels between all of the units. The
black-levels can be easily reduced by simply using a less bright projection
lamp. The Screen Gain listed in Table 1 can also be lowered. Manufacturers
seldom take these approaches because it also reduces the peak brightness of the
display, which is often perceived by consumers as more important than the
black-level (see below). A better way to reduce the black-level is to use an
adjustable iris aperture (manual or automatic) that varies both the black-level
and peak brightness at the same time. Generally, with low ambient lighting the
black-level is very important while the peak brightness is not very important.
With high ambient lighting the reverse is true. A manual iris adjustment allows
a viewer to find the best compromise for their particular situation and
preferences. None of these units had such a control, although they all would
benefit tremendously from one. Note that HDTVs with an iris have variable
black-levels, so their black-level specifications may be misleading. Use
caution when comparing their published black-levels (and resulting Contrast
Ratios) to the values measured here.
Peak Brightness
When
it comes to brightness many people feel that brighter is better (more on that
below). In fact, all of these HDTVs have more than enough brightness for most
viewing environments, with the possible exception of a very sunny room. But in
order to get good picture quality the ambient room light needs to be
controlled, and for a home theater it should be very carefully controlled in
order to obtain the excellent contrast and color and gray-scale accuracy that
these sets provide. So for a well designed home theater these sets are all way
too bright. Unfortunately, when you turn down the brightness (with the Contrast
Control) the image quality also decreases because the digital range used for
the producing the image also decreases and that increases noise, false
contouring and other digital artifacts. To make matters worse, the black-level
remains constant. So if we were grading on brightness, the brightest set would
get the lowest score (see below).
None-the-less,
for both manufacturer’s specifications and showroom sales peak brightness is
unfortunately the most important figure after price. One way to increase it is
to raise the Screen Gain, which increases brightness by concentrating the light
in the direction of the audience, but this also reduces the viewing angles and
often introduces speckle, hot spots, and other artifacts into the picture. The
easiest way to increase screen brightness is simply to use as much of the
projection lamp’s light as possible. This generally gives the white light from
the unit a greenish or bluish tint and many HDTVs come preset that way in order
to make them look as bright as possible. But for accurate color reproduction
the color of white needs to match the standard specified for TVs and HDTVs,
which is known as D6500 or D65 and corresponds to the color
of natural daylight at noon. (The D is for daylight and it has a blue sky
component added to the spectral light distribution of a laboratory black-body
raised to an absolute temperature of 6500 degrees Kelvin.) Temperatures greater
than 6500 K are too blue and most TVs and HDTVs come from the factory set from
8,000 K to 12,000 K with a bluish tint (like cool-white fluorescent bulbs).
Lowering the Color Temperature to match the D6500 standard almost always
results in a significant decrease in brightness (luminance) because some
portion of the lamp’s luminance must be thrown away to make the adjustment.
Color
Temperature alone cannot uniquely specify the color of white because it’s a
single one-dimensional parameter, while color is two-dimensional, so it
requires two parameters (see below). Colors that lie close to, but not exactly
on the black-body curve can still be assigned a Color Temperature value that
produces the closest color match to a black-body. This is referred to as a
Correlated Color Temperature. Table 1 shows the
Correlated Color Temperature and Peak White Luminance measured for each of the
units using the Konica Minolta Spectroradiometers. The JVC Consumer and eLCOS
units were significantly hotter than the D6500 standard and they were also the
two brightest units. In fact, they are among the very brightest monitors I have
ever measured. (These calibrated luminance measurements should not be directly
compared to most manufacturer’s published specifications because they almost
always set every control to maximum in order to print the largest possible
brightness value, but that yields unacceptable image quality. Factors of two
increases in the published brightness specification are possible this way. See Parts I and IV.)
When
Brillian sent an engineer to do a factory level recalibration of their 1080
unit we were able to examine the effect of shifting the Color Temperature on
peak brightness. (To do this optimally requires that the device profiles be
remeasured from scratch using the low-level panel driver electronics rather
than using the user or service controls, which operate using the front-end
electronics.) Changing the Color Temperature from 6545 K to 7281 K increased
the peak white luminance from 457 to 509 cd/m2. Going up to 8000 K
or beyond would result in much larger increases in brightness.
Some
TVs have two or more Color Temperature settings, often marked as High and Low
or Warm and Cool. The High or Cool setting generally produces 9300 K or above,
while the Low or Warm setting should get you much closer to D6500. I was
intrigued when I saw that the JVC Consumer unit has a dedicated remote control
button that activates their “TheaterPro D6500K” settings. The manual explains
it this way: “TheaterPro D6500K Color Temperature technology function makes
sure that the video you watch is set to the standard Color Temperature.” The
product brochure further explains why “D6500K” is so important in much greater
detail. Unfortunately, both of the Konica Minolta Spectroradiometers measured
7881 K, which is very far from the promised D6500. The manual and brochure
departments should talk to the factory more often…
The Retail
Brightness Factor
Why
are these TVs so bright? Why are the manufacturers putting in bigger lamps and
special higher gain screens to make these already way too bright TVs even
brighter? I know they’ve all read the earlier articles, so why are they doing this?
Sadly, the reason is that in a retail setting brightness is frequently a
deciding sales factor. So, like it or not, the manufacturers have to build
their sets to be as bright as possible in order to be commercially successful.
I
spoke at length about this with Steven Lopez, manager of the Nashua New
Hampshire store of Cambridge SoundWorks (a specialty AV chain based in New
England). Steven expanded upon what the manufacturers had already told me, “The
unfortunate truth to selling TVs on the sales floor is that bright sets attract
the mass consumer. The most accurate sets may not be the most appealing. The
brightest units simply make the other nearby sets look anemic and old, kind of
like the tired CRT they are replacing. Often that’s enough to tip the scales in
a sale, regardless of the price range involved.” He also explained that in
order to give all of the sets a fair chance they need to be carefully arranged
on the sales floor so as to minimize the impact of this brightness factor. (As
an interesting historical aside, Cambridge SoundWorks is one of the companies
founded by legendary speaker designer Henry Kloss, who also founded Acoustic
Research, KLH and Advent. In 1972 he produced the world's first home projection
television, the 3-CRT Advent VideoBeam 1000 with a 7-foot curved front
projection screen. He actually left KLH and started Advent in order to develop
the projector, but when he ran out of money he started manufacturing Advent
speakers in order to finance the projector’s development. He later received an
Emmy Award for that revolutionary product.)
TVs
that are optimized for the showroom floor at the factory will not look their
best in a home theater. It’s another example of the tail wagging the dog. Since
these LCoS HDTVs are internally processor and data table driven, a perfect
solution would be to have three completely independent factory digital
calibrations stored in the already available memory within a set - each with a
different transfer function, gamma, screen uniformity calibration table, level
of gray-scale compression, and Color Temperature. Note that merely adjusting
the normal user and service controls cannot accomplish the same thing (because
they operate using the front-end electronics). One set of values would be
optimized for the showroom floor, one for bright ambient light home theaters
(day use), and a third for low ambient light home theaters (night use). Another
great option would be to offer more than one screen: a higher gain version for
locations that need high brightness, such as retail stores and sunny living
rooms, and a lower gain screen that will produce a dimmer but higher quality
image with fewer artifacts.
Contrast Ratio
Since
the black-level and peak brightness are interdependent for any given display, a
really important figure of merit is the ratio of these two values. This is
called the Contrast Ratio of the display and it tells us the maximum range of brightness that the
display can produce – and the higher the better. (It’s
also frequently called the full-field, full on/off, or sequential contrast but
a better term would be Dynamic Range because, in my opinion, the term contrast should be
reserved for measurements on a single image, not on different screens). A high
Contrast Ratio is especially important in imaging and home theater
applications, where, for example, bright/day scenes and dark/night scenes both
need to be rendered accurately. The higher the Contrast Ratio the better the
display will be able to reproduce wide differences in scene brightness. More
importantly, for a given peak brightness a higher Contrast Ratio will produce a
darker black-level.
The last
row of Table 1 lists the Contrast Ratio for each unit. The values range from
936 [to 1] for the JVC Consumer unit, which is very good for its price class,
up to a very impressive 4425 for the eLCOS unit. These are the highest values
I’ve ever measured for a non-CRT display. With an iris aperture (below) it’s
quite likely that the measured values would have been even higher. The JVC
Professional unit was a prototype. When it was
returned to JVC, they confirmed the Contrast Ratio measurement in Table 1. When
they refurbished the unit and put in a brand new projection lamp the Contrast
Ratio increased to 2606. The final production version should have a Contrast
Ratio of 3000.
Although
you’ll see higher values published in the spec sheets for many HDTVs, they are
generally for overdriven displays pushed to the extreme and not for the
calibrated and accurate gray-scales, gamma functions and specific Color
Temperatures measured here. As an example, the “wide-open” Contrast Ratio
values for the Brillian and eLCOS units both exceed 5500 (without an iris
aperture). These Contrast Ratio values actually indicate that the manufacturers
are being incredibly conservative with the Contrast Ratio specifications they
provided for the LCoS Panels by themselves in Table 2 of Part A. The values
listed there should be roughly 50 to 100 percent larger than the full-field
Contrast Ratios that we measured at the screen (because the projection optics
reduces the panel’s contrast).
Another
element that can (artificially) jack up the Contrast Ratio is a dynamic iris
that is wide open for the Peak White measurement and fully closed for the
Black-Level measurement. That’s how some HDTVs are able to advertise Contrast
Ratios of 10,000 or more, but those figures can be very misleading and should
not be compared to the values measured here.
Iris Control
If
the brightness of an HDTV is too high, it should be lowered, otherwise viewers
will likely suffer eyestrain and possibly even headaches. If sunlight is a
factor then you may need different settings for daytime and nighttime. The
traditional way to lower the brightness of a TV is with the Contrast Control,
which varies the level of the video signal. This works quite well for CRTs,
which are pure analog devices with extremely dark black-levels, but it works
rather poorly for all of the other display technologies because their
black-levels are much higher and they all use digital signal processing
internally (whether or not the input signal is analog or digital). There are
two good methods for lowering brightness: one is to supply less power to the
projection lamp, a second and even better one is to use an iris aperture
somewhere in the optical path to cut down the light throughput. (Yet another
method, which is equivalent to lowering the lamp power, and that will be
available with the JVC Professional unit, is the use of variable crossed
polarizing filters to lower the effective lamp brightness. This has all of the
advantages of an iris aperture except that the Contrast Ratio will remain
constant, rather than increase, as discussed below.)
The
Peak Brightness listed in Table 1 was set to digital intensity level 255 for
all of the Shoot-Out tests. While the nominal peak video level is digital level
235 (Reference White), source
material is allowed to stray into the headroom area of 236 to 254 for very
bright highlights (technically, level 255 is reserved and not allowed for
digital video, so it’s processed as level 254). Most HDTVs will be set up at
the factory to reach Peak White at level 235 in order to maximize picture
brightness, but for accurate image reproduction the gray-scale needs to be
extended so that Peak White is reached at level 255. This will reduce picture
brightness (luminance) by 17% (assuming a Gamma of 2.2, see below). Here we’re
considering the reverse situation: the luminance at Reference White, level 235,
in Table 2 is 17% lower than Peak White, level 255, in Table 1. For a
moderately lit room, there are a number of industry recommendations for a
Reference White Luminance – they range from 80 to 170 cd/m2, which
is still a lot dimmer than these HDTVs. To be extra conservative in the
discussion that follows I have picked the largest of these values. Table 2
compares the results between adjusting the luminance to 170 cd/m2 using
an Iris and a Contrast Control.
Table 2 : Adjusting the
Luminance using an Iris or Contrast Control
|
Brillian 720
|
JVC Consumer
|
Brillian 1080
|
eLCOS-JDSU
|
JVC Professional
|
Luminance
at Level 235 for
Reference White
|
371 cd/m2
|
456 cd/m2
|
377 cd/m2
|
420 cd/m2
|
730 cd/m2
|
193 cd/m2
|
Black-Level for
170 cd/m2 if
using Contrast Control
|
0.24 cd/m2
|
0.59 cd/m2
|
0.12 cd/m2
|
0.12 cd/m2
|
0.20 cd/m2
|
0.11 cd/m2
|
Black-Level for
170 cd/m2 if
using an Iris
|
< 0.11 cd/m2
|
< 0.22 cd/m2
|
< 0.05 cd/m2
|
< 0.05 cd/m2
|
< 0.05 cd/m2
|
< 0.10 cd/m2
|
Contrast Ratio for
170 cd/m2 if
using
an Iris
|
> 1870
|
> 936
|
> 3807
|
> 4239
|
> 4425
|
> 2132
|
Contrast Ratio for
170 cd/m2 if
using
Contrast Control
|
858
|
349
|
1717
|
1717
|
1030
|
1873
|
Digital Levels for
170 cd/m2 if
using
Contrast Control
|
70 percent
|
64 percent
|
70 percent
|
66 percent
|
52 percent
|
94 percent
|
The
advantage of an Iris is that it lowers the black-level along with Peak or
Reference White, while the black-level remains constant when using the Contrast
Control (2nd and 3rd rows). As a result, the hard won
Contrast Ratio values all fall substantially from the Table 1 values when using
a Contrast Control (5th row). When using an Iris the black-level
actually falls faster than the Peak or Reference White luminance due to
improvements in the optical path, so the Contrast Ratio actually goes up when
using an Iris (4th row). In many cases it will go up by 50 or even
100 percent or more, so the effect can be quite substantial. This tradeoff of
brightness for Contrast Ratio is used in many high-end projectors.
Another
major problem with using the Contrast Control to decrease brightness is that
the number of available digital intensity levels decreases as well. With fewer
levels, false contouring and other digital artifacts increase. This effect is
shown in the last row of Table 2, which shows the percentage of available
digital levels that are available when using the Contrast Control to lower the
brightness. If the brightness is lowered further these effects become even
larger.
Some HDTVs
have a dynamic iris that automatically adjusts the iris aperture after
analyzing the picture content. The iris closes down for dark scenes, which
improves the black-level where it matters the most, and opens back up again for
bright scenes. However, a dynamic iris will introduce gray-scale artifacts
(which are image errors) because virtually all dark scenes have some bright
content that will be attenuated by the iris, so a manual iris is definitely
preferred when an accurate gray-scale that is free of iris artifacts is
desired. Also, some dynamic irises operate relatively slowly, taking half a
second or more to make their adjustment, while others operate within a frame
time. In most cases it’s possible to disable a dynamic iris and use a manual
iris control only. In either case, the Black-Level and Contrast Ratio (either
measured or listed in spec sheets) will be inflated by the iris aperture and
should not be compared to the true Contrast Ratio for the display.
Display
Contrast
Display
Contrast determines how well the optics and screen preserve the differences in
brightness that occur within an image. Internal reflections can cause light
from the bright areas of the image to bleed and contaminate the dark areas so
they don’t get as dark as they should be. This reduces the contrast within any
given image, but its actual impact depends on the structure of the particular
image being shown. A standard way to measure Display Contrast is to use a black
and white checkerboard test pattern and measure the luminance at the center of
the white blocks and then the black blocks and finally compute their ratio.
These values are always less than the Contrast Ratio values because its white
and black levels are measured on separate screens, so there isn’t any bleed
between them. Generally, the smaller the blocks the greater the bleed. We’ve
done this for a 4×4 checkerboard, which is a standard pattern, and then for a
much finer 9×9 checkerboard to see how much more the contrast falls when the
blocks are reduced by an additional factor of 5 in area. Note that this
measurement is tricky because a similar contamination effect (called Veiling
Glare) also affects the instrument used in the measurements. We used heavy
black felt masks to eliminate this common source of error in checkerboard
contrast measurements. The results are listed in Table 3.
Table 3 : Checkerboard
Contrast and Gamma
|
Brillian 720
|
JVC Consumer
|
Brillian 1080
|
eLCOS-JDSU
|
JVC Professional
|
4 × 4 Checkerboard
|
162
|
107
|
204
|
202
|
63
|
9 × 9 Checkerboard
|
140
|
82
|
140
|
130
|
47
|
Gamma
|
2.13
|
2.06
|
2.18
|
2.18
|
2.19
|
The values are all
substantially less than the Contrast Ratio measurements listed in Table 1 due
to internal reflections within the light engine optical components, the cabinet
interior and the screen.
The interpretation of these
results is not straightforward. In principle, there is no question that the
higher the Checkerboard Contrast the better, but the JVC Professional unit got
the lowest values by far yet it consistently received the highest picture
quality scores in the Jury Panel evaluations (Part C). The same was
true in Part I,
where the CRT also had by far the lowest Checkerboard Contrast but the highest
picture quality.
Clearly the laboratory Checkerboard Contrast data do not
have a direct correspondence with the eye’s perceived picture quality. There are two reasons for this: (1) the optical system of
the eye also suffers from similar optical aberrations because of the tissue
based lens and the aqueous humor and vitreous humor that fill the interior of
the eye, which further degrades the image on the retina, and (2) the eye is
part of a visual processing system that is designed to extract as much useful
visual information from an image as possible, and not an instrument for
comparing quantitative luminance values in an image. So the eye and brain work
to compensate for any image degradation on the retina in order to extract as
much visual information as possible from an image. What you actually see in
your mind is the brain processed image, not the raw data.
Both of these effects work to establish a threshold level
interpretation for Checkerboard Contrast. Values less than the threshold are
detected as a degraded image by the eye, and values above the threshold all
effectively appear to be equivalent. So, there is no question that if the
Checkerboard Contrast falls too low the eye will at some point take full notice
of the effect – we just didn’t encounter it in any of the testing for this
7-part Shoot-Out series. These issues are further discussed under Display
Contrast in Part I.
The
only time that I could readily see that the JVC Professional unit had the
lowest Checkerboard Contrast was during movie credits that had just a couple of
lines of white text on an otherwise completely black screen. The area between
the lines of text wasn’t as dark as in the other units, but you really needed
to be looking for this effect to notice it. In a typical visually complex
cinema image this effect would be even less noticeable. It’s important to note
that this unit was a prototype. When the unit was returned to JVC, they
confirmed the 4×4 Checkerboard measurement in Table 3. When they refurbished
the unit and put in a brand new projection lamp the 4×4 Checkerboard Contrast
increased to 113. It should climb even more in the final production units (the
current estimate is about 140) because the cabinet interior will have a much
blacker lining and it will have an improved screen.
Note that Contrast Ratio and
Checkerboard Contrast are two entirely distinct display specifications. Each
tells us something important and different about the display, and both are very
useful. However, based on the above discussion Contrast Ratio is much more
important than Checkerboard Contrast for homer theater applications. On the
other hand, for computer images with fine graphics and text, the reverse is
true (see Part I).
Gray-Scale
Accuracy and Gamma
We
have been discussing the extremes of display brightness: the black-level and
peak intensity. Here
we’re going to carefully examine all of the intensities in between, which is
referred to as the display’s Gray-Scale or its Gamma Curve (technically the Transfer
Function). This is the signature of a display; it’s what gives the display its
own unique look and performance characteristics. The functional form of the
gray-scale has a major effect on the display’s color and gray-scale accuracy. A
non-standard Transfer Function will introduce errors in brightness, contrast,
hue and color saturation in an image (see Part II for a
quantitative discussion). While every display technology has its own native
gray-scale, signal processing electronics within the display is needed to
produce a gray-scale that matches that of reference CRT monitor, which is an
industry standard used in television and movie post-production. An HDTV that
accurately follows this standard will produce an image that is identical to
that seen in a studio by the creators of professionally produced content –
generally the director and cinematographer or videographer.
It turns
out that the gray-scale is not linear as most people presume, but is instead
logarithmic (mathematically it’s actually called a power-law, which behaves
linearly on logarithmic scales) because that’s how standard CRTs behave, and
also because that corresponds well with the eye’s own logarithmic response
(which is also a power-law). If a display has the correct behavior then the
gray-scale will appear as a straight line on a graph where both scales are
logarithmic, which is referred to as a “log-log” plot. All of the display
controls had to be adjusted very carefully for these measurements, especially
the black-level. Figure 1 shows the Screen Brightness (luminance) in cd/m2
for each of the displays as a function of the signal intensity level expressed
as a percentage of maximum. The value for 100 percent represents Reference
White, digital signal intensity level 235. The measurements were made with the
Konica Minolta CS-200 Spectroradiometer (because it’s very fast at low light
levels) and a DisplayMate Window Test Pattern. The open symbols plotted on the
graph are the measured data points. Note that the
values for the Brillian 720 and the CRT were shifted downward by 25 percent for
clarity.
FIGURE 1
Caption: Figure 1. The Gamma
Curve or Transfer Function for each of the HDTVs. Note that the values for the
Brillian 720 and the CRT were shifted downward by 25 percent for clarity.
From
Figure 1 you can see that all of the LCoS HDTVs (as well as the reference CRT
monitor) have a Gamma Curve (Transfer Function) very close to the ideal – a
straight line on a log-log plot. This is a dramatic improvement over what we’ve
seen in the recent past (compare with Figure 1 in Part II) and is
another reason why these LCoS HDTVs are delivering excellent picture quality.
(See Part II for
an in-depth discussion of Gamma.) The slope of the Transfer Function in the
log-log plot is known as the Gamma of the display. If the line is not perfectly
straight then the Gamma varies with intensity, which is undesirable. The
industry standard, which is based on CRT technology, is a value of 2.20. Table
3 lists the Gamma values measured for each of the units from a fit to the data
in the most important region of 100 to 30 percent signal intensity. They are
all very close to the 2.20 standard, except the JVC Consumer unit, which is a
bit too low, although its curve steepens somewhat at the dark end.
Color Accuracy
The color
coordinates of the red, green and blue primary colors in each display defines
the gamut of colors that it can reproduce. All of the colors that the display
produces are combinations of the primary colors that it uses. In principle, the
wider the color gamut the better, and many manufacturers are now prominently
advertising their extended color gamuts. However, variations in the primaries
change all of the displayed colors in an image. So, in practice, it’s much more
important to use standard primaries in order to maintain the color accuracy of
the reproduced images. Wider color gamuts decrease color accuracy and should be
avoided except in specialized imaging applications – for example in medical or
military applications. If you do get an HDTV with an extended color gamut it
will be necessary to reduce the color gamut back to the standard values by
using the color Saturation Control or other color management functions within
the unit, so it’s a pretty useless feature in HDTVs. In fact, it’s undesirable,
and is just being used as a marketing gimmick. Look for an HDTV that advertises
the Rec.709 and Rec. 601 standard primary colors and avoid, or be very
suspicious of any HDTV that brags about its extended color gamut, particularly
a comparison to the original large NTSC color space, which is now non-standard,
obsolete, and will likely lead to on-screen color errors.
You will
occasionally hear that extended color gamuts are useful for high ambient
lighting conditions, which wash out the on-screen colors (in addition to the
gray-scale). That won’t help because it would require a special correction that
depends on brightness (darker colors require a much greater correction than
brighter colors). On the other hand, some source material may have insufficient
color saturation, so it might occasionally be desirable to turn up the color
saturation. If that’s the case then it’s very important that the primary color
triangle for the extended color gamut be an exact proportional enlargement of
the standard gamut color triangle (see below).
For High
Definition content the standard colors are specified in the ITU-R BT.709
standard (Rec.709). We measured the primary colors with the Konica Minolta
Spectroradiometers and have plotted them on a 1976 CIE Uniform Chromaticity
Scale diagram with u’,v’ coordinates in Figure 2, together with the Rec.709
primary colors, which are marked by the black triangle. (See Part II for an
in-depth discussion of this topic.)
FIGURE 2
Caption: Figure 2. The 1976 CIE
Uniform Chromaticity Diagram for the High Definition Primaries for the HDTVs
together with the Rec.709
standard (in black). Note that the color management for the Brillian 1080 unit
(blue lines) had not yet been completed.
From Figure
2 you can see that almost all of the primaries are just a bit more saturated
than the Rec.709 standard, which is fine because a small adjustment of the
color Saturation Control will bring them into good agreement with the standard.
This is also a dramatic improvement over what we’ve
seen in the recent past (compare with Figures 4 and 5 in Part II) and is yet
another reason why these LCoS HDTVs are delivering excellent picture quality. The
red primary for the Brillian 1080 unit was much too red. This was quite evident
during the visual testing (Part C) and we
partially compensated for it by turning down the color Saturation Control. It’s
important to note that this was a prototype and Brillian had not yet completed
its optical color management design for the unit. This is straightforward and
the production 1080 units should perform just as well as the Brillian 720. The
red primary for the JVC Professional unit was a trifle too weak, which was due
to the unit's older projection lamp. A new lamp or an optional filter will move
the red primary a bit further out beyond Rec.709.
The
primaries in Figure 2 are only for High Definition signals and sources. When an
HDTV is displaying a DVD or other Standard Definition content, a different set
of primaries are needed. They are based on a different standard, and were not
measured for the Shoot-Out. When displaying Standard Definition content, the
HDTV needs to automatically switch its primary colors through a color
management process that electronically remixes the red, green and blue primary
color signals (see Part
II).
What’s Coming
Next
In Part C we'll continue
with a revealing Test Pattern analysis, followed by a description of the
extensive Jury Panel testing and then provide individual Assessments for each
of the units, including Jury evaluations and comments. Part D will have an
overall Assessment of LCoS technology, followed by detailed technical
performance comparisons between all of the major display technologies: CRT,
LCD, Plasma, DLP, and LCoS, and we’ll finish with a discussion of the most
exciting new developments in display technology that will be the subject of
future articles in this series.
Article Links
Series
Overview
Part A: Introduction
to LCoS Technology
Part B: LCoS Color
and Gray-Scale Accuracy
Part C: Test Pattern
and Jury Panel Evaluations
Part D: Comparison
with CRT, LCD, Plasma and DLP
Sidebar: LCoS
HDTV Manufacturers
Sidebar:
Shoot-Out Hardware and Software
Acknowledgements
Over 75
people were involved with the Shoot-Out: about half were participating
manufacturers and the other half were Panelists (see Part C) that came to
evaluate the HDTVs.
Special Thanks: A number of people made important contributions that warrant
a special mention: special
thanks to Dr. Edward F. Kelley of the NIST (National Institute of Standards and
Technology) for many interesting discussions and for generously sharing his
expertise. Special thanks to Dave Migliori for his
excellent photography of the Shoot-Out with its difficult lighting layout and
viewing angles. Special thanks to Julia Soneira and Lauren Soneira for
helping to produce the Shoot-Out, which turned out to be a much larger
operation than I had anticipated. And finally, very special thanks to Hope
Frank (Brillian), David McDonald (eLCOS), Terry Shea (JVC Consumer) and Rod
Sterling (JVC Professional) for the tremendous amount of work that they put in
coordinating their company’s efforts, which was crucial for making the
Shoot-Out a success.
HDTV Manufacturers: Brillian Corporation: Hope Frank
(Vice-President), Chad Goudie, , Gil Hazenschprung, Dr. Robert Melcher (Chief Technology Officer), Dr.
Matthias Pfeiffer, Jack Waterman. eLCOS Microdisplay
Technology: Dr.
David J. Cowl, Roland Lue, David McDonald. JDS
Uniphase: JVC (Consumer Division): Dan McCarron, Terry Shea, Fumi Usuki. JVC Professional
Products: Jack Faiman (Vice-President), Dr. David Hakala (Chief Operating
Officer).
Equipment Manufacturers: Anchor Bay Technologies: Gefen: Hagai Gefen (President),
Linda Morgan, Khasha Roholahi.
CinemaQuest: Alan
Brown (President). Konica Minolta Instrument
Systems Division: Tom Kwon and Maria Repici. Microsoft
Windows Digital Media Division: Silicon Optix: Gary
Chin, Ney Christensen, Darren Gnanapragasam,
Justin Lam, Gopal Ramachandran, Derek Yuen.
About the Author
Dr. Raymond Soneira
is President of DisplayMate Technologies Corporation of Amherst, New Hampshire,
which produces video calibration, evaluation, and diagnostic products for
consumers, technicians, and manufacturers. See www.displaymate.com. He is a research
scientist with a career that spans physics, computer science, and television
system design. Dr. Soneira obtained his Ph.D. in Theoretical Physics from Princeton
University, spent 5 years as a Long-Term Member of the world famous Institute
for Advanced Study in Princeton, another 5 years as a Principal Investigator in
the Computer Systems Research Laboratory at AT&T Bell Laboratories, and has
also designed, tested, and installed color television broadcast equipment for
the CBS Television Network Engineering and Development Department. He has
authored over 35 research articles in scientific journals in physics and
computer science, including Scientific American. If you have any comments or questions about the article, you can contact
him at dtso.info@displaymate.com.
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