| Video Noise January 2003
Matters of Resolution
High-definition TV is better than conventional TV for a
number of reasons: It supports surround sound, it can display a wider range of colors,
widescreen is the standard format, and the resolution of images is higher.
Resolution is a tricky thing for HD images. Conventional
television tubes have to place the red, green, and blue color phosphors side by side.
Alternatively, the Rs, Gs, and Bs might be arranged in a triangle of dots. Projection
systems have a distinct advantage over direct-view television tubes because they can place
up to nine pixels in the space of one pixel on the direct-view television tube. How is
this possible? Projection sets lay red, green, and blue light onto a single small spot on
the screen. The beams are projected at the same spot on the screen, right on top of each
other. Direct-view TV tubes must have separate spots for red, green, and blue because a
phosphor produces the light you see. The size of the R spot on a direct-view TV will
support an entire RGB pixel for a projection set and the same goes for the green and blue
spots. In actual practice, projection systems usually produce somewhat fewer than nine
pixels for every one pixel in a tube, but they still have quite a theoretical advantage,
resolution-wise.
Not all projection sets are created equal
There are two kinds of projection products: fixed pixel and
non-fixed pixel. D-ILA, LCD, and DLP are all fixed-pixel displays. They must create every
image using a set number of pixels across their width, as well as a set number of pixels
high. The pixels are always in the same location and always the same size. Plasma screens
are also fixed-pixel devices, but are not projection devices.
Most fixed-pixel displays do not yet have enough pixels to
meet the maximum 1920 x 1080 HD-resolution standard, or even the next step down, 1280 x
720. The fixed-pixel devices have more pixels every year, but they are still a bit shy of
maximum HDTV resolution. Thus, images displayed on fixed-pixel displays must undergo
sophisticated processing. The processor has to upsample images with resolution lower than
that of the fixed-pixel display or make the image physically smaller, which is not usually
desirable. It must carefully downsample images with more resolution than the fixed-pixel
device for display.
The only devices presently capable (in theory) of
displaying maximum HDTV resolution in home equipment are non-fixed-pixel projection
systems using CRTs. In actual practice there are few, if any, CRT projectors that can
produce the full 1920 x 1080 image resolution of HDTV. Rear-projection systems are limited
by cost constraints on the tubes they can use, their electronics, and optical components.
Front-projection systems come closest to full HD resolution, but you will be up in the
heady $25,000-plus range for a product that can claim to be very close to the 1920 x 1080
specification. You can imagine how much compromise there will be in a $2000, 50"
rear-projection HD set.
I want my HD
Today, you probably won’t get maximum HD resolution at
home. Each technology has limitations. Direct-view HDTVs with large tubes have a better
image than standard TVs, but their horizontal resolution is only marginally higher than
the best standard TVs. They can only make the phosphor spots so small before tubes become
impractical -- the picture would be so dim you wouldn’t see it. The smaller the
phosphor spots, the less light each one can emit.
Sets with large direct-view tubes can look good, especially
brightness-wise, but they don’t have the eye-popping detail of expensive CRT
projection systems. Inexpensive fixed-pixel displays using LCD and DLP are plagued with
artifacts. In this context, "inexpensive" means under $10,000! I have been side
by side with others who fail to see the obvious (to me) artifacts from DLP, D-ILA, and LCD
projection systems, but given the opportunity, I can get almost anybody to see them. Once
seen, the artifacts are difficult to ignore. Projection products with multiple CRTs can be
challenging to align and keep in alignment. Projection CRTs are fairly easy to damage if
they run too bright for any length of time. The worst case I’ve heard of so far was a
six-month-old set that already had the Disney logo burned into the tubes.
The more money you can throw at the resolution problem, the
easier it is to solve. That’s not terribly surprising. When I had $3000 to spend on a
new video display, I had a choice between a DLP front-projection device with limited
display capabilities and resolution much lower than HD; a direct-view TV with a 36"
tube and resolution similar to the DLP projector; or several rear-projection sets around
55", all with higher resolution. The DLP device required a dark (black) room and was
loaded with artifacts in many scenes, especially scenes with motion. The 36" tube was
much brighter. It could potentially last for 14 years like the last fancy TV I bought, and
it had about the same resolution as the DLP projector. The rear-projection sets were
obviously brighter in the middle of the screen and obviously darker in the corners; there
were convergence and geometry errors; and any room light degraded the picture.
One year later, the DLP projector was upgraded. The new
model costs 10% more but has resolution high enough to be interesting: 848 x 600, still
lower than the top two HDTV resolutions. Unfortunately, images are still riddled with
pixel noise, especially when there is motion on the screen. The 36" TV was
discontinued and replaced with a newer model with the same price, same screen size, and
same resolution but with added features I wish I had. The rear-projection sets all
improved this year, getting a little better in almost every way, but not good enough to
erase the uneven illumination problems, the geometry issues, or the sensitivity to room
light. So this year my choice would still be the 36" direct-view set even though it
means giving up some resolution and screen size. I wish I had more resolution, but I want
it in a package that’s more flexible and reliable than the current alternatives.
How and when will full HD-resolution video come home?
My prediction is that lasers will be the display technology
that brings reliable (sort of) full HD resolution to the home market. Lasers are adaptable
to both rear-projection and front-projection applications. Spot sizes can be managed
actively -- spots can be larger or smaller as resolution of the image changes from source
to source. Lasers have no problems with phosphors burning like CRTs, and none of the
limitations of fixed-pixel displays. Control circuitry for lasers is already well
developed and reliable. There is little lost light when lasers travel 18 to 30 feet across
a room to a screen. At the power levels needed for home viewing, heat shouldn’t be
much of an issue. Daylight viewing should be possible by cranking up the power in bright
rooms. Lasers made in large quantities can be very inexpensive.
Laser light has a single frequency of light in the beam.
This is ideal for reproducing beautiful images with more colors than you will ever have
seen in other image-creation technologies. Image size won’t be much of an issue. You
want it to cover your entire wall? No problem.
But there are issues with lasers. They have a finite life,
for one thing. Hit their limit and they stop working. Blue lasers have been the limiting
factor. Until recently, they could not even be made. Now that they can be made, they are
far behind red and green lasers in lifespan and cost. The blue laser is likely to stop
working first and will cost many times more than the red or green laser to replace. It is
reasonable to expect this disparity to be whittled away over the next decade or so. The
first laser display devices are not going to be inexpensive. It will take some time for
costs to get down to direct-view CRT levels, perhaps a decade or more. Alignment will
remain tricky, but not as tricky as CRT alignment. There will be a lot more control over
laser position, more precision, and opportunities for some self-adjusting that will get
things very close, if not perfect.
The struggle to reach full high-definition resolution
leaves us in the HD age without affordable displays that can show us all HD has to offer.
On the other hand, the improvements HD offers are clear enough to see with any HD-capable
product. We’ll have to be satisfied with these improvements until true HD-resolution
displays become available at popular consumer prices. That will happen, but
don’t hold your breath!
...Doug Blackburn
db@hometheatersound.com |
