Direct view
This
includes flat panel and CRT TVs.
A
CRT produces a bright, sharp image that can be viewed from any angle in a fully
lit room. The largest are about 38”
(measured diagonally). You may find
fitting more than 2 people in front of one of these to be a problem. Wide screen CRTs come in cases that are
exceptionally deep. Because of their
size and weight, direct view CRTs are going extinct.
Plasma and LCD flat panel TVs are available. Plasmas are large and expensive, while LCDs are small and cheap (“cheap” means $500 right now).
Rear projection
Rear
projection sets come in sizes 40” to 80”.
Most are based on DLPs, but there are other competing technologies.
A
CRT set typically would have three CRTs (red, green, and blue) hidden inside a
box. They point upward into a flat
mirror and then onto a diffuser screen.
The CRTs have big lenses that focus the image at the screen. Adequate intensity is a problem, so the
diffuser screen is designed to not radiate light in directions where there are
no viewers. A fully bright image is
visible to about 45°
to the left and right, but only to about 10°-15° above and below any spot on the
screen.
Typically, if you sit closer than 3 times the screen height, you will not be able to see the top and bottom of the screen at full brightness. Although 3 times the screen height is a correct distance, if your head moves vertically even a few inches, you may lose brightness at the top or bottom of the screen. If you don’t mind the loss of brightness you can watch standing up.
Room
lighting must be controlled, but a blackened room is not necessary. Indirect ceiling lighting works well, but the
image of any lamps can usually be seen in the large, flat screen. Ditto for windows. You will probably want dark shades for
windows that reflect in the screen. If
the salesman tells you the screen is non-reflective, check it out for yourself.
Front projection
For larger than 80”, front projection is the way to go. Usually the projector is ceiling mounted or on a shelf behind the viewers. Since the screen is white, a completely blackened room is necessary. Otherwise the blacks will be gray.
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Display Technologies
CRT -- CRTs are used in direct view, rear projection, and front projection TVs.
CRT advantages:
1. Best color fidelity
2. Excellent blacks, fairly bright whites.
3. Smooth blending of adjacent pixels. (Some people think this is important.)
4. Some draw both 1080i and 720p, avoiding errors in format conversion.
CRT disadvantages:
1. Brightness drops as screen size increases.
2. Few truly achieve 1920x1080 resolution, although some get very close. Projection TVs employing 9-inch CRTs get the closest.
3. Constant need for convergence adjustments (making red, green, and blue coincide perfectly).
4. Focus adjustment required every few years.
Many experts think CRTs still produce the best pictures. Few CRT sets will draw both 1080i and 720p. Most will draw one and require the tuner to convert the other.
Digital (Fixed-pixel) displays -- All of the technologies below are digital technologies. They never need adjusted for focus or convergence. But they have only one native format and must convert all the others. Most digital displays will re-digitize analog outputs such as component video.
Some people don’t like fixed-pixel displays because adjacent pixels are not smoothly blended. Sometimes there is a noticeable black border around each pixel, which is called the screen door effect. But if these things bother you, you are sitting too close.
Plasma -- These flat panel displays are large and very bright. The disadvantages are high price, poor blacks, and short lifespan (dimming with age). Plasma is not a good choice for video games.
DLP -- (digital light processor, also call DMD for Digital Micro-mirror Device). This is a large chip with about a million tiny mirrors on its surface. The chip can tilt each mirror to vary the amount of light reflected off of it. DLPs are used in rear and front projection TVs. Advantages: very bright, good blacks, never suffers burn-in. Disadvantage: the rainbow effect (explained below). DLP chips are made only by Texas Instruments.
LCD -- (Liquid Crystal Display) Polarized light shines through a sheet of glass on which is deposited a liquid crystal array. Each pixel changes polarization to either pass or block the light. LCDs are used in front and rear projectors. Large LCDs are used in flat panel displays. Disadvantage: Black is not as dark as on a CRT. On cheaper models, pixel response can be a little slow, causing blurred motion.
LCoS and D-ILA -- (Liquid Crystal on Silicon, by Toshiba and RCA, and Direct-drive Image Light Amplifier, by JVC) These are reflective chips with a polarizing LCD layer. Each pixel changes polarization to either reflect or block a light beam. Disadvantage: Black is not as dark as on a CRT. These chips are used in front and rear projector TVs.
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Set configurations
Monitor -- This is a set without a tuner, and usually without any audio. This configuration is often preferred either because external OTA receivers are thought to be better or because the owner plans to rely on cable or satellite. Many of these sets are presently being used just for viewing DVDs.
Integrated TV -- This is a complete HDTV set with NTSC and ATSC tuners and also some speakers. An external tuner can always be added.
HD-Ready TV -- This is a TV with an NTSC tuner. The set will draw HD images at 1080i or 720p if an external tuner is added. The FCC has mandated an end to this category (in effect requiring an ATSC tuner with any device that has an NTSC tuner) but there are still a few of these around.
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What is burn-in?
The
phosphors used in CRTs and plasma displays become less bright with usage. The phenomenon is a lot like “tire
wear”. If you drive fast, the
wear-per-mile increases, but there is some wear at any speed. The speed of a car corresponds to white
in a TV image.
CRT
burn-in used to be rare, but the demand for brighter images has made
manufacturers less conservative. Now
CRTs that have been showing a Windows desktop for a couple years will often
show a lightly burned-in task bar when the screen is painted all white. The CRTs in big-screen TVs are pushed even
harder, especially in the largest sets.
All
CRT and plasma sets dim with usage.
Making the screen age evenly is the user’s responsibility. The user must ensure that a fixed, unmoving
shape is not displayed for many hours, or that shape will slowly become burned
into the screen.
LCD,
LCoS, and DLP sets do not suffer burn-in.
(Some LCD and LCoS sets exhibit “image retention”, but it goes away in
an hour or so.)
How serious a problem is burn-in?
Many
CRT sets are nearly immune to burn-in, but you can’t know if yours is one of
those. There is no website that tells
which sets are prone to burn-in. But
such information would not be a guarantee.
Ultimately
you must accept the fact that burn-in can happen to your set. You must watch for it and must not delay
taking action if you see it. A very mild
burn-in can be made to go away, but a more severe burn-in requires replacing
the CRTs, probably a $2000 service call for a big TV.
Network
logos and scrolling “ticker tapes” on news channels can cause burn-in. The black sidebars on 4:3 programs are a big
threat if you watch mainly 4:3 programs.
Some manufacturers will state that the warranty is void if the user
watches with black bars more than 15% of the time. That would mean that you are allowed hundreds
of hours of black bar viewing over the life of the product. But still you must watch for burn-in and act
quickly if it shows up. Other manufacturers
refuse to cover any burn-in in their warranties.
(The
author watches 4:3 programming with black bars about 30% of the time. His set shows no burn-in after 7 years of
this.)
One
measure of the likeliness of burn-in is the screen size to CRT size
ratio. That is, a 60-inch set that has
9-inch CRTs is less likely to burn-in than a same sized set with 7-inch
CRTs. Direct view CRTs rarely suffer
burn-in.
Also
note that black is less of a problem than white. It might take hundreds of hours watching 4:3
material for burn-in to occur. But a
station logo containing some fully intense white can cause burn-in in less than
ten hours.
Should I buy one of these sets?
Adults
who understand the consequences can generally avoid serious burn-in. But kids present some uncertainty. If you have kids and a limited budget then
you should keep in mind: If you buy a
CRT set with an incomplete warranty then you are taking some risk. It is generally a mistake to connect a video
game to a plasma or projection CRT set.
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The rainbow effect
One-chip
DLP sets employ a rotating color wheel.
Thus the three colors are delivered to the screen sequentially. Suppose the image is white text on a black
background. If you shift your gaze
rapidly across the image, the white lines will decompose into the primary
colors (until your eyes stop moving).
Most people don’t notice this, and most of the people who do learn to
ignore it. But a few people can’t get
past being distracted by it.
Set
makers can reduce this problem by changing the colors faster (using color
wheels with 6, 9, or 12 color segments).
The rainbow effect is eliminated in three-chip DLP sets, which have no
color wheel.
Some
CRTs have a similar problem. The
author’s set employs a green phosphor that stays lit 4 times as long as the red
and the blue. A rapid eye shift will
reveal some flashes of green in an image that has only white text on black.
.
This page is part of “An HDTV Primer”, which
starts at www.hdtvprimer.com