How the Display Reacts to Changing Images
The video scenes being displayed in a home theater are constantly changing - there are bright scenes, then dark scenes, and
then images moving rapidly across the screen. Since the eye has such a wide sensitivity to luminance levels, adapting to and displaying this wide range of scenes can be very demanding on the video display. This section goes into some of the related issues for the various technologies, and how the manufacturers have adapted in their attempts to provide the highest fidelity possible.
[Note that there are several links to WalVisions test patterns here. When one of these patterns is selected, it will open in a new window; to then view the pattern in nearly full screen, maximize the window
by using the F11 (full screen) key.]
Lamp-Based Projector Issues:
One of the biggest performance drawbacks for lamp based systems is that the lamp is always ON, and when very dark scenes are required it's very difficult to block all that lamp light from reaching the screen. You might notice that film projectors also shares this same characteristic - the screen in the movie theater never goes completely black! This washout of a black, or nearly black image shows up
as a reduction in the full field contrast specification. While CRT projectors can have contrast ratios that are effectively infinite (they
can produce NO light at true black), lamp based projectors have contrast ratios running from just a few hundred to one to ten
of thousands to one for some LCOS projectors.
Techniques used to lower the black level, and thus increase these contrast ratios, include Dynamic Iris and Automatic Lamp controls. The idea behind these features is to sense the average scene level, and when darker images are present to then reduce the amount of light available, either by reducing the power to the lamp, or by blocking some of the lamp light with an iris in the light path. If you reduce the amount of lamp light for the display, then to maintain correct brightness levels there must be a simultaneous video signal adjustment made to make sure the luminance of the image parts are maintained. For instance, if there is an area of the image that is at a 50% level, and then the lamp output is decreased by 50%, that image area,
if uncorrected, will be at a 25% level. To properly correct, that image area video level should be increased to 100% to result in an accurate 50% final level. But what about an image area that started at 60%? This is likely a problem in this example since you probably will not be able to increase the video level to 120%. In this case you may see a either a "crushing" effect where all levels above 50% become approximately the same luminance, or you may see a lowering of those luminance levels resulting in a level that is dimmer than intended.
The actions of dynamic iris and automatic lamp controls can evaluated using the
Dynamic Brightness test pattern (WV-17), which cycles the average picture level from a low level to a nearly full white level. To get the full impact of the pattern, make sure to view in a full field, or the at least in the maximum size possible. While the pattern cycles through the levels, carefully watch the gray bars in the lower left corner - these bars should not change at all, other than some possible washout due to light scattering in the projector or reflected light from the room. There also should be no noticeable brightness "bounce" just after any of the transitions.
Note that some projectors have fixed, or non-automatic, lamp and iris controls, and they work to decrease light output,
but both for the white and black levels. Thus while these fixed controls will decrease the black levels, they will not increase the contrast ratio as they cause the white levels drop by the same proportion. Of course, this is still effective and desirable if you are viewing in a well light-controlled area.
LCD Flat Panel Issues:
Conventional LCD flat panel displays also have an always
on "back light" as a light source, so here again when
full black is required, some light invariably leaks
through to keep the panel illuminated to some relatively
low level. While the standard LCD display
illumination source is a CCFL (cold cathode fluorescent
lamp), LEDs (light emitting diodes) now provide an
alternative illumination that can be actively
controlled. These LED arrays can be rapidly turned
on or off, or adjusted to intermediate levels.
This provides an ability to reduce illumination based on
the scene brightness requirements, basically equivalent
to the lamp based projector dynamic irises and automatic
lamp controls as discussed above.
LED BLU (back light units) are comprised of an array
(either one or two dimensional) of LED illumination
units, local areas of the image can be selectively
controlled independent of other areas. Thus in a
single image there can be full illumination to some
areas and reduced illumination in other areas. If
you have an LED driven LCD display, you can check
dynamic dimming effects using the Dynamic
Brightness test pattern (WV-17). If your
display has local dimming, the Expanding Ring
test pattern (WV-15) may be useful to check for
local area dimming/brightening as the expanding ring
travels through the various zones.
For comments about deinterlacing, see the Video
Motion Aliasing: The most common motion effect is aliasing due to insufficient frame rate - and the most commonly noticed scene in films is the wheel that apparently rotate backwards. This is effectively caused by not taking enough pictures per second - the parts of the wheel are captured at fixed times in a "strobing" fashion. If the captured images aren't taken quickly enough compared to the wheel motion, when played back the parts of the wheel may appear to go in reverse, go in slow motion, or even appear to stop! This is mostly a problem with film sources (movies) that shoot at 24 frames per second, and unfortunately there is little that can be done to correct the problem in the projector. To good news is that at 60 frames per second the problem becomes minor, and more and more material is being recorded at that rate. As the motion picture industry moves from recording on film to video, we expect improvements in motion, contrast, gamma and possibly even resolution.
Response Time, Image Lag: As there an object moves from one area of the screen to another, sometimes there may be a slight lag or "trail" due to slower response time. In a video display, this can be seen most readily when credits at the end of a movie scroll up the screen. If present, this is
usually due to excessive response time in the video display, which
can show up as detectable delay for the new image to appear, and/or for the old image to disappear. Look for this characteristic in LCD based displays, and in some CRT displays that may use "slow" phosphors. You may be able to see some effects using the
Expanding Ring test pattern (WV-15). Look for the disappearing sections of the ring to not immediately totally disappear, but to stay partially illuminated for a brief moment. Note that the motion here is pretty basic, and it may appear jerky - this is expected.
"Strobe" Effects: In CRT and DLP displays, the complete image never totally appears at one time. In CRTs the image is drawn top to bottom at the "field rate" (30 or 60 times per second), and only one spot on each CRT is receiving power at any instant. While the image does remain for a short period due to phosphor persistence, usually only a horizontal band, usually about one tenth of the screen height, is producing light at any one time. With single chip DLP displays, there is the color wheel which only permits one color to be displayed at any instant on the screen, leading to the "rainbow effect" that can be seen with eye motion. Use the
Rainbow Finder test pattern (WV-14), and carefully follow the LOOK HERE sign while using your peripheral vision to notice what happens to the center vertical column. With a DLP, you will likely see it break up into red, green and blue columns. With a CRT, you will probably see the center column images distinctly offset to the sides of the main column, at an angle, and possible looking a little yellow. On the other hand, with LCD and LCoS displays, which generally react more slowly and the entire display can be addressed at once, the center column should look pretty normal.
DLP Comment: All DLP projectors illuminate each pixel by alternating shining maximum light and then no light, switching many thousands of times per second. If the pixel is to be illuminated to a 50% level, then the pixel will be fully illuminated for 50% of the time, and fully black the other 50%. Thus if you could take a screen snapshot that lasted one millionth of a second, you would see only full white or full black pixels over the entire image - and you might not even be able to make out details of any part of the image! Fortunately the eye cannot respond nearly this quickly, so the mirror switching is not seen and we see the average level. If there is a time that this effect can be seen during normal viewing, you might see it in very dark areas with rapid eye motion - as your eye scans rapidly across such an area you may be able to see some brighter patterns of pixels. You may also be able to see such patterns by taking a high speed (1/1000 second or faster) digital picture of a of a DLP display showing an image with a low level gray area - the
Gamma Low test pattern (WV-34) is a good pattern to us for this purpose.
CRT and CRT Projector Issues:
CRT displays use high voltage power supplies (HVPS) to provide the energy to the electron beam
(this is the energy that is converted into light by the phosphors in the CRTs). If more light is required, more power is required from HVPS.
But problems can arise at the higher power levels, where the HVPS may
not be able to keep up with the additional demands and loose
regulation (change its output voltage level). The most common effects seen are a change in image size, a bending
of the image in the horizontal direction, a change in luminance level, or some loss of focus. These reactions can be checked with the
Growing Fields test pattern (WV-16).