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Front Projection - Home Theater
Front projection provides the ultimate viewing
experience to parallel the commercial movie theater. In many
ways, the front projection home theater can improve on certain
limitations of a commercial theater.
The following pages will give you some background on
what to consider for a home theater projection system.
Handling a variety of video sources
Current video projectors for home theater should be
capable of handling basic video (NTSC / Standard Definition) as well
as High Definition content. NTSC video is a 4:3 aspect ratio,
interlaced analog signal with 525 total lines of resolution. Often
referred to as 480i which means that 480 horizontal lines are visibly
displayed to "paint" a single image (frame) in 2 scans; the first scan
laying out 240 odd numbered lines and the 2nd scan interlacing the
remaining 240 even numbered lines.
1080p is the highest resolution which a projector
might currently display with 1080 horizontal lines painted
progressively one after another from top to bottom of the screen.
You will have to make a decision, based on the video sources you have available and expect to have available over the
life of the projector, as to whether you accept an upper limit of
720p, 1080i or 1080p for your projector choice.
The resolution of standard DVD is 480 lines of
vertical resolution and the maximum total horizontal resolution
is set at 720 lines. A good scaler, either in a High Definition DVD
player or in the projector, can improve the appearance of these DVDs
and a Blu-ray or HD DVD player, for optimum image quality, will require a projector with the
capability of displaying 1080 lines of vertical resolution.
Even though the projector may be capable of high
definition display as well as standard definition video
up-scaling, beware that noise and imperfections of video sources such
as VHS tape will be magnified on a large screen to the point that they
are very distracting. That being the case, if you are primarily
watching standard definition material, a front projector may not be
the appropriate vehicle.
Connections - a.k.a. Inputs and Cabling
Various video sources will output different signal
formats and require the appropriate cabling to optimize the image via
the best input into the projector. For example, a VHS VCR will
provide a better image if the composite video signal is input to the
projector via RCA jacks rather than a coaxial cable (F-connector)
input. If passing the signal through an audio-video receiver the
receiver output to the projector should likewise be equal to or better
than the best signal being fed into the receiver. Hence, if an HDMI
cable is fed from a Blu-ray/HD DVD player to the receiver, an HDMI
cable should be used to relay this signal to the projector.
You should keep in mind a couple of points with regard to digital cables:
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Lengths of digital cable over 15 feet will require
an amplifier.
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When you are using a digital cable to carry an
HDTV signal, if the source material is carrying an HDCP enabled
signal, your display device will also have to be able to accept an
HDCP enabled source. If not, you will have to use analog component
cables.
Ambient Light
The difficulty with a front projection system is that the screen
doesn't discriminate about what light it reflects back to your eye.
Lights on in the room or light coming in through windows will combine with the reflected light of the projector
to wash
out the projected image. A typical home theater projector will have a
brightness rating in the range of 800-1500 lumens. This is fine
for a partially or fully darkened room; however, a brightly lit room
isn't an appropriate environment for a front projector.
You might ask, "Why can't I buy a data-grade
"business" projector which has a higher brightness rating of
2000 or more lumens?" Quite simply, this is an option;
however, you may regret the sacrifices you'll have to make, especially
if you are very discriminating about the image quality. Here is
a sampling of the resolution sacrifice:
Data projectors typically display only a 4:3 aspect ratio.
They will display a widescreen image but within this 4:3 format
causing loss of vertical resolution.
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An SVGA (800 x 600) projector will provide an 800 x
450 resolution when using it in 16:9 mode. Black
bars will appear on top and bottom of the image
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An XGA (1024 x 768) projector will provide a 1024 x 575
resolution when using it in16:9 mode. Black
bars will appear on
top and bottom of the image.
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An SXGA (1365 x 1024) projector will provide a 1365 x
766 resolution when using it in16:9 mode. Black
bars will appear on top and bottom of the image.
As well, a DLP data projector will typically sacrifice
color saturation and color accuracy in comparison to a home theater
projector.
Room size considerations
One of the primary reasons expressed for having a
front projector, rather than a flat-screen panel, is to have a very
large image to emotionally involve the viewer. To create this large
image, a considerable distance is required from the projector to the
viewing screen. A "short-throw" lens can be used to decrease
this distance or the projector can be installed behind the screen
(rear projection); however, both solutions are quite costly.
Scalers
In short, a scaler is an electronic device that
changes a signal from one resolution into another resolution.
Scalers are built-in to all projectors, plasma screens, LCD screens
and rear projection displays. They are required as a display outputs
just one resolution, yet it will accept multiple types of video
signals at various resolutions. Known as "fixed resolution
displays", they can only display a finite fixed resolution known
as the display's "native" resolution. It matters not
what resolution signal is sent to them. Video displays input multiple
resolutions like SXGA, XGA, SVGA, NTSC and HDTV, and convert those
signals to the native resolution of the display.
Most difficult, is the scaling down of a high
resolution image to a lower resolution. This requires discarding
pixels while trying to preserve the quality of the original image.
This scaling down process is actually harder than scaling up an image
where pixels of information have to be added.
Deinterlacing
Simply adding two interlaced fields together to
produce a contiguous single frame will result in jagged edges when
there is motion between frames. These are referred to "jaggies"
and are an indicator of poor deinterlacing. A good deinterlacer will
use motion adaptive deinterlacing which compares one field to another
to look for mismatches caused by motion and correct them. The best
deinterlacers will compare pixel changes from the previous frame and
to the next frame to make the appropriate adjustment.
A deinterlacer must be able to distinguish between
film sourced content and video content. Where there is a
transition between film and video material, the deinterlacer should
easily switch to the appropriate deinterlacing method. Chips
that are reliable deinterlacers are made by manufacturers such as
Silicon Image and Faroudja.
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