| Video Noise May 2004
Viewing Distance: Home Theater’s Forgotten
Problem
Most people buy the biggest screen
they can afford, or the biggest that will fit into their home theater. But this often
leads to unexpected results. The big problem is that the resolution of conventional NTSC
(or PAL) TV is not very good compared to HDTV. If you sit too close to the screen, NTSC
images look bad -- scan lines are visible, bleeding colors are visible, and images are
loaded with motion artifacts. By comparison, HDTV has spectacular resolution. The problem
is that if you sit far enough away from the screen to make NTSC images look acceptable,
you’ll miss out on a lot of the detail of HDTV broadcasts.
Here’s an example: If your display is a 34"
(measured diagonally) direct-view, 16:9 HD monitor, the recommended viewing distance for
conventional TV programming is almost 12’. Switch to HDTV and the recommended viewing
distance shrinks to about 6’. And if there were a 34" direct-view TV that could
resolve 100% of the maximum HDTV resolution of 1920x1080 display pixels, the optimum
viewing distance would be just 4.5’. I know of no home theaters equipped to move the
display closer to or farther from the viewers, or where the seats could be moved closer or
away from the display, depending on whether you were watching conventional TV, DVD, 720p,
or 1080i programming -- for each of these would have a different optimum viewing distance,
based on their resolution capability.
Let’s move up to the popular 57" screen size. For
conventional TV, the recommended viewing distance would be almost 20’. But many of
the rooms with 57" displays are not 20’ deep. That means the owner will always
see conventional TV larger (closer) than it should be to hide all the defects. But
switch to 1920x1080 HDTV at full resolution (a few of the newer DLP rear projectors can
produce this resolution), and you’ll want to be sitting just 7.5’ from the
screen. Really!
If you’re into larger projection screens -- for
example, 100" -- you’ll want to be 34’ away for conventional TV and
13’ away for full-resolution, 1920x1080 HDTV. DVD resolution will put you at 20’
to 25’ for optimum image quality. Front-projection systems sometimes have zoom lenses
that can accommodate different image sizes for different video resolutions. This means the
viewer can sit in one location while expanding or shrinking the image size to compensate
for low-resolution sources, or to take advantage of higher-resolution images.
With direct-view and rear-projection displays, there are
only two ways to adjust the viewing distance: move the display or move the viewer. If you
try to move yourself around your home theater to fully or partially compensate for low-
vs. high-resolution images, your speakers won’t move with you. The distances of the
speakers from the listening position will change drastically -- which means that each time
you change your position, you’ll have to use the setup menu to enter new distances
and set new levels for each speaker. If you move the video display so that the picture is
now closer to you than the speakers, the sound will not match the screen location.
Neither solution is very practical. Most people end up
compromising on a single viewing distance that’s too close for conventional TV and
too far away for HDTV. This compromise is usually accidental -- arrived at because of
spousal imperatives, or because that’s just the way things worked out, given the
room’s dimensions.
Keep all this in mind when you do a close-viewing
experiment with HDTV. Most speakers require a significant amount of space in front of them
before the drivers’ outputs combine to produce a well-blended sound. If you sit too
close to the speakers, the sounds from the tweeter, midrange, and woofer won’t be
completely integrated into a single sound: rather than the output of a single speaker,
you’ll hear the separate outputs of the speaker’s various drivers. The smaller
the speaker, the less distance will be needed for its drivers to integrate properly. The
larger the speaker, the more distance will be needed for its drivers to integrate and
sound good.
The 7.5’ ideal viewing distance for showing a
1920x1080 HDTV image on a 57" display capable of that resolution will make it very
difficult to position the center-channel speaker. Placed atop the display, the
center’s sound will mostly go right over the audience’s heads. Rear-projection
sets won’t have space below the display. Plasmas and large LCDs might have space
below the display, but the optimum listening axis would be at knee or shin level, not ear
level. The taller the main speakers, and the more drivers they have, the less likely
they’ll be to produce well-integrated sound 7.5’ in front of them.
If the size of your room dictates that you need to sit
about 9’ from your video display and your front speakers are 9’ or 10’
away, you’ll need a screen in the range of 80" diagonal, making your only
choices for HDTV display either front projection or Samsung’s yet-to-be-released
80" plasma. This all assumes that you optimize for the 1080i format with 1920x1080
pixels of resolution. So far, the highest progressive-scan resolution for HDTV is 720p,
1280x720. You could use this resolution as a compromise and be able to downsize the screen
size to 68" or so and keep the 9’ viewing distance -- still quite a large
screen, all things considered.
In three years or so, the HD-DVD format will be launched,
most original network programming will be in high-definition, and there will be many more
HD cable and satellite channels. That might be a good time to re-evaluate your viewing
position, and move closer to the screen to take advantage of the high resolution of HDTV.
Just don’t forget to readjust the sound to match your new viewing position.
...Doug Blackburn
db@hometheatersound.com |
