| Video Noise January 2007
What You Don't Know about
Fixed-Pixel Displays
The types of fixed-pixel high-definition display
include DLP, D-ILA (aka HD-ILA), LCoS, SXRD, LCD, and plasma. They can be thin panels you
can hang on a wall, compact rear-projection displays, or front projectors with separate
screens. There is so much market hype about fixed-pixel displays today that most people
already have one or more, or are thinking seriously about getting one.
Conversion
If you’re used to the instant response of a
late-model, standard-definition display, a fixed-pixel HD display might surprise you. The
new HD displays must be able to reproduce images of many different resolutions on a screen
with a fixed number of pixels. If you get one of the newer flat panels with 1920x1080
pixels, that means that every image it displays must be converted to 1920x1080 pixels --
except, of course, programming that originates in that resolution. All standard-definition
and 720p HD (ABC, Fox) broadcast programming, as well as SD DVDs, must be converted from
their lower resolutions to the display’s full 1920x1080. If your new video
display’s native resolution is 720p or 768p, lower-rez signals must be upconverted,
and higher-rez 1080i programming (CBS, NBC, HD DVD, Blu-ray, HD videotape)
downconverted, to 720 or 768.
Slowing down
The processing that handles these up/downconversions is not
nearly as fast as the analog tuners in late-model SDTVs. If you use your HD display’s
internal tuner to receive broadcast or cable channels, you’ll notice delays when you
change channels that may be a lot longer than you’re used to. If you’re
accustomed to surfing 80 channels in 40 seconds to see what’s playing, you’ll
have to slow down to 10 to 20 channels a minute.
The delay is longer for several reasons. The first is the
video processing that up- or downconverts each channel to the fixed-pixel display’s
native resolution. (The signal processing must also process progressive-scan and
interlaced sources.) Second, today’s video displays contain three or four tuner
modes: analog broadcast, analog cable, digital broadcast, digital cable. Plus,
there’s the possibility of Cable Card support.
Because the new displays are truly digital devices, analog
broadcast and analog cable channels must be converted to digital signals internally by the
display. Every time you switch to a new channel, the tuner has to figure out if it’s
an analog or a digital signal; if it’s analog, the tuner must then convert it to
digital, then figure out if the signal is interlaced or progressive, then convert the
signal to the display’s 1080i, 1080p, or 720p/768p native resolution. This all takes
time. Video displays that are "popularly priced" rarely include superfast video
processing. Faster processing will come as chipsets become more standardized and are
produced in greater quantities.
Alternatives
Channel-switching delays can be so long that some people
use a satellite or cable TV set-top box to bypass the display’s internal tuner and
speed up their channel surfing. These converter boxes typically pass along programming at
whatever resolution it originated in: the box converts analog to digital, and the
display’s tuner converts the various signals’ resolutions to the display’s
own native resolution. This splits the work between box and display, and channel changes
can happen quite a bit faster -- if the box and display are linked by a digital
video connection. But if you use a three-cable component-video link between the box and
display, you may find yourself back in the channel-change slow lane.
Check ’em out at the store
For the same reasons, if you have a lot of different video
sources and switch between them often, you may have a similarly slow response. So when
shopping for a video display, request a demonstration of each display’s speed of
channel and input switching. If the display you like best has a slow reaction time but is
otherwise worthy of your system, at least there won’t be any unpleasant surprises
when you get it home.
...Doug Blackburn
db@hometheatersound.com |
