Scanning and Scaling (Sound & Vision
Buyer’s Guide 2003)
Until recently, the basic technology behind television images was
pretty cut and dried. In our analog system, the video stream
consists of 30 complete frames per second, a frame being a
complete still picture created by 480 active scan lines running
horizontally across the screen. (The total number of lines in a
frame is 525, but that includes lines in what is known as the
vertical blanking interval, or VBI, which carries no picture
information.) Each frame is split into two fields, each of which
contains half the scan lines. The fields are transmitted and
displayed sequentially, so that the first field of a frame is
completely scanned, and then the lines of the second field are
scanned between those of the first. This is known as interlaced
scanning.
Interlacing is surprisingly effective. However, if you compare a
scene shot and displayed in standard analog format, known as
480i, with the same scene displayed using progressive scanning –
in which all the lines in each frame are scanned sequentially
instead of being divided into two interlaced fields – the
progressive-scan version will look cleaner and smoother. That
format is called 480p (the number indicates the active scan lines,
and the “I” or “P” indicates the scanning method).
When DVDs are made from movies, the images on the disc are
stored at the native frame rate of film, which is 24 frames per
second (fps). DVD players convert the video from 24 fps to 30 fps,
but that’s not the only way to get the job done. DVD movies can
be displayed progressively at 60 fps if the player is designed to
provide such an output and the TV set is designed to handle it – as
more and more sets are, because 480p is one of the standard-
definition formats in the DTV system. (Another is plain old 480i.)
The DTV standard also defines two high-definition formats, 720p
and 1080i, that provide higher resolution – meaning, greater
detail. An HDTV set must, therefore, be able to accommodate
inputs in a number of scan formats and in both 4:3 and 16:9
aspect ratios for standard-definition signals (4:3 is not used for
high-definition broadcasts). That’s where things get hairy.
It’s possible to design a CRT display to handle all of those formats
directly, which is what high-end CRT front projectors typically do.
But since it’s cheaper to convert some formats to others than to
make a full-bore multi-scanning monitor, most rear-projection and
direct-view CRT sets take the conversion approach. And in the
case of fixed-pixel displays, such as LCD, DLP, and plasma, all
incoming signals must be converted to a progressive-scan format
that exactly matches the display’s pixel array.
Most digital CRT sets work at 480p and 1080i and convert every
other incoming signal to one of those native formats. That usually
means 480i gets bumped up to 480pand, if the set has a built-in
HDTV tuner, 720p gets unconverted to 1080i. (Because 720p
actually has the highest data bandwidth and horizontal scan rate, it
is easier from the display-design standpoint to convert it “up” to
1080i than to step 1080i “down.”) The process of converting
between scan formats is known as scaling.
Unfortunately, good scaling is hard to do, and bad scaling can look
really, really bad. Historically, good scalers have been very
expensive, even if all they did was line double 480i to 480p. And
the very best scalers, from companies such as Faroudja and Runco,
are still very pricey. The good news is that the growing need for
video scaling has let to substantial progress further down the food
chain – a trend that will surely continue. Still, before you buy any
set with built-in scaling – a category that includes all DTV sets and
all fixed-pixel displays – cast a critical eye on how it looks with a
variety of input signals.
Pay special attention to what the set does with ordinary analog
signals from cable or broadcast TV, which tend to give crummy
scalers the biggest fits. Look particularly at what happens around
the edges of moving objects. (Problems are often most apparent
on slowly moving objects in the back ground.) Jagged or fuzzy
edges or halos around objects are a bad sign. Poor handling of
analog TV signals might not matter much ten years from now, but I
t could make you pretty unhappy in the meantime. You should
also make sure the scaler provides 2:3 pull down detection and
compensation for film-originated programs that have been
converted from 24 fps to 30 or 60 fps.
