Resolution Has Become
the Key to Video Performance
Compared to audio, video has never been very
specs-conscious. Historically, few people have bought television sets on the basis of
their performance, possibly because technical variations from one set to another used to
be far less dramatic than differences in size and finish. Twenty years ago, all TV sets
were basically lousy, but they were about equally lousy; today's monitors are far better,
but they're all much better. As a result, few conventional TV manufacturers bother
to publish much information on the technical aspects of their products.
More sophisticated video equipment, especially in the DTV
realm, does come with specifications as a rule, but because consumers have not "grown
up" with them, they are often more confusing than elucidating. Still, knowing a
component's technical characteristics can be a very useful aid in making a buying choice,
particularly now that video has reached the stage where there are real differences between
various delivery media.
One spec in particular deserves clarification:
"resolution", which is as important to video as frequency response is to audio
-- and, in fact, the two are closely related, although expressed in very different terms.
For instance, the distinctions between the VCR formats and DVD, say, are mainly described
in terms of resolution; ditto digital camcorders and the pizza-sized satellite dish.
Great. But what is it?
Generally, resolution is an expression of a picture's
clarity; its ability to reproduce fine detail. In a sense, it's related to focus -- an
out-of-focus picture suffers from loss of detail -- but resolution is much more specific:
it expresses not only the degree of "in-focusness" but also its direction.
Television deals with vertical and horizontal information in entirely different ways, so
the measure of resolution may well be different depending on the orientation of the image.
Unlike most electronic specifications, which are expressed
in decibels or ohms or Hertz or whatever, video has adopted the photographic concept of
"lines" for resolution. Imagine a picture with the whole left-hand side of the
screen black and the whole right side white. If that were the most the system were capable
of, it would be said to have a resolution of one line (that is, one black line and one
equally wide white space). A resolution of two would be two black lines alternating with
two equal white spaces. The maximum number of such pairs of lines that the system can
reproduce -- actually in a chunk of screen the same width as the screen is high -- before
they run completely together is its resolution. Note that this is often quite subjective
-- at the higher numbers, viewers may disagree as to when the system slips from
distinctness to blur.
Sometimes resolution is stated in terms of "picture
elements" or pixels, although this is really only appropriate when talking about LCD
or plasma displays, which are actually divided into individual pixels. It takes two pixels
-- one black, one white -- to make a line of resolution, so you can just divide the pixel
numbers in half to get lines.
In theory, vertical resolution should be very
straightforward in video. The inherent nature of television is that the picture is made up
of 525 horizontal lines (in the NTSC system), painting a picture on the screen from top to
bottom. A small number of these lines are used for other purposes -- closed-captioning,
for instance, and some color information -- but about 480 make up the picture itself, and
it is theoretically possible to alternate them black and white. This would mean a vertical
resolution of 240 lines, and although this is not a particularly good figure, it is
a maximum, at least when it comes to analog TV.
Because of this, few conventional television specification
sheets bother to list vertical resolution, so any figures you encounter refer to
horizontal resolution, whether labeled as such or not. This may occasion some confusion
for the uninitiated: "Why does laserdisc only offer 430 lines? I thought all TV
worked at 525 lines!" Different lines.
Vertical resolution is frequently discussed when it comes
to digital television, because there are three standards -- 480, 720, and 1080 lines --
but in these cases, the resolution is determined by the system chosen, not by the
competence of the set designer.
Good horizontal resolution has always been difficult to
achieve because it requires very good high-frequency response (usually referred to in
video terms as wide bandwidth). The scanning dot that makes up a TV picture moves across
the screen at a constant rate; the closer picture elements are, the higher the frequency
required to reproduce them. As with audio, transient response is important in order to
make the edges sharp, and this places even greater demands on the bandwidth capabilities
of a video system.
Until the home-theater boom, relatively scant attention was
paid to providing more than minimal horizontal resolution. There was little consumer
demand for better performance in this respect, partly because relatively few buyers
understood it, but mainly because increased high-frequency response usually meant greater
sensitivity to video noise. Most people preferred to sacrifice some clarity rather than
have a “snowy" picture, so TV manufacturers tended to provide a pleasing, but
not technically perfect, picture.
The coming of color TV made matters even worse. The main
criterion in establishing a color standard was that it be compatible with existing
black-and -white sets; to do this, the system's developers used a slice of the
high-frequency part of the picture signal to carry the color information, and so that this
would not interfere with the main picture, response was restricted to avoid this
non-picture information. The result was a ceiling on the potential horizontal resolution
(although this could be offset by the "comb filter" included in most
top-of-the-line sets). VCR makers perpetuated the trend in the interests of economy by
designing units that offered what they believed to be the minimum acceptable level of
horizontal resolution (they were probably right in 1975, but no longer).
Gone are the days when the most you could expect of an
off-air signal -- about the highest quality available for most of television's history --
was about 300 horizontal lines, so any set that could reproduce that was good enough. Now
DVDs routinely top the 500-line mark, and HDTV goes way beyond that. Resolution has
definitely become something we have to pay attention to.
...Ian G. Masters
ian@mastersonaudio.com
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