Sony TVP-08, Training Manual

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53
DRC - Digital Reality Creation
Another picture quality issue has been with us since the dawn of televi-
sion. Deeply ingrained in our television system is the question of visible
scanning lines.
All about scanning lines
A television picture is “painted” across the CRT screen by an electron
beam that scans on a horizontal line from left to right. Once the beam
reaches the ridge edge, it shuts off and returns to the left edge to start
another line. All told, there are some 525 scanning lines in the American
NTSC (National Television Standards Committee) television system, and
they create a new television picture or “frame” some 30 times a second.
In reality, you don’t see the full 525 lines on the screen. Over 40 lines are
consumed by the Vertical Blanking interval. This leaves roughly 480 lines
for the actual picture. And you don’t even see the 480 lines all at once.
Each video frame is divided into two “fields”, which last for 1/60
th of a
second. The first field is composed of all the odd-numbered lines (1, 3, 5
and so on). The second field “fills in” with the even-numbered lines. This
technique of alternating odd and even fields is called “interlacing.” The
NTSC system is often referred to as “525/60” (for 525 total scanning lines
and 60 fields per second). It is also called “480I” (for 480 net scanning
lines, interlaced).
Over the years, the 480i system has worked remarkably well. But with
only 240 lines on-screen at any one time, the scanning lines can become
painfully obvious, particularly when you’re sitting close to a large-screen
display.
Problem: Visual scanning lines
Originally, television engineers designed the NTSC system so that the
picture would appear seamless when viewed from a distance of 8 times
the picture height. This worked well in an era when the biggest commer-
cially available screens were 12 inches diagonal. But in today’s big-screen
era, viewers tend to set far closer to their televisions in order to get wrapped
up in the action. Under these conditions, the scanning lines become
blatantly visible.
One solution: Line doublers
Demanding home theater enthusiasts, videophiles and video profession-
als have long sought a cure for this problem. One solution is to double
the number of scanning lines with a circuit called a line doubler. Sony has
been an active supplier of line doublers, particularly for professional video
projectors.