Extron Electronics IN1776QZX User Manual

Page 16

IN1776QZX OPERATION MANUAL - REV 1.4 11/27/99

ETTING THE BEST IMAGE FROM YOUR SCAN CONVERTER

Beginning with a quick overview on the differences between computer video signals and
conventional video signals, this section concludes with several tips which will help you get the
best looking images from your scan converter.

REGULAR VIDEO AND HIGH RESOLUTION COMPUTER VIDEO - WHAT’S
THE DIFFERENCE?

Users are sometimes surprised when the scan converted image displayed on a conventional video
monitor has lower quality than the original VGA or MAC signal displayed on their computer
monitor. There are several reasons for the image quality loss, the main one being the vast
differences between the two types of signals. VGA / MAC to video scan converters such as the
IN1776QZX have the difficult job of converting a non-interlaced, high resolution image with
precise colors into an interlaced, lower resolution image with a more limited choice of colors.

Resolution
When compared to regular video signals you see on your home TV, computer video signals
usually have a higher resolution (a higher number of pixels or picture elements make up the
image). This means that the scan converter must actually throw away some of the VGA or MAC
picture information and detail in order to transfer the image to a conventional monitor. This
problem is more noticeable in the United States and other countries using the NTSC video
standard because the system has 100 fewer lines of picture information than the PAL video
standard.

Color
Computer video systems maintain separate signals for red, green, and blue allowing more
accurate color reproduction than conventional video monitors which combine all the color
information into one signal. In addition to less accurate color, there is generally a narrower
choice of colors available with conventional video monitors. Therefore, some colors shown on a
computer monitor simply cannot be reproduced on a conventional video screen, especially when
sent as a composite video or an S-Video signal.

Scan Methods
The other big difference between the two video systems is the way the picture is scanned on the
screen. A high-resolution computer video signal is usually non-interlaced. This method (also
called progressive scan) “paints” all of the lines in a single pass from the top to the bottom of the
screen. A conventional TV video monitor displays an interlaced video signal. This means that
all of the odd numbered lines are displayed first followed by all of the even numbered lines. The
two pictures made up of odd lines and even lines are displayed so quickly in succession that the
human eye combines the two into a single picture.

When a non-interlaced computer video signal is converted to an interlaced signal and displayed
on a conventional TV monitor/projector, flicker may be observed, especially on parts of the
image made of very thin lines (1 pixel) or patterns composed of thin lines.