Algolith HD to SD Downconverter XVC-1001-DC - Manual (Page 33)

XVC-1001-DC Guide to installation and operation
33
Technology Overview
Aspect Ratio
The aspect ratio of an image is its width divided by its height. Aspect
ratios are mathematically expressed as x:y and x×y. Two common
videographic aspect ratios are 4:3, universal for standard-definition video
formats, and 16:9, universal to high-definition television and European
digital television. Other cinema and video aspect ratios exist, but are used
infrequently.
Converting formats of unequal ratios is done by either cropping the
original image to the receiving format’s aspect ratio, by adding horizontal
mattes (letterboxing) or vertical mattes (pillarboxing) to retain the original
format’s aspect ratio, or by distorting the image to fill the receiving
format’s ratio.
De-Interlacing
De-interlacing is the complex process that converts a traditional interlaced
video source like common analog television signals, into the progressive
scan format required by modern high definition displays. This is the job of
a video processor, and the process itself is called de-interlacing.
The words ‘interlaced’ and ‘progressive’ arise from the days of CRT or
‘picture-tube’ televisions, which form the image of each frame on the
screen by scanning an electron beam horizontally across the picture tube,
starting at the top and working its way down to the bottom. Each horizontal
line ‘drawn’ by the beam includes the part of the picture that falls within
the space occupied by that line. If the scanning is interlaced, the electron
beam starts by drawing every other line (all the odd-numbered lines) for
each frame; this set of lines is called the odd field. Then it resets back to
the top of the screen and fills in the missing information, drawing all the
even-numbered lines, which are collectively called the even field. Together,
the odd and even fields form one complete frame of the video image.
However, recording which is performed in an interlaced manner consists
of two source fields that make up a complete frame. These two fields
are not recorded at the same time. Each frame is recorded as an odd
field from one point in time, and then as an even field recorded 1/50th or
1/60th of a second later. So, if an object in the video has moved in that
fraction of a second, simply combining fields causes errors in the image
called “combing” or “feathering” artifacts.
The simplest approach to avoid these artifacts is to ignore the even fields.
This is called a non-motion adaptive approach. In this method, when the
two fields reach the processor, data from the even fields are completely
ignored. The video-processing circuitry recreates the missing lines by
averaging pixels from above and below. While there are no combing
artifacts, image quality is compromised because half of the detail and
resolution have been discarded.