Video - A Practical Guide to Video and Audio Compression - page 77

5

Video

5.1 On the Matter of Video

During this segment of our journey we will examine the video source material that you
might want to compress. We will start with the frame size and layout, and then move on
to analog-to-digital conversion and synchronization of the video, all of which is related to
the content of a line of picture information. The chapter is rounded out by considering
color information and how it is converted from RGB to luma and chroma.

5.2 Video Is Not Film

Television is much more complicated than film. It involves several interlocking concepts
that interact with one another. The way the picture is represented electronically and the
techniques for color encoding influence how it is converted from an analog form into a
digital form. This also has implications for how the compression works. The technicality
of how video works is a book-length topic in itself. In order to cover it in just a chapter, a
lot of the fine detail has to be put to one side.
The imaging mechanisms for film and TV are different. The technology requires that
certain processing be done to deliver a satisfactory broadcast within the available band-
width.
Television pictures are composed of a series of horizontal lines arranged in a
sequence that scans down the entire screen from top to bottom. This is called a raster . The
number of lines and how they are ordered in the scanning sequence depend on the kind
of display being driven and where in the world the TV signal is being broadcast.
European TV is closer to movie film projection than the American system because the
display frame rates are similar, but it is still different.
If only it were possible to go back in time and reinvent some of these things. Just
changing the frame rate of projected film or adjusting the way that pictures are dis-
played to use progressive scanning instead of interlacing on TV sets would make the
world a much simpler place for video compressionists. The following factors are all

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