Scan conversion

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For conversion of vector images to raster images, see Rasterisation.

Scan conversion or scan rate converting is a technique for changing the vertical / horizontal scan frequency of video signal for different purposes and applications. The device which performs this conversion is called scan converter.

The application of scan conversion is wide and covers video projectors, cinema equipment , TV and video capture cards, standard and HDTV televisions, LCD monitors and many different aspects of picture and video processing.


Contents

[edit] Mechanisms and methods

Scan conversion process, not only needs to make changes in synchronization (sync) frequencies, changes in picture information data rate are also mandatory in most cases. There are two distinct methods for doing the process:

  • Analog Methods (Non retentive, memory-less or real time method)

This conversion is done using large numbers of delay cells and is appropriate for analog video.

  • Digital methods (Retentive or buffered method)

In this methods , picture is stored in a line or frame buffer with n1 speed (data rate) and is read with n2 speed, several picture processing techniques are applicable when the picture is stored in buffer memory including kinds of interpolation from simple to smart high order comparisons, motion detection and … to improve the picture quality and prevent the conversion artifacts.

[edit] How to realize

The process in practice is applicable only using integrated circuits in LSI and VLSI scales. Timing, interference between digital and analog signals, clocks, noise and exact synchronization have important roles in the circuit. Digital conversion method needs the analog video signal to be converted to digital data at the first step. A scan converter can be made in its basic structure using some high speed integrated circuits as a circuit board however there are some integrated circuits which perform this function plus other picture processing functions like scissoring, change of aspect ratio and … an easy to use example was SDA9401 from Micronas.

[edit] Some examples

  • In many LCD monitors there is a native picture mode, however the monitor can display different graphical modes using a scan converter.
  • In a 100 Hz/120Hz analog TV, there is a scan converter circuit which converts the vertical frequency (refresh rate) from standard 50/60Hz to 100/120Hz to achieve a low level of flicker which is important in large screen (high inch) TVs.
  • An external TV card receives the TV signals and converts them to VGA or SVGA format to display on monitor.
  • In most graphic cards there is an output for TV, here there is a conversion from computer graphical modes to TV standard formats.
  • The two recent examples show how the scan conversion is a bridge between TV and computer graphics technology , the first three examples are applications of up conversion and the last example is a down conversion , usually the first conversion type is more difficult.

[edit] See also

[edit] References

  • Television Engineering Handbook (K. Blair Benson) ISBN : 0070047790
  • A technical introduction to digital video. (Charles A. Poynton) ISBN : 0-471-12253-X
  • Television and Video Systems. (Charles G. Buscombe) ISBN : 013903014X
  • Printed Circuits Handbook. (Clyde F. Coombs) ISBN : 0070126097
  • Handbook of Filter Synthesis (Anatol I. Zverev) ISBN : 0471986801

[edit] External links