Converting from one TV system to another

Converting from one TV system to another
Main article: Television standards conversion

Converting between different numbers of lines and different frequencies of fields/frames in video pictures is not an easy task. Perhaps the most technically challenging conversion to make is from any of the 625-line, 25-frame/s systems to system M, which has 525 lines at 29.97 frames per second. Historically this required a frame store to hold those parts of the picture not actually being output (since the scanning of any point was not time coincident). In more recent times, conversion of standards is a relatively easy task for a computer.

Aside from the line count being different, it's easy to see that generating 60 fields every second from a format that has only 50 fields might pose some interesting problems. Every second, an additional 10 fields must be generated seemingly from nothing. The conversion has to create new frames (from the existing input) in real time.

There are several methods used to do this, depending on the desired cost and conversion quality. The simplest possible converters simply drop every 5th line from every frame (when converting from 625 to 525) or duplicate every 4th line (when converting from 525 to 625), and then duplicate or drop some of those frames to make up the difference in frame rate. More complex systems include inter-field interpolation, adaptive interpolation, and phase correlation.