Toshiba Tv kephiba

[Szali Sandor]

Interlace is achieved very simply in the master timing generator for a 
television system by making the total number of lines in the picture odd: 
the field flyback must then occur halfway through a line on alternate 
fields, which produces the required effect - see figure below for a 625 line 
interlaced raster. It is for this reason that all analogue electronic tv 
systems have used an odd number of lines. It is also necessary for the field 
timebase and vertical deflection circuitry to operate at a frequency twice 
the picture repetition rate; i.e. for the European system, which has a 
picture repetition frequency of 25 pictures/second or 25 Hz, the vertical 
deflection operates at 50 Hz. Furthermore, in order to get perfect 
interlace, all vertical deflection cycles and waveforms must be identical.

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