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Firstly the colour system NTSC was already well established in the US when the PAL system was introduced in the mid 60's,
Although the PAL system was based on the NTSC system, it was refined by 3 importent technoligies.
1. A swinging burst is used to change the phase of the R-Y signal phase plus & minus 45 degrees on every 2nd line (Alt Lines)
2. A chroma delay line is employed to avarage out any phase errors in the chroma signal.
3. A half line frequency or ident signal is used to switch the phase of the R-Y signal on every 2nd line.
To have introduce now would have made all existing colour TV's in the US obsolete.
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