It’s simply a maths based fudge, design to stop two elements of the picture interfering which each other. Back in the mid 50s, technology was limited and adding a subcarrier, to ‘invisibly’ piggy back on the colour information meant that the two signals produced a series of inter-modulation products and the picture was a bit sparkly to look at with lots of high frequencies being created by the sum and difference products of the luminance and chrominance paths. It’s not really right to say it was slowed down, but it kind of works. They simply lowered the horizontal line rate from from 15,750 to 15,734 lines per second – and if you do the maths behind it – this creates the weird frame rate. So simply a fudge that worked. We’ve lived with the results ever since.
