It's 315/88. Chosen to be a half-integer multiple of the line rate. The line rate was also adjusted so that the audio subcarrier is a multiple of the line rate.

It's 315/88. Chosen to be a half-integer multiple of the line rate. The line rate was also adjusted so that the audio subcarrier is a multiple of the line rate. The audio subcarrier is 4.5 MHz. So it's really 4.5 MHz, multiplied by 35/44. In practice, you might buy a 3.579545 MHz crystal oscillator and then use dividers and PLLs to make all the other frequencies you need. The actual error is going to be based on the oscillators you use, but for the past few decades, we've just been using ordinary crystal oscillators and that means something like 50ppm, maybe (for cheap ones). Crystal oscillators were available prior to the invention of NTSC. Quartz oscillators achieved 0.1ppm accuracy at some point in the 1920s.