Comment by meatmanek

Wikipedia for NTSC alludes to a couple reasons why you'd want your refresh rate to be based on your power line frequency:

> Matching the field refresh rate to the power source avoided intermodulation (also called beating), which produces rolling bars on the screen. Synchronization of the refresh rate to the power incidentally helped kinescope cameras record early live television broadcasts, as it was very simple to synchronize a film camera to capture one frame of video on each film frame by using the alternating current frequency to set the speed of the synchronous AC motor-drive camera.

(I suspect shows that were pre-recorded and telecined for broadcast would've also been filmed at 30fps using a synchronous AC motor.)

> In early TV systems, a master voltage-controlled oscillator was run at twice the horizontal line frequency, and this frequency was divided down by the number of lines used (in this case 525) to give the field frequency (60 Hz in this case). This frequency was then compared with the 60 Hz power-line frequency and any discrepancy corrected by adjusting the frequency of the master oscillator.

I think later TVs would've just synchronized to the received signal.

https://en.wikipedia.org/wiki/NTSC#Resolution_and_refresh_ra...