About video converters - a little need here...

[old_tv_nut]

Some mostly irrelevant math which you may nevertheless find interesting:

The exact NTSC subcarrier frequency is 455/2 x 15750 x 1000/1001.
The factor of 1/2 means there is an odd number + 1/2 cycles (227.5) per scan line time.
If you factor these numbers and calculate the exact frequency in lowest terms, it is
3579545 + 5/11 Hz.
The largest prime factor in this chain is 13, which is small enough to make stable tube frequency dividers.

In PAL-M, the subcarrier is offset by 1/4 cycle per horizontal scan time instead of 1/2 cycle, using
909/4 x 15750 x 1000/1001.
The exact frequency works out to 3575611 + 127/143 Hz.
The large prime factor of 101 in this chain only became practical with the advent of solid state digital counters.

These exact frequencies are obtained in equipment that is synchronized to atomic clock or GPS time, although the analog broadcast standards have a subcarrier tolerance of +/- 10 Hz to accommodate equipment based on oven-stabilized crystal oscillators.

Home equipment based on un-heated crystals will have a wider tolerance of subcarrier frequency, and in the case of some gear such as home VCRs, may not have the scan rate locked to the subcarrier.