are there so many different TV standards?
As technology became enhanced it was no longer necessary to keep this relationship between the number of frames per second and the mains frequency and when the Americans introduced colour, they changed their frame frequency to 59.94 Hz while the mains frequency remained at 60Hz.
However, the world remains divided into countries that use 60 frames per second, primarily the USA and Japan, and those that have 50 frames per second, which is nearly everyone else. There is a corresponding difference in the number of horizontal lines which make up the picture - 60Hz systems tend to use 525 lines, and 50Hz systems use 625 lines.
The difference in
the frame rate is the biggest incompatibility between standards.
Amongst the countries based on 50Hz systems, PAL (Phase Alternating Line) has been the most widely adopted. PAL is not the only colour system in widespread use with 50Hz; the French designed a system of their own - primarily for political reasons to protect their domestic manufacturing companies - which is known as SECAM, standing for Sequential Couleur Avec Memoire. SECAM was widely adopted in Eastern Block countries to encourage incompatibility with Western transmissions - again a political motive. SECAM is further split between several variants.
In general, since the field and scan rates are identical, you can expect to get a monochrome picture from a PAL video recording replayed on SECAM equipment, and vice versa.
services provide for the conversions between these competing formats.