I have placed a similar thread on the UK VRAT forum:
http://vintagetvandradio.myfreeforum...p=56930#p56930 but I'd like to get the USA point of view too.
As I understand it video in the USA has been carried as 140 units peak to peak on a 75R co-axial cicruit. This greatly simplifies connecting different equipment. The size of the unit has changed over time, from 10mV (gives 1.4Vp-p) down to about 7.2mV currently. Not sure about how many units have been allocated to pedestal/setup and whether this has always been present on 525 signals. These values are a little different to those used in Europe but not sufficiently different to cause much difficulty.
Going back to 1936 and the BBC TV station at Alexandra Palace the
Black Book shows that a variety of much higher levels were used. Much of the kit was directly connected without going to the trouble of matching to co-ax.
Leaving aside the fact that 1V to 1.4V p-p into 75R is not a terribly convenient level for valve kit, when and how did this standard emerge? I have a copy of Fink's 1952
Television Engineering but I haven't yet found a reference to baseband video levels, despite extensive discussion of the transmitted video waveform.