Video signal levels
Video signals used in TV production
For all standard TV video signal systems the standard video level was 1 V p-p in 75 ohms, for video + sync. However, waveform monitors were marked in 140 "IRE units", 40 for sync, and 100 for video.
Under the original RS-170 standard, video was 1.4 Vp-p, with the white level 1.0V up from blank (which is the reference) and the sync tip 0.4V down (negative) with respect to the same reference level. And from blank to white was 100 IRE units.
The RS-170 levels have pretty much been replaced in most applications by the later RS-343 standard levels. Under this standard, the entire signal (including the sync pulses) was reduced to 1.0 Vp-p across 75 ohms. However, the same 100/40 division above and below the blank level was retained. This results in the reference white level being 0.714 V positive from blank, and the sync tips 0.286 V negative from blank. The BLACK level, under this standard, is slightly up from blank, having an 0.054V "setup" or "pedestal".
Standard practice in Europe has also been to use a 1.0V p-p signal, but they simplified life and set white to +0.700V and sync to -0.300V, again with respect to the blanking level, and did away with "setup" (i.e., black and blank are the same level).
PC video levels
From European 1 Vpp system we got one common PC video standard, which uses 0.700V p-p video (with separate syncs). Unfortunately, we also saw the development of ANOTHER common PC video practice, which was to simply use RS-343 with the setup chopped out - resulting in 0.660V p-p for the video.
All of these are close enough to work about equally well on most equipment and monitors, but there are several "standards" for video floating around.