[old_tv_nut]

By the way, if the DC component was not broadcast, it would of course not be present at the detector, and the whole exercise would be somewhat futile (although it could have some effect by clamping the darkest thing in the scene as black).

Iconoscope camera tubes had inherent AC coupling. The DC level had to be made up by clamping the darkest parts of the scene to not go below black, plus there was a "shader" (a live person) adjusting it along with the spurious shading across the image that the iconoscope was prone to. Someone who does camera electrical alignment is still called a shader today, although they are not doing it all real-time during use.