[old_tv_nut]

Some more things:

The CT-100 had DC restoration near the end of the video chain, at the CRT. This compensated for the inevitable drift of DC level in the tube video circuits. When it was decided that DC restoration was too expensive for the later consumer chassis (CTC-5 and onward), DC coupling was substituted. Using 80-85% DC coupling meant that the drift was reduced by 15 or 20%, which the older engineers told me was believed worth it. DC level in early broadcasts drifted or changed from program to program too, and a similar argument was used by receiver makers to justify reducing the DC coupling to less than 100%.

Two of the greatest improvements in video pictures over the years have come from precise control of DC (black) level at both transmit and receive sides, and increased contrast ratio of displays, that is, reduced room light reflection.