David Looser wrote:

The real benefit of -ve modulation was AGC. With -ve modulation sync tips
correspond to 100% modulation and make an easy source for the AGC bias. With
+ve modulation sync tips are at zero carrier which obviously is useless for
AGC. Instead the back-porch has to be used and many different weird and
wonderful circuits were devised to "gate out" the signal voltage during the
back porch. Due to the need to keep costs down manufacturers increasingly
turned to "mean-level AGC" in which the video signal itself was simply
low-pass filtered to form the AGC bias. This lead to receiver gain being
varied by the video content, so the black on low-key scenes was boosted
whilst the whites in high-key scenes were reduced leading to a general
greyness to everything. To me it looked awful but as the Great British
Public kept buying these sets (and they were cheaper to build) mean-level
AGC became the norm for B&W UK domestic TV receivers. One great advantage of
colour was that mean-level AGC could not be used, to give correct colour
values colour sets *had* to display a picture with a stable black-level.

We have a PAL TV set that displays bright white as black. :-)

Geoff.

--
Geoffrey S. Mendelson, N3OWJ/4X1GM
My high blood pressure medicine reduces my midichlorian count. :-(