charles wrote:
In article , David Looser
wrote:

[Snip]

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.

I remember hunting at the "Radio Show" in 1964 for a dual standard set that
had a proper black level clamp. I succeeded, but with difficulty.

I seem to remember there was an upmarket dual-standard Murphy with the
channel buttons on the top surface that had it. Those sets were
available in a variety of cabinet colours, pink, mauve, orange, etc,
such was the gaiety of the times.

When you changed channel on those sets there was a black screen
initially, then the picture sort of faded in.

The sets had better than average sound as well.

Bill