In article , Bill Wright
wrote:
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.

I think that I bought an Ekco. 4 preset buttons for VHF and another 4 for
uhf with a change=over switch between.

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