Re: How good were 1950's/1960's Band 1 TV receivers?


"Andy Wade" <spambucket@xxxxxxxxxxxxxxxxxxx> wrote in message
news:46af2aa1$0$31718$db0fefd9@xxxxxxxxxxxxxxxxx
Mark Carver wrote:

I can't remember seeing black level clamping on many b/w sets, 625 or
405, though I'm a bit too young to remember 405 lines that well.

My memory is that most of the earlier (50s & 60s) B/W sets tended to have
sync-tip DC restoration (diode clamp) after the demodulator (always an
envelope detector in those days of course) and were then DC-coupled
through a pentode video o/p stage to the CRT. However the great majority
had mean-level AGC, gated AGC being found only on distinctly up-market
sets.

By the mid-70s, when B/W was on its way out, the inevitable
cost-engineering had led to AC coupling throughout, with no attempt at
clamping at all. Programme producers in that era, ISTR, avoided dark
scenes which would display as indifferent mid grey on most viewers'
equipment. That era really was the nadir of TV set design, with some
really horribly unreliable components in use, plus all the nastiness of
dual-standard switching still being around.

The other point about impulse interference being less visible on
negative modulation pictures, well is it in practice ? Surely
overshoot within the receiver's circuity would give any black dot a
nice white edge ?

I'm sure you're right, c.f. the recent thread ("dodgy lamp ...") on the
same subject in uk.tech.digital-tv.

When I was studying TV receiver theory in about '69 the "black rather
than white spots" argument was definilatly given as the reason
for negative mod, but the klystron operating point does seem just
as plausible. Klystrons and TWTs didn't make much of an
impact on the syllabus AFAIR.

--
Graham.
%Profound_observation%


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