Re: Who remembers how bad analogue television was?

Sylvia Else <sylvia@email.invalid> wrote:

Leave aside the ghosting, which could largely be addressed by having a
decent antenna.
 
But my memory of a Philips Colour TV (1984ish) was that it had rubbish
automatic gain control (AGC), and odd interactions between brightness
and picture position.
 
The AGC should have been based on the amplitude of the sync pulses,
which was 30% of the total. I'm sure this could have been done, but my
experience was that instead it was based on the average amplitude of the
demodulated signal. A black image containing large white text, such as a
title screen, would show a clear darkening to the sides of the text,
while being decidedly grey over the rest of the screen.

I think you may be mis-remembering; something similar to the fault you
describe was prevalent on B&W televisions which were built down to a
price.  Some of it was caused by average AGC and some was due to lack of
DC coupling, or skimped DC restoration, in the video amplifier.  Some of
the better sets used back-porch AGC and, for the enthusiast, add-on
circuits were published in Wireless World (designed by Mothersole, I
think).

From the beginnings of colour television the designers recognised that
all three video amplifiers had to be DC coupled but the AGC was much
simpler because they used inverted modulation, so a sync pulse
corresponding to 100% modulation was always available.  I can't imagine
Philips would have produced a model with such gross errors as you
describe,.  Was your own set faulty or was this a common insurmountable
problem caused by NTSC and positive modulation on the system in use in
the U.S. at the time?

In Europe, Philips and Mullard (their UK valve-making subsidiary)
published large quantities of material to aid set designers and help
them get the best out of their range of valves.  I read it several years
before the colour television service started in England and it included
details on DC coupling and AGC. (The BBC did a lot of their preliminary
experimental work using NTSC - but eventually decided to use PAL for the
public broadcast system).


--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk