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From: Martin Brown <'''newspam'''@nonad.co.uk>
Newsgroups: sci.electronics.design
Subject: Re: Who remembers how bad analogue television was?
Date: Thu, 27 Feb 2025 20:45:07 +0000
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On 27/02/2025 19:58, KevinJ93 wrote:
> On 2/26/25 8:52 PM, Sylvia Else 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 suspect this same poor AGC was responsible for a shift in the 
>> detection of the sync pulse such that the text would be moved to the 
>> right of its proper position, which could result in distortion of the 
>> letters as the average brightness varied line by line.
>>
>> In the early days of television, using thermionic valves, it was 
>> probably a miracle that these things worked at all, but surely in the 
>> transistor age, something better could have been provided.
>>
>> Were studio monitors any better, anyone know?
>>
>> Sylvia.
> 
> Are you sure that 1984 date is correct? By 1970 in the UK colour TVs 
> used transistor signal processing stages and many had already changed to 
> transistors for the power stages such as line and frame output as well 
> as using chopper stabilised power supplies.

The first two colour TVs I recall owned by friends or family were about 
the time of Apollo 8 in 1968. Memorable for the Earth rise shot. Both 
were entirely valves and my uncle's caught fire leaving a nasty brown 
burn mark on their wool carpet and smoke damage on the ceiling.

The earliest was at a school friends house and was in pastel shades pre 
Nd glass. It was in colour but only just... Joe 90 launch was the first 
programme I can recall watching there in colour. Test cards in shops 
don't count.

I'd believe 1974 as a date for hybrid colour TVs that almost worked 
correctly and didn't need a service engineer visiting them every other 
week. By 1980 I'm pretty sure they were almost entirely semiconductor based.

> Typically they used some form of gated AGC to not be affected by the 
> video modulation.

I recall sound on vision being a bit of a problem too with certain types 
of check or dogtooth suits or other high contrast periodic fabrics.

-- 
Martin Brown