AGC in English 405 lines positive modulation sets...
 

Most early 405 lines TV's didn't have AGC, they could only receive a single channel in low band, (band 1) & if the signal faded so did the picture. Some posh sets had an AGC system that used the back porch as a signal strength reference as that was the only part of the signal that stayed constant, but it was fairly complicated to do , the sync pulses couldn't be used as they were at zero % modulation. When ITV on high band (band 3) fired up in the mid 1950's most set makers used a mean level AGC system, this used the average picture content to adjust gain, however the gain would vary depending on picture content = bright scenes would be reduced in brightness & dark scenes increased in brightness & the black level would be all over the place. You'd think this would be disastrous on the picture, but us Brits got used to it & accepted it as normal, a lot of setmakers also abandoned DC coupling/restoration & went for pure AC coupling. Dual standard 405/625 sets also used mean level AGC, it was only when single standard 625 UHF sets using negative modulation only were made in late 60's/early 70's that sync tip AGC became normal....

Interesting. Of course, the iconoscope (or equivalent) cameras used early-on were inherently AC coupled and had a floating black level anyway. It is conceivable (not sure if it was done) to clamp on the dark parts of the image; This would prevent shadows going blacker than black, but would make the darkest areas black even if they were really gray.

A section on an iconoscope film camera in Fink's "Television Engineering" (1952) says (describing the control amplifier):
"The second video stage also feeds a rectifier which develops a bias at the black level. This bias is added to the bias fof the follower [sic] clipper stage, so that the clipper always limits at the black level. The black level control is disabled during the blanking intervals..."

This seems to correspond to setting the darkest part of the image to black.

Mean level AGC, especially with AC coupling, was often known as Always Grey Control.

At the studio end, it was the job of the racks engineers to keep black level where it should be. Working an Emitron/Iconoscope camera was a difficult job. Not only black level but also tilt and bend to correct for brightness distortions within the picture.

For a while (I think non-keyed AGC fell out of use in the second half of the 50s) non-keyed AGC was common here in the USA. It probably worked marginally better here since sync/black level here has always been at the top end of carrier strength....so no matter what the video was the sync pulses gave some extra consistency to the average.

"Always grey control" - love it!

If you google "TV shader job" Google leads to "camera shading jobs" and "studio video technician," so the term "shader" has been a sticky one, it seems, even though no one has done that instantaneous manual correction for a long time now.

Frances 819 & 625 line systems used positive video modulation as well as Britains, I'm presuming they used mean level AGC for their black & white sets too, I'm presuming they used gated back porch AGC for their SECAM colour sets, unless someone knows different..

Edit... Belgium also used positive modulation 819 & 625 systems till the late 1960's/early 1970's...

I bet Jhalphern knows France's video modulation polarity.

It must have been an interesting time for Belgium's TV repair shops when broadcasters switched modulation polarity...

All the TV system parameters are conveniently available here: https://en.wikipedia.org/wiki/Broadc...vision_systems

The following used +ve vision:
A: UK 405
C: 625. Early, discontinued in 1977
E: France 819. Full BW
F: Belgium 819. Reduced BW
L: 625. Last surviving +ve system. France until 2011.

Belgium probably had the most need for complex multistandard sets as they could receive signals from several countries.

+ve vision systems all used AM sound though this wasn't an engineering requirement. +ve systems can't use intercarrier audio as the vision carrier falls to zero at sync tips.

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It must have been an interesting time for Belgium's TV repair shops when broadcasters switched modulation polarity...[/QUOTE]

Here in England we have a history of messing about with & swapping TV systems.... Until the mid 1950's all TV's were only able to receive low band (band 1) as that's where the only broadcaster (the BBC) lived using channels 1 to 5. When ITV fired up region by region in the mid/late 50's using channels 8, 9, 10 & 11 (channels 6,7,12 & 13 were not used for TV till the 1960's, I don't know why they were not) people who wanted to watch the new channel had to buy a converter or new TV. I remember we had a BBC only TV till 1959, I used to go round next door to watch cartoons on ITV... In 1964 BBC2 fired up on UHF with a new line system of 625 using negative picture modulation & FM sound so a UHF to VHF converter wouldn't work, they made dual standard sets that were complicated unreliable monsters. in the 2000's/2010's we changed to digital but you could buy digi-box converters for about 15/20 pounds so you didn't need to buy a new TV if you didn't want to...

But negative modulation haves any advantages over positive modulation?
All bistandard sets (405 & 625 lines) where so unrelaible? They made some color ones too.

-ve modulation allows intercarrier sound. I'm sure there are other advantages too but can't think of them at the moment.

The UK's dual standard era started in 1964. At that time most TVs were not especially reliable. Having a standards switch made things worse. Early colour sets in the UK (1967) were all dual standard and likewise unreliable. In 1969 we had the first 625 only sets in the UK. Reliability improved for 2 reasons. One was getting rid of the system switch, the other was a general improvement in technology.

Read somewhere (can'r remember where or how true it was) that in the 405 days BBC & ITV tried to avoid making programs with very dark or very bright scenes because of the mean level AGC in most sets. Of course when showing films or imported programs they couldn't do this, wondering if they adjusted the telecine gubbins to make things appear better with mean level AGC?

Another thought I've just had... Did any American (or any other countries) TV's that used negative modulation have mean level AGC? I'm presuming most had gated sync tip AGC, but maybe a cheapskate or British import?

Ooh yes, we had mean level AGC. Our early post war sets had no AGC and the user was expected to control RF/IF gain manually with the " contrast" control. Later various AGC circuits were tried and many were signal mean over a few frames time these designs were common into the first half of the 50's. In the mid 50's gated sync AGC (in the USA we call it keyed AGC) started taking over in the mid 50's.