AGC in English 405 lines positive modulation sets...

[Colly0410]

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....