AGC in English 405 lines positive modulation sets...

[ppppenguin]

With -ve systems you don't need gated/keyed AGC. You can just use the peak level. If the transmitted signal is accurate then peak level and black level have a fixed relationship.

[Telecolor 3007]

I wish me an early British color tv set.
Most people in Romania don't know that in Great Britain where manufactured tv sets.

[old_tv_nut]

Originally Posted by ppppenguin

With -ve systems you don't need gated/keyed AGC. You can just use the peak level. If the transmitted signal is accurate then peak level and black level have a fixed relationship.

True. Gating is not necessary, but it improves immunity to impulse noise (like car ignition noise, which used to be a problem). The effects of impulse noise could also be reduced with a noise inverter stage that canceled impulses that went too much blacker than black.

In positive modulation systems, impulse noise would appear as white flashes in the picture instead of black.

[Colly0410]

Read somewhere that some British 625 single standard UHF only sets didn't have gated/keyed AGC as car ignition interference is so low at UHF. I remember seeing low band (band 1) ignition interference when on vacation in Barcelona (on ch E4) & Miami (on ch's 2, 4 & 6) in 1988 & 89. I hadn't see ignition interference since we went from 405 VHF to 625 UHF in the early 1970's...

Some 405 positive modulation sets had an interference limiter: it was supposed to make the bright white flashes go to mid grey IIRC, if you turned it too far up the picture would have sort of negative'ish look to it, it's a bit hard to describe the look...