AGC in English 405 lines positive modulation 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.