[paul cotter]

Most one saw on those tv screens was random electronic noise as the agc ramped up the gain in the absence of a signal.

[cpu68 (OP)]

Most one saw on those tv screens was random electronic noise as the agc ramped up the gain in the absence of a signal.

You may be partially right.

[paul cotter]

Very easy to prove with one of those old tv sets. Disconnect the aerial(antenna) and there will be no observable change in the "snow" on the screen, hence it is internally generated in the high nineties %.