[WillPS]

IBA transmitters had ‘silence’ detectors - if they detected no audio for a period (2 minutes from memory) they would automatically transmit a locally generated apology caption. So to prevent this, as the transmission staff were putting on their hats and coats before leaving the building they would output black/bars/test card and tone to the transmitter as they rushed out to their cars.

This example from late-1981 shows the blue caption kicking in less than 30 seconds after loss of video.

www.youtube.com 

Presumably if the transmitter was manned an on site engineer could have put the slide up before the silence detector did it automatically?