[Stooky Bill]

Depends on the definition of dead air and the era.

If its dead air as in transmitting something that just happens to be a black screen for whatever reason, that may require somebody to intervene to get something else on air.  Usually an OB has packed up or something or a source has suddenly become unavailable but the channel itself is still "on air". Case in point:  The One Show once had a fire alarm, they showed a VT, evacuated, the VT ran out and there was a black screen for a while until presentation took over.

Prior to I believe 1991 the IBA transmitters if they lost their source would show you a blue screen generated by the transmitter, though sometimes the sound would come through but not the picture for some reason.  Some examples on YouTube.

In the digital era not sure what happens if the Freeview transmitters lose their source, but I presume its just a frozen picture.

There's two different things; there's loss of content and loss of signal. 

If there's black or silence but there's still valid video then that's a loss of content. There are detectors throughout the TX chain that trigger alarms after a certain duration of silence or frozen video. So the incident today would have set off alarms in control rooms in both Red Bee and the BBC. In the case of the latter a lot of alarms as there's many versions of BBC1, so it'll trigger them for every encoder of every region and every path that gets it to air! This is the reason why the breakdown captions and overnight slates both move and have some audio. Incidentally, those silent adverts for Trade Centre UK do trigger silence alarms, which is one reason why you wouldn't want anything automatic to step in - sometimes silence is intentional

Loss of video signal - i.e. the SDI video, or the transport stream is missing or corrupt. This is something that is also detected, but will automatically switch between main and reserve paths... assuming the other one is good. These sort of switches do so instantaneously and often are barely noticable on air 

So if something fails to run or an OB fails then that's something that'll need to be sorted by where the channel is coming from - everything's synchronised so it'll be good valid video. However if I went and pulled out a cable carrying one path of BBC1, or reconfigured something so it's non-spec video then that would also alarm but the chain would auto switch cut away from the fault   


In terms of transmitters, there's no way they could detect content. A TV transmitter is fed with an MPEG transport stream of several video and audio services and metadata all multiplexed together. It's just data, it doesn't care if BBC1 is black and silence and there's very little that it could do anyway. 

It's different to the analogue days where each channel had an independent set of transmitters at each site that were fed with baseband video, there they could do some detection and if the TV company was sending nothing on every path then it could put up a caption. 

Radio has the backup tape (which can on occasion reveal its age and the last time it was updated) due to the medium, as its usually the station itself is still "on air", just transmitting the audio equivalent of nothing.  I believe as as its an audio medium it has no other output.  Few examples where the fire alarm has gone off and cut the studio output to prevent that from going out on air, but equally some examples of where it has gone out on air and they've managed to segway gracefully into the backup material.  Example:  Radio 5.

Radio is simpler, only one thing to detect: 'is there audio, or is there no audio?' Also it's still transmitted in analogue so it's possible to have a silence detector at an FM or MW transmitter. But like in TV, if one station in a DAB multiplex goes silent then that's not going to be detected, it has to be done at the station while it's still exists as audio. 

The  BBC and presumably other places like Global have the silence detection at the control room at their studios as they're feeding multiple transmitters and platforms. So if Radio 2 or Classic FM goes silent for a while there's just one silence detector that triggers one emergency 'tape', they're not triggering hundreds round the country, which would be a nightmare to manage.  Transmitters have reserve feeds and detection to switch to them if they lose their signal completely.

Also you wouldn't want automatic emergency tape that's too sensitive or too keen to switch back in case the problem is intermittent, imagine having the emergency tape start playing for a few seconds, stopping and then restarting again after 2 minutes when the audio is lost again

Going cleanly into the backup 'tape' is only possible when the studio has a copy of it, otherwise they have to give it x number of seconds of silence first. Getting out cleanly when it's the emergency tape is done by feeding that into the studio and then putting the studio back on air. There's a great example of both of these happening one morning to Chris Moyles when he was on Radio 1 - he put it on manually from the studio first time round and no-one noticed, but had to let it crash to air when it was needed again.