I'm sure Hugh will be along to say what happens at the BBC, but here's a portion of a post from the PitBull Guitars' recording forum where Marcel, who has worked on radio hardware for most of his life, describes the general setup in Australia.
"In the early days of broadcasting (back when AM radio was King) it was found that over driving transmitters was a very bad thing which apart from adding unwanted distortion to the voice of the DJ also had the problem of making it very difficult for the listener to tune in their often quite basic radio. Other problems included 'splatter' where one radio station could be heard on multiple places on the old school dial. So to alleviate this it was decided and internationally legislated way back in great grandads day that hard audio limiters in the audio chain would be mandatory and ALC (Automatic Leveling Control) would be an recommended option that each station could implement at their discretion.
Fast forward to the '70's and the whole broadcast audio limiting/ALC changed into a race into who could sound the best/loudest. A plethora of new solid state devices that optimised the audio for transmission were devised and implemented. This all came under the thinking that a smaller and cheaper to operate transmitter if modulated correctly could sound as loud as a transmitter 10 times the size and cover as many miles or kilometres as one twice the size, so it was a cheap way of boosting presence on the dial, to sound bigger, better, more awesome all for less money....
FM radio and TV didn't escape the madness. They too have their own versions driven by the desire to sound the best/loudest while staying within the national Federal broadcasting regulations.
So what do they do?. Well essentially the hard limiter by legislation must be there, often in the form of a fast attack and fast release 20:1 compressor with the threshold set at 95% modulation as the last element just before being sent to the transmitter, purely to prevent over modulation. But the ALC part is where it gets interesting. The automatic levelling part is for the most part again is a 20:1 compressor that has a fast attack and very very slow release and a threshold set quite low in the 10 to 30dB below 0VU range used with the intention of averaging all the audio to a predetermined volume. As part of the averaging the audio is broken down into frequency ranges that are also averaged. The number of frequency groups is determined by the type/style of typical program material, but typical breakdowns are usually between 4 and 8 frequency groups. If you think of a 2way or 3way crossover in your typical loud speaker it's a similar type breakup of frequencies being done, one or two bass frequency groups and one or two mid frequencies and one or two treble frequency groups. All the individual frequency groups are ALC'd individually before being recombined into the one signal that is sent to the hard limiter and then the transmitter....
Setting up one of these 'audio processors' is a black art reserved for the station technical engineer under the verbal direction of the station management team. Nobody 'plays' or fiddles with the station audio processor as it does invariably affect how the station sounds to all its listeners.
So if you were ever wondering why your CD or MP3 sounds so vastly different on the radio than from your home stereo, that is why.... The processor does change the 'sound' of the audio passing through it...
If you're interested in seeing a very popular model of these 'audio processors' in operation then follow the link which will take you to a YouTube list of demonstrations.... https://www.youtube.com/results?sear...=orban+optimod
The photos are of the one I had the joy of adjusting just the other day... "
So basically very heavy multi-band compression/limiting followed by another limiter to ensure nothing above a regulated maximum signal level.
Here's a link to the original post. http://www.buildyourownguitar.com.au/forum/showthread.php?t=8762&p=169417&viewfull=1#post169417