The main types of processor encountered in the radio broadcast chain are:
Microphone Processor
Transmission Processor
Both are audio processors and the principles are similar; to alter the sound in ways that improve its reception or intelligibility.
The basic processing that audio signals undergo is of gain, the change the volume level, and equalisation - adding or reducing the bass, mid or treble frequencies. Contolling the LEVEL of gain is vital for several reasons, not the least of which is to make listening a more pleasurable experience for listeners. Constantly varying the volume of a receiver is a nusance.
More importantly, allowing sudden and large variations in level can damage items of equipment later in the broadcast chain, including valves or semiconductoirs in the Power Output stages.
The two parts of the audible spectrum, bass and treble, which are often seen on domestic equipment and as 'mid range, often seen in musicians PA equipment, are in fact very much 'generalisations'. For professional audio processing, it's preferable to split the audible spectrum into several bands, perhaps as many as ten. Audio processing tweaks and modifications are then applied to each part of the spectrum individually.
As well as equalisation, a signal may be expanded, in which the volume is increased during quiet periods, or limited, which adds a ceiling to the highest sounds and stops them going beyond a certain threshold. A signal may also be companded, which is to supress the high sounds and squeeze in all the minima and maxima, effectively squashing the sound.
The amount of processing added to a signal may make it very dense, so it sounds LOUD to the human ear. Adding too much processing however while making the sound 'loud and proud' on a radio dial in fact also makes the ear get very tired of hearing it after a while. this is called 'listener fatigue' and often makes people switch off a radio station.
A commonly seen but very misundestood control on any audio processing is GATING. This is simply some circuitry muting the audio signal when it is below a certain threshold, making the signal quieter as it reduces 'background' noise. The 'noise floor' is the general mush and low level sounds that do not add any information to the audio.