All fm radio aid systems have Automatic Gain Control or AGC. This is required by all national radio regulatory authorities in order to control the use of valuable radio spectrum.
There are many forms of AGC. Basic hearing aids use 'peak clipping' (also known as hard limiting) and more advanced hearing aids use 'compression'. fm radio aid systems use 'soft limiting' in which changes of sound level at the input are directly reflected at the output up to a certain value known as the 'knee point'. For example, at an input sound level of 60dB SPL, a 5dB increase of sound at the input results in a 5dB increase of output. Above the 'knee point' the AGC circuit controls the gain so that the output remains virtually constant.
There is a very great difference between the hard limiting used in some hearing aids that causes much distortion and the soft limiting used in radio aid systems that does not. Compression is a process in which quiet sounds are made louder and loud sounds are made quieter either side of a particular level, but it is usually a linear process where there is no 'knee point'.
