Optimod is an "intelligent" form of compression - primarily used in radio in order to limit dynamic range. The reasons for it's use were:

1. It's easier to listen to a compressed signal on a portable radio, especially if there's high levels of background sound - like in a car or workplace.

2. It's cheaper to transmit a compressed signal - transmission is a reverse process where the loudest sound needs the smallest power to transmit it..

3. Folks were more likely to listen to the loudest radio station when tuning in - therefore if the level was kept to as near +12dB as poss more folks would listen..

4. Most pop music is compressed anyway - though this is a chicken and the egg jobbie - was radio compressed heavily before the music?

The same thing is used a lot on TV now as well - though not necessarily Optimod.. The Well Known Soap I used to work for had a frequency-dependant compression system - brought in mainly because the continuity announcements and trails between the programmes were significantly louder than the progs themselves! We were quite gentle with it - allowing about 16dB of dynamic range.. Radio 1 is nearer to 3dB!