The U.S. 75 microsecond Pre-Emphasis Curve used for FM Broadcasts in the Americas and South Korea.

 

 

 

 

 

 

 

 

 

 

What is Pre-Emphasis?

Pre-Emphasis is an early form of noise reduction.  It is where the high audio frequencies are boosted by some amount and transmitted over a delivery system.  At the receive point, the high frequencies are attenuated (De-Emphasized) at a porportional amount as the boost.  By doing this, noises in the transmission system are also reduced by that amount.

This type of noise reduction is used in analog tape and phonograph recordings, as well as radio broadcasts.  This is why high frequency materials (such as “esses” and cymbols) are sometime difficult to handle through these mediums.

The chart above shows the mandated limits for broadcasting on the radio bands here in the United States.Here is what pre-emphasised audio sounds like:

This is the same audio clip with De-Emphasis applied, just like what happens through your FM radio.

These clips are also heavily processed so you can hear some of the effects of heavy clipping on the pre-emphasized audio.