The concept of Pre emphasis and De emphasis is a broad subject, however I will try to give you the basic concept behind the advantages, and necessities of using it in FM.

In common narrowband two way fm communications,  Pre emphasis follows a 6 dB per octave rate. This means that as the frequency doubles, the amplitude increases 6 dB. This is usually done between 300 - 3000 cycles. Pre emphasis is needed in FM to maintain good signal to noise ratio. Why is it necessary? Common voice characteristics emit low frequencies higher in amplitude than high frequencies. The limiter circuits that clip the voice to allow protection of over deviation are usually not frequency sensitive, and are fixed in level, so they will clip or limit the lows before the highs. This results in added distortion because of the lows overdriving the limiter. Pre emphasis is used to shape the voice signals to create a more equal amplitude of lows and highs before their application to the limiter. The result is that the signal received is perceived louder due to more equal clipping or limiting of the signal, but probably more important is the increased level of the higher frequencies being applied to the modulator results in a better transmitted audio signal to noise ratio due to the highs being above the noise as much or more than the lows. So what is the original reason for Pre Emphasis? Read on and skip back to this explanation of the original reason.

Transmitters that employ a true FM modulator require a pre emphasis circuit before the modulator fore the true FM modulator doesn't automatically pre emphasize the audio like a transmitter that uses a phase modulator. A separate circuit is not necessary for pre emphasis in a transmitter that has a phase modulator because the phase modulator applies pre emphasis to the transmitted audio as a function of the phase modulator.

The receivers De emphasis circuitry takes the unnatural sounding pre emphasized audio and turns it back into its original response. Pre emphasized (discriminator) audio is however available directly from the audio demodulation (discriminator) circuitry.

In linking systems, many choose to eliminate the emphasis circuitry to allow better representation of retransmitted signals. Since the signal has already been pre emphasized (by the user that is transmitting,) and since the receiver you are listening to takes care of the de emphasis.... it doesn't need to be done over and over again.
Some loss of quality does exist, but quality is better maintained by a flat system. A flat audio response system is one which has equal output deviation for the same input deviation, no matter what the applied audio frequency is within reason. Reasonable audio frequency response would be from 50 cycles to about 3500 to 5000 cycles in a system not filtering PL. Audio response in a system filtering PL would be around 250 to about 3500 to 5000 cycles. Upper cut off frequency would be determined mainly on acceptable use of available bandwidth.

Available Bandwidth.
Injecting discriminator audio back into an FM modulator without any limiting or low pass filtering is bad news, plain and simple.  On UHF, you've probably been able to get away with it without excessively bothering either of your adjacent channel neighbors, but on 2m, especially with 15 kHz spacing, you'd be asking for a lynching.

Without low pass filtering, all of the high frequency energy (hiss) that comes from the discriminator from a noisy user, if not low pass filtered, will deviate your transmitter in excess of 5 kHz, in addition to pushing the sidebands out further than they would be if the AF was cut off at/about 3 kHz.  Do this to see what I mean.  Set your repeater up for 1:1 input to output ratio (like, put in a signal that is deviated 3 kHz by a 1 kHz tone, and set your Tx audio gain to get 3 kHz out of the transmitter). Now open your repeater receiver squelch wide open.  You should see your transmitter is now deviating somewhere around 8 or 9 kHz (presuming you have enough audio headroom through the controller).  Under this test condition, the combination of the excessive deviation and the lack of high frequency filtering will make your signal somewhere around 30 kHz wide instead of 16 as it should be.  The only thing "limiting" the occupied bandwidth at that point is the dynamic range of the audio circuits and the natural high frequency rolloff of the discriminator's output noise.  Look at it on a spectrum analyzer if you don't believe me.

Obviously that's a worst case scenario, but the fact remains that you should have brick wall limiting at 5 kHz (a little lower at 15 kHz channel spacing), and low pass filtering at 3 kHz (a little higher if you want on 25 kHz channels).

A repeater can be built to utilize a flat audio response to maintain quality through the system. This is fairly easy in a system using a true FM modulator. Usually modification to the controller is necessary to allow it, especially ones that have speech or a phone patch. Systems using a phase modulator require de emphasis before the modulator because of the automatic pre emphasis of the phase modulator, and for this reason... it is easier to utilize flat audio modifications and maintain quality audio in a system employing a true FM modulator in the transmitter.
 
 

Kevin K. Custer  W3KKC   January 19 1999