Pre-emphasis is a boosting of the high-end of the audio base-band
Early transmitters were PM (phase modulated), not FM, so they had an
inherent 6dB/octave pre-emphasis. When FM transmitters came along,
the audio had to be pre-emphasized to maintain compatibility with
Pre-emphasis has nothing to do with signal-to-noise ratio.
If FM had always been used, there never would have been pre-emphasis
The following is a network which will give the required amount of
pre-emphasis. This network is dubbed a '75 microsecond' pre-emphasis
network because of the time constant created by the product of the
resistor value times the capacitor value. A '50 microsecond'
pre-emphasis network could be created by making both resistors
50k-ohm in value. This would be the standard amount of pre-emphasis
used in Europe.
The 6dB gain point is shown in the graph. This is the point where the
voltage gain, Vout/Vin, equals 2. The slope of the curve on the right
side of the plot is equal to 6dB/octave.