The reason that preemphasis is needed is that the process of detecting a frequency-modulated signal in a receiver produces a noise spectrum that rises in frequency (a so-called triangular spectrum).
Preemphasis increases the magnitude of the higher signal frequencies, thereby improving the signal-to-noise ratio.
At the output of the discriminator in the FM receiver, a deemphasis network restores the original signal power distribution.
FM improvement factor is the quotient obtained by dividing the signal-to-noise ratio (SNR) at the output of an FM receiver by the carrier-to-noise ratio (CNR) at the input of the receiver.
FM improvement threshold is the point in an FM (frequency modulation) receiver at which the peaks in the RF signal equal the peaks of the thermal noise generated in the receiver.