what is the function of pre-emphasizer and dee-emphasizer in Stereophonic FM Receiver?
The de-emphasis is used to flatten the noise acquired during transmission. White noise in the RF input of the receiver does not come out of the FM demodulator as white noise. The de-emphasis filter is designed to flatten this noise spectrum and turn it in to white noise.  However, it also affects the signal itself, so the emphasis filter is required to get a flat signal with the de-emphasis.