Pre Emphasis

Pre emphasis is nothing but enhanging the message signal 
and it is used for Transmission time.

De Emphasis

De Emphasis nothing but the comprese the message signal and 
it is used for receiving time.

Pre-emphasis

" Improving the signal to noise ratio by increasing 
the magnitude of higher frequency signals with respect 
to lower frequency signals"

De-emphasis


" Improving the signal to noise ratio by decreasing 
the magnitude of higher frequency signals with respect 
to lower frequency signals"

PRE-EMPHASIS :-

In an FM system the higher frequencies contribute more to
the noise than the lower frequencies. Because of this all FM
systems adopt a system of preemphasis where the higher
frequencies are increased in amplitude before being used to
modulate the carrier. The transfer function sketched above
is used for a preemphasis circuit for FM signals in the FM
band. The Time T = 75µs. For FM systems in the FM band m ~ 5
resulting in a S/N improvement of 19dB. With preemphasis
this can be increased by 4dB for a total of 23dB. 





                DE-EMPHASIS :-

At the receiver the higher frequencies must be deemphasised
in order to get back the origunal baseband signal. The
transfer function of the deemphasis circuit is shown above.