De-emphasis
It is common for FM broadcasting to introduce pre-emphasis at the transmitter; high-frequency audio signals are boosted in comparison with middle and low frequencies. The purpose is to swamp most of the noise, the greater part of which tends to be high frequencies. At the receiving end, the signal must be subjected to de-emphasis by attenuating the high- frequency treble, content in a similar but opposite way to the emphasis at the transmitter. Emphasis is carried out by the use of a filter and is expressed in terms of the time constant of the filter. In the UK, a pre-emphasis of 50 Jl s
is used. A de-emphasis filter with a time constant of 50 Jl s must therefore be used in the receiver to restore the audio frequencies to their original relative levels.