Preemphasis and Deemphasis

Cuthbert Nyack
When an FM system is compared to an AM system with a modulation index of 1 operating under similar noise conditions, then it can be shown that the FM signal has a signal to noise ratio which is 3m2 better than the FM system. m here is the modulation index or deviation ratio for the FM signal.

Preemphasis Transfer Function

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.

Deemphasis Transfer Function

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.
Passive circuits are shown below. For deemphasis RC = 75µs. For the Preemphasis CR1 = 75µs. To get unit gain at low frequencies an opamp with gain(R1 + R2)/R2 must be added. The pole should be set at a frequency higher than the maximum modulating frequency.

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COPYRIGHT © 1996, 2012 Cuthbert Nyack.