PRE-EMPHASIS & DE-EMPHASIS:
refers to boosting the relative amplitudes of the modulating voltage for higher
audio frequencies from 2 to approximately 15 KHz.
means attenuating those frequencies by the amount by which they are boosted.
However pre-emphasis is done at the transmitter and the de-emphasis is done in
the receiver. The purpose is to improve the signal-to-noise ratio for FM
reception. A time constant of 75µs is specified in the RC or L/Z network for
pre-emphasis and de-emphasis.
1. Pre-Emphasis Circuit:
transmitter, the modulating signal is passed through a simple network which
amplifies the high frequency, components more than the low-frequency
components. The simplest form of such a circuit is a simple high pass filter of
the type shown in fig (a). Specification dictate a time constant of 75
microseconds (µs) where t = RC. Any combination of resistor and capacitor (or
resistor and inductor) giving this time constant will be satisfactory. Such a
circuit has a cutoff frequency fco of 2122 Hz. This means that frequencies
higher than 2122 Hz will he linearly enhanced. The output amplitude increases
with frequency at a rate of 6 dB per octave. The pre-emphasis curve is shown in
Fig (b). This pre-emphasis circuit increases the energy content of the
higher-frequency signals so that they will tend to become stronger than the
high frequency noise components. This improves the signal to noise ratio and
increases intelligibility and fidelity.
2. De-Emphasis Circuit: