The process of extracting the audio signal from the modulated wave is known as demodulation or detection. The wireless signals consist of radio frequency (high frequency) carrier wave modulated by audio frequency (low frequency). The diaphragm of a telephone receiver or a loud speaker cannot vibrate with high frequency. So it is necessary to separate the audio frequencies from the radio frequency carrier wave.
Simple demodulator circuit
A diode can be used to detect or demodulate an amplitude modulated (AM) wave. A diode basically acts as a rectifier, i.e., it reduces the modulated carrier wave into positive envelope only.
An AM wave input is shown in Fig. 16. It appears at the output of the diode across PQ as a rectified wave (since a diode conducts only in the positive half cycle). This rectified wave after passing through the RC network does not contain the radio frequency carrier component. Instead, it has only the envelope of the modulated wave.
In the actual circuit the value of RC is chosen such that 1/fc << RC, where fc is the frequency of carrier signal.