Q. What is Message Demodulation?

The major advantage of the conventional (standard) DSB AMsignal transmission is the ease with which the signal can be demodulated. Themessage signal is received by passing the rectified signal through a low-pass filterwhose bandwidthmatches that of themessage signal. The combination of the rectifier and the low-pass filter is known as an envelope detector, which is almost universally used. A standard AMenvelope detector consisting of a diode and an RC circuit (which is basically a simple low-pass filter) and its response are shown in Figure.

During the positive half-cycle of the input signal, the diode is conducting and the capacitor charges up to the peak value of the input signal. When the input falls below the voltage on the capacitor, the diode becomes reverse-biased and the input becomes disconnected from the output. During this period, the capacitor discharges slowly through the load resistor R. On the next cycle of the carrier, the diode conducts again when the input signal exceeds the voltage across the capacitor. The capacitor charges up again to the peak value of the input signal, and the process is continued. The time constant RC must be selected to follow the variations in the envelope of thecarrier-modulated signal. Generally, it is so chosen that

The simplicity of the demodulator has made conventional DSB AM a practical choice for AM radio broadcasting. A relatively inexpensive demodulator is very important in view of the several billions of radio receivers. With only a few broadcast transmitters and numerous receivers, the power inefficiency of conventional AMis justified. Thus, it is cost-effective to construct powerful transmitters and sacrifice power efficiency in order to have simpler signal demodulation at the receivers.