| Modulation Feature Focus |
R.V.Shyam Prakash, IIT, B Contributing Editor |
| Radios have come a long way from the bulky boxes to the compact pocket transistors on which we listen to the latest score of the India-Pakistan match. However, the fundamental process of transferring information from one point to another through the use of radio waves remains essentially the same. The essential problem with radio waves is their attenuation with distance, leading to loss in information and hence a small range. This problem is overcome using an amplifier at the input and also using an interesting process called Modulation. The main task of radio engineering is to transmit information over a distance. This is effected by means of signals, emitted by suitable aerial systems connected with the transmitter. These signals are capable of travelling through space separating the sender from the recipient of information. Such signals are high-frequency oscillations also called Radio Waves. The information to be transmitted must be transformed into high-frequency oscillations or just a carrier. The selected frequency wo of this oscillation depends on the distance between the transmitter and the receiver, the conditions of propagation and some other economical and technical factors. But, in any case, the frequency wo, must be higher than the upper frequency of the message to be transmitted. In general, a signal carrying information can be presented in the form: a(t) = A(t) cos (wot + q(t))...................(1) Which can be simplified to: a(t) = A(t) cos (y(t)).........................(2) The process of controlling one or several parameters of a high frequency signal is called modulation. Depending on the parameter being |
changed (the amplitude A or the angle y), two of the fundamental types of modulation are known, as amplitude modulation (AM) and angle modulation. The latter is itself divided into frequency modulation (FM) and phase modulation. The individual signals to be transmitted are modulated with different carrier frequencies. The modulated signals are then summed and transmitted simultaneously over the same communication channel. Through this process the individual input signals are allocated distinct segments of the frequency band. To recover the individual channels in the process, there are two basic steps: 1. Band pass filtering to extract the modulated signal corresponding to a specific channel. 2. It is followed by a demodulation process to recover the original signal. Modulation is extensively used in communication systems. Angle modulation have a number of advantages over amplitude modulation, for example, the modulated signal has a constant peak amplitude and thus allows the transmitter to operated at peak power always. For these reasons, in public broadcasting and a variety of other contexts, FM reception is typically better than AM reception. FM however, requires greater bandwidth than AM does. |