Communications Technology Handbook, 2nd Edition

22 Modulation and demodulation

Information for communications purposes often falls within the low frequency sonic range and the transmission of such information using sound waves is virtually impossible. The low velocity of propagation through air (330 m/s) would produce long time delays and the signal attenuation would severely restrict the length of the communications path. Increasing power output would only add to the general level of interference between the channels sharing the relatively narrow bandwidth, and this in turn restricts the number of channels that are simultaneously available. These problems are overcome by superimposing the information on to a higher frequency signal that can be radiated as an electromagnetic wave. This process ...