THE ELEMENTS OF AN ELECTRONIC COMMUNICATION SYSTEM

Electronic communication is, at its core, the transmission of sound, text, pictures or data through a medium -- typically free space for long-distance links -- and every practical communication system, whatever it carries, is built from the same handful of functional blocks joined together in a block diagram running from the original information all the way to its recovery at the far end. (1) Information (the baseband or input signal) is the speech, music, picture or computer data to be sent. (2) The input transducer, the classic example being a microphone, converts this information -- in whatever physical form it starts out (sound, light, pressure) -- into an equivalent electrical baseband signal. (3) The transmitter feeds this electrical signal into the communication channel, and internally consists of an amplifier (boosting the transducer's inherently weak output), an oscillator (generating the high-frequency sinusoidal carrier wave, whose energy -- being proportional to frequency -- is very high), a modulator (superimposing the baseband signal onto this carrier to produce the modulated signal), and a power amplifier (raising the modulated signal's power so it can cover a large distance). (4) The transmitting antenna then radiates this modulated radio signal into space in all directions, travelling as an electromagnetic wave at the speed of light, $3\times10^8$ m/s. (5) The communication channel carries the signal from transmitter to receiver with as little noise or distortion as possible, and is either wireline (point-to-point, using wires, cables or optical fibres -- e.g. telephone, intercom, cable TV -- unsuitable for long distances because of the physical connection) or wireless (using free space itself as the medium, via a transmitting antenna -- e.g. mobile, radio/TV broadcasting, satellite communication -- and so the natural choice for long-distance links). (6) Noise is any undesirable electrical signal that interferes with and attenuates the transmitted signal; it may be man-made (automobiles, welding machines, electric motors) or natural (lightning, solar and stellar radiation, other environmental effects), and while it can never be completely eliminated, various techniques can reduce it. (7) The receiver picks up the transmitted signal at a receiving antenna (which converts the incoming EM wave into an RF electrical signal), then runs it through a demodulator (which extracts the baseband signal back out of the modulated carrier), followed by further amplification and detection. (8) Repeaters -- essentially a combined transmitter and receiver, the best example being a communication satellite -- receive a weakened signal, amplify it, and retransmit it on a fresh carrier frequency to extend the overall r …