Commonly used terms in electronic communication system

A number of standard terms recur throughout the study of any electronic communication system, and it is worth defining each of them clearly before going further. A signal is the information -- such as music or speech -- converted into an electrical form suitable for transmission; in a radio station, for example, a microphone converts sound and speech into this electrical signal before it is sent out into space. A signal can be either analog or digital (Fig. 13.7): an analog signal is one that varies continuously (in voltage or current), with a wave being the fundamental example of an analog signal -- sound and picture signals in ordinary TV are analog in nature. A digital signal, by contrast, can take on only two discrete values (such as +5 V and 0 V), a square wave being the standard example.

A transmitter converts the signal produced by an information source into a form suitable for transmission through a channel and subsequent reception. A transducer is any device that converts one form of energy into another -- a microphone, which converts sound energy into electrical energy, is a transducer, and so, in the reverse direction, is a loudspeaker, which converts electrical energy back into sound energy. A receiver receives the message signal at the output of the channel and reconstructs it into a recognisable form of the original message, ready to be delivered to the end user of the information.

Noise is any random, unwanted signal that corrupts the desired signal; the source generating it may be located either inside or outside the communication system, and every effort is normally made to minimise the noise level present. Attenuation is the loss of signal strength that occurs as a signal propagates through the channel, arising because the channel distorts, reflects and refracts the signal as it passes through. Amplification is the reverse process -- deliberately raising the strength of a signal using an electronic circuit called an amplifier. Range is the maximum (largest) distance between a source and a destination over which the signal can still be received with sufficient strength. Bandwidth is the range of frequencies over which a given electronic circuit operates efficiently.

Modulation is the process, carried out at the transmitter, of superimposing a signal onto a high-frequency carrier wave so that it can be sent over long distances (this idea is developed fully in section 13.6). Demodulation is the reverse process: recovering the original signal back out of a modulated wave, normally performed at the receiver. A repeater is a combined transmitter-and-receiver unit: it receives an incoming signal, amplifies it, and retransmits it onward to the next repeater in a chain (Fig. 13.8), a mechanism used specifically to increase the effective range of a communication system beyond what a single transmitter-receiver link could achieve on its own. …