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| Undesirable Effects in the Course of Signal Transmission |
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| Attenuation is undesirable because it reduces signal strength at the receiver. More serious, however, is distortion interference, and noise, which appear as alternations of the signal shape. Although such contaminations may occur at any point, the standard convention is to blame them entirely on the channel. The transmitter and receiver are always treated as ideal. |
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| Distortion is waveform perturbation caused by imperfect response of the systems to the desired signal itself. Unlike noise and interference, distortion disappears when the signal is turned off. If the channel has a linear but distorting response, then distortion may be corrected, or at least reduces, with the help of special filters called equalisers. |
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| Interference is contamination by extraneous signals from human sources, other transmitters, power lines and machinery, switching circuits, etc. Interference occurs most often in radio systems whose receiving antennas usually intercept several signals at the same time. Radio-frequency interference (RFI) also appears in cable systems if the transmission wires or receiver circuitry picks up signals radiated from nearby sources. Appropriate filtering removes interference to the extent that the interfacing signals occupy different frequency bands than the desired signal. |
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| Noise refers to random and unpredictable electrical signals produced by natural processes both internal and external to the system. When such random variations are superimposed on an information-bearing signal, the message may by partially corrupted or totally obliterated. Filtering reduces noise contamination, but there inevitably remains some amount of noise that cannot be eliminated. |
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