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Nyquist filter
A Nyquist filter is an electronic filter used in TV receivers to equalize the video characteristics. The filter is named after the Swedish–US engineer Harry Nyquist (1889–1976).
== VSB ==
In analogue TV broadcasting the visual radio frequency (RF) signal is produced by amplitude modulation (AM) ; i.e., the video signal (VF) modulates the carrier. In AM two symmetric sidebands appear, containing identical information. So the RF bandwidth is two times the VF bandwidth. For example, the RF bandwidth of a VF signal with a bandwidth of 4.2 MHz, is 8.4 MHz. (System M) In order to use the broadcast band more efficiently, one sideband can be suppressed. However, it is impossible to suppress one sideband completely without affecting the other. Furthermore, a very sharp edge filter characteristic causes intolerable delay problems. So as a compromise, a standard filter is used which reduces a considerable portion of one side band (lower side band in RF) without causing extensive delay problems. Such a filter is known as vestigial side band filter (VSB).〔Gordon J.King: ''Beginner's Guide to Colour Television'', Newnes technical Books,Butterworth & Co. London, 1973 pp.99〕
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