The Nyquist-Shannon Sampling Theorem is the basis for all digital sampling of analog signals. Its named for Harry Nyquist, whose work on telegraph technology was instrumental in the later work by Claude Shannon in 1949. It's also often referred to as just the Nyquist Sampling Theorem or simply the Sampling Theorem.
Nyquist Frequency
The importance of the sampling theorem is that following it allows analog signals, such as audio or video, to be reproduced digitally and then converted back to analog again later. One of the key requirements for accurate conversions (sampling) is the necessary Sample Frequency. The sample frequency must be more than twice the frequency of the original signal. In order to reproduce sounds as high as 20kHz you must take at least 40,001 samples per second ( 20,000 x 2 + 1 ). Likewise, in order to Capture a 3MHz VHS signal you must sample at no less than 6,000,001Hz ( 3,000,000 x 2 + 1 ).
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