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Im studying the DSP book by Steven W. Smith. On page 41 he covers the Nyquist frequency. He makes up an example by writing the following.

... consider an analog signal composed of frequencies between DC and 3kHz.

What does that mean? My guess is that in this context DC is equivalent to zero. Why does he use DC here?

Sorry if this is totally obvious.

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    $\begingroup$ DC ... direct current $\endgroup$ – Matt L. Dec 3 '16 at 15:47
  • $\begingroup$ this is actually a good question that is about a nomenclature problem involving obsolete electrical engineering terms that should be deprecated. but i must confess, i have also used the term "DC" for 0 Hz. $\endgroup$ – robert bristow-johnson Feb 8 at 20:33
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You are right, DC means zero frequency. The term probably originates from electrical engineers' use of the terms DC and AC.

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When the current is "direct", it is basically a constant with no change over time, therefore having zero frequency. Alternating current on the other hand, changes over time and therefore having nonzero frequency components

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