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could someone help me explain downsampling in the context of Nyquist–Shannon sampling theorem and tell the difference between Aliasing and downsampling.Also is downsampling and undersampling the same ?Thanks in advance :)

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closed as unclear what you're asking by Marcus Müller, Laurent Duval, hotpaw2, MBaz, Matt L. Jan 18 '17 at 7:49

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ So, this question is actually unclear. Because: "Aliasing" is an effect, and "downsampling" is an operation. Both are well-defined – us reproducing the wikipedia definitions here clearly doesn't help you, so please edit your question to explain what of the definitions you don't understand. Same applies for "undersampling". $\endgroup$ – Marcus Müller Jan 17 '17 at 21:05
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    $\begingroup$ To add on Marcus comment, it would be quite interesting for us that you could provide hints of what you already understood, and what is missing $\endgroup$ – Laurent Duval Jan 17 '17 at 21:34
  • $\begingroup$ The Question was unclear to me as well , when asking Alias vs downsampling , but in my lecture slides it stated 2.1.3 Alias vs. down sampling • Generated by sampling frequencies which don‘t fulfill the sampling criteria, see before. − We talk about alias, when spectral components overlap − When the sampling theorem is not fulfilled but no overlapping spectrums are generated we talk about down sampling $\endgroup$ – D.Makk Jan 18 '17 at 9:40
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"Alias" is to "downsampling" what "a broken leg" is to "jumping from a 40-feet height". An effect from a cause, with a very probable risk in general, but could be harmless if you are lucky.

I am not sure there is a clear shared difference between uses of downsampling and undersampling. To me, used as a verb, "to downsample" may suggest a rate reduction by an integer or a fraction. While "to undersample" could be more general (at some sample rate below its Nyquist rate). Some definitions equate undersampling and bandpass sampling.

Used as an adjective as in "the signal is downsampled" or "the signal is undersampled", I understand different meaning. In the first one, I suppose a voluntary action. In the second one, this can be interpreted as an observation: the signal has been sampled at a rate lower than expected.

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