Nyquist frequency and sampling frequency

[pinkcashmere]

Homework Statement

$N= \frac{2 f_s}{f_{lowest}}$$\frac{f_s}{2} - \frac{f_s}{N}$

Homework Equations

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The Attempt at a Solution

I have these equations listed on a formula sheet but I do not know what they are used for. It is from a chapter titled "Discrete Sampling and Analysis of Time Varying Signals". Can someone explain what these would be used for?

 

[BvU]

Your textbook, perhaps ? Or your notes ?

 

[haruspex]

Hi pinkcashmere,
This is a topic I know nothing about, so I tried to brush it up from the net.
Couldn't find anything that squared with that equation or that expression. I felt perhaps the "lowest" was a reference to the lowest alias frequency, but couldn't get that to work.
I did notice that two authoritative looking pages were somewhat confused regarding aliasing. Looks like one copied extracts from the other and copied the error. Seems to me the term alias frequency can mean two slightly different things:
- where the sampling frequency is more than double the actual frequency, it is a higher frequency that produce the same samples.
- where the sampling frequency is less than that, it is that frequency less than half the sampling frequency that would produce the observed samples.
The error is that the text discusses aliasing as though it refers to the first case, then shows a folding diagram illustrating the second case.