FFT resolution lines

[Slawek]

The FFT function returns Nyquist (half) of input points. How do I get 400 lines of resolution out of 1024 points? Do I just disregard other 112?
Thanks

[CoastalMaineBird]

In an ideal world, a 1024-point FFT results in 512 valid values, at frequencies from 0/N * Fsamp up to (N/2-1) / N * Fsamp.
Of course, an ideal world would also have a filter with 100% pass at the nyquist frequency, and 0% at any frequency above it.

T'ain't no such beast.

The usual reason for using 400 lines instead of 512 (or 800 instead of 1024, etc.) is the imprecision of the input filters - you don't really know if their cutoff is at the nyquist or above it or below it.

Even with digital filters, you have a slope to them, so 400 out of 512 is a convenient place to assume that the pass-through is 100% (hasn't started falling off yet).

So yes, just run the FFT array through an array-subset function and keep the lower 400 lines.