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Given a block, $x[n]$, of M samples.

Calculating abs(fft(x)).^2 returns the power spectrum of that block through the use of a $M$-point FFT.

I can calculate the same using Goertzel's algorithm.

At which $M$ will it be more efficient to use an FFT instead of Goertzel?

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    $\begingroup$ What do you mean by power of N ? is it $2^N$ frequency points? That's to say, you want to compute a $2^N$-point DFT of M-point sequence x[n], using either an FFT or the Goertzel algorithm..? $\endgroup$
    – Fat32
    Commented Oct 5, 2021 at 22:45
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    $\begingroup$ By power of N I mean the power of N frequency points. I want to either compute an N-point FFT of an M-point sequence which returns N frequencies OR I want to compute it using Goertzel. My question is: For which N is an FFT more efficient than using Goertel? $\endgroup$
    – james3849
    Commented Oct 6, 2021 at 8:10
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    $\begingroup$ @james3849 You didn't answer Fat32's question. If N = 4 what is the "power of N"? $\endgroup$
    – Richard Lyons
    Commented Oct 6, 2021 at 10:52
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    $\begingroup$ @RichardLyons Sorry for not being clear. Read it as "power of N frequencies" and not "power of N". In other words, I am interested in calculating the power of N frequency bins. $\endgroup$
    – james3849
    Commented Oct 6, 2021 at 12:14
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    $\begingroup$ Ha ha! I thought your statement Let's say I want to run a frequency analysis that returns the power of N equally spaced frequencies. indicated that you run a DFT that calculates (returns) power of $N$, (that's to say $a^N$ for some $a$ not indicated) equally spaced frequency points. That's why I asked for clarification of the term power or N... $\endgroup$
    – Fat32
    Commented Oct 6, 2021 at 12:47

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If you implement the Goertzel algorithm P times to detect P different spectral samples, Goertzel is more efficient (fewer multiplies) than the N-point FFT when P < log2(N).

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