[FFmpeg-user] 'showspectrumpic' filter: scaling of the frequency axis and fft windows size

Wolfgang Hugemann auto at hugemann.de
Fri Sep 13 19:05:23 EEST 2019

> Because used fft accepts only power of 2 sizes.

Well, this is clear to me - but what size do you use? My use of the term
'window' may be unclear in this context: I meant the window size for the
fft, i.e. how many samples are used in one fft run? I take from your
code that the window size is two times the output windows width (? ...
rounded to ...?).


I already tried scaling the frequency axis with 2^n, i.e. 4096 Hz or
8192 Hz, by that did not help. I think there is something basically
wrong with the scaling of the frequency axis if you use log.

Furthermore, the value that you supply for parameter 'stop' seems to be
divided by roughly 2 (?).

>There is code already for more advanced fft but it is not optimized,
>and thus not yet used.

To my knowledge, all such calculations use fft, i.e. 2^n window sizes,
mostly with about 50% overlap (as you also do for most windows in your

Wolfgang Hugemann

More information about the ffmpeg-user mailing list