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

[Wolfgang Hugemann]

Hello,

today, I stepped across a bug I reported last year, September 13th,
which still seems to exist in the current version of ffmpeg:

Obviously, the frequency scaling of the attached spectrum is not
logarithmic, i.e. the frequency ratio of the labels is not constant:

655.47 / 641.80 = 1.02130
.
.
.
313,67 / 300.00 = 1.04557

I downloaded the source code at Github, and the bug seems to be located
in avf_showspectrum.c, somewhere in the lines 748 - 751:

for (y = 0; y < h; y += 40) {
  float range = s->stop ? s->stop - s->start : inlink->sample_rate / 2;
  float bin = s->fscale == F_LINEAR ? y : get_log_hz(y, h,
inlink->sample_rate);
  float hertz = s->start + bin * range / (float)(1 << (int)ceil(log2(h)));

Deplorably, I am not able to fix the code on my own and I see no
possibilty to report the issue on Github.

Can somebody help me with this issue? How does bus reporting work in the
ffmpeg project?

Grretings from Münster, Germany
Wolfgang Hugemann

Am 13.09.2019 um 15:35 schrieb Wolfgang Hugemann:
> Today, I have been experimenting with the 'showspectrum' filter, mostly
> with 'showspectrumpic'.
> 
> I think there is a bug in the log scaling of the frequency axis if (and
> only if) it doesn't start at zero: The ratios of the frequency axis'
> labelings is not constant, they grow with larger values.
> 
> I had a quick look at the source code and gained the impression that the
> fft windows size is based on the height of video created (?). At least
> it cannot by specified in the filter options.
> 
> Kind regards
> Wolfgang Hugemann


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