So I went to watch a video on YouTube: https://www.youtube.com/watch?v=7fSq0_Ljb_s
And about a minute in, I noticed some strange high frequency in both audio channels. Here’s a sample:
Not sure what this is, I decided to download it to investigate further:
$ youtube-dl -F 'https://www.youtube.com/watch?v=7fSq0_Ljb_s'
One of the output lines is:
140 m4a audio only tiny 129k , m4a_dash container, mp4a.40.2@129k (44100Hz), 84.38MiB
$ youtube-dl -f 140 'https://www.youtube.com/watch?v=7fSq0_Ljb_s' $ ffmpeg -i '【GAMABOOKS_コラボ企画】星空古本市開催！【新人VTuber】-7fSq0_Ljb_s.m4a' -ss 02:00 -t 30 gamabooks_cut.wav $ octave-cli octave:1> [y, Fs] = audioread('gamabooks_cut.wav'); octave:2> Y = fft(y); octave:3> plot(abs(Y));
Here’s our high frequency. Now zoom in to find the FFT index number:
The peak is at index 338543. Convert it to hertz (and note the 1-index):
octave:4> (338543-1) / length(y) * Fs ans = 1.1285e+04
So now we just need a way to suppress this frequency. Searching “mpv audio filter”
online tells us that
mpv --af=help will list all audio filters available. Looking
through the list, I found:
bandreject Apply a two-pole Butterworth band-reject filter.
So I just need to find out how to configure this filter. Searching online, I eventually came across some scripts that referenced the FFmpeg manual, specifically, the “equalizer” filter: (emphasis mine)
Apply a two-pole peaking equalisation (EQ) filter. With this filter, the signal-level at and around a selected frequency can be increased or decreased, whilst (unlike bandpass and bandreject filters) that at all other frequencies is unchanged.
Sounds like what I want. So to try it, with reference to the manual (the bandwidth and attenuation factor were determined by trial-and-error):
$ mpv --af=equalizer=f=11285:t=h:width=5:g=-100 gamabooks_cut.wav
Sounds a lot better. Now download the video:
$ youtube-dl -f 136 'https://www.youtube.com/watch?v=7fSq0_Ljb_s'
And play it:
$ mpv 【GAMABOOKS_コラボ企画】星空古本市開催！【新人VTuber】-7fSq0_Ljb_s.mp4 --audio-file=【GAMABOOKS_コラボ企画】星空古本市開催！【新人VTuber】-7fSq0_Ljb_s.m4a --af=equalizer=f=11285:t=h:width=5:g=-100