---
layout: post
title: "Using spectral analysis to watch a YouTube video"
date: 2022-08-29 01:49:02 +0100
---
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:

<audio controls src="/blog/images/2022-08-29-spectral-analysis-gamabooks-youtube-video.2.wav"></audio>  
[(Signature)][sig2]

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

So:

    $ 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));

![fft spectrum][img0]  
[(Signature)][sig0]

Here's our high frequency. Now zoom in to find the FFT index number:

![fft zoomed][img1]  
[(Signature)][sig1]

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][eq],
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

Done!


[eq]: https://ffmpeg.org/ffmpeg-filters.html#equalizer
[img0]: /blog/images/2022-08-29-spectral-analysis-gamabooks-youtube-video.0.png
[sig0]: /blog/sig/2022-08-29-spectral-analysis-gamabooks-youtube-video.0.png.sig
[img1]: /blog/images/2022-08-29-spectral-analysis-gamabooks-youtube-video.1.png
[sig1]: /blog/sig/2022-08-29-spectral-analysis-gamabooks-youtube-video.1.png.sig
[sig2]: /blog/sig/2022-08-29-spectral-analysis-gamabooks-youtube-video.2.wav.sig