Playing YouTube Video with Volume Normalized in GNU/Linux

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YouTube videos' loudness varies. This can be annoying. If the system volume is quiet, we won't hear some videos. If it is loud, we are worried about sudden loud sounds. That is because many videos' sounds haven't been processed. In GNU/Linux, we can do the processes on our computer in real-time system-wide.

Nowadays most GNU/Linux distributions use PulseAudio as the sound server. We will do the trick base on it.

Install the filters/plugins

We need the filters or plugins to do real-time sound analysis and compression.

On Debian/Ubuntu

# apt install swh-plugins

On Fedora/CentOS/RHEL

# dnf install ladspa-swh-plugins

swh-plugins is a suite of LADSPA (Linux Audio Developer's Simple Plugin API) plugins collected by Steve Harris.

Make sinks

pactl load-module module-ladspa-sink sink_name=compressed master=SINK_NAME_OR_INDEX plugin=sc4_1882 label=sc4 control=0,101.125,401,-15,3,3.25,0
pactl load-module module-ladspa-sink sink_name=normalized master=compressed plugin=fast_lookahead_limiter_1913 label=fastLookaheadLimiter control=10,0,0.5075

SINK_NAME_OR_INDEX is the name or index of the "sink" which plugins should be applied to.

In PulseAudio, "sinks" are what "clients" send the audio to. A client can be an audio player, a web browser or any other audio application. The default sink is most probably the audio hardware.

To view sinks: 

pacmd list-sinks

"master=SINK_NAME_OR_INDEX" can be omitted so that the default sink will be selected instead.

Parameters

"control=" sets the parameters for the plugin. Each parameter is separated by a "," with no " " between them. Some of the values given in the commands are empirical and suit unprocessed audio of YouTube videos, Others are the default.

SC4
Parameter Title Min Max Default
1st RMS/peak 0(RMS) 1(peak) 0
2nd Attack time (ms) 1.5 400 101.125
3rd Release time(ms) 2 800 401
4th Threshold level (dB) -30 0 0
5th Ratio (1:n) 1 20 1
6th Knee radius (dB) 1 10 3.25
7th Makeup gain (dB) 0 24 0
Fast Lookahead limiter
Parameter Title Min Max Default
1st Input gain (dB) -20 20 0
2nd Limit (dB) -20 0 0
3rd Release time (s) 0.01 2 0.5075

Details for the plugins can also be checked with "analyseplugin", for example: 

analyseplugin /usr/lib/ladspa/dyson_compress_1403.so
For some distributions, the plugins are under "/usr/lib64/ladspa/".

Apply Changes

To make the plugins take effect, set the default sink to "normalized" (the sink we made) with the GUI or with this command: 

pactl set-default-sink normalized

Make the Changes permanent

We can either autostart these commands or make the file "~/.config/pulse/default.pa" and add the following content. 

.nofail
.include /etc/pulse/default.pa
load-module module-ladspa-sink sink_name=compressed plugin=sc4_1882 label=sc4 control=0,101.125,401,-15,3,3.25,0
load-module module-ladspa-sink sink_name=normalized master=compressed plugin=fast_lookahead_limiter_1913 label=fastLookaheadLimiter control=10,0,0.5075
set-default-sink normalized

Rollback changes

With these filters, we compress the dynamic range and raise (gain) the volume while prevent clipping. This works well for unprocessed audio of YouTube videos, but for processed audios like music files or movies, you probably want to disable these filters.

To set back to the default audio sink, either via the GUI or with the following commands: 

pactl set-default-sink SINK_NAME_OR_INDEX
The index of the default sink (hardware) starts from 0.

To remove the filters: 

pactl unload-module MODULE_NAME_OR_INDEX

To view the modules (filters): 

pacmd list-modules

References

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