notes/audio.md

title
Audio

Bluetooth

Bluez

• Using `gdbus` to introspect or retrieve properties

gdbus introspect --system --dest org.bluez  --object-path /org/bluez/hci0/dev_6C_C4_D5_6C_CF_19
gdbus call --system --dest org.bluez  --object-path /org/bluez/hci0/dev_6C_C4_D5_6C_CF_19 --method org.freedesktop.DBus.Properties.GetAll org.freedesktop.DBus.Properties

Information on codecs

• https://www.soundguys.com/understanding-bluetooth-codecs-15352/

• https://habr.com/en/post/456182/

• https://www.nytimes.com/wirecutter/blog/what-you-need-to-know-about-bluetooth-audio/

• https://www.bluetooth.com/learn-about-bluetooth/recent-enhancements/le-audio/le-audio-specifications/

• https://www.bluetooth.com/blog/best-recorded-bluetooth-presentations-from-2020/

Transport payload sizes observed for SBC

• SBC-XQ @ 552 Kbps: 622 bytes
• SBC-XQ @ 512 Kbps: 574 bytes
• SBC-XQ @ 453 Kbps: 678 bytes

Random

Clean up audio with Audacity

Consider an mkv file recorded with OBS, do the following

• Extract the audio from video file

  ffmpeg -i infile.mp4 -acodec copy outfile.aac
  

  The option -acodec copy implies to not re-sample the audio.

• Use Audacity's noise reduction filters. Export as m4a.

• Merge audio and video again

  ffmpeg -i infile.mp4 -i denoised-audio.m4a -vcodec copy -acodec copy -map 0:0 -map 1:0 denoised-video.mp4
  

• Increase the volume using ffmpeg

  ffmpeg -i infile.mkv -vcodec copy -filter:a "volume=5.000000" infile-outputlouder.mkv
  

  Note that this command seems to change the format in mkv file from aac to ogg.

  To keep the codec same namely AAC, use the following

  ffmpeg -i infile.mp4 -vcodec copy -filter:a "volume=3.000000" -c:a aac -b:a 320k infile-outputlouder.mkv
  

  See the link https://trac.ffmpeg.org/wiki/Encode/AAC