The sound of many headphones can be improved by applying equalization (EQ). The problem on a smartphone is to find a way to apply EQ globally to all sound.
On Android, there are some apps that can do this, like Poweramp Equalizer, Wavelet or Sonarworks SoundID, however in some cases manual work and developer knowledge is needed to give special permissions.
On iOS, it seems no 3rd party app gets access to the global audio stream and the only solution I found people use is an external hardware DAC with EQ support.
As a workaround, would it be possible to create an app that acts as a streaming endpoint device using a protocol like AirPlay/DLNA/UPnP/ ChromeCast?
Then the app would get access to the audio stream by selecting it as output device in e.g. Spotify. EQ (and other effects) could be applied by the app, and then the audio could be output to the headphones.
I found no apps that do this, so I guess there may be some limitation that makes this impossible?
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