Stereo support

[hiroshihorie]

Automatically enable stereo recording when device supports it.

[pblazej]

@hiroshihorie I started playing with that, however:

• it will require switching to the bottom/back mic 🎤
• for some reason, the supported modes are different from the Apple example

(lldb) po dataSources
▿ 3 elements
  - 0 : <AVAudioSessionDataSourceDescription: 0x302658f30, ID = 1835216945; name = Bottom>
  - 1 : <AVAudioSessionDataSourceDescription: 0x302658f40, ID = 1835216946; name = Front>
  - 2 : <AVAudioSessionDataSourceDescription: 0x302658f60, ID = 1835216947; name = Back>
(lldb) po newDataSource
<AVAudioSessionDataSourceDescription: 0x302658f60, ID = 1835216947; name = Back>
(lldb) po newDataSource.supportedPolarPatterns
▿ Optional<Array<AVAudioSessionPolarPattern>>
  ▿ some : 3 elements
    ▿ 0 : AVAudioSessionPolarPattern
      - _rawValue : Omnidirectional
    ▿ 1 : AVAudioSessionPolarPattern
      - _rawValue : Subcardioid
    ▿ 2 : AVAudioSessionPolarPattern
      - _rawValue : Stereo
(lldb) po dataSources[0].supportedPolarPatterns
▿ Optional<Array<AVAudioSessionPolarPattern>>
  ▿ some : 1 element
    ▿ 0 : AVAudioSessionPolarPattern
      - _rawValue : Omnidirectional
(lldb) po dataSources[1].supportedPolarPatterns
▿ Optional<Array<AVAudioSessionPolarPattern>>
  ▿ some : 4 elements
    ▿ 0 : AVAudioSessionPolarPattern
      - _rawValue : Omnidirectional
    ▿ 1 : AVAudioSessionPolarPattern
      - _rawValue : Cardioid
    ▿ 2 : AVAudioSessionPolarPattern
      - _rawValue : Cardioid
    ▿ 3 : AVAudioSessionPolarPattern
      - _rawValue : Stereo
(lldb) po dataSources[2].supportedPolarPatterns
▿ Optional<Array<AVAudioSessionPolarPattern>>
  ▿ some : 3 elements
    ▿ 0 : AVAudioSessionPolarPattern
      - _rawValue : Omnidirectional
    ▿ 1 : AVAudioSessionPolarPattern
      - _rawValue : Subcardioid
    ▿ 2 : AVAudioSessionPolarPattern
      - _rawValue : Stereo

whereas for the SDK:

(lldb) po session.inputDataSources
▿ Optional<Array<AVAudioSessionDataSourceDescription>>
  ▿ some : 2 elements
    - 0 : <AVAudioSessionDataSourceDescription: 0x3028ea5b0, ID = 1835216946; name = Front>
    - 1 : <AVAudioSessionDataSourceDescription: 0x3028e90d0, ID = 1835216947; name = Back>
(lldb) po session.inputDataSources![0].supportedPolarPatterns
▿ Optional<Array<AVAudioSessionPolarPattern>>
  ▿ some : 2 elements
    ▿ 0 : AVAudioSessionPolarPattern
      - _rawValue : Cardioid
    ▿ 1 : AVAudioSessionPolarPattern
      - _rawValue : Cardioid
(lldb) po session.inputDataSources![1].supportedPolarPatterns
▿ Optional<Array<AVAudioSessionPolarPattern>>
  ▿ some : 1 element
    ▿ 0 : AVAudioSessionPolarPattern
      - _rawValue : Subcardioid

the session mode and options are pretty much the same 🤷

[gulzardev]

Is there a way to enable stereo even if the mic doesn’t support it? @pblazej
From LKRTCAudioDeviceModule maybe ?

The use case is capturing app audio from a broadcast extension. However, it downmixes to a mono channel. From the broadcast extension, I receive stereo audio. I tried modifying the SDP, but it didn’t work.

[pblazej]

@gulzardev thank you for reporting! I'll try to reproduce that with broadcast 🤞

[pblazej]

@hiroshihorie I tried to revisit this topic:

• stereo capturing is probably not worth the effort given the limitations of noise cancellation etc.
• stereo broadcast, sound effects, music streaming definitely is worth exploring

We discussed the idea of "bypassing ADM" completely, I made a quick and dirty experiment here and there. The stereo playout is not working yet, probably there are some missing pieces.

My idea is that in case we decide to go this way, we can:

• simplify a lot of ADM-coupled internals like AudioManager.shared.mixer.capture(appAudio:) in manual mode vs just push
• create a better abstraction for the upcoming sound effects API
• create a better abstraction from totally custom use cases, like the internet radio, "bring your own capturer" kind of API

TL;DR I think the decoupling itself brings a lot of added value here.