notes/pulseaudio.md

title
Projects

AAC observations

The following are the observations from the Wireshark capture traces regarding the issue observed while using AAC with SRS-XB33.

1. Android and SRS-XB33

   • Remote sends GetAllCapabilities command and gets SBC, MPEG-2/4 AAC and LDAC as supported in the response.

   • In case of AAC, the following flags are set on Android side

     • MPEG-2 AAC LC
     • Sampling rate of 44.1 KHz
     • VBR supported
     • 2 channels

   • The SetConfiguration gets send with the following parameters set

     • MPEG-2 AAC LC
     • Sampling rate of 44.1 KHz
     • VBR supported
     • 2 channels

2. PulseAudio and SRS-XB33

   1. Initial Connection

      • The device sends a Discover request, followed by GetAllCapabilities and then SetConfiguration.

      • For the GetAllCapabilities, MPEG-2,4 AAC and SBC are returned by bluez.

      • The GetAllCapabilities command, seems to be send twice from the device and in both cases return the exact same capabilities. This happens for both AAC and SBC.

      • The SetConfiguration command send is send by SRS-XB33 and has the following parameters set

        • MPEG-4 AAC LC
        • Sampling rate of 48 KHz
        • 2 channels
        • VBR not supported

   2. Reconnection

      In the reconnection case, the SetConfiguration command has the following parameters set.

      • Sampling rate of 44.1 KHz
      • MPEG-4 AAC LC
      • 2 channels
      • VBR not supported

Codec switching

• Currently the following profiles exist

  1. headset_audiogateway
  2. headset_headunit
  3. a2dp_source
  4. a2dp_sink

• For backchannel, two additional profiles are needed

  1. a2dp_{sourcewithbackchannel}
  2. a2dp_{sinkwithbackchannel}

• Each profile has it's own codec. However, one problem to solve is, one codec for one direction and a different codec for the other direction. For eg. aptX-LL uses aptX for playback and SBC for recording. How to solve this? Is introducing the backchannel profiles, be sufficient to take care of this?

Under the hood

• https://gavv.github.io/articles/pulseaudio-under-the-hood

Bluetooth testing

./simple-endpoint 2C:DB:07:58:F2:F9 ldacsource
./test-device connect 4C:BC:98:80:01:9B "0000110b-0000-1000-8000-00805f9b34fb"
env GST_DEBUG=3 gst-launch-1.0 audiotestsrc ! ldacenc ! a2dpsink transport=/org/bluez/hci0/dev_4C_BC_98_80_01_9B/sep10/fd0

Module loop back

• https://endless.ersoft.org/pulseaudio-loopback/