---
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<sub>audiogateway</sub>
    2. headset<sub>headunit</sub>
    3. a2dp<sub>source</sub>
    4. a2dp<sub>sink</sub>

-   For backchannel, two additional profiles are needed

    1. a2dp<sub>sourcewithbackchannel</sub>
    2. a2dp<sub>sinkwithbackchannel</sub>

- 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

``` bash
./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/