How to send Canned Audio Data over an SCO Connection?

[HarryKrause]

Hello Bumble dev's,

I am trying to edit the bumble\examples\run_hfp_handsfree.py file to send HFP audio to a paired (bonded+connected) HFP Gateway (AG) device, but I can't seem to get it to work.

Here's what I'm attempting to do:

I pair the AG to the HF, connect them, start a WiFi call using Google Meet between the AG and a secondary phone, and then see that the SCO connection is established. I've edited the example code to then read from a .wav file, encode the data as an HCI_SynchronousDataPacket , and send the encoded packet from the HF using Host.send_hci_packet() . But I always get the following warning from the USB transport:

DEBUG:bumble.host:### HOST -> CONTROLLER: SCO: handle=0x0135, ps=0, data_total_length=8, data=494e464f49534654
WARNING:bumble.transport.usb:unsupported packet type 3

It then continues to attempt to send SCO packets but I do not see anymore of the above warnings and I never hear anything on the secondary phone (receiver on the WiFi call).

DEBUG:bumble.host: ### HOST -> CONTROLLER: SCO: handle=0x0135, ps=0, data_total_length=8, data=0d0000004c617666
DEBUG:bumble.host: ### HOST -> CONTROLLER: SCO: handle=0x0135, ps=0, data_total_length=8, data=36302e332e313030
DEBUG:bumble.host: ### HOST -> CONTROLLER: SCO: handle=0x0135, ps=0, data_total_length=8, data=00006461746100a0
DEBUG:bumble.host: ### HOST -> CONTROLLER: SCO: handle=0x0135, ps=0, data_total_length=8, data=4b02000000000000...

From these logs, I'm unable to determine if (1) I'm encoding the SCO packets incorrectly, (2) I'm using send_hci_packet() incorrectly, or (3) bumble does not support sending SCO packets via HCI.

Could someone please let me know if sending SCO packets via HCI is supported or not? If so, then is there an example that I can reference for how to do so? I've been referencing the run_hfp_gateway.py example, because it seems to send SCO packets via HCI using a .wav file.

My configuration is:
OS: Windows 11

HFP HF device
HW: Intel Wireless Bluetooth (built-in to my laptop).
Drivers: WinUSB v6.17600.16385.

HFP AG device
HW: Samsung Galaxy A51
OS: Android 13

I've attached a HFP_HF.log for additional context as needed. Thanks in advance!

[barbibulle]

The reason this doesn't work is that the USB transport implementation here doesn't yet support sending/receiving SCO packets to the controller (the main reason for this is that SCO over HCI is not supported by all controllers, since usually the audio exchanged for HFP gets routed from a separate line from the controller to some audio subsystem on the host, typically i2s).
However, what you're trying to do is a perfectly valid use case, and should be supported. I do have a local branch where I started adding support for that, I will try to finish it up this week and push a PR.
NOTE: other transports may support sending SCO over HCI, like for example going through the Android HCI HAL via a proxy, see https://google.github.io/bumble/extras/android_remote_hci.html for more details on that, if you have an Android phone to use for that purpose.

[HarryKrause]

@barbibulle Thanks for the quick response! We will look into the Android HCI HAL work-around as you've linked, but the USB transport support for SCO packets is definitely the path that I would prefer to use (one less phone needed for the test set-up). I will await the PR for your local change and test / review once it's ready to go.

[barbibulle]

One more quick note on this: not all Bluetooth controllers support SCO packets over HCI, so even with the support for it in the usb transfer implementation, it isn't guaranteed it will work with your particular hardware. We'll find out when you try it.