BT Audio Sink

[TheOldPresbyope]

Are there any error error messages, logs or diagnostics from the Echo?

-Tim

Nah. Amazon hides all the messy stuff from us.

I've gotten back to my Amazon Echo and tried connecting again while monitoring just the Bluetooth AVDTP packets ($ sudo hcidump avdtp).

Everything in the negotiations looks the same as it does when I connect one of my Android devices until the point where the initiator (e.g., Echo) tells the acceptor (e.g., the moOde Bluetooth stack) which initiator and acceptor stream endpoints will be connected together and with what settings. I've posted the detailed dumps to pastebin (https://pastebin.com/2Db2GiHr) to keep this post short. 

In a nutshell, although the same endpoints are chosen, the Android devices select 44.1kHz for the connection and the Echo selects 48kHz. One would think this would be fine with the moOde Bluetooth stack because it's already advertised this capability

Code:

Media Codec - SBC
     16kHz 32kHz 44.1kHz 48kHz

Then when the stream is actually opened, the Android devices increment the transaction number and begin transmitting media packets as payload type (pt) = 96 with synchronization source (ssrc) = 0 while the Echo resets the transaction number to 0 and begins transmitting media packets as pt=1 with ssrc=1. I don't know how the transaction number is used. The ssrc setting seems like it should be irrelevant in this context.*

The payload type may be the smoking gun here. Recall the trouble with PC Bluetooth connections. I'll try disabling the type check in bluez-alsa again. If that works, I need to reopen an old issue on Arkq's bluez-alsa github repo. He has argued (and I think with good justification) that the A2DP spec calls for payload type=96 (usage dynamically defined by apps) and checks for it. According to the RTP Profile for A/V, the payload type=1 is reserved.

Regards,
Kent

*From the AVDTP spec

SSRC field is only used for multicast application, because SSRC field identifies the source-node ID on the specific Media transport session. 

I see ssrc is defined but not used in the bluez-alsa repo.