Interpret AVRCP packets

Question

After some mucking about, I have got a pybluez script to connect to an AVRCP profile on various devices, and read the responses.

Code snippet:

addr="e2:8b:8e:89:6c:07"  #S530 white
port=23
if (port>0):
    print("Attempting to connect to L2CAP port ",port)
    socket=bluetooth.BluetoothSocket(bluetooth.L2CAP);
    socket.connect((addr,port))
    print("Connected.")
    while True:
      print("Waiting on read:")
      data=socket.recv(1024)
      for b in data:
         print("%02x"%b,end=" ")
      print() 
    socket.close()

The results I'm getting when I press the button on the earpiece are as follows:

Attempting to connect to L2CAP port  23
Connected.
Waiting on read:
10 11 0e 01 48 00 00 19 58 10 00 00 01 03 
Waiting on read:
20 11 0e 00 48 7c 44 00 
Waiting on read:
30 11 0e 00 48 7c 46 00 
Waiting on read:
40 11 0e 00 48 7c 44 00 

After careful reading of the spec, it looks like I'm seeing PASSTHROUGH commands, with 44 being the "PLAY" operation command, and 46 being "PAUSE" (I think) I don't know what the 10 11 0e means, apart from the fact that the first byte appears to be some sort of sequence number. My issue is threefold:

1. I don't know where to find a list of valid operation_ids. It's mentioned in the spec but not defined apart from a few random examples.
2. The spec makes reference to subunit type and Id, (which would be the 48 in the above example) again without defining them AFAICT.
3. There is no mention of what the leading three bytes are. They may even be part of L2CAP and nothing directly to do with AVRCP, I'm not familiar enough with pybluez to tell.

Any assistance in any of the above points would be helpful. Edit: For reference, the details of the AVRCP spect appears to be here: https://www.bluetooth.org/docman/handlers/DownloadDoc.ashx?doc_id=119996

Answer 1

The real answer is that the specification document assumes you have read other specification documents.

The three header bytes are part of the AVCTP transport layer: http://www.cs.bilkent.edu.tr/~korpe/lab/resources/AVCTP%20Spec%20v1_0.pdf

In short:

0: 7..4: Incrementing transaction id. 0x01 to 0x0f
   3..2: Packet type 00 = self contained packet
     1 : 0=request 1=response
     0 : 0=PID recognized 1: PID error
1-2: 2 byte bigendian profile id (in this case 110e, AVRCP)

The rest is described in the AVRCP profile doc, https://www.bluetooth.org/docman/handlers/DownloadDoc.ashx?doc_id=119996

I don't find the documentation to be amazingly clear.

I have provided a sample application which seems to work for most of the AVRCP devices I have been able to test:

https://github.com/rjmatthews62/BtAVRCP