Question

There is a problem when using Google Navigation on Android with a Bluetooth device. Navigation sends the audio for the turn-by-turn instructions over the A2DP stream. I have a Motorola T605 Bluetooth car kit and it supports HFP and A2DP. I often just listen to the old fashioned car radio though. In this scenario I never hear turn-by-turn directions because my live A2DP stream is being sent to the radio AUX input but I am listening to the radio instead. My app has a feature where it reads out SMS messages to me. I have it set up so you can pick the stream you want to use for this. I prefer using AudioManager.STREAM_VOICE_CALL since it uses a dedicated separate speaker on the T605.

I have looked into a few ways to handle this:

1) Reroute notifications to Bluetooth SCO instead of A2DP. I have not fund a way to do this. I am able to send TTS messages I create over this path and it works great (I have an SMS reader built into my app). I tried activating SCO but the turn-by-turn instructions stream moves to the phone internal speaker instead of the voice call stream like I would prefer.

2) Capture the navigation messages and echo them back over the SCO path. I can’t find a way to intercept or capture the navigation stream though. Does Google Navigation use the TTS engine or its own methods to create the voice output? It would be great to just get the directions in a text string since I could easily send that to the TTS engine and route it where I want like I do with SMS strings.

You can see my app source here: http://code.google.com/p/a2dpvolume/

Any thoughts?

Answer 1

Unfortunately I think you’re SOOL when it comes to rerouting the navigation messages to BT SCO. I would expect the navigation app to use either the TTS or NOTIFICATION stream type to play the messages, and both of those stream types follow the MEDIA routing strategy (for NOTIFICATIONs that is at least typically true when there’s no active voice call).

Although the behavior is up to each vendor to decide, my guess is that you’ll find that in most implementations streams that follow the MEDIA routing strategy won’t ever be routed to BT SCO, except during an ongoing voice call that is routed to BT SCO.

The best you could do in terms of routing is to force the routing to the loudspeaker, or to “anything but A2DP” (which means wired headset if one is attached, or the loudspeaker otherwise).

Here’s how you could do that (I haven’t verified that this works on every phone out there):

Class audioSystemClass = Class.forName("android.media.AudioSystem");
Method setForceUse = audioSystemClass.getMethod("setForceUse",
    int.class,
    int.class);
// 1 == FOR_MEDIA, 10 == FORCE_NO_BT_A2DP (FORCE_SPEAKER would be 1).
setForceUse.invoke(null, 1, 10);