I need to be able to send and receive audio over bluetooth and it seems A2DP is the thing that does this
Using the code given here, I managed to get my phone to detect and pair with the remote bluetooth device, then establish an A2DP connection which sent all the audio output of the android device to the remote one via bluetooth
I was previously able to do such two way data communication with the same remote bluetooth device via RFCOMM (the android phone needed to be the one initiating the communication though, I could not do it through the remote device though I know it is possible). Once the RFCOMM link was established, the android device could send data to the remote device, as well as receive data from it
What I need now is something similar, but for audio (preferrably using A2DP, but I am open to using whatever else, as long as it gets the job done). While snooping around SO I've got hints that receiving audio data on phone (at least an unrooted normal one, rooting is not an option for me) via A2DP is not possible (see for example, here). But I have also seen headsets that allow making phone calls entirely through bluetooth (well, wirelessly actually, I am assuming it must be bluetooth). For example, for an incoming call, you can just press a button on the bluetooth earpiece, then you can talk as well as listen to the other party. If a phone cannot be used as an audio sink, how do such headsets do it?
I need some ideas on how I should go about writing an android app to send as well as receive audio data via bluetooth (for simplicity lets consider it similar to the Bluechat sample program, but involving audio instead of text). Is it possible at all (through java)? Is it possible only through NDK? How do the devices on the market do it? Is it a different profile than A2DP?
PS: My test devices are Samsung Galaxy 2 (phone) with Gingerbread and Asus Nexus 7 (tablet) with KitKat. Incidentally the remote bluetooth device (it is a development board) comes with a sample project that is supposed to turn the board into an A2DP audio source. if I run this sample project on the board, using the phone's and tablet's built-in bluetooth settings, I can sort of pair with the remote board, but the android devices do not receive any audio.
There are two different A2DP profiles, A2DP source and A2DP sink. The A2DP source profile is supported by Android thus allowing you to stream music from your device to a sink like a bluetooth speaker. To my knowledge the A2DP sink profile is not enabled, which is why you're unable to stream music to your mobile phone and have it act as a speaker. I only know this from having googled around some time ago so things may have changed.
It is possible to enable the A2DP sink profile by editing /etc/bluetooth/audio.conf and rebooting your phone. Unfortunately this has the side-effect of disabling the A2DP source profile and thus cannot be used in a widely distributed app. The other downside is that there is not much of a java software stack for working with that profile so you'd have to do quite a bit of work to get things rolling.
To answer your question as to why a headset is able to connect to the Android, that is because headsets use a different profile called Headset profile (HSP), commonly used by headsets, or Handsfree profile (HFP) commonly used in cars. Both profiles have two sides: a client side and the gateway side (mobile phone). The audio quality of both of these profiles is considered voice quality. 8 or 16kHz sample rates and a crappy codec. There is also a lot of protocol for answering/making calls, etc... I'm not sure if you'd even get audio to transmit without an active connection to the phone network.