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I was wondering whether it is possible to capture audio data from other sources like the system out, FM radio, bluetooth headset, etc. I'm particularly interested in capturing audio from the FM radio and already investigated all possibilities including trying to sniff the raw bluetooth communication between the phone and the radio device with no luck. It's too bad Android only allows recording audio from the MIC.

I've looked at the Android source code and couldn't find a backdoor to allow me to do that without rooting the device. Do you, at least, have any idea how to use other devices (maybe access somehow /dev/audio) say via NDK or even better - Java (maybe Reflection?) to trick the system to capture the audio stream from say, the FM radio. (in my case I'm trying to develop the app for the HTC Desire)

PS. And for those of you who are against using undocumented APIs, please don't post here - I'm writing an app that will be for my personal use or even if I ever publish it I will warn the user of possible incompatibilities.

Jonas
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Martin Marinov
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1 Answers1

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I've spent quite some time deciphering the audio stack, and I think you may try to hijack libaudio. You'll have trouble speaking directly to the hardware (/dev/*) because many devices use proprietary audio drivers. There's no rule in this regard.

However, the audio hardware abstraction layer (HAL) provided by /system/lib/libaudio.so should expose the API described at http://source.android.com/porting/audio.html

The Android system, and especially audioflinger, uses this libaudio HAL to find available devices, deal with routing, and of course to read/write PCM data.

So, you could hijack the interaction between audioflinger and libaudio, by renaming the later, and providing your own libaudio which decorates the real one. Doing so, you should be able to log what happens and very possibly intercept FM radio output, provided that this is not directly handled by the hardware.

Of course, all this requires rooting. Please comment if you manage to do this, that interests me.

olivierg
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  • Hm, it might be possible but as I mentioned above I don't like rooting (I haven't rooted my phone yet, which is not really a problem). Any way, I'll wait a little bit more and if there are no other suggestions how to do it without rooting I may try yours (and accept your solution). – Martin Marinov Jan 08 '11 at 08:12
  • Apart from rooting, the only backdoor I can think about is to try and sniff the shared memory between audioflinger and individual clients (audio_track_cblk_t). But I really doubt this is possible, and it won't give you access to the global system output anyway. – olivierg Jan 11 '11 at 10:00
  • All Fm supported OEMs will give you one audio source id. By using that we can record FM. But the real problem here is Audio source ID is hidden. I tried with some random numbers, surprisingly 9 working for some devices like Samsung and Motorola, but not all of them. I am still searching way to find the Audio source id for all OEMs. – gkondati May 28 '15 at 21:52