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I am intending to record stereo audio on an Android 4.4.2 device. However the audio being recorded via a simple recording App (using AudioRecord) does not match the configuration supplied. I would hope to see error messages in logcat if the device is using default configuration values, but I can see that the supplied values appear to be accepted by AudioHardware, and AudioPolicyManagerBase.

The current configuration for:

recorder = new AudioRecord(Media.Recorder.MIC,
                           sampleRate,
                           AudioFormat.CHANNEL_IN_STEREO,
                           AudioFormat.ENCODING_PCM_16BIT,
                           audioBufferSizeInBytes);

Changing the Media.Recorder.AudioSource has been raised an option for trying to resolve this issue; But this has not changed how the Android stack behaves- Except to (understandably) fail to load the recorder when the configuration is invalid.

Changing SampleRate has shown also produced no visible change in the output- both 44.1kHz, and 16kHz are valid options, however both produce 16kHz audio when examined. The output audio also appears to be one channel of audio upmixed to stereo.

TinyALSA/Tinycap is available to capture the audio, and this appears to behave as expected.

Could this be an issue within the Android Stack? Or is this more likely to be an issue with the code supplied by the OEM?

Steve Bailey
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1 Answers1

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The reason for the downmixed audio in this case was that the Speex Codec was being used in the HAL to downmix and de-noise the stereo input.

This was configured under:

<android source tree>/hardware/<OEM>/audio/audiohardware.h

The alternatives to this problem would be to route the audio out of ALSA, and around the android stack via a unix domain stream socket. This would be accessible in the application layer with androids localSockets.

Steve Bailey
  • 106
  • 7