Android phone sample rate DSP puzzle

Problem solved! (mostly)

Someone in the Android Developers forum suggested that I open my audio
channel with an option that supposedly bypasses AGC and filtering.  It
is called

  MediaRecorder.AudioSource.VOICE_RECOGNITION

in the Android SDK.  When I did that the aliasing artifacts vanished.
All I see now is one nice peak in the FFT regardless of frequency.  So
apparently there was something in the AGC or noise cancellation
processing that introduced the problem.  I guess the only thing I am
sacrificing now is AGC, but for my app I don't think that is a
problem.

-Robert Scott
 Hopkins, MN

On Tuesday, February 9, 2016 at 9:34:26 PM UTC-5, Robert Scott wrote:
> Problem solved! (mostly)
> 
> Someone in the Android Developers forum suggested that I open my audio
> channel with an option that supposedly bypasses AGC and filtering.  It
> is called
> 
>   MediaRecorder.AudioSource.VOICE_RECOGNITION
> 
> in the Android SDK.  When I did that the aliasing artifacts vanished.
> All I see now is one nice peak in the FFT regardless of frequency.  So
> apparently there was something in the AGC or noise cancellation
> processing that introduced the problem.  I guess the only thing I am
> sacrificing now is AGC, but for my app I don't think that is a
> problem.
> 
> -Robert Scott
>  Hopkins, MN

FYI: a 2011 post on the android-platform forum:

https://groups.google.com/forum/#!topic/android-platform/2ZJFBjJOZV0

Kevin