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Compression / AGC in 3.0 SDK: How to turn off?
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Compression / AGC in 3.0 SDK: How to turn off?


  • Subject: Compression / AGC in 3.0 SDK: How to turn off?
  • From: Andrew Smith <email@hidden>
  • Date: Fri, 15 May 2009 13:13:03 -0600

I am using Audio Units on the iPhone to do audio & acoustics testing. I have several apps running using this.

I have found that if I compile against the 3.0 SDK, I get what looks like compression, or automatic gain control (AGC). I'm using an Apple headset mic (the new one) on a 2G Touch. As the level in the room goes down, it's like someone is turning up the input gain. As the level goes up (like if I make a sound), the sensitivity goes down. This is not subtle, its like 20-40 dB, depending on the sound level in the room.

If I compile to 2.2.1, I do not see this behavior. For an audio testing app, this is not desired behavior. In fact, it makes these apps unusable. Does anyone know if there is a way around this, like an audio session property that I can turn off? Or do I need to file a bug report?

thanks in advance,
Andrew Smith
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