iOS实现VoIP可能会用到的一些技术

iOS 4后的Voice Processing I/O Unit(Audio Unit的一种)可以直接支持回声消除，自动增益控制等功能，详细的可以参考以下链接。

1. Audio Unit Hosting Guide for iOS

https://developer.apple.com/library/ios/documentation/MusicAudio/Conceptual/AudioUnitHostingGuide_iOS/Introduction/Introduction.html#//apple_ref/doc/uid/TP40009492-CH1-SW1

2. Audio Unit Properties Reference

https://developer.apple.com/library/ios/documentation/AudioUnit/Reference/AudioUnitPropertiesReference/Reference/reference.html#//apple_ref/doc/constant_group/Voice_Processing_I_O_Audio_Unit_Properties

3. How to use “kAudioUnitSubType_VoiceProcessingIO” subtype of core audio API in mac os?

http://stackoverflow.com/questions/10792469/how-to-use-kaudiounitsubtype-voiceprocessingio-subtype-of-core-audio-api-in-ma

4. How to control VoiceProcessingIO

http://lists.apple.com/archives/coreaudio-api/2010/Jul/msg00056.html

5. VoiceProcessingIO - echo cancellation issues

http://web.archiveorange.com/archive/v/q7bubz4SYiUN3FMSYEJ6

6. VoiceProcessingIO Sample code

https://github.com/hfink/matchbox/blob/master/Xcode/SimodOne/SOVoiceRecorder.m