How does noise cancellation work in android?

I came across this relatively old post which describes how impressively Nexus One's noise cancellation works and I was wondering where can I find more information about its implementation in the OS software.

In particular:

1. How much of it is done using software and how much of it 开发者_C百科is done in hardware?
2. Which modules in the Android source code are responsible for noise cancellation?
3. Can I control its behavior via Android's API? (if so, which ones)
4. Does it also work with the microphone in the headset that comes with Nexus One (4-pin 3.5mm jack) or does it work with the built-in microphone only?

I only know the answer for the Nexus One, but:

1. It's done in hardware.
2. Not sure.
3. Nope.
4. Maybe?

For the N1, it works using a second microphone in the back, and comparing the two signals. I don't know exactly how this process is done (hardware or software), but I know there isn't an API for it. Also, it probably doesn't work for the external headset, since there's no second sound source to compare the first one to (unless the headset has two mics too, but I don't think it does).

About the Nexus One:

1. All hardware only configuration in software.
2. Sound drivers and sound system but only configuration.
3. No API possibly some prop configuration but I haven't been able to get that to work.
4. No, longer reply following.

I haven't found any indication that it uses the other microphone to do noise reduction for the headset. It wouldn’t make much sense either as it would most likely just try to cancel out with the noise from your pocket.

For most other android phones and for headset on the Nexus One I'm pretty sure that there is only some sort of filter to reduce input of sound that is not speech.

I have done some research on this that I tried to get some help with on the android porting and dev lists. There is a little further info:

http://groups.google.com/group/android-porting/browse_thread/thread/fe1b92065b75c6da?pli=1

With the reservation that I haven't looked at the latest and greatest versions of android.