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You need any microphone with a 3.5mm jack (standard headphone size). I've been able to get any microphone I've tried to work with an app called Spectrum.
 
does this meant that the app store pages for recorder and shazam will start saying compatible with iphone and ipod touch(2nd gen)?
 
Hopefully, but nothing has happened yet. Obviously it takes the same commands as the iPhone to use an external mic. (the app I posted was old), so they just need to take off the block they have on the apps that stops iPT users from installing/using the mic. capabilities.
 
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