Siri, Apple's widely advertised voice-activated "intelligent assistant," has so far been limited to the latest iPhone 4S hardware after Apple's acquisition. Though observers have come up with various reasons for the restriction, a newly revealed piece of the puzzle suggests the issue is related to hardware after all. According to recent SEC filings from technology start-up Audience, Apple incorporated an improved version of its background noise filtering technology directly into the A5 processor used in the iPhone 4S—technology that improves Siri's speech recognition capabilities.

Siri was originally a third-party app for the iPhone that ran on devices as old as the iPhone 3GS. Apple later bought the company behind Siri, and integrated the tech directly into iOS 5, which was released to the public in October of 2011. Siri is now only available on the iPhone 4S, however, and Apple subsequently pulled the old app from the App Store when the 4S was released.

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