We all know that Siri is available exclusively on the iPhone 4S. There were a lot of speculations why Apple didn’t release it for the iPhone 4 as well, because as we can see with Spire, it works perfectly fine. CNET now reported that the A5 processor used in the iPhone 4S contains a special noise-reduction circuitry:

Apple’s A5 processor includes noise-reduction circuitry licensed from a start-up called Audience, and a chip analyst believes that fact resolves an iPhone 4S mystery and explains why the iPhone 4 lacks the Siri voice-control system.

Audience revealed details of its Apple partnership in January, when it filed paperwork for an initial public offering (IPO) of stock. Teardown work from iFixit and Chipworks revealed a dedicated Audience chip in the iPhone 4, but the iPhone 4S integrates Audience’s “EarSmart” technology directly into the A5 processor, the company’s S-1 filing said.

While this is a nice explanation, it still doesn’t explain why Siri isn’t available on the iPad 2, as it has the same A5 processor as the iPhone 4S.

And because we can see that Siri works just fine on the iPhone 4 without the noise-reduction circuitry, it seems more likely that Apple just wanted to use Siri to sell the iPhone 4S, not due to hardware restrictions.