If you’ve wondered why you can’t just get Siri as a software upgrade on the iPhone 4, it’s because, as Linley Gwennap of the Linley Group explains, you’d need a chip upgrade as well.

Apple (News - Alert) Insider recently wrote that the older version of the phone has a chip made by Audience (News - Alert). The newer one does too, but it’s an improved version, capable of handling “far-field speech,” where you can hold your iPhone further away from your mouth and still get the point across.

As Apple Insider explains, last spring the site iFixit (News - Alert) discovered that Apple was using the Audience A1026 chip, adding a second mic to cut out background noise and improve overall sound quality. They worked great, too -- "the iPhone’s audio cancellation capabilities are very impressive, outperforming every non-Audience powered cell phone we’ve tried,” the site noted.

Audience has since improved the “earSmart” technology, and according to recent S1 filings, the company revealed that "We sell our products to Foxconn International Holdings, Ltd. and its affiliates (collectively, Foxconn) and Protek (Shanghai) Limited and its affiliates (collectively, Protek), each a major CM that produces mobile phones containing our processors almost exclusively for Apple."

Audience officials describe earSmart as “based on pioneering work to reverse-engineer processes of human hearing and replicate that brilliance onto a tiny voice processor,” to “bring auditory intelligence to mobile devices... working like the human ear, earSmart processors ‘hear’ and distinguish sounds, isolate and enhance the voice signal and suppress surrounding noise.”

So that’s why you can’t get Siri on the older iPhones. It’s a chip thing.

TMC’s tech blogger, Tom Keating (News - Alert), wrote earlier this week that “many iPhone 4 users were none to happy with not getting Siri and assumed Apple was simply trying to force users to upgrade to the 4S. You would think Apple would have just come straight out and admitted there was a technical reasoning (Audience chip), as opposed to allowing their loyal customer base assume the worst of Apple.”

Siri’s an exciting technology in the geek community. Last week, Epipheo even released a video titled “Why Siri Can Take Over the World.” The key difference, the video says, is that Siri isn’t just voice recognition, “she understands language,” and she can apply logic. Epipheo CMO and co-founder Jon Collins said “I use Siri all the time and I've seen how it has changed my life already.”

Google (News - Alert) thinks it’s a pretty big threat. At least that’s what Chairman Eric Schmidt told the U.S. Senate recently. So it has to be true, right?