According to CNET, Audience, a company responsible for a chip in the iPhone 4 and iPhone 4S that reduces voice noise, has a newer chip in the iPhone 4S called “EarSmart” that makes Siri possible.
From CNET:[quote]Audience also said in its filing that its iPhone 4-era technology was good only when the phone was held near the speaker’s mouth. Audience’s noise-reduction technology built into the iPhone 4S is better, though. “This situation helps explain why Apple does not offer Siri as a software upgrade on the iPhone 4. Although the older phone includes an Audience chip, the company has since improved its technology to handle ‘far-field speech,’ which means holding the device at arm’s length rather than directly in front of the mouth,” Gwennap said.[/quote]
The EarSmart technology works by processing audio, enhancing and isolating “primary voice signals”, and then suppressing the surrounding noise. Long story short, without the EarSmart chip in the iPhone 4S, Siri has a hard time distinguishing exactly what it is you’re trying to say. The technology works on the iPhone 4, as hacks have shown, but the results aren’t nearly as flawless as they are on the iPhone 4S.
It sounds pretty concrete now that Siri will not be available on the iPhone 4 any time soon, considering the phone lacks important Audience technology to power the feature.