Now we know why Siri is on the iPhone 4S, but not the iPhone 4. Stephen Shankland at CNET reported that it has to do with Apple’s partnership with Audience. The Apple A5 processor has noise-reduction circuitry licensed from Audience.
Audience’s partnership with Apple was revealed in January when they filed paperwork for their IPO. There is an Audience chip in the iPhone 4, but the iPhone 4S has Audience’s EarSmart technology directly in the A5 processor.
Apple signed an agreement to pay royalties for Audience’s technology used in the A5 processor in the “three months ended December 31, 2011.” This suggests that the iPhone 4S version of the A5 supports Audience’s technology.
Apple is a licensee for the next-generation intellectual property, but they are not contractually forced to use it. iPhone 4’s technology from Audience also uses advanced noise-reduction, but it was in their second-generation product that Audience was able to to perfect recognition and reduction at an arm’s length, which allowed for features like Siri’s voice control.
The way that the iPhone 4S uses Audience’s EarSmart on the A5 processor explains why the iPhone 4 does not have Siri. There have been some hacks for putting Siri on older devices, but they are not as organic as the real technology on the iPhone 4S.