Best explanation I have yet read for why Siri is only available on iPhone 4S

Via the Linley Group:

Our previous coverage of Apple’s custom-design A5 processor pointed out that the die size of the processor is much larger than that of competing application processors. We speculated that Apple included some custom logic to improve the performance of its products.

A recent S-1 filing from Audience confirmed this speculation and explains exactly what that custom logic does. For those unfamiliar with the company, Audience has developed impressive technology that removes most or all of the background noise when someone places a cell-phone call from a restaurant, airport, or other noisy location. The company initially offered this technology in a small chip, which Apple included in the iPhone 4, generating sizable revenue for the startup.

To reduce system cost and eliminate the extra package required for the Audience chip, Apple cut a deal to integrate the noise-reduction technology directly into its A5 processor, which appears in the iPhone 4S. This technology is critical for the new phone because not only does it improve call quality, it blocks out background noise when users provide voice commands to Siri, the intelligent assistant built into the iPhone 4S. Without this noise reduction, Siri would be unusable even with a modest amount of background noise.

This situation helps explain why Apple does not offer Siri as a software upgrade on the iPhone 4.