In a Google Plus post, Guillaume explains that the Snapdragon 800 MSM8974 processor inside the Nexus 5 is capable of always-on listening, just like the Moto X. The chip contains a processor dedicated to processing voice commands, without keeping the CPU active, and therefore with low battery consumption. Guillaume says this core can be activated through software modifications, and the video above is the proof of concept.
Guillaume says that the functionality is “far from being stable, and not releasable in the current state,” but the developer hopes to make it available to the public in the future. Here’s how he explains his work in technical terms:
The S800 also has dedicated processing power. Even if it's not shown in the video, it is using the capabilities of the WCD audio codec to wake up the audio DSP when the mic reaches a certain level. The audio is then matched against a predefined sound model, which will wake up the CPU above a certain match threshold. The CPU is otherwise inactive.
This is not a kernel-level mod, but the modifications are nonetheless extensive:
The kernel didn't need any change, but all the audio chain needed changes (audio HAL, audio_policy, and various libs to rebuild and add). Unfortunately I can't release those changed files.
Touchless Control on the Moto X is probably the most distinctive feature of the device, allowing users to wake up the device without touching it, with a simple “OK, Google Now” voice command. The Moto X runs on Motorola’s proprietary X8 chip, which contains a specialized signal-processing core that allows continuous listening for commands. But the Snapdragon 800 chip is capable of the same trick, as this promotional video from Qualcomm shows:
We’ll definitely keep an eye for any development on this front. If Guillaume Lesniak manages to enable accurate, battery-friendly on the Nexus 5, Google’s device (and potentially other Snapdragon 800 powered phones) will become even more attractive.