Absolute Power


iPhone 4S Power Efficiency Improvements

This blog originally posted on the Low Power Engineering Community 11/3/11.

I’ve had my iPhone 4S for a few weeks now, and have gotten to know it pretty well. Mine is the 64GB model, mostly because I want to play around with the new 1080p video recording capabilities, and don’t want to worry about running out of storage space all the time.

I’ve read many reports online about runaway battery consumption with the new 4S, but I haven’t noticed any huge change vs. my iPhone 4 in battery behavior. I still charge it every night and most of the time when I’m at my desk in my office.

From a hardware perspective, the iPhone 4S pretty much delivers as expected. The dual-core A5 processor zips along with plenty of headroom, smoothing out many of the rough spots in everyday usage that have started to creep in since the iOS 4.3 update. The camera upgrade boosts both still image and video (1080p) performance into the realm of most modern point-and-shoot cameras, although the lack of any optical zoom is still a big limitation. On the other hand, the wide availability of camera and photo enhancement apps pretty much make up for the lack of optical zoom. Battery capacity has increased minimally, but not enough to make any practical difference.

From the power efficiency standpoint, by far the most interesting new hardware feature is the new dual-antenna design, which eliminates the infamous “death grip” effect of the iPhone 4 and improves cellular reception in general. Combined with the new communications chip that boosts data rates (for GSM networks), the new radio setup is worth looking at—especially since we’ve seen that the radio can contribute even more to the energy equation than the display.

The runaway star on the software side with the iPhone 4S and iOS 5.0 is Siri. Reminiscent of a cross between HaL and the famous Star Trek “Computer,” Siri listens, seems to think, and generally does a better-than-expected job of carrying out your wishes. While still clearly early in the development cycle, Siri feels to me like the beginning of a paradigm shift where we may actually become just a productive without a keyboard as with one. I’ve dictated quite a few e-mail and text messages with Siri (sometimes while driving!), and accuracy is very good—or very bad. That to me is an indication of evolving and improving AI on the recognition side. And as is usual for Apple, the real genius of the Siri interface is the simplest part of it: You simply hold your phone up to your head to start talking to Siri. To everyone else, it just looks like you’re answering a phone call! I wish I thought of that…

Running the 4S though my usual battery (pun intended) of power tests running the Star Trek movie resulted in the following:

The results were surprising in several respects. First, the 4S seemed to be extremely efficient in the “Max Battery” mode. It played through the entire movie consuming just 0.6 Wh of energy. That would be more than 18 hours of continuous movie playing, although you can’t see much at the lowest brightness setting. This is almost a 40% improvement in energy efficiency vs. the iPhone 4! The new lower power A5 chip is likely at the heart of this result. These days video decoding is an almost-routine task, and can probably easily be handled on one of its cores.

Turning the display to full brightness (the “Max Brightness” test) shows the expected result. The energy cost of running the display at full brightness is about 0.6 Wh for the two-hour movie. The display on the 4S isn’t notably different than on the iPhone 4, so this is expected.

Turning up the sound to maximum (“Max Movie” mode) shows one other interesting change. On the iPhone 4, there was virtually no change in energy consumption between the runs with the sound muted or with the sound at maximum. On the iPhone 4S there is definitely a measurable difference, both in energy consumption as well as in the perceived loudness of the sound. In fact, with the sound at maximum, the tiny speakers in the 4S produced enough sound to make it annoyingly loud as I was trying to do some other work. I had to resort to my “manual” muting method (putting a piece of tape over the speaker) to conduct my tests.

Well, I’m about out of space and time for this post. Next time I’ll describe the results of testing the new iPhone 4S dual-antenna and modem chip setup. These results are very interesting.

In late breaking news, Apple just announced a software update, iOS 5.0.1, which among other things “fixes bugs affecting battery life.” No big surprise, as the complexity of today’s smartphones rivals any other computing platform—and dwarfs the others when it comes to power management. In particular, the interaction between hardware and software to minimize energy consumption is very difficult to model and predict, but the ramifications on battery life are immediate and sometimes ugly.

And by the way, if you are interested in learning more about low power hardware design, my colleague Josefina Hobbs is hosting a new series of short videos covering everything from introductory concepts to selected advanced low power design topics. Check them out here.

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