Despite its superficial similarity to the iPhone 4, three landmark features set the iPhone 4S apart from its predecessor: Siri, the new 8 megapixel camera, and the much more powerful A5 processor. The iPhone 4S also comes with a few more subtle design tweaks that differentiate it from the iPhone 4, and you might miss some of those changes if you didn't have an iPhone 4 and 4S sitting side-by-side for a comparison.
Phil Schiller actually did talk about the new antenna in the iPhone 4S during the most recent Apple event, but the changes to the iPhone's antenna aren't really obvious until you have the handset in your hands. One of those changes literally requires you to hold the device in your hand before you'll see it: the dreaded "death grip" of the iPhone 4 is gone in the iPhone 4S.
A well-known and widely (over)publicized shortcoming of the iPhone 4's antenna design caused it to attenuate wireless signals when held a certain way. Bridging the black gap on the lower left side of the iPhone's antenna band would cause signal strength to drop for some users. This so-called "Antennagate" dominated headlines about the iPhone 4 for months, and Apple eventually addressed the issue by providing free bumper cases to affected users.
Since I use my iPhone left-handed and without a case, with the iPhone 4 I always had to be mindful of how I held it. 3G signal strength at my home wasn't the best with my old wireless provider -- I'd get one or two bars if I was lucky -- so accidentally bridging that gap would cause my signal strength to drop to zero within a minute or less.
That's no longer an issue with the iPhone 4S. In fact, to get the signal strength to drop at all I have to hold the phone in a very unnatural two-fisted grip that requires bridging all four of the black antenna gaps at once. The "death grip" is a thing of the past.
The improved antenna design also equates to much faster 3G speeds, improved call quality, and lightning-fast Wi-Fi signal acquisition. 3G download speeds on my iPhone 4S are anywhere from two to four times faster than my iPhone 4 on the same network, and the murky "bottom of the ocean" call quality I got on the iPhone 4 was replaced with crystal-clear voice quality on the iPhone 4S. The newest iPhone also latches onto a Wi-Fi signal much faster than my iPhone 4 ever did, with essentially zero delay in connecting to networks it's connected to before.
Many critics lambasted the iPhone 4's antenna design in the wake of "Antennagate," but the iterative update of the iPhone 4S antenna shows that Apple has shaken out any deficiencies in the design.
One consequence of the antenna re-design is those antenna gaps have been shifted around. Rather than the asymmetrical three-gap design of the GSM iPhone 4, the iPhone 4S has four gaps arranged with bilateral symmetry, essentially identical to the gaps on the CDMA version of the iPhone 4. As a result, the mute switch and volume controls for the iPhone 4S have been shifted downward toward the dock connector by about a millimeter compared to the iPhone 4, similarly to how the CDMA iPhone 4's controls were altered.
As a result, many (if not most) cases designed for the GSM model iPhone 4 won't fit the iPhone 4S, including Apple's own bumper cases. Cases designed for the CDMA iPhone 4 should fit the iPhone 4S just fine. So should "universal" cases designed for both the GSM and CDMA versions of the iPhone 4; these cases feature slightly larger cutouts for the mute switch and volume buttons, so they should fit the iPhone 4S without issue.
Quieter Vibration Motor
Apple has swapped out the old vibration motor in the GSM model iPhone 4 with one that's either similar or identical to the one in the CDMA iPhone 4. The new motor features a smoother vibration which doesn't rattle the phone itself quite so much. The result is a much quieter vibration, one that you might not even hear from more than a few feet away if your iPhone's lying on a table; if you're holding the phone in your hand, you might barely hear the vibration at all.
Depending on your needs, that might not necessarily count as an improvement. If you loathe ringtones and instead listen for your iPhone rattling across a table or other flat surface for your notifications, this change may disappoint you. Personally, I appreciate the new vibration, because the old one always sounded as though it was trying to shake the phone to shreds.
The quieter motor in the iPhone 4S doesn't mean the phone's vibrations are weaker than those of the iPhone 4. I held the 4S in one hand and the 4 in my other; the strength of vibrations felt essentially the same, with the only difference being a distinctively loud BZZZZT coming from the iPhone 4.
The speaker on the iPhone 4S is NOTICEABLY LOUDER compared to the one in the iPhone 4. In some cases it's almost too loud; high-pitched notification sounds from apps like Tweetbot definitely have a piercing effect on my eardrums with the alert volume turned up all the way, which was never an issue with the quieter iPhone 4. The obvious upshot of the louder speaker is it'll be easier to hear ringtones and other sounds when you're in an environment with lots of ambient noise.
Despite that increase in overall volume, the speaker in the iPhone 4S also seems to produce sounds with higher clarity than the iPhone 4's speaker. Audio that used to overdrive the iPhone 4 speaker and make it sound "clippy" now sounds much clearer on the iPhone 4S speaker. Music and games sound much better on the iPhone 4S, but the improved speaker clarity also means some ringtones or other media may actually sound worse than on the iPhone 4. For example, I sourced some of my custom ringtones from low-fidelity mp3 audio files; while they sounded fine on the iPhone 4 speaker, the flaws in recording quality are much more obvious on the iPhone 4S speaker, with very audible background hiss in some cases.
Audio playback through both the iPhone 4S speaker and attached headphones produces a much less trebly sound than the iPhone 4. Music playback quality on the iPhone 4S also sounds subtly improved even through the included Apple-branded earbuds. Based on these sonic differences in music playback between the iPhone 4 and iPhone 4S, I suspected the iPhone 4S had a different audio processor; information in iFixit's teardown confirmed that though the audio codec chip is from the same manufacturer (Cirrus Logic) as the iPhone 4, the part number for the iPhone 4S is different. It's likely the updated audio codec chip is also part of the reason the iPhone 4S speaker sounds clearer than the iPhone 4.
Your personal tastes may vary from mine. It's possible we could review the same hardware and come to entirely different conclusions about whether the iPhone 4 or 4S sounds "better" over headphones. There's no mistaking the improvements in sound quality through the built-in speaker on the iPhone 4S, though; it sounds markedly improved compared to the iPhone 4.
The iPhone 4S is the first iPhone to support Bluetooth 4.0, a low-energy Bluetooth spec that allows devices to sync via Bluetooth while consuming far less power than traditional Bluetooth devices. This low-power version of Bluetooth doesn't support voice -- headsets and other voice devices still have to transmit over one of the higher-power specs -- but it does support transmission of data from other peripherals like heart rate monitors, watches, and input devices like keyboards or game controllers.
The applications for health and fitness-related gear are virtually endless. As of now, many third-party accessories require a proprietary receiver to send data to an iPhone, or else they transmit via one of the older, more power-hungry Bluetooth specs. Using Bluetooth 4.0 would allow these devices to communicate directly with the iPhone, without the need for an intervening dock connector dongle, and such devices could have tremendously increased battery life compared to those currently on the market.
One example of a device that could greatly benefit from Bluetooth 4.0 is Jawbone's Up health monitoring band. The Up monitors a great deal of user health data, but syncing that data to the iPhone requires plugging a connector into the iPhone's headphone jack. Using Bluetooth 4.0 could theoretically allow a device like the Jawbone Up to be in constant communication with the iPhone 4S without the need for users to sync data manually.
Another possible implementation of Bluetooth 4.0 would be a wristwatch that can display certain kinds of information transmitted to it from a synced iPhone (notifications, for example), and send basic commands back to it, such as controlling music playback. This has been a popular dream of geekier users ever since the current iPod nano debuted; once it became clear the nano could be used as a watch, people almost immediately leapt to the next-level idea of using the nano to control another device and/or display data transmitted to it.
Not many Bluetooth 4.0 devices exist on the market as of yet, but expect to see a lot of them hitting the market once the standard becomes more widely adopted. The low-power spec allows devices powered by standard watch batteries to run with lifetimes measured in months to years rather than the hours to days of battery life current devices get.
The iPhone 4S is the first iPhone to support mirroring to an Apple TV via AirPlay, at 720p resolution. It also supports video mirroring or video out at 1080p resolution via Apple's Digital AV Adapter or VGA Adapter, the same connectors that allow for 1080p mirroring on the iPad 2. Video mirroring to an Apple TV will make it much easier to share content with people sitting nearby, and it also has great applications for gaming.
None of these features are anywhere near as headline-grabbing as Siri, but the minor details that you might not even notice are often where Apple's products excel. The iPhone 4S is no exception.