Way back when I was a young nerd, the first electronic gadget I owned was a digital LED watch. These were the first digital watches from before the rise of LCD displays. You had to press a button to make the time light up in the form of menacingly red digits—having the time displayed continuously would have taken too much power. Several LCD watches followed during the 1980s, but my watch-wearing days came to an abrupt end when I got my first cell phone.

However, with the advent of the Pebble smart watch and all the iWatch rumors, I can see myself wearing a watch again in the near future. Maybe not all day every day, but certainly when on the go, so I can keep my iPhone tucked away safely but still interact with my digital life.

But what would a smart watch built by Apple be like? Time to put on our speculation spectacles.

Radios

First of all, the iWatch is unlikely to have 4G support. Or 3G. Or any G. Why? An iPhone 5 with its 1434 milliampere-hours (mAh) battery can handle standby times of "up to" 9 days—if you let the phone lie on your night stand without touching it, that is. The Pebble watch has a battery about a tenth the size of the iPhone's, so that would make for about a day's worth of 3G standby time. A cellular connection just takes too much power. It's not going to happen. A 3G iWatch would also be more expensive to build and use. (Yay, another data plan!)

Wi-Fi is not impossible, but it's unlikely—dealing with different Wi-Fi networks and intermittent connectivity is just too onerous. Without the need to transfer big files, Bluetooth is fast enough and much more energy efficient, especially Bluetooth 4.0, which Apple has conveniently been building into its recent Macs and iOS devices. Bluetooth also works directly from device to device without a need for Wi-Fi base stations.

Without its own connection to the Internet, the iWatch won't be able to do very much by itself. Like the Pebble watch, it will undoubtedly be an accessory for the iPhone. It may be an accessory for other devices in the Apple ecosystem, such as iPods, iPads, Macs, and Apple TVs. No need to limit the market for the iWatch to just iPhone users.

On the other hand, by having Wi-Fi onboard, the iWatch could operate independently from an iOS device, which would make the device much more attractive to those without iPhones, such as kids.

Power

Many people feel that the need to charge a watch frequently would make an iWatch a non-starter. Yes, this is certainly a downside, but I'm pretty sure people will live with this if the features are compelling. Several days to a week of battery life seems a reasonable tradeoff. However, it would be great if Apple could incorporate a battery that can be topped off quickly, perhaps by using supercapacitor technology. A supercapacitor doesn't hold nearly as much energy as a battery, but it has the advantage that it can be charged in seconds.

I'm not sure whether Apple would reuse the Lighting connector, use something MagSafe-like that allows for charging while wearing the watch, or adopt some form of wireless charging. Each solution has pros and cons.

The Pebble only has Bluetooth connectivity and a black-and-white LCD screen (although the company calls it e-paper); its 150 mAh battery that lasts seven days with regular use is charged using a connector not unlike a MagSafe one.

Display

Although it's unlikely that an iWatch would play video or fast-moving games, a responsive display is still needed to allow the contents of the screen to track finger movements and play other animations. As such, the display would very likely be LCD-based rather than use e-ink or another non-LCD display technology. OLEDs also don't seem like a viable option, because they need to actively emit light to be visible. They can do this pretty efficiently in dim environments, but OLEDs still use too much power in bright sunlight.

Maybe Apple's hardware magicians can figure out a way around this, but I don't see how the screen could be a regular color LCD display. Although the light has to pass through a polarizer twice in a monochrome LCD display, those displays can be made bright enough to be useful without a backlight. With color LCD displays this is much harder, because now there are also color filters in the way, further reducing the ambient light that can be reflected back.

The third-generation iPod uses a black-and-white LCD display, and it only turns on the backlight when the user presses the right button. The fifth-generation iPod has a color LCD display that stays on during audio playback, but as soon as you interact with the device, it turns on the backlight. With the backlight off, the screen has very little contrast—sometimes enough to be useful, sometimes not. The iPhone doesn't even bother keeping the display on when the backlight is off. As I found out wearing that LED watch, needing both hands to see what time it is gets old fast.

It's possible that Apple would opt to automatically turn on the backlight for a color LCD display based on sensed arm movements. However, I'm not sure that can be made sufficiently reliable. Also, having a display that's always on so it can always display the current status of the world seems like a perfect complement to the iPhone, which goes dark after a minute or two when unused.

However, this is not to say that the iWatch's display can't be colorful. Apple could steal borrow be inspired by the Philips Hue LED lights and have RGB LEDs for the backlight, allowing the backlight to take on any color desired.

If the LCD screen has a sufficiently fast response time, it would even be possible cycle through red, green, and blue backlit images in quick succession to produce a color image, similar to how DLP projectors operate.

If the display is indeed based a monochrome LCD, it could support reading of linear barcodes and thus make Passbook a whole lot more useful. Linear barcodes are usually read by scanning them with a laser and then determining how much laser light reflects back. Color LCDs don't reflect much light, but they emit their own, and thus linear barcodes on a color LCD screen can't be read by most scanners.

Looking at Lennart Ziburski's concept of a smart watch, I am struck by the circular display that he uses in his post. This is exactly the kind of risky design and manufacturing choice that Apple could and would make, embracing style over pedestrian considerations such as ease of manufacturing.

User interface

Zuburski suggests a scroll wheel/ring around the display for navigation. I don't see that happening. Scroll wheels are great on largish iPods, because they allow for easy one-thumbed operation, with the fingers of the same hand holding the device. With a watch, finer movements are harder, because your right hand and left wrist can easily move relative to each other unintentionally.

So it's important that most interaction with the iWatch work without requiring too much precision. For instance, it could work by allowing users to swipe and tap a touchscreen with one or two fingers. The addition of double tapping would allow for about a dozen different gestures, which should be sufficient to control the iWatch's most commonly used functions and switch between them. For less common functions, a number pad taking up pretty much the entire touchscreen is probably the limit for such a small device—unless you want to use a stylus. Add a home button for iOS-like familiarity and to get the iWatch unstuck when it gets confused.

The iWatch could have a microphone to talk to Siri, but remember that Siri actually lives in Apple's data centers, so Siri-powered functionality would have to be funneled through an iPhone or other iOS device. I doubt that Siri would speak back through the iWatch; that would require a relatively good (and thus large) speaker, it could possibly use too much power, and it would be disruptive to others. The watch is right in front of you when you talk to it, so it's easy enough to look at the screen. But if you're wearing headphones connected to your iPhone, Siri could talk back.

The iWatch would of course vibrate when notifications come in—preferably silently. Bruce "Tog" Tognazzini (developer of the original Apple Human Interface Guidelines in 1978) speculates that the iWatch may spur a Morse code renaissance among kids, allowing them to message each other during class with impunity. Hey, stranger things have happened.