What's inside your iPad? Tablet tech explained
13th Jun 2011 | 10:46
Processors and graphics inside iOS, Android and other tablets
Tablet technology explained
What makes one tablet a winner and another a bucket of bolts? Let's start with a story.
Once upon a time, there was a little English company called Acorn Computers. Through the early 1980s, it had quite a bit of success with an 8-bit home computer named the BBC Micro. You may remember it.
For a replacement, Acorn decided that none of the then-available processors - such as the Motorola 68000 and Intel 80286 - were suitable. So it decided to make its own, as you do.
The year was 1985 and Acorn developed a 25,000 transistor RISC processor called the ARM v1. It used it to power the now defunct Archimedes computer, and that same ARM core - with a few enhancements - is what now powers more than 75 per cent of all embedded 32-bit devices made in the 21st century, 90 per cent of all mobiles in the last five years, and the majority of tablets, including the iPad.
This amazing success is partly to do with how ARM Holdings sells the processors, in that it doesn't. It licenses the design IP so anyone out there can create their own ARM-powered processor or hybrid system. So the A4 processor in the iPad is ARM powered, is the same as the Galaxy Tab, sits at the heart of Nvidia Tegra and also inside almost all smartphones.
The original ARM v1 had no pipelines, no cache and no MMU. The latest v7 design is used in a bewildering array of different processors and consumer devices, uses the Cortex-A line and is the first multi-core capable design from ARM.
There are two main variants you'll want to keep an eye out for. The Cortex-A8 is a single-core implementation that uses a 13-stage pipeline, up to 32KB L1 cache and 1MB L2 cache, an MMU and an optional FPU, and has a maximum clock speed of 1GHz. You'll find it in the original iPad A4 chip and the HTC Desire.
The more recent and better Cortex-A9 is a quad-core capable variant with a 13-stage out-of-order pipeline, up to 64KB L1 cache and 8MB L2 cache, an MMU, an optional FPU and a maximum clock speed of 2GHz. A dual-core version of this is inside the iPad 2 A5 chip and the Motorola Xoom.
A cheaper version is the Cortex-A5, which has no L2 cache and a maximum speed of 1GHz or 600MHz for the low-power variant.
For 2012, ARM has the Cortex-A15 lined up. It promises at least a 40 per cent leap in speed per clock over current designs.
The other players
Of course, ARM isn't the only name in the game; both Intel and AMD are hammering their own stakes into the market.
The AMD Fusion ship has already set sail in the Acer Iconia W500 tablet, which uses the AMD Fusion C-50 system-on-a-chip (SoC) processor.
Running at 1GHz with two x86 cores and a 512KB L2 cache, it still draws 9W of power, which might be good for a Windows laptop but is nowhere near the sub-1W that ARM devices can achieve. Hence the shorter, four-hour battery life while playing video, versus 10 hours on an iPad.
But its main design goal is to drive Windows systems - it sports full DirectX 11 graphics, though lacks the CPU and GPU grunt to run anything recent of note.
On the Intel side, tablets fall into two groups: full Sandy Bridge models and Atom-powered units. The Asus Eee Slate EP121 is a perfect example of a 'fully featured' Core i5 Windows tablet. It costs £999 and has a two-hour battery life. But, then, tablets like this aren't supposed to compete with consumer ones. They're business tools for business solutions, such as medical imaging in a networked environment.
The big hope for Intel is its Atom range, which we're all familiar with thanks to the lock-down netbook design. Unlike the AMD Fusion and Sandy Bridge Core, architects for the Intel Atom took out a heap of features to make it less power hungry, primarily giving it an in-order pipeline, a limited 512KB L2 cache, a single-channel memory controller and no SSE4, among other things.
Even though the original Atom N2x0 only drew 2.5W, it needed an additional system chipset that consumed 9.3W, totalling 11.8W.
It was the Pineview SoC models - which integrated graphics into the Atom N450 single-core and N550 dual-core models, dropping total power to 5.5W and 8.5W - that made these more acceptable, boosting netbook battery life towards the eight-hour level.
The Atom N550 is found in the new Dell Inspiron Duo Sparta convertible netbook/tablet but, again, battery life struggles to make the four-hour mark.
However, for Intel the game won't really start until the first Atoms using the 32nm production process launch by the end of 2011. Its Medfield variant will initially pop up in phones, but a 32nm Atom will arrive in tablets and offer full 1080p HD playback.
Nvidia Tegra 2 SoC
One familiar name that's starting to win friends in tablets is Nvidia, with its Tegra 2 SoC. It's an ideal example of what many other brands do in the mobile space: take the ARM core and build a system around it. That's how the A4 and A5 processors work.
The Tegra 2 is a 1GHz dual-core SoC built around the ARM Cortex-A9 core, using a unified 1MB of L2 cache and separate 32KB instruction and 32KB data caches for both cores.
Interestingly, alongside these sits a separate ARMv7 processor. This is a far simpler model that can deliver ultra-low power requirements, primarily when playing audio and video, helping extend battery life by shutting down the larger Cortex-A9 cores.
Talking of which, Tegra 2 boasts both a full 1080p hardware decoder and encoder. The former is designed to handle both local and streamed content, and is optimised for Flash video encoded with h.264 but also VC-1 and MPEG-4 video. Power consumption while playing 1080p video is less than 400mW.
The encode engine is designed to handle 1080p video at 30fps from an integrated camera, and both units can work in unison to deliver HD video conferencing. Alongside the video is a dedicated audio processor able to handle MP3 playback using less than 30mW of power.
An image signal processor is designed to take the raw output from any integrated cameras up to 12MP in resolution and 30fps, applying real-time image enhancement such as white balance and noise reduction. The output can then be saved as an image, or as a stream ready for compressing and broadcasting.
There's also a host of additional I/O requirements that Nvidia squeezes onto the silicon to handle security protocols and encryption, USB, HDMI, SPI touch controller, memory, NAND flash storage, PCIe and interfaces for radio. Many other models, such as the Texas Instruments OMAP3xxx series, Qualcomm Snapdragon and Samsung Hummingbird, provide similar abilities.
The one missing element is graphics. Mobile graphics are no different to desktop ones. It's just that, instead of a 200W, power-sucking card, you need a sub-1W solution that fits on a SoC.
Tablet graphics explained
Returning to the Nvidia Tegra 2, it uses what's called the ultra-low power (ULP) GeForce GPU. Think DirectX 9 and a card with four pixel shaders and four vertex shader cores. It's capable of 5x Coverage Sample Anti-aliasing (CSAA) and 16x anisotropic filtering. Running at 300MHz and with each core executing two FLOPS per clock, that's a peak output of 4.8GFLOPS. But because these aren't unified shaders, it would take careful optimisation to get the best out of them.
Remember PowerVR? That's the de facto standard in the mobile world. Its tiled-based deferred rendering in a power-limited environment has delivered better performance per watt than Z-buffer alternatives.
The current range in almost all SoC solutions is the PowerVR SGX5xx series. Usually running at 200MHz, all the models use a Universal Scalable Shader Engine (USSE). It's able to perform geometry, pixel and GP-GPU operations. Each pipeline is highly flexible and able to complete two FLOPS per clock, with the latest generation of USSE2 cores issuing twice as many instructions, increasing that to four FLOPS per clock.
The PowerVR SGX535 in the Apple A4 and Intel Atom has a pair of USSEs able to run two FLOPS at 200MHz for a maximum 1.6GFLOPS. The Galaxy S has a PowerVR SGX540, offering four USSEs for a maximum 3.2GFLOPS.
That's no problem for the Nvidia Tegra 2, until you hit the latest generation of multi-core PowerVR SGX54x models. The iPad 2 makes use of a dual-core SGX543MP2 with a total of eight USSE2 units, each capable of four FLOPS for an impressive 12.8GFLOPS.
The PS Vita will have a quad version of this running at 543MHz, with the technology scaling up to 16 cores. Nvidia has the Tegra 2 running Quake 97 per cent faster than the Galaxy S, but it should do - the Galaxy S is a 1GHz single-core ARM Cortex-A8 using the 3.2GFLOPs PowerVR SGX540. A Tegra 2 running a single core runs 62 per cent slower, never mind if it also has a third less graphical power.
Against the iPad 2, the Tegra 2-powered Motorola Xoom performs at about a third of the speed. At best, it gives two thirds of the performance. But direct comparison is tricky due to resolution and OS differences.
Specific 3D performance hasn't been something tablets have been sold on. Apple gave a nod to it when the iPad 2 was launched, claiming nine-times the graphical performance over the iPad, but Nvidia is trying to drive this home in a real sense. We might finally be seeing an end to the 'Angry Bird period' of mobile gaming.
The company has its Tegra Zone app, which helps promote Tegra-optimised games such as Samurai II: Vengeance THD, Dungeon Defenders and Monster Madness. But beyond this, the tablet gaming scene remains embryonic, with a heavy mix of indie games and a few AAA titles.
Oddly enough, DirectX 9 and DirectX 10 hardware support is here - largely because it's a pre-requisite for Windows Phone 7 devices. It's already available on some PowerVR models, such as the SGX535 and SGX545, but all current mobile games are developed for OpenGL ES 2.0, which is supported by Android and Apple iOS.
A key element in game production is a mature development engine. If you want exciting 3D games then you want developers to have access to the best tools. The good news is that there's certainly not a shortage of free or big-name kit out there.
Due to its popularity, Apple iOS has gained the most attention, but Android is certainly catching up, with development tools supporting both. Cocos2D is a free 2D aid that's spawned a load of games. iTorque is a well-known and long-running environment, is low-cost and offers 2D and 3D kits.
While you may have come across Unity for Flash games, it also provides relatively low-cost 3D development tools. If you're hoping for big-name support, Epic already has the Unreal Engine 3 on Apple iOS. You can try it out with Epic Citadel from the App Store.
While id Software beavers away with RAGE, it's also been developing a mobile id Tech 5 Apple iOS engine that spawned the RAGE HD game. John Carmack, technical director at id, has publicly commented that it plans to create an Android version as well.
But it's not all about what's inside a tablet. We should step back and take some time admiring the tantalisingly slim exteriors - all the tablets on the market have a slimline build. There's no doubt that the iPad 2 is the most perfectly engineered, but the Asus Eee Pad Transformer is close.
The second thing the iPad and Transformer get spot on is the screen. The former has a less than stellar 1,024 x 768 resolution - good enough, but you can tell that alternatives such as the Galaxy Tab and Asus Transformer, both 1,280 x 800, are sharper and perfect for HD content.
Responsiveness is also vital. All tablets should be capacitive, which means the screen is coated with a conductive material, rather than relying on resistive screens that use pressure to function.
Battery technologies are key too. Li-Ion batteries are relatively expensive, but even they're cheaper than the Li-Polymer alternatives found in the iPad and Asus Transformer. Battery capacity is usually measured in milliamp hours (mAh), but companies have started using a watts per hour (Wh) measure, which is the capacity times the operating voltage.
The iPad has a 25Wh battery, the Asus a 24Wh one and the Acer W500 a 36Wh unit. Obviously, the amount of power offered by the battery is only part of the story. Despite its larger battery, the Acer W500 only manages half the runtime of the other two.
Tablet operating systems
Considering that the majority of tablets sold seem to be running very similar hardware, the OS you choose is a large differential between devices.
At the start of 2011, the choice was limited: Apple iOS, Android 2.x or Windows 7, where iOS easily won because it was designed from the ground up to be a touch-controlled tablet OS. Android 2.x almost worked, but it was engineered for small-screened phones, and doesn't support the app store on larger tablet screens.
Samsung and Dell did good work masking this with the Galaxy and Streak, but the host of knock-off Android devices suffered massively. Of course, Windows 7 supports multi-touch, but it's not designed to be its primary input method, so all tablets struggle in one way or another.
The big win for Windows models is that you can run all of your favourite Windows software. Unfortunately, this tends not to be touch-orientated, which undermines the proposition somewhat.
New for 2011, there's Android 3.0, which is specifically designed for tablets and does an excellent job of catching up with iOS from a feature and functionality perspective. If you're a little sick of Apple's controlling nature and dependence on iTunes, it's going to make an excellent alternative.
BlackBerry will also be entering the market with its all-new PlayBook tablet and Tablet OS. Though it will initially lack the number of apps the other platforms have, the company should create a compelling device.
The tablet world is still very much in flux, but is maturing rapidly. From our perspective, Apple iOS devices are beautiful but are also locked into an Apple/iTunes ecosystem, which we find both distasteful and limiting.
Android tablets offer far more choice and variety, even if that includes cheap and nasty options. The BlackBerry PlayBook is an unknown quantity so far, as dedicated Windows 8 tablets will be come 2012. But with prices going as low as £199, tablets certainly can be fun things to play with.
First published in PC Format Issue 253
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