The success of the iPhone has triggered the adoption of touchscreen systems in a wide range of mobile devices, and a bevy of new gadgets equipped with capacitive sensing technology have now hit the market. MOTO has years of experience developing products that use capacitive touch, and we’ve had the opportunity to test many of the latest devices. Our conclusion: All touchscreens are not created not equal.
It takes finesse to create a touchscreen system that’s pleasant to use, because touchscreens require seamless integration between hardware components, software algorithms, and user-interface design. If a manufacturer cuts corners or flubs any of the critical elements, the user’s experience with a touchscreen product is likely to suffer.
Simple and True
Although we usually use sophisticated tools to test touch screen accuracy, MOTO has also developed a simple technique anyone can use to evaluate the resolution and accuracy of a touchscreen device. All you need is a basic drawing program (download one if necessary), a steady hand, and a few straight lines drawn very slowly on the screen.
This video shows what happened when we recently took several touchscreen systems out for a test drive:
The Virtue of Slow
Why do you need to draw slowly? On a good touchscreen, users can draw clean straight lines, even while going very slowly, so the graphics that appear on screen accurately represent what was physically drawn.
On inferior touchscreens, it’s basically impossible to draw straight lines. Instead, the lines look jagged or zig-zag, no matter how slowly you go, because the sensor size is too big, the touch-sampling rate is too low, and/or the algorithms that convert gestures into images are too non-linear to faithfully represent user inputs.
Also, even on a single device, the amount of pressure and the part of the finger you use on the screen has an impact on how well it senses. A good touchscreen device will produce linear output regardless of whether you’re using the full pad of your finger, or just the dry corner of your cuticle. When comparing devices, make sure to use even pressure across all of them.
If you want to show the most extreme case, draw very lightly with the corner of your finger. The artifacts will increase significantly, showing which device is really the best with a weak signal. This is important because quick keyboard use and light flicks on the screen really push the limits of the touch panel’s ability to sense.
Here you can see the results of our test:
Take careful note of the performance at the edges of the screen. The performance at the edge is challenging to tune, and separate from the basic “waviness” test. The iPhone tracks all curve very strongly as you approach the edge of the screen, despite a straight finger trajectory. This is especially obvious at the bottom, where the iPhone has a sensitivity problem.
The Droid Eris [Nexus One] is actually the clear winner for edge performance — the signal tracks right off the edge of the screen very consistently.
 As of time of first writing, we hadn’t tested the Nexus One. It does slightly better than the Eris. In fact, they both use the same touch controller IC.
A Quest for High Signal-to-Noise Ratio
To create a superior touchscreen experience, it’s essential to develop a touchscreen sensor that has the highest possible signal-to-noise ratio, or SNR. When a manufacturer gets it right, the device tracks touch inputs almost as if they were connected to physical objects in the real world. Get it wrong and consumers end up with inferior touchscreen systems that are inaccurate, insensitive, and absolutely infuriating to use for typing.
Key drivers of SNR include:
- Conductive sensor material
- Substrate material
- Substrate thickness
- Distance from display (the biggest noise source)
- Sensing waveform
- Sensor pattern
- Sensor pitch
- Analog sensing circuitry
- Sample rate
Touchscreens are a catalyst for innovation and a powerful way for device manufacturers to differentiate their products in an intensely competitive marketplace. But as our demonstration shows, there’s a right way and a wrong way to deploy the technology. MOTO has worked with capacitive touch interfaces for more than 15 years, and here are some essential dos and don’ts for anyone entering the field:
- Don’t skimp on materials. With touchscreen hardware, manufacturers get what they pay for — and consumers will notice the difference.
- Allow ample time to develop your algorithms. Don’t treat touchscreen algorithms as an element of component sourcing; for best results, create a distinct touch development track under your own roof to make sure your products are both responsive and accurate.
- Closely integrate touchscreen hardware, software, and user interaction development, and do so as early as possible in the product development process. Never treat them as separate tasks.
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