To Answer the Phone, Scratch Your Jeans
The sound of a fingernail raking across a table or a board may be enough to drive most people crazy. But get past that annoyance and it could become a way to answer your phone, silence a call or turn up the volume.
Scratch Input, a computer input technique developed by researchers at the Human-Computer Interaction Institute at Carnegie Mellon University, uses the sound produced when a fingernail is dragged over the surface of any textured material such as wood, fabric or wall paint. The technology was demonstrated at the Siggraph graphics conference this year.
“It’s kind of a crazy idea but a simple one,” says Chris Harrison, one of the researchers on the project. “If you have a cellphone in your pocket and want to silence an incoming call, you don’t have to pull it out of your pocket. You could just drag your fingernail on your jeans.”
As researchers study how people can interact in simpler and more innovative ways with computers and gadgets, going beyond the traditional keyboard, mouse and keypad has become important. Earlier this year, Harrison and his team demonstrated a touchscreen where pop-up buttons and keypads can dynamically appear and disappear. That allows the user to experience the physical feel of buttons on a touchscreen.
Scratch Input is another way to explore how we can interact with devices, says Harrison. Harrison, along with a colleague Julia Schwarz, and his professor Scott Hudson started working on the idea a year ago. Scratch Input works with almost any kind of surface except glass and a few other materials that are extremely smooth.
“With this we can start to think of every flat surface as an potential input area,” says Daniel Wigdor, user experience architect at Microsoft and curator of the emerging technology demos at Siggraph. “Imagine a cellphone with a mini projector. You can now turn an entire surface into a screen for the projector and use the surface to control it.”
Scratch Input works by isolating and identifying the sound of a fingernail dragging on an area.
“All the sound happening in the environment like people putting coffee cups on the table, cars going by or children screaming, we know what frequencies they are in,” says Harrison.
A fingernail on a surface produces a frequency between 6000 Hz and 13,000 Hz. Compare that to voice, which is typically in the range of 90 Hz to 300 Hz, or noise from a refrigerator compressor or air conditioning hum, which is in the range of 50 Hz or 60 Hz.
“It makes it easy for us to throw away all the other acoustic information and just listen to what your nail sounds like,” says Harrison.
Harrison and his team used that principle to rig up a system for Scratch Input. They attached a modified stethoscope to a microphone that converts the sound into an electrical signal. The signal is amplified and connected to a computer through the audio-input jack.
“If mass produced, this sensor could cost less than a dollar,” says Harrison.
Scratch Input also supports simple gesture recognition. Tracing the letter ‘S,’ for instance produces an acoustic imprint that the system can be trained to identify. The idea has its limitations. For instance, many letters that are written differently, sound very similar such as M, W, V, L, X or T. Scratch Input cannot accurately distinguish between these gestures. But still Harrison says the system can respond with about 90 percent accuracy.
Another problem is that the system cannot determine the spatial location of the input, says Wigdor. “For instance, with volume control, it can hear your finger spin in the appropriate gesture but the system can’t see it so sometimes it does not have enough information to react.”
Despite the limitations, the technology holds enough promise to make it into the hands of consumers, says Wigdor. “It is exciting because it is so low cost,” he says. “This idea has the potential to go beyond just a research project.”
Check out this video demo of Scratch Input:
To Answer the Phone, Scratch Your Jeans 
, necessity is the mother of invention 
The optimist proclaims that we live in the best of all possible worlds;
