The humble accelerometer shows its worth again, with Stanford University computer music student Nicholas J. Bryan turning an iPhone into a DJ tool that allows analog turntables to control digital music. MOPHO DJ uses the iPhone's accelerometer to track its position on the turntable, picking up a DJ's scratches and transmitting that wirelessly to a nearby computer which processes MP3s in real-time.
While there have been so-called time-coded vinyl (TCV) systems in the past, they have all required specially-marked records and hardware to track their movement. MOPHO DJ differs in that it simply requires an accelerometer or gyroscope-equipped device, like an iPhone or iPod touch, and uses that motion tracking to do the same thing. That makes it cheaper and easier to implement; Bryan uses a disc of perspex with a rubber mat mounted to seat the iPhone, but you could feasibly use a regular record.
Because the direct movement of the turntable is being tracked, you can still use all its manual controls - such as pitch-adjustment - while the iPhone's display shows the real-time waveform of the audio currently playing. Even if you don't have a turntable you'll still be able to use MOPHO DJ in the "untethered performance" mode, simply moving the phone in mid-air so as to scratch records.
Bryan details MOPHO DJ in his paper [pdf link] and intends to release the iPhone app and accompanying computer software in the near future.