The electromagnets allow each string's vibration to be shaped in real time. However, traditional MIDI keyboards represent notes as discrete onsets and releases. Though a few keyboards support aftertouch capability, current interfaces generally lack the fine-grained control needed to continuously shape each note.
Left: Modified Moog Piano Bar with USB interface for continuous key position. Right: optical operation of the Piano Bar.
We have built a new keyboard sensor, based on a modified Moog Piano Bar, which measures the continuous position of each key. Pairs of LEDs and photodiodes measure the reflected light off each key surface, which varies inversely with the key's distance from the sensor. A 12-bit analog-to-digital converter samples each key 600 times per second, transmitting the values to a host computer over USB.
In addition to capturing traditional key presses with high spatial and temporal resolution, the sensor makes possible several extended keyboard techniques, including partial key presses, light taps or sweeps on the keys, and vibrato gestures. On the acoustic piano, a felt washer separates each key from the key bed; increased force on the key compresses the felt, creating a slight displacement. Thus, our optical sensor can capture pressure-based aftertouch information as well.
By mapping continuous key motion to actuator behavior, the performer gains continuous control over each note. We are currently exploring the large array of possible mappings, with the goal of finding ones that are intuitive for trained pianists while offering nuanced control over each musical dimension. One example is shown in this video: