For lack of seeing a comment discussing this option: the Theremin can be used as a device to track gestures, position and movements of a conductors hands or baton. It is based on an electric field which is perturbed by hands, controlling frequency and amplitude of the generated sound.
It would require some signal processing to adapt it to a singer, either by passing the theremin sounds into an earplug or modifying the signal to input a sensing device, like a rotor, small current or pinching device.
Slashdot Mirror
For lack of seeing a comment discussing this option: the Theremin can be used as a device to track gestures, position and movements of a conductors hands or baton. It is based on an electric field which is perturbed by hands, controlling frequency and amplitude of the generated sound. It would require some signal processing to adapt it to a singer, either by passing the theremin sounds into an earplug or modifying the signal to input a sensing device, like a rotor, small current or pinching device.