Much of the Hot Hand sound-effect technology's capability comes from the use of the ADXL320 microelectromechanical-system-based accelerometer.
The device is capable of sensing ±5 g's and works by virtue of a tiny silicon plunger that is free to move over a range of a few microns, with the motion tracked by a capacitive sensor that provides an analog output corresponding to the plunger's acceleration. Two plungers are used for two-axis sensing. The plungers are metallic and suspended on polysilicon "springs." The device contains the capacitive sensors, signal- conditioning and self-testing circuitry all on the same chip.
For the Hot Hand application, Source Audio puts the ADXL320 accelerometer chip in a ring worn on the pick hand. The ring is connected via a coaxial cable to an electronics box containing the SA601, a system-on-chip that includes a customized SigmaDSP. The SA601 is said to be the first SoC for processing guitar effects.
The guitar is plugged into the electronics box, the output from which is fed to the guitar amplifier. When the guitarist's picking hand moves, the signals indicating its acceleration along the x and y axes parallel to the guitar body are sent to the electronics box, where the SigmaDSP uses the control inputs to achieve the sound effect.
"You get little, subtle motion just by picking and moving your fingers, slightly more by rotating your wrist, or even more effect by moving your hand at the elbow," said Jesse Remignanti, vice president of engineering at Source Audio.
The SigmaDSP mixed-signal device includes dual 24-bit analog-to-digital converters, an audio DSP and dual 24-bit digital-to-analog converters. It has a 56-bit fixed-point multiplier-accumulator, on-board data and program memory, clock-generation circuitry and built-in algorithms for common audio-processing functions.
On the control side of the system-on-chip are three A/Ds, to which are fed the x- and y-axis signals from the accelerometer chip (the third input is for an expression pedal).
The digitized control signals feed the Hot Hand motion-sensing electronics, to which the DSP refers as it runs audio algorithms. A separate D/A converter emulates the expression pedal output.
One knob on the controller box selects the desired effect (from a menu of 11), and a second knob sets the starting or ending sweep frequency for the built-in effects modulator. A third, sensitivity knob on the box allows the guitarist to adjust how much effect is actuated by hand motions, ranging from pick to strum to flail. The sensitivity knob can also be turned through 360° to enter "invert mode," indicated by an LED, where the effects are reversed--switching the "pedal up" and "pedal down" of a traditional wah-wah or volume pedal.
"The pick setting detects very small motions and is fast enough to follow every note you pick, whereas flail does smoothing on the signal so it does not respond to quick jittery motions but only to very deliberate flicks of the hand and arm," said Remignanti. "We also have a headband you can wear that is fun to use to vary the sweep of a filter with the angle between your head and the ground."
The Hot Hand can be calibrated for specific users and will retail for $299. The company plans a wireless version, as well as models with different guitar effects and control possibilities. A MIDI version that can control external effects processors and keyboards is also on the drawing board.