A seven-bit digital multiple voice coil loudspeaker and constant current driver has been constructed and evaluated. Two's compliment coding offers the best compromise between complexity, excessive power consumption and distortion due to base-line drift. The effect of motion impedance and mutual coupling in a multiple coil system is found to magnify the effective impedance of an individual coil by its weighting in the binary representation. This effect places an unreasonable demand on the output impedance of the driver unless damping is controlled, which can most effectively be achieved by a shorted voice coil. Optimum damping occurs when the damping resistance is equal to minus the resistance of an individual coil.
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