A 7-bit digital multiple-voice-coil loudspeaker and a constant current driver have been constructed and evaluated. Two's complement coding offers the best compromise between complexity, excessive power consumption, and distortion due to baseline 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 be achieved most effectively by a short-circuited voice coil. Optimum damping occurs when the damping resistance is equal to the negative resistance of an individual coil.
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