Efficient DSP based detection of DTMF tones

The author describes a novel approach for the efficient DSP (digital signal processing)-based processing of DTMF (dual-tone multifrequency) tones in which the DTMF detector is structured as the cascade of a band separation filter and followed by a simple bank of single-tone detectors. Efficiency is then achieved by implementing the band separation filter at the lowest possible sampling rate, i.e. 1000 Hz, through a recursive decimator. The decimators used consist of all-pass parallel branches and simulate doubly terminated lossless lattice networks through the wave linear transformation, and they can be shown to implement a subset of elliptic transfer functions. Thus, an analytic solution exists for the set of coefficients yielding the optimum solution in the equiripple sense. It is shown that the complete band separation and detection operation can be accomplished with a computational load of 5.25 multiplications and 11.375 add