Dtmf detector system and method which performs static and dynamic

Abstract

An improved dual tone multifrequency (DTMF) or multitone signal detector which uses both static and dynamic thresholding techniques to provide an increased functional dynamic range, improved speech immunity for improved detection and reduced error, and simplified signal to noise ratio control. The DTMF detector receives signals from the transmission media, calculates energy values for the different frequencies, and then determines maximum values of the energy values for each of the frequency groups, referred to as M(1) and M(2). The DTMF detector then performs both static and dynamic thresholding according to the present invention to ensure valid tone detection. The static thresholding compares each of the M(1) and M(2) values with a static threshold value to ensure that the maximum energy values have a minimum energy to warrant detection. Since both static and dynamic thresholding are performed according to the present invention, the static threshold value is preferably set low, thus allowing an increased functional dynamic range. The dynamic thresholding computes the ratio of the maximum value in each sub-array to each of the other values in the respective sub-array, wherein the remaining values in the sub-array are presumed to be noise. This ratio essentially computes the signal to noise ratio (SNR) of the received signals. These ratios are then compared with a second threshold. If the maximum energies are not greater than the static threshold, or any of the ratios is not greater than the second threshold value, then a SNR error is set, and no detection is indicated. The present invention improves the DTMF detector's functional dynamic range and noise immunity.