Method and apparatus for generating digital dual frequency signals

Abstract

A dual tone multiple frequency digital tone generator for a PCM/TDM electronic telephone system. A digital representation of a dual tone signal is generated from a multi-bit command character by determining the phase angle of two frequencies specified by the command character relative to an independent time reference signal, converting paired phase angles to amplitude signals representative of the amplitude of a sine wave having each calculated phase angle, and summing the amplitude representations to generate a value corresponding to the digital representation of the frequency pair at that particular instant. Individual phase angles are calculated by generating a multi-bit digital character corresponding to the phase angle of a sinusoidal reference signal of the particular frequency. This character is preferably generated by multiplying a multi-bit digital character having n-bits representing the fractional part of the number of seconds elapsed from some reference time by a multi-bit digital character having m bits representing the frequency of the sinusoidal signal, m and n both integers. The resulting m + n bit product consisting of an m-bit integral part and an n-bit fractional part is examined and the p most significant bits to the right of the decimal point are selected, p being an integer. The value of n determines the accuracy of the product relative to the absolute value of the phase angle, while the value of p determines the resolution of the phase angle representation. Both n and p are preselected in accordance with the required accuracy of the frequency representation.