JPS551597B2 - - Google Patents

Abstract

1383621 Excitation frequency detector INTERNATIONAL BUSINESS MACHINES CORP 6 March 1973 [28 March 1972] 10914/73 Heading H4R Fundamental frequency of a speech signal is determined by sampling the speech signal and generating digital values representative of each sample, determining the absolute value of the difference between each successive pair of sample values, continuously summing the absolute differences to obtain for each sample "i" a function "S(i)" and detecting variations in the slope of that function to indicate the time at which the mouth cavities are excited, i.e. the time at which differences between successive sample values start to increase after a period in which they have been decreasing. As described speech is sampled at 100 Ásec intervals and digitally coded in pcm coder 1. Differences between successive samples are provided by subtractor 7 fed directly from coder 2 on one input and via a one sample period delay 4 on the other input. The absolute value of the difference is provided by unit 10. Adder 11 and register 13 enable a running total to be provided for the absolute differences, while shift registers 17 and 19, both of which provide twenty sample periods delay, allow the comparison, in subtractors 20 and 21, of the running totals at three sampling points spaced twenty sampling periods apart, to give an indication of the slopes before and after the middle sampling point. The slopes are compared in subtractor 23 to provide on line 26 the magnitude of the difference in slopes, and on line 27 the sign of the difference in slopes. When line 27 is positive, corresponding to an increase in slope of the sum of the difference signals which occurs at the commencement of the excitation period, AND gate 28 is enabled passing the signal on line 26 to line 29. The slope difference signal is filtered in digital filter 30, to remove small high frequency variations, and applied to comparator 44 and register 42, via AND gate 43 which is enabled only when the comparator 44, which compares current slope and the slope stored in register 42, provides an output indicating that the current slope is larger than the stored slope. When the opposite condition occurs the value then stored in register 42 is held and the comparator 44 output signal is fed through inverter 47, and gate 48 to bi-stable circuits 52, 53, to produce a short pulse on the output of AND gate 57 indicating the start of the excitation period. When the sign of the slope difference signal on line 49 first goes positive, prior to the commencement of an excitation period, the register 42 and bi-stable 52, 53, are reset ready to detect the following maximum slope difference indicating start of the excitation period. The units shown dotted are provided to give a variable threshold to the maximum slope difference value detector by enabling and gate 48 only when the maximum detected by the comparator 44 bears a predetermined relation, e.g. more than half, the previous maximum detected.