FR2054410A5 - - Google Patents

Abstract

1,264,060. Pitch extractors. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 6 July, 1970 [9 July, 1969], No. 32717/70. Heading H4R. [Also in Division G4] In a fundamental frequency detector, particularly for use as a pitch frequency detector in a vocoder, the signal is converted to a number of pulses whose positions correspond to the peaks of the input signal, and the pulse signal is applied to a cascade arrangement of period filters having mutually different characteristic time durations. As described a speech signal is rectified and passed through a low pass filter before feeding a peak enhancer in which peaks obtained by peak rectification are added to the enhancer input signal to provide a signal with very pronounced peaks which are used to trigger a pulse generator to provide a train of pulses each occurring at the positions of the peaks of the enhanced signals. The pulse signals are fed to a cascade arrangement of period filters each of which comprises a monostable circuit triggered by the positive edge of an incoming pulse signal, and which resets a predetermined time after the most recent positive edge. The period filters are connected so that each has a period before reset which is a predetermined amount, e.g. 10%, greater than the preceding filter. In operation the first stage which is not reset provides an indication of the fundamental frequency of the signal input. In order to distinguish sub periods in the input signal from the main periods, a pulse stretcher is provided in respect of each period filter stage. The pulse stretcher comprises a circuit element similar to a period filter connected by an "AND" gate to the output of the respective period filter stage and to the output of the preceding filter stage. The stretcher is operated if a starting signal, i.e. a positive-going edge, is available from the previous filter stage while a positive signal is maintained in the associated filter stage. The output from the stretcher may then be arranged to operate lamps or a digital device to indicate the period established. A further development, Fig. 6, includes an additional "OR" gate, e.g. 644, arranged between the period filter stages, e.g. 44, 46, which prevents gaps in the operation of the stretchers when the fundamental frequency is slowly increasing. In addition the stretcher is fed via an OR gate, e.g. 442, with signals from the stretchers of preceding, related, and following stages, and the output signal is derived from the stretcher output via an "AND" gate, e.g. 345, fed with the output of the related stretcher and the inverted output of the succeeding stretcher, in order to ensure that only one output terminal is active at any one time.