A speech synthesis system uses a programmable digital divider to generate desired formant frequencies. The divide factor is controlled by an MPU. The output of the divider is passed through a sinewave shaper and then through an electronically controlled amplitude amplifier to provide an audio output. The system is based upon the realization that alternating between two formant frequencies at the pitch rate generates a sound comparable to generating two formants simultaneously and adding algebraically.

Speech synthesizing apparatus is provided using digital coded values specifying parameters of a sound to be reproduced. The apparatus has a voiced sound generator using digital values specified by input parameters to represent three formant waveforms respectively which are sampled at substantially constant intervals, the parameters each expressing in relation to the sampling interval the frequency of the formant concerned. At each sampling, the input parameters are arranged to derive for each formant waveform respectively a value representative of the instantaneous amplitude of that waveform in relation to the others, the three values obtained then being combined to produce a resultant value representing an instantaneous amplitude output of a voiced sound component waveform. This output waveform is subjected to modification by attenuation and the addition, if required, of an unvoiced sound component before being passed through an analogue converter to a sound reproducing transducer. The sampling rate is chosen to produce a succession of outputs at sufficiently short intervals to approximate continuous speech after being subjected to a low pass filtering operation. The apparatus has provision for the interlaced sampling of a number of channels.

Speech synthesizing apparatus is provided in which different sounds are required for a plurality of channels. The channels are polled in turn on a cyclic scanning basis, the scan for each channel occupying a channel sampling period during which an instantaneous value is generated for the amplitude of a voiced sound component. Values representing instantaneous amplitudes of each of a plurality of unvoiced sounds are separately generated in turn during the period occupied by the scanning of all the channels and these values are stored. A selected one of the unvoiced sound values is extracted from the store as required for a particular channel during the sampling period for that channel, so that an updated value for the unvoiced sound is available on each occasion that that particular channel is sampled. The voiced and unvoiced sound values are then combined during the appropriate sampling periods. The required sound combinations are specified by parameters expressed in digital form for each channel separately.

A hearing aid system and method includes apparatus for receiving a spoken speech signal, apparatus coupled to the receiving apparatus for determining at successive intervals in the speech signal the frequency and amplitude of the largest formants, apparatus for determining at successive intervals the fundamental frequency of the speech signal, and apparatus for determining at successive intervals whether or not the speed signal is voiced or unvoiced. Each successively determined formant frequency is divided by a fixed value, greater than 1, and added thereto is another fixed value, to obtain what are called transposed formant frequencies. The fundamental frequency is also divided by a fixed value, greater than 1, to obtain a transposed fundamental frequency. At the successive intervals, sine waves having frequencies corresponding to the transposed formant frequencies and the transposed fundamental frequency are generated, and these sine waves are combined to obtain an output signal which is applied to a transducer for producing an auditory signal. The amplitudes of the sine waves are functions of the amplitudes of corresponding formants. If it is determined that the speech signal is unvoiced, then no sine wave corresponding to the transposed fundamental frequency is produced and the other sine waves are noise modulated. The auditory signal produced by the transducer in effect constitutes a coded signal occupying a frequency range lower than the frequency range of normal speech and yet which is in the residual-hearing range of many hearing-impaired persons.

Artificial speech is produced with minimized memory requirements by using a bank of digital oscillators to produce voiced sounds by combining multiple harmonies of a fundamental-frequency sine wave, and using only one of these oscillators to reproduce stored nonsinusoidal waveforms for unvoiced sounds. Sufficient dynamic range is achieved with a minimum number of oscillators by generating only every other harmonic at the higher frequencies. All harmonics are derived from a single stored digitized sine wave by using stored sets of skip counts and amplitude codes corresponding to various voiced sounds to be produced.