Patent GB-1519004-A (GB)
Title: Voice synthesizer
IPC: G10L13/00
Filed: 1975-07-02
Abstract: 1519004 Speech synthesizer SCITRONIX CORP 2 July 1975 [8 July 1974] 27832/75 Heading H4R In a speech synthesizer in which input signals specify selected phonemes to be output from a vocal tract model and the input signals are converted to control signals to control the frequency of tunable filters and the outputs of amplitude control devices, the control signals are in the form of variable duty cycle waveforms. The arrangement allows easy conversion of digital control signals to analogue form. As described phoneme selection signals, e.g. from a computer or keyboard etc, are fed in on lines 10 Fig. 1 to memories 12 and 14 to provide analogue control signals on lines 16a, 16b to control the various parameters of the vocal tract model, the signals being in the form of serialized digital signal quantities in which the various bits in the repeating series are time weighted according to a binary progression, so that the signals have an average value which is the analogue equivalent of the digital input quantity addressing the respective output. The signals on the lines 16a and 16b are fed via filters 24a and 24b. The filters 24a have such long response times relative the phoneme interval that the parameters specified are seldom actually reached, and the filter responses are variable with the speech rate. The filters 24b also produce a damped response to a step input but to a lesser degree than filters 24a. The output signals from the filters 24a and 24b, which are slowly varying d.c. levels, are converted to fixed frequency varying duration pulses in converter 26 before application to the vocal tract model to control formant frequencies, excitation function, amplitude, spectral contour, and spectral shape etc. The output speech rate can be controlled manually by rate control 40, and the rate also varies from phoneme to phoneme under the control of the stored information, control being effected by variation of the slope of a ramp voltage generated in the timer 46 which in turn controls a delay on the vocal amplitude control parameter, and the time delay of the vocal spectral contour, closure, bandwidth, and the fricative amplitude control parameters, and also controls the response times of filters 24a so that the response times track with the phoneme intervals. The vocal tract model in shown in Fig. 2 in which the formant frequency filters Fl to F5 are connected in series. Fig. 2 also shows a typical control channel from the memory 14 to duty cycle generator 26. Such a control channel is shown in greater detail in Fig. 3 in which the phoneme address is given by selection of the binary weighted input lines 1 to 32. In addition the decoder 84 is fed with timing waveforms MSB and LSB. The time distribution of the four selected bits for each parameter output from the decoder 84 is controlled by the waveforms MSB and LSB so that the first digit of the parameter control signal selected by the input address lasts for 8 timing periods, the second digit lasts for 4 timing periods, the third for 2 and the last for 1. The effect is of a serialized and binary weighted duty cycle signal on the output line 81 the average value of which is controlled between 0 and 15 in increments of 1. Buffer amplifier 82 controls the peak to peak excursion of the waveform to 0 to 5 volts. The output of amplifier 82 is applied to a filter 24 whose characteristic is controlled by the transition rate signal applied to the gates of f.e.t.'s 87 and 92, which signal is a width modulated pulse at a 20 kHz repetition frequency. The output signal of the filter is a varying d.c. which is applied to a comparator 106 supplied also with a sawtooth waveform on its second input to provide width modulated pulses as the parameter control signal. Also described is the method of variably delaying the excitation function to match the filtering between adjacent phonemes, or phoneme constituents Figs. 4 and 5 (not shown) and a noise waveform generator using a shifter register to generate a pseudo random waveform Fig. 6 (not shown).