Patent GB-1318985-A (GB)
Title: Audio response apparatus
IPC: G10L13/02
Filed: 1970-10-09
Abstract: 1318985 Speech synthesizer NIPPON TELEGRAPH & TELEPHONE PUBLIC CORP 9 Oct 1970 [7 Feb 1970 (2)] 48065/70 Heading H4R In an apparatus for synthesizing speech from samples stored, e.g., on a magnetic drum, which are extracted under the control of a computer to provide continuous speech, e.g. for answering calls in a telephone system, the speech samples are stored in the form of partial autocorrelation coefficients between two closely adjacent time instants of the speech signal, which define the spectrum function of the speech, and autocorrelation coefficients between more remotely separated samples which provide excitation source information. The partial autocorrelation coefficients are obtained by correlation of the differences between estimated values of the signal level and the actual signal level at two instants. The estimated values of the signal level are determined from sampled values of the signal level obtained between the two instants using the least squares method. As described, the partial correlation coefficients are determined using the networks of Fig. 3, each stage producing one partial correlation coefficient at terminal 15 for speech applied to the series connection of stages at input 6. The auto-correlation device 16, which comprises a number of delay stages 27, Fig. 5, with the signals between the delay stages being correlated with the input signal in multipliers 28 and low-pass filters 29, after determination of the delay stage giving maximum correlation, provides an indication of the excitation function, a steady output from a particular stage indicating a voiced sound while an unvoiced sound will cause a random appearance of the correlation maxima. Sets of partial correlation coefficients and excitation function descriptions are determined at 15 millisecond intervals and stored on a magnetic drum with the sets for different words or speech elements being interleaved, with each speech element or word occupying 1À98 seconds, and a number of tracks being provided to increase the storage capacity. In the speech synthesizer, Fig. 7, a computer, in response to signals from a telephone system, controls the reading out of stored signals corresponding to the particular speech elements to be fed back into the telephone system. The serial signals from the storage drum 60 selected by the computer are converted to parallel form in converters 63-1 to 63-8 and fed into buffer memories 64-1 to 64-8, the information being in five bit codes for each coefficient. From the buffer memory the signals are fed to the synthesizer 65, shown in detail in Fig. 6, where the excitation function signals are applied to operate the pulse generator 30 or white noise generator 31 and effect amplitude control of the excitation signal while the partial autocorrelation coefficients are fed to terminals 41 of the digital filter stages 35a-35n which operate on the excitation function passing therethrough to provide synthesized speech samples at the output terminal 42 to be fed to the output buffer memory 66. The samples at this point are still in digitally coded form and are converted to analogue form in d to a converter 67. The capacity of the speech synthesized is such that it can handle 64 channels in time division multiplex and the output from the d to a converter 67 is therefore fed to the common input of gates 68-1 to 68-64 which are enabled in sequence to switch the speech samples to the appropriate channels 1 to 64 through low-pass filters 69-1 to 69-64 which convert the pulse amplitude modulated signals to continuous form.