Patent GB-1310036-A (GB)
Title: Signal synthesizer
IPC: G10L13/06
Filed: 1970-10-22
Abstract: 1310036 Speech synthesiser LTV ELECTRO-SYSTEMS Inc 22 Oct 1970 [22 Oct 1969] 50267/70 Heading H4R In a speech synthesizer an input signal comprises a succession of frames of digital words, each frame comprising one word, defining the fundamental frequency of a speech signal, and a number of other words, defining the energy in respective predetermined frequency bands of the original speech signal. The frequency information word is used to produce digital signals indicative of the fundamental frequency and its harmonics up to a predetermined upper frequency limit. The digital signals corresponding to the various frequencies are combined with the respective amplitude information words to derive digital signals corresponding to instantaneous values of a sine wave of the corresponding frequency and amplitude. The derived digital signals in respect of each frame are summed, preferably equalized in level by reference to the number of harmonics contained, and the resulting digital signal converted to analogue form to provide the output speech signal. As described with respect to Fig. 1 the input control signal at 14 comprises a frame of 54 bits, six bits denoting the pitch frequency followed by fifteen groups of three bits and one group of two bits denoting the energy level in sixteen bands across the speech spectrum. A serial to parallel converter 18 feeds the incoming pitch frequency information to a pitch frequency register 26 and the amplitude signals to amplitude register 30. The pitch signals in register 26 are converted to signals defining the pitch frequency in binary form in converter 28 and the resulting signals are stored in store 29 for the remainder of the frame. During the remainder of the frame the pitch signal is fed to adder 35 and accumulator 36 which, at times defined by a clock signal from timing control 12, successively adds the pitch signal to itself a number of times to identify each of the pitch harmonics. A magnitude comparator 50 compares the various pitch harmonics with signals from the table of channel bandwidths 70 so that it is determined which pitch harmonics are associated with each of the energy level signals in the envelope register 30. A complete comparison cycle is completed in 1/256 th of the period of the period of the frequency corresponding to the pitch signal in the register 26 and a complete cycle of comparisons is signalled by a "K" pulse on line 52. The "K" pulse, as well as triggering the operation of output circuits 84, 85, 86, also triggers a K counter 80 which has a counting range of 256 so that the counter cycles at the pitch frequency. The actual state of the count of the K counter 80 is fed to the adder 77 and K-H accumulator 75 so that the signal on the output of the K-H accumulator increases at a steady rate between K pulses, the rate of increase varying with the state of the K pulse counter 80, and being reset by each K pulse. The signal appearing at the output of accumulator 75 is fed, via adder 78, to the table of amplitude modulated trigonometric functions 90 where, together with amplitude information for the various speech frequency analysis bands, appropriate binary numbers are selected corresponding to instantaneous values of each of the harmonics of the pitch frequency. These values are added and stored in 85, multiplied, in 84, by a scaling factor, to compensate for the change in level dependent on the number of harmonies present, and converted to analogue form by D to A converter 86, to provide an output sample of synthesized speech. During unvoiced sounds the pitch frequency information signal will be all zeros, this is detected by unvoiced detector 33 to cause energization of a noise generator 127  which injects, via adder 78, random numbers into the digital signal from the K-H accumulator. At the same time a number corresponding to a pitch frequency of 128 Hz is fed into the store 29 so that the "K" counter and K-H accumulator operate as for 128 KHz pitch, and the result is a natural sounding unvoiced signal output.