Device for sound synthesis

Abstract: A device for sound synthesis intended to generate a desired acoustic signal comprising - a first signal source (1) intended to emit during operation a periodic signal having a given repetition frequency as representation of the voiced parts of the desired acoustic signal, - a second signal source (2) intended to emit during operation an aperiodic signal or a noise signal as representation of the unvoiced parts of the desired sound signal, - a combination circuit (3) intended to combine the signals of the two signal sources with each other, and - a filter circuit (5) having a variable transmission function intended to process the combined signal to the desired output signal. The device is further provided with a third signal source (2, 14) intended to emit during operation a modulated noise signal consisting of a train or sequence of noise plops of comparatively short duration, whose temporal envelope is synchronous with the temporal envelope of the said periodic signal and which invariably have at least approximately the same energy, which modulated noise signal is supplied during operation together with the signal of the first signal source (2, 11) to the combination circuit (3, 17).

Description: The invention relates to a device for sound synthesis for generating a desired acoustic signal comprising a first signal source for emitting a periodic signal having a given repetition frequency as representation of the voiced parts of the desired acoustic signal, a second signal source for emitting an aperiodic signal or a noise signal as representation of the unvoiced parts of the desired sound signal, a combination circuit for combining the signals of the two signal sources with each other, and a filter circuit having a variable transmission function for processing the combined signal to the desired output signal. Such a device has been described, by J. Makhoul et alii in the article "A mixed-source model for speech compression and synthesis", published in Proceedings of 1978, I.E.E.E. International Conference on Acoustics, Speech and Signal Processing, April 10-12m 1978, Tulsa, Oaklahoma. In this device, besides the said signal sources, the combination circuit and the variable filter circuit, a low-pass filter connected between the first signal source and the combination circuit, and a high-pass filter connected between the second signal source and the combination circuit are also used. A similar device has been described by S.H. Kwon and A.J. Goldberg in the article "An enhanced LPC vocoder with no voiced/unvoiced switch", published in I.E.E.E. Transactions on Acoustics, Speech and Signal Processing, Vol. ASSP-32, No. 4, 1984, p. 851 ff. In this known device, besides the said components a controlled amplifier is provided behind both the first signal source and the second signal source. Both amplifiers are controlled by a signal originating from the filter circuit having a variable transmission function in such a manner that the combination circuit can be reduced to a simple hybrid circuit. All these known devices intend to generate a speech signal with good perception quality. In practice, none of the known devices reaches a speech quality which is fully satisfactory. The invention has for its object to indicate the manner to attain a substantial improvement with respect to the known devices. Therefore, the invention is characterized in that the device is provided with a third signal source for emitting a modulated noise signal comprising a train or sequence of noise bursts, whose temporal envelope is synchronous with the temporal envelope of the periodic signal and which invariably have at least approximately the same energy, which modulated noise signal is supplied together with the signal of the first signal source to the combination circuit. In the known devices, stationary noise is added to the voiced periodic signal. It has been found that a listener to the ultimate acoustic signal produced by the known devices gets the impression as if the noise signal originates from a separate source that is different from the source emitting the periodic signal, so the perception quality is comparatively poor. This situation is improved by adding a high-pass or a low-pass filter as described by Makhoul, but this device still requires improvement. When according to the invention noise is added in the form of a sequence or train of noise bursts, whose temporal envelope satisfies the aforementioned condition and which invariably have (at least approximately) the same energy, a perceptive fusion of the noise with the voiced periodic signal is effectively obtained, as a result of which a considerable improvement of the perception quality is attained. Although the aforementioned prior art more particularly relates to devices for generating speech signals, the present invention is not limited thereto. The device according to the invention can be used successfully for synthesizing, for example, musical sounds. By way of example, mention may be made of the sound of a German flute, which sound has a "hoarse timbre". In the known music synthesis techniques, this hoarse character is obtained by adding comb-filtered noise or by adding inharmonic components to the start of the sound. However, the use of the invention leads to a much more satisfactory result. In connection with the general applicability of the invention, it should be noted that in this description the term "voiced" relates to non-noisy signal parts and the term "unvoiced" relates to noisy signal parts. According to a further developed embodiment of the device according to the invention, the two noise sources are combined with each other. By itself, IEEE Transactions on Audio and Electroacoustics, Vol. AU-16, no. 1, March 1986, pages 40-5-, New York, USA, N.R.Dixon et al.: "Terminal analog synthesis of continuous speech using the diphone method of segment assembly" features a combination noise generator. However, this particular reference does not give any hint in the direction of the principal elements of the present invention that lie on the improvement of the perceptual quality of speech and other sounds. The invention will now be described more fully with reference to the accompanying Figures. Fig. 1 shows a device known from the prior art, Fig. 2 shows a first embodiment of a device according to the invention, and Fig. 3 shows a second embodiment of a device according to the invention. The device shown in Fig. 1 comprises a first signal source 1 intended to emit during operation a periodic signal, more particularly a pulse train having a given repetition frequency F o . The device further comprises a second signal source 2 intended to emit an aperiodic signal, more particularly a noise signal. The outputs of the two signal sources 1 and 2 are connected to the inputs of a combination circuit, which is indicated in outline in Fig. 1 by means of a switch 3, which is controlled by a VUV signal. This VUV signal determines whether a voiced sound segment or an unvoiced sound segment has to be generated. The output signal of the combination circuit 3 is supplied to an amplifier stage 4 having a variable amplification factor G. The signal G influences the amplitude of the combined signal as a function of time. The output signal of the amplifier stage 4 is supplied to a variable filter 5, to which the filter coefficients C can be supplied from the outside. This filter circuit consists in practical embodiments of a cascade arrangement of a number of second-order subfilters intended each to modulate one of the formants or resonance frequencies, which can occur within the band-width range chosen. Fig. 2 shows a first embodiment of a device according to the invention. Like the device of Fig. 1, the device of Fig. 2 is also provided with a first signal source 11 intended to emit a periodic signal having a given repetition frequency F o , a second signal source 12 intended to emit during operation an aperiodic signal or a noise signal, a combination circuit 13, in this case in the form of a summator, and a filter circuit 15, which also in this case is provided with a number of subfilters and intended to form the different formants in the band-width range chosen. In conformity with the invention, the device of Fig. 2 is further provided with a third signal source 14, which emits a train or a sequence of noise plops, whose envelope is synchronous with the temporal envelope of the signal emitted by the first signal source 11. In other words: the noise plops or trains of noise emitted by the source of noise 14 occur at a repetition frequency F o and moreover all the noise plops have at least substantially the same energy. The output signals of the signal sources 11, 14 are combined with each other in the summator 17 and are amplified or attenuated, if required, in an amplifier stage 18 and the amplified or attenuated signal is supplied to the combination circuit 13. The combination circuit 13 also receives the noise signal from the source of noise 12, the amplitude of which noise signal can also be influenced via an amplifier/attenuator stage 19. In the same manner as in Fig. 1, the output signal of the combination circuit 13 is also supplied to a variable filter circuit 15, whose filter coefficients C can be supplied from the outside. The synthetic acoustic signal is supplied to the output 16. By means of the device according to the invention, a much more natural sound is produced than is possible with the devices according to the prior art. With the use of the device for generating synthetic speech signals, vowels are produced having such a (hoarse) timbre that even in ideal conditions (for example when listening to the speech signal via a high-quality headphone) the vowels cannot or can substantially not be distinguished from the natural vowels giving in general a more or less hoarse impression. With the use of the device, for example, for music synthesis, a music signal is also obtained having such a "hoarse" timbre giving a natural impression that even the trained listener cannot or can substantially not distinguish this synthesized signal from a music signal produced by a real musical instrument. In other words: the device according to the invention brings about a perceptible timbre variation in such a sense that the timbre becomes "more noisy" or "more hoarse". The noise plops can be obtained in that the output signal of a source of noise emitting a noise signal having the same energy content as a function of time is passed through a filter which is constructed so that the filtered signal has an energy varying in time according to a predetermined envelope. It is then to be preferred that the instant in the period at which the energy of the noise is maximal coincides more or less with the instant in the period at which the energy of the periodic signal is maximal. The Applicant has carried out practical experiments in which the envelope used is a cosine square window, but within the scope of the invention other filter types may also be used, for example a Gaussean filter, a Hamming filter, a Hanning filter, a Tukey filter, etc. Another embodiment of the device according to the invention is shown in Fig. 3. In Fig. 3, the two sources of noise 14 and 12 of Fig. 2 are combined to a single source of noise 24. This source of noise 24 emits a noise signal modulated in time, the temporal envelope of this noise signal having a repetition frequency F o so that the temporal envelope of the noise plops occurring in this noise signal is synchronous with the temporal envelope of the periodic signal emitted by the first signal source 21. This first signal source 21 is again comparable with the source 11 in Fig. 2. The output signal of the first signal source 21 is subjected to a low-pass filter operation in the filter circuit 22, is then amplified or attenuated in the amplifier/attenuator 28 and is supplied to the combination circuit 23. The output signal of the noise generator 24 is subjected to a high-pass filter operation in the filter circuit 27, is then amplified or attenuated in the amplifier/attenuator 29 and is also supplied to the combination circuit 23. The output signal of the combination circuit 23 is supplied again to a filter stage 25, whose filter effect depends upon the externally supplied filter coefficients C and the ultimate synthetic acoustic signal is supplied to the output 26. It should finally be noted that in Fig. 1 an amplifier stage is used having a variable amplification factor G. A similar amplifier stage may of course also be included in Fig. 2 and Fig. 3. In Figures 2 and 3, such an amplifier stage would have to be included between the combination circuit 13 and 23, respectively, and the filter circuit 15 and 25, respectively. It is also possible in this case to construct the combination circuit 13 and 23, respectively, so that the variable amplification function is realized therein. It should further be noted that only in the embodiment of Fig. 3 use is made of a low-pass filter 22 and a high-pass filter 27. Such filters may also be used, if required, in the embodiment of Fig. 2, in which event these filters are connected in series with the amplifier stages 18 and 19, respectively, or are integrated, if possible, in these amplifier stages 18 and 19.

Claims: A device for sound synthesis for generating a desired acoustic signal comprising a first signal source (1) for emitting a periodic signal having a given repetition frequency as representation of the voiced parts of the desired acoustic signal, a second signal source (2) for emitting an aperiodic signal or a noise signal as representation of the unvoiced parts of the desired sound signal, a combination circuit (3) for combining the signals of the two signal sources with each other, and a filter circuit (5) having a variable transmission function for processing the combined signal to the desired output signal, characterized in that the device is provided with a third signal source (14) for emitting a modulated noise signal comprising a train or sequence of noise bursts, whose temporal envelope is synchronous with the temporal envelope of the periodic signal and which invariably have at least approximately the same energy, which modulated noise signal is supplied together with the signal of the first signal source (2,11) to the combination circuit (3,17). A device as claimed in Claim 1, characterized in that the modulation of the noise signal supplied by the third signal source is such that the instant in the period at which the energy in the noise signal is maximal coincides at least approximately with the instant in the period at which the energy of the periodic signal is maximal. A device as claimed in Claim 1 or 2, characterized in that the second and the third signal source are combined with each other.

Inventors: HERMES DIRK JAN, HERMES, DIRK JAN
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1990
Main classification: G10L3
IPC codes: G10L19, G10L13