System and method for modeling speech spectra

A system and method for modeling speech in such a way that both voiced and unvoiced contributions can co-exist at certain frequencies. In various embodiments, three spectral bands (or bands of up to three different types) are used. In one embodiment, the lowest band or group of bands is completely voiced, the middle band or group of bands contains both voiced and unvoiced contributions, and the highest band or group of bands is completely unvoiced. The embodiments of the present invention may be used for speech coding and other speech processing applications.

FIELD OF THE INVENTION

The present invention relates generally to speech processing. More particularly, the present invention relates to speech processing applications such as speech coding, voice conversion and text-to-speech synthesis.

BACKGROUND OF THE INVENTION

Many speech models rely on a linear prediction (LP)-based approach, in which the vocal tract is modeled using the LP coefficients. The excitation signal, i.e. the LP residual, is then modeled using further techniques. Several conventional techniques are as follows. First, the excitation can be modeled either as periodic pulses (during voiced speech) or as noise (during unvoiced speech). However, the achievable quality is limited because of the hard voiced/unvoiced decision. Second, the excitation can be modeled using an excitation spectrum that is considered to be voiced below a time-variant cut-off frequency and unvoiced above the frequency. This split-band approach can perform satisfactorily on many portions of speech signals, but problems can still arise, especially with the spectra of mixed sounds and noisy speech. Third, a multiband excitation (MBE) model can be used. In this model, the spectrum can comprise several voiced and unvoiced bands (up to the number of harmonics). A separate voiced/unvoiced decision is performed for every band. The performance of the MBE model, although reasonably acceptable in some situations, still possesses limited quality with regard to the hard voiced/unvoiced decisions for the bands. Fourth, in waveform interpolation (WI) speech coding, the excitation is modeled as a slowly evolving waveform (SEW) and a rapidly evolving waveform (REW). The SEW corresponds to the voiced contribution, and the REW represents the unvoiced contribution. Unfortunately, this model suffers from large complexity and from the fact that it is not always possible to obtain perfect separation into a SEW and a REW.

It would therefore be desirable to provide an improved system and method for modeling speech spectra that addresses many of the above-identified issues.

SUMMARY OF THE INVENTION

Various embodiments of the present invention provide a system and method for modeling speech in such a way that both voiced and unvoiced contributions can co-exist at certain frequencies. To keep the complexity at a moderate level, three sets of spectral bands (or bands of up to three different types) are used. In one particular implementation, the lowest band or group of bands is completely voiced, the middle band or group of bands contains both voiced and unvoiced contributions, and the highest band or group of bands is completely unvoiced. This implementation provides for high modeling accuracy in places where it is needed, but simpler cases are also supported with a low computational load. The embodiments of the present invention may be used for speech coding and other speech processing applications, such as text-to-speech synthesis and voice conversion.

The various embodiments of the present invention provide for a high degree of accuracy in speech modeling, particularly in the case of weakly voiced speech, while at the same time enduring only a moderate computational load. The various embodiments also provide for an improved trade-off between accuracy and complexity relative to conventional arrangements.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

Various embodiments of the present invention provide a system and method for modeling speech in such a way that both voiced and unvoiced contributions can co-exist at certain frequencies. To keep the complexity at a moderate level, three sets of spectral bands (or bands of up to three different types) are used. In one particular implementation, the lowest band or group of bands is completely voiced, the middle band or group of bands contains both voiced and unvoiced contributions, and the highest band or group of bands is completely unvoiced. This implementation provides for high modeling accuracy in places where it is needed, but simpler cases are also supported with a low computational load. The embodiments of the present invention may be used for speech coding and other speech processing applications, such as text-to-speech synthesis and voice conversion.

The various embodiments of the present invention provide for a high degree of accuracy in speech modeling, particularly in the case of weakly voiced speech, while at the same time enduring only a moderate computational load. The various embodiments also provide for an improved trade-off between accuracy and complexity relative to conventional arrangements.

FIG. 1is a flow chart showing the implementation of one particular embodiment of the present invention. At100inFIG. 1, a frame of speech (e.g., a 20 millisecond frame) is received as input. At110, a pitch estimate for the current frame is computed, and an estimation of the spectrum (or the excitation spectrum) sampled at the pitch frequency and its harmonics is obtained. It should be noted, however, that the spectrum can be sampled in a way other than at pitch harmonics. At120, voicing estimation is performed at each harmonic frequency. Instead of obtaining a hard decision between voiced (denoted, e.g., using the value 1.0) and unvoiced (denoted, e.g., using the value 0.0), a “voicing likelihood” is obtained (e.g., between the range from 0.0 to 1.0). Because voicing in nature is not a discrete value, a variety of known estimation techniques can be used for this process.

At130, the voiced band is designated. This can be accomplished by start from the low frequency end of the spectrum, and going through the voicing values for the harmonic frequencies until the voicing likelihood drops below a pre-specified threshold (e.g., 0.9). The width of the voiced band can even be 0, or the voiced band can cover the whole spectrum if necessary. At140, the unvoiced band is designated. This can be accomplished by starting from the high frequency end of the spectrum, and going through the voicing values for the harmonic frequencies until the voicing likelihood is above a pre-specified threshold (e.g., 0.1). Like for the voiced band, the width of the unvoiced band can be 0, or the band can also cover the whole spectrum if necessary. It should be noted that, for both the voiced band and the unvoiced band, a variety of scales and/or ranges can be used, and individual “voiced values” and “unvoiced values” could be located in many portions of the spectrum as necessary or desired. At150, the spectrum area between the voiced band and the unvoiced band is designated as a mixed band. As is the case for the voiced band and the unvoiced band, the width of the mixed band can range from 0 to covering the entire spectrum. The mixed band may also be defined in other ways as necessary or desired.

At160, a “voicing shape” is created for the mixed band. One option for performing this action involves using the voicing likelihoods as such. For example, if the bins used in voicing estimation are wider than one harmonic interval, then the shape can be refined using interpolation either at this point or at180as explained below. The voicing shape can be further processed or simplified in the case of speech coding to allow for efficient compression of the information. In a simple case, a linear model within the band can be used.

At170, the parameters of the obtained model (in the case of speech coding) are stored or, e.g., in the case of voice conversion, are conveyed for further processing or for speech synthesis. At180, the magnitudes and phases of the spectrum based on the model parameters are reconstructed. In the voiced band, the phase can be assumed to evolve linearly. In the unvoiced band, the phase can be randomized. In the mixed band, the two contributions can be either combined to achieve the combined magnitude and phase values or represented using two separate values (depending on the synthesis technique). At190, the spectrum is converted into a time domain. This conversion can occur using, for example, a discrete Fourier transform or sinusoidal oscillators. The remaining portion of the speech modelling can be accomplished by performing linear prediction synthesis filtering to convert the synthesized excitation into speech, or by using other processes that are conventionally known.

As discussed herein, items110through170relate specifically to the speech analysis or encoding, while items180through190relate specifically to the speech synthesis or decoding.

In addition to the process depicted inFIG. 1and as discussed above, a number of variations to the encoding and decoding process are also possible. For example, the processing framework and the parameter estimation algorithms can be different than those discussed above. Additionally, different voicing detection algorithms can be used, and the width of each frequency bin can be varied. Furthermore, the modeling can use only the mixed band, or it is possible to use many bands representing the three different band types instead of using one band of each type. Still further, the determination of the voicing shape can be performed in other ways than that discussed above, and the details of the synthesis approach can be varied.

The various embodiments of the present invention provide for a high degree of accuracy in speech modeling, particularly in the case of weakly voiced speech, while at the same time enduring only a moderate computational load. The various embodiments also provide for an improved trade-off between accuracy and complexity relative to conventional arrangements.

Devices implementing the various embodiments of the present invention may communicate using various transmission technologies including, but not limited to, Code Division Multiple Access (CDMA), Global System for Mobile Communications (GSM), Universal Mobile Telecommunications System (UMTS), Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Transmission Control Protocol/Internet Protocol (TCP/IP), Short Messaging Service (SMS), Multimedia Messaging Service (MMS), e-mail, Instant Messaging Service (IMS), Bluetooth, IEEE 802.11, etc. A communication device may communicate using various media including, but not limited to, radio, infrared, laser, cable connection, and the like.

FIGS. 2 and 3show one representative mobile telephone12within which the present invention may be implemented. It should be understood, however, that the present invention is not intended to be limited to one particular type of mobile telephone12or other electronic device. The mobile telephone12ofFIGS. 2 and 3includes a housing30, a display32in the form of a liquid crystal display, a keypad34, a microphone36, an ear-piece38, a battery40, an infrared port42, an antenna44, a smart card46in the form of a UICC according to one embodiment of the invention, a card reader48, radio interface circuitry52, codec circuitry54, a controller56and a memory58. Individual circuits and elements are all of a type well known in the art, for example in the Nokia range of mobile telephones.

Software and web implementations of the present invention could be accomplished with standard programming techniques with rule based logic and other logic to accomplish various actions. It should also be noted that the words “component” and “module,” as used herein and in the claims, is intended to encompass implementations using one or more lines of software code, and/or hardware implementations, and/or equipment for receiving manual inputs.