Abstract:
A method of encoding speech analyzes a digitized speech signal to determine excitation parameters for the digitized speech signal. The method includes dividing the digitized speech signal into at least two frequency bands, performing a nonlinear operation on at least one of the frequency bands to produce a modified frequency band, and determining whether the modified frequency band is voiced or unvoiced. The nonlinear operation is an operation that emphasizes a fundamental frequency of the digitized speech signal so that the modified frequency band signal includes a component corresponding to the fundamental frequency even when the at least one frequency band signal does not include such a component.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to improving the accuracy with which excitation parameters are estimated in speech analysis and synthesis. 
     Speech analysis and synthesis are widely used in applications such as telecommunications and voice recognition. A vocoder, which is a type of speech analysis/synthesis system, models speech as the response of a system to excitation over short time intervals. Examples of vocoder systems include linear prediction vocoders, homomorphic vocoders, channel vocoders, sinusoidal transform coders (&#34;STC&#34;), multiband excitation (&#34;MBE&#34;) vocoders, and improved multiband excitation (&#34;IMBE&#34;) vocoders. 
     Vocoders typically synthesize speech based on excitation parameters and system parameters. Typically, an input signal is segmented using, for example, a Hamming window. Then, for each segment, system parameters and excitation parameters are determined. System parameters include the spectral envelope or the impulse response of the system. Excitation parameters include a voiced/unvoiced decision, which indicates whether the input signal has pitch, and a fundamental frequency (or pitch). In vocoders that divide the speech into frequency bands, such as IMBE (TM) vocoders, the excitation parameters may also include a voiced/unvoiced decision for each frequency band rather than a single voiced/unvoiced decision. Accurate excitation parameters are essential for high quality speech synthesis. 
     Excitation parameters may also be used in applications, such as speech recognition, where no speech synthesis is required. Once again, the accuracy of the excitation parameters directly affects the performance of such a system. 
     SUMMARY OF THE INVENTION 
     In one aspect, generally, the invention features applying a nonlinear operation to a speech signal to emphasize the fundamental frequency of the speech signal and to thereby improve the accuracy with which the fundamental frequency and other excitation parameters are determined. 
     In typical approaches to determining excitation parameters, an analog speech signal s(t) is sampled to produce a speech signal s(n). Speech signal s(n) is then multiplied by a window w(n) to produce a windowed signal s w  (n) that is commonly referred to as a speech segment or a speech frame. A Fourier transform is then performed on windowed signal s w  (n) to produce a frequency spectrum S w  (ω) from which the excitation parameters are determined. 
     When speech signal s(n) is periodic with a fundamental frequency ω o  or pitch period n o  (where n o  equals 2π/ω o ), the frequency spectrum of speech signal s(n) should be a line spectrum with energy at ω o  and harmonics thereof (integral multiples of ω o ). As expected, S w  (ω) has spectral peaks that are centered around ω o  and its harmonics. However, due to the windowing operation, the spectral peaks include some width, where the width depends on the length and shape of window w(n) and tends to decrease as the length of window w(n) increases. This window-induced error reduces the accuracy of the excitation parameters. Thus, to decrease the width of the spectral peaks, and to thereby increase the accuracy of the excitation parameters, the length of window w(n) should be made as long as possible. 
     The maximum useful length of window w(n) is limited. Speech signals are not stationary signals, and instead have fundamental frequencies that change over time. To obtain meaningful excitation parameters, an analyzed speech segment must have a substantially unchanged fundamental frequency. Thus, the length of window w(n) must be short enough to ensure that the fundamental frequency will not change significantly within the window. 
     In addition to limiting the maximum length of window w(n), a changing fundamental frequency tends to broaden the spectral peaks. This broadening effect increases with increasing frequency. For example, if the fundamental frequency changes by .increment.ω o  during the window, the frequency of the m th  harmonic, which has a frequency of mω o , changes by m.increment.ω o  so that the spectral peak corresponding to mω o  is broadened more than the spectral peak corresponding to ω o . This increased broadening of the higher harmonics reduces the effectiveness of higher harmonics in the estimation of the fundamental frequency and the generation of voiced/unvoiced decisions for high frequency bands. 
     By applying a nonlinear operation, the increased impact on higher harmonics of a changing fundamental frequency is reduced or eliminated, and higher harmonics perform better in estimation of the fundamental frequency and determination of voiced/unvoiced decisions. Suitable nonlinear operations map from complex (or real) to real values and produce outputs that are nondecreasing functions of the magnitudes of the complex (or real) values. Such operations include, for example, the absolute value, the absolute value squared, the absolute value raised to some other power, or the log of the absolute value. 
     Nonlinear operations tend to produce output signals having spectral peaks at the fundamental frequencies of their input signals. This is true even when an input signal does not have a spectral peak at the fundamental frequency. For example, if a bandpass filter that only passes frequencies in the range between the third and fifth harmonics of ω o  is applied to a speech signal s(n), the output of the bandpass filter, x(n), will have spectral peaks at 3ω o , 4ω o , and 5ω o . 
     Though x(n) does not have a spectral peak at ω o , |x(n)| 2  will have such a peak. For a real signal x(n), |x(n)| 2  is equivalent to x 2  (n). As is well known, the Fourier transform of x 2  (n) is the convolution of X(ω), the Fourier transform of x(n), with X(ω): ##EQU1## The convolution of X(ω) with X(ω) has spectral peaks at frequencies equal to the differences between the frequencies for which X(ω) has spectral peaks. The differences between the spectral peaks of a periodic signal are the fundamental frequency and its multiples. Thus, in the example in which X(ω) has spectral peaks at 3ω o , 4ω o , and 5ω o , X(ω) convolved with X(ω) has a spectral peak at ω o  (4ω o  -3ω o , 5ω o  -4ω o ). For a typical periodic signal, the spectral peak at the fundamental frequency is likely to be the most prominent. 
     The above discussion also applies to complex signals. For a complex signal x(n), the Fourier transform of |x(n)| 2  is: ##EQU2## This is an autocorrelation of X(ω) with X*(ω), and also has the property that spectral peaks separated by nω o  produce peaks at nω o . 
     Even though |x(n)|, |x(n)| a  for some real &#34;a&#34;, and log |x(n)| are not the same as |x(n)| 2 , the discussion above for |x(n)| 2  applies approximately at the qualitative level. For example, for |x(n)|=y(n) 0 .5, where y(n)=|x(n)| 2 , a Taylor series expansion of y(n) can be expressed as: ##EQU3## Because multiplication is associative, the Fourier transform of the signal y k  (n) is Y(ω) convolved with the Fourier transform of y k-1  (n). The behavior for nonlinear operations other than |x(n)| 2  can be derived from |x(n)| 2  by observing the behavior of multiple convolutions of Y(ω) with itself. If Y(ω) has peaks at nω o , then multiple convolutions of Y(ω) with itself will also have peaks at nω o . 
     As shown, nonlinear operations emphasize the fundamental frequency of a periodic signal, and are particularly useful when the periodic signal includes significant energy at higher harmonics. 
     According to the invention, excitation parameters for an input signal are generated by dividing the input signal into at least two frequency band signals. Thereafter, a nonlinear operation is performed on at least one of the frequency band signals to produce at least one modified frequency band signal. Finally, for each modified frequency band signal, a determination is made as to whether the modified frequency band signal is voiced or unvoiced. Typically, the voiced/unvoiced determination is made, at regular intervals of time. 
     To determine whether a modified frequency band signal is voiced or unvoiced, the voiced energy (typically the portion of the total energy attributable to the estimated fundamental frequency of the modified frequency band signal and any harmonics of the estimated fundamental frequency) and the total energy of the modified frequency band signal are calculated. Usually, the frequencies below 0.5ω o  are not included in the total energy, because including these frequencies reduces performance. The modified frequency band signal is declared to be voiced when the voiced energy of the modified frequency band signal exceeds a predetermined percentage of the total energy of the modified frequency band signal, and otherwise declared to be unvoiced. When the modified frequency band signal is declared to be voiced, a degree of voicing is estimated based on the ratio of the voiced energy to the total energy. The voiced energy can also be determined from a correlation of the modified frequency band signal with itself or another modified frequency band signal. 
     To reduce computational overhead or to reduce the number of parameters, the set of modified frequency band signals can be transformed into another, typically smaller, set of modified frequency band signals prior to making voiced/unvoiced determinations. For example, two modified frequency band signals from the first set can be combined into a single modified frequency band signal in the second set. 
     The fundamental frequency of the digitized speech can be estimated. Often, this estimation involves combining a modified frequency band signal with at least one other frequency band signal (which can be modified or unmodified), and estimating the fundamental frequency of the resulting combined signal. Thus, for example, when nonlinear operations are performed on at least two of the frequency band signals to produce at least two modified frequency band signals, the modified frequency band signals can be combined into one signal, and an estimate of the fundamental frequency of the signal can be produced. The modified frequency band signals can be combined by summing. In another approach, a signal-to-noise ratio can be determined for each of the modified frequency band signals, and a weighted combination can be produced so that a modified frequency band signal with a high signal-to-noise ratio contributes more to the signal than a modified frequency band signal with a low signal-to-noise ratio. 
     In another aspect, generally, the invention features using nonlinear operations to improve the accuracy of fundamental frequency estimation. A nonlinear operation is performed on the input signal to produce a modified signal from which the fundamental frequency is estimated. In another approach, the input signal is divided into at least two frequency band signals. Next, a nonlinear operation is performed on these frequency band signals to produce modified frequency band signals. Finally, the modified frequency band signals are combined to produce a combined signal from which a fundamental frequency is estimated. 
     Other features and advantages of the invention will be apparent from the following description of the preferred embodiments and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a system for determining whether frequency bands of a signal are voiced or unvoiced. 
     FIGS. 2-3 are block diagrams of fundamental frequency estimation units. 
     FIG. 4 is a block diagram of a channel processing unit of the system of FIG. 1. 
     FIG. 5 is a block diagram of a system for determining whether frequency bands of a signal are voiced or unvoiced. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1-5 show the structure of a system for determining whether frequency bands of a signal are voiced or unvoiced, the various blocks and units of which are preferably implemented with software. 
     Referring to FIG. 1, in a voiced/unvoiced determination system 10, a sampling unit 12 samples an analog speech signal s(t) to produce a speech signal s(n). For typical speech coding applications, the sampling rate ranges between six kilohertz and ten kilohertz. 
     Channel processing units 14 divide speech signal s(n) into at least two frequency bands and process the frequency bands to produce a first set of frequency band signals, designated as T 0  (ω) . . . T I  (ω). As discussed below, channel processing units 14 are differentiated by the parameters of a bandpass filter used in the first stage of each channel processing unit 14. In the preferred embodiment, there are sixteen channel processing units (I equals 15). 
     A remap unit 16 transforms the first set of frequency band signals to produce a second set of frequency band signals, designated as U 0  (ω) . . . U K  (ω). In the preferred embodiment, there are eleven frequency band signals in the second set of frequency band signals (K equals 10). Thus, remap unit 16 maps the frequency band signals from the sixteen channel processing units 14 into eleven frequency band signals. Remap unit 16 does so by mapping the low frequency components (T 0  (ω) . . . T 5  (ω)) of the first set of frequency bands signals directly into the second set of frequency band signals (U 0  (ω) . . . U 5  (ω)). Remap unit 16 then combines the remaining pairs of frequency band signals from the first set into single frequency band signals in the second set. For example, T 6  (ω) and T 7  (ω) are combined to produce U 6  (ω), and T 14  (ω) and T 15  (ω) are combined to produce U 10  (ω). Other approaches to remapping could also be used. 
     Next, voiced/unvoiced determination units 18, each associated with a frequency band signal from the second set, determine whether the frequency band signals are voiced or unvoiced, and produce output signals (V/UV 0  . . . V/UV K ) that indicate the results of these determinations. Each determination unit 18 computes the ratio of the voiced energy of its associated frequency band signal to the total energy of that frequency band signal. When this ratio exceeds a predetermined threshold, determination unit 18 declares the frequency band signal to be voiced. Otherwise, determination unit 18 declares the frequency band signal to be unvoiced. 
     Determination units 18 compute the voiced energy of their associated frequency band signals as: ##EQU4## ω o  is an estimate of the fundamental frequency (generated as described below), and N is the number of harmonics of the fundamental frequency ω o  being considered. Determination units 18 compute the total energy of their associated frequency band signals as follows: ##EQU5## 
     In another approach, rather than just determining whether the frequency band signals are voiced or unvoiced, determination units 18 determine the degree to which a frequency band signal is voiced. Like the voiced/unvoiced decision discussed above, the degree of voicing is a function of the ratio of voiced energy to total energy: when the ratio is near one, the frequency band signal is highly voiced; when the ratio is less than or equal to a half, the frequency band signal is highly unvoiced; and when ratio is between a half and one, the frequency band signal is voiced to a degree indicated by the ratio. 
     Referring to FIG. 2, a fundamental frequency estimation unit 20 includes a combining unit 22 and an estimator 24. Combining unit 22 sums the T i  (ω) outputs of channel processing units 14 (FIG. 1) to produce X(ω). In an alternative approach, combining unit 22 could estimate a signal-to-noise ratio (SNR) for the output of each channel processing unit 14 and weigh the various outputs so that an output with a higher SNR contributes more to X(ω) than does an output with a lower SNR. 
     Estimator 24 then estimates the fundamental frequency (ω o ) by selecting a value for ω o  that maximizes X(ω o ) over an interval from ω min  to ω max . Since X(ω) is only available at discrete samples of ω, parabolic interpolation of X(ω o ) near ω o  is used to improve accuracy of the estimate. Estimator 24 further improves the accuracy of the fundamental estimate by combining parabolic estimates near the peaks of the N harmonics of ω o  within the bandwidth of X(ω). 
     Once an estimate of the fundamental frequency is determined, the voiced energy E v  (ω o ) is computed as: ##EQU6## Thereafter, the voiced energy E v  (0.5ω o ) is computed and compared to E v  (ω o ) to select between ω o  and 0.5ω o  as the final estimate of the fundamental frequency. 
     Referring to FIG. 3, an alternative fundamental frequency estimation unit 26 includes a nonlinear operation unit 28, a windowing and Fast Fourier Transform (FFT) unit 30, and an estimator 32. Nonlinear operation unit 28 performs a nonlinear operation, the absolute value squared, on s(n) to emphasize the fundamental frequency of s(n) and to facilitate determination of the voiced energy when estimating ω o . 
     Windowing and FFT unit 30 multiplies the output of nonlinear operation unit 28 to segment it and computes an FFT, X(ω), of the resulting product. Finally, an estimator 32, which works identically to estimator 24, generates an estimate of the fundamental frequency. 
     Referring to FIG. 4, when speech signal s(n) enters a channel processing unit 14, components s i  (n) belonging to a particular frequency band are isolated by a bandpass filter 34. Bandpass filter 34 uses downsampling to reduce computational requirements, and does so without any significant impact on system performance. Bandpass filter 34 can be implemented as a Finite Impulse Response (FIR) or Infinite Impulse Response (IIR) filter, or by using an FFT. Bandpass filter 34 is implemented using a thirty two point real input FFT to compute the outputs of a thirty two point FIR filter at seventeen frequencies, and achieves downsampling by shifting the input speech samples each time the FFT is computed. For example, if a first FFT used samples one through thirty two, a downsampling factor of ten would be achieved by using samples eleven through forty two in a second FFT. 
     A first nonlinear operation unit 36 then performs a nonlinear operation on the isolated frequency band s i  (n) to emphasize the fundamental frequency of the isolated frequency band s i  (n). For complex values of s i  (n) (i greater than zero), the absolute value, |s i  (n)|, is used. For the real value of s 0  (n), s 0  (n) is used if s 0  (n) is greater than zero and zero is used if s 0  (n) is less than or equal to zero. 
     The output of nonlinear operation unit 36 is passed through a lowpass filtering and downsampling unit 38 to reduce the data rate and consequently reduce the computational requirements of later components of the system. Lowpass filtering and downsampling unit 38 uses a seven point FIR filter computed every other sample for a downsampling factor of two. 
     A windowing and FFT unit 40 multiplies the output of lowpass filtering and downsampling unit 38 by a window and computes a real input FFT, S i  (ω), of the product. 
     Finally, a second nonlinear operation unit 42 performs a nonlinear operation on S i  (ω) to facilitate estimation of voiced or total energy and to ensure that the outputs of channel processing units 14, T i  (ω), combine constructively if used in fundamental frequency estimation. The absolute value squared is used because it makes all components of T i  (ω) real and positive. 
     Other embodiments are within the following claims. For example, referring to FIG. 5, an alternative voiced/unvoiced determination system 44, includes a sampling unit 12, channel processing units 14, a remap unit 16, and voiced/unvoiced determination units 18 that operate identically to the corresponding units in voiced/unvoiced determination system 10. However, because nonlinear operations are most advantageously applied to high frequency bands, determination system 44 only uses channel processing units 14 in frequency bands corresponding to high frequencies, and uses channel transform units 46 in frequency bands corresponding to low frequencies. Channel transform units 46, rather than applying nonlinear operations to an input signal, process the input signal according to well known techniques for generating frequency band signals. For example, a channel transform unit 46 could include a bandpass filter and a window and FFT unit. 
     In an alternate approach, the window and FFT unit 40 and the nonlinear operation unit 42 of FIG. 4 could be replaced by a window and autocorrelation unit. The voiced energy and total energy would then be computed from the autocorrelation.