A method for generating speech feature signals and an apparatus for carrying through this method

Abstract: A method and an apparatus is disclosed for generating speech feature signals characterizing speech signals by generating a set of spectral parameters characterizing speech signal, weighting the set of spectral parameters, and generating speech characterizing feature signals based on the weighted set of spectral samples. If speech signals are received they are blocked into frames and then in combination a linear predictive coding and cepstral recursion analysis is performed on the blocked signals to produce mel-LPC cepstrum feature signals. The mel-LPC cepstrum features are then compared to a recognition model.

Description: This invention relates to a method for generating speech feature signals characterizing speech signals according to claim 1. Furthermore the present invention relates also to an apparatus for generating such speech feature signals according to claim 12. Background Art From the literature Shirai et al., "Phoneme Recognition in Continuous Speech using Feature Selection Based on Mutual Information", Euro Speech 1989, pp. 370-373, a method for generating speech feature signals is known, which speech feature signals characterize speech signals. This known method comprises the steps of generating a set of spectral parameters characterizing speech signals, weighting the set of spectral parameters; and generating speech characterizing feature signals based on the weighted set of spectral samples. A similar method and apparatus for carrying through the method is known from the literature Buhrke et al., "Application of Vector Quantized Hidden Markov Modeling to Telephone Network Based Connected Digit Recognition", ICASSP 1994, pp. 1/105-108. Speech signals, which are transmitted through a telecommunication network, such as a long distance exchange network, often may be affected acoustically by unknown variable conditions. These conditions may significantly deteriorate the performance of an automatic speech recognition (ASR) system that is typically included in such a network. Such conditions may include, for example, ambient noise, channel interference and the utilization of different sound detection equipment. A typical ASR system performs speech recognition by comparing or matching feature data representative of transmitted speech signals to data patterns representative of phonetic units, known as training recognition models. The recognition models are typically obtained by monitoring the transmission of predetermined speech signals having known phonetic characteristics through the same network connection path that actual speech signals such as those occurring during a telephone conversation will be transmitted to an ASIR system. In many instances, the presence of unknown variable conditions in a network path may cause acoustical mismatches between the recognition models and the testing data. These acoustical mismatches may cause deterioration in speech recognition performance. Acoustical mismatches are likely to occur, for example, when speech signals transmitted through a network path are enhanced for improving speech quality to human perception. When such enhancement occurs, speech recognition performance may become suboptimal where the recognition models used in speech recognition processing have not been generated or trained on a network connection path, which includes similar speech signal enhancement circuitry. Current speech signal processing techniques that attempt to provide an ASR system robustness in matching features representative of detected speech signals with recognition models typically do not sufficiently compensate for acoustical mismatches which may result as discussed above. Summary of the Invention It is an object of the present invention to provide an improved method and an improved feature extractor apparatus for generating speech feature signals, which are leading into an improved speech recognition performance. According to the inventive method this object is solved by the features of claim 1. Improved embodiments of the inventive method result from the subclaims 2 to 11. In connection with the inventive apparatus the above object is solved by the features of claim 12. Improved embodiments of the inventive apparatus result from the subclaims 13 to 18. The inventive method an feature extractor apparatus is in particular suitable for use in implementing a compensation that provides superior robustness to speech recognition performance in networks with an enhancement, where, for example, the recognition models have not been generated or trained on a network connection path with similar enhancement circuitry. One aspect with which the present invention can be utilized is the speech signal enhancement, which is compensated for during the performance of feature extraction processing at the front end of a speech recognition system, which includes the inventive feature extractor and also a speech recognizer. The feature extractor computes cepstral features of the enhanced speech signals using a combination of linear predictive coding (LPC) and mel-filter analysis. The extractor compensates for the enhancement to the time varying speech signals after they are transformed to a frequency domain representation. According to this technique, first the time varying enhanced speech signals are sampled. These samples are collected into frames and then converted from time to frequency domain representations using Fourier transformation. Then, the power spectrum for each frame is subjected to selectively weighted mel-filter bank processing, where weightings selectively compensate for the effect of the enhancement on the spectral characteristics of the speech signals. Subsequently, cepstral features are obtained by computing autocorrelation coefficients from spectral representations of the compensated enhanced speech signals, and then by performing LPC analysis and cepstral recursion. Further features and advantages of the present invention will become readily apparent to those of ordinary skill in the art by reference to the following detailed description and accompanying drawings. Brief Description of the Drawings FIG. 1 illustrates a block diagram of an improved telecommunication network suitable for performing speech recognition processing on enhanced speech signals in accordance with the present invention. FIG. 2 is an illustrative block diagram of a speech signal processor, which may be included in the system of FIG. 1 for performing speech recognition by computing Mel-LPC cepstrum features of enhanced speech signals in accordance with the present invention. FIG. 3 is an illustrative block diagram of an exemplary compensated feature extractor. FIG. 4 is a flow diagram of a method for performing speech recognition in accordance with the present inventive technique of computing Mel-LPC cepstrum features of enhanced speech signals. Detailed Description FIG. 1 shows a block diagram of an exemplary telecommunication network 100 suitable for performing speech recognition processing on enhanced speech signals in accordance with the present inventive technique. Referring to FIG. 1, the network 100 may suitably comprise a plurality of toll offices, such as toll offices 105 and 110, which may be interconnected, in any of a variety of well known fashions, to one another to provide long distance voice and data connections for its subscribers, such as telephone users, associated with station sets S1 and S2. A local central office 50 connects the station S1 to the toll office 105, and a local central office 75 connects the station S2 to the toll office 110. The network 100 may suitably further include a speech enhancement device or enhancer 107 in the connection path between the toll offices 105 and 110 for enhancing the quality of speech signals before they are received by a subscriber, such as a telephone user. For example, the enhancer 107 may include suitable circuitry, such as echo cancelling circuitry, or other circuitry, for increasing or decreasing the power level of speech signals. See U.S. Patent 5,195,132 for a more detailed description of the implementation of speech signal enhancement in a network, which involves increasing the power levels of select frequencies. In accordance with the present invention, a speech signal processor 102 is additionally included in the network connection path to receive speech signals from the enhancer 107 and provide for speech signal recognition signal processing including feature extraction, as more fully discussed below. A telephone connection from the station S1 to the station S2 is suitably established through the local central office 50, the toll office 105, the enhancer 107, the processor 102, the toll office 110 and the local central office 75 according to conventional techniques which will not be described herein. It is to be understood that the network 100 may be any well-known telecommunication network, such as the AT&T network. In prior art telecommunication networks, the computation of parameters, known as features, for characterizing time varying speech signals was highly sensitive to the introduction of acoustic variations to speech signals by, for example, speech signal enhancement. This condition was particularly noted for the computation of preferred features, such as cepstral features. As a result, the inclusion of such speech signal enhancement in a network connection path severely degraded the performance of automatic speech recognition (ASR) processing where the recognition models utilized for matching feature data representative of the transmitted speech signals were trained on a network path that did not include similar or identical speech signal enhancement circuitry. In accordance with the present invention, the processor 102 included in the network 100 suitably operates to minimize error levels when speech recognition is performed on transmitted speech signals, which may have been enhanced by the speech signal enhancer 107 before further routing to the ASR processing device, such as the processor 102. The processor 102, essentially, performs enhancement compensation operations that afford improved robustness to cepstral feature-based speech recognition processing. Alternatively, the processor 102 in the network 100 may operate in accordance with the present inventive technique to minimize error levels when speech recognition is performed on transmitted speech signals which have not been enhanced before being routed to the processor 102, or have been enhanced by an enhancer which introduces an enhancement to speech signals that differs from the enhancement being compensated for by the processor 102. For purposes of illustration, it is assumed that the recognition models used in performing feature matching during speech recognition processing at the processor 102 have been trained over a network path that does not include speech signal enhancement circuitry. For purposes of illustration, the enhancer 107 introduces speech signal enhancement to speech signals transmitted from the station S1 to the station S2 during routing through the network to the processor 102. It is assumed that, for the transmitted speech signals routed to the processor 102 which have been enhanced by the enhancer 107, information concerning the nature of the enhancement is available for implementation at the processor 102, in the manner described below, to allow for suitable compensation of the enhancement. For purposes of clarity and brevity, a knowledge of speech recognition processing techniques which involve the use of linear predictive coding (LPC) and mel-filter analysis for extracting cepstral features from data representative of the spectral characteristics of speech signals is assumed. See Rabiner, Lawrence and Juang, Biing-Hwang, Fundamentals of Speech Recognition" (1993), pp. 103-107, 112-117 and 183-190. In short, LPC processing involves extracting features from speech signals by source-tract separation and modeling of spectral peaks as a linear combination of past speech samples. Mel-filter analysis involves extracting features from speech signals, based on knowledge of human perceptivity of the acoustic spectral components of speech signals, by advantageously sampling the complete spectrum of speech signals in terms of subjective pitch of pure tones at so-called mel-scale intervals. These mel-scale interval values are conventionally implemented as filter banks, known as mel-filter banks, for suitably modifying spectral components of speech signals to improve overall speech recognition. In one aspect of the present invention, LPC analysis is combined with mel-filter bank analysis for computing cepstral features that compensate for the introduction of enhancement to transmitted speech signals, where the compensation is implemented when the enhanced speech signals have been suitably transformed to frequency domain representation. As more fully described below, the computation of Mel-LPC cepstrum features includes a mel-filter bank analysis followed by LPC analysis, where autocorrelation coefficients are computed based on a perceptual frequency scale and where smoothing of the power spectrum is performed using LPC techniques. For purposes of illustration, the enhancer 107 includes enhancement circuitry similar to that described in U.S. patent 5,195,132, cited above (hereinafter TrueVoice sm enhancement), for enhancing speech signals being routed to the processor 102 from the station S1, and speech recognition processing is performed at the processor 102 using recognition models trained using speech signals that have not been enhanced. FIG. 2 shows an exemplary embodiment of the processor 102 which may perform arithmetic operations for computing Mel-LPC cepstrum features to provide for highly robust speech recognition for speech signals enhanced by TrueVoice sm enhancement at the enhancer 107. Referring to FIG. 2, the processor 102 suitably includes individual functional blocks for performing mel-cepstral computations and feature matching for transmitted speech signals including or without enhancement. The functions these blocks represent may be provided through the use of either shared or dedicated hardware, including but not limited to hardware capable of executing software. In a preferred embodiment, the functional blocks in the processor 102 comprise a compensated feature extractor 202 and a recognizer 204, which includes a training recognizer model memory 206 as RAM. The extractor 202 and the recognizer 204 are suitably connected to a memory 208. The memory 208 includes programmed instructions, which may be suitably performed for implementing the present inventive technique. As explained below, the functional blocks of the extractor 202 and the recognizer 204 provide for the processing of the speech signals transmitted via the toll office 105 or 110 in accordance with implementation of the present inventive technique. It is to be understood that the operations performed by the functional blocks may be similarly performed using a single shared processor. Such a processor may comprise a standard digital signal processor and would include read only memory or other appropriate memory for storing software and performing the operations discussed below. Further, the present invention technique may be implemented using firmware or by discrete components implemented in an integrated circuit. Other suitable embodiments may be readily implemented by those of ordinary skill in the art. For example, while as shown in FIG. 2, the compensated feature extractor 202 is connected to receive its input from enhancer 107, it could be connected directly to a toll such as toll 105 without any intervening enhancement. FIG. 3 shows, in detail, an exemplary embodiment of the extractor 202 which may perform the operations for compensating for enhancement to speech signals, which are described below in connection with FIG. 4. Referring to FIG. 3, the extractor 202 may suitably comprise a receiver 211, a preemphasizer 212, a frame blocker 213, a Hamming multiplier 214, a frequency domain converter 215, a compensator 216 and a cepstral feature computer 217. These functional blocks suitably perform operations for implementing the present invention technique as more fully discussed below in connection with FIG. 4. It is to be understood that the operations performed at each of these functional blocks may be implemented using a combination of software and/or hardware as well as by software alone. Referring to FIG. 2, the compensated feature extractor 202 is essentially a front end processor in an ASR system that performs operations for computing Mel LPC cepstral features of the time varying speech signals routed from the toll office 105 based on spectral or frequency domain representations of the characteristics of these signals. The Mel-LPC cepstral features ultimately computed by the cepstral feature computer 217 of the extractor 202 describe the broad acoustic properties of different phonetic units, which are conventionally utilized for speech recognition. In particular, the compensator 216 and the cepstral feature computer 217 combine LPC and mel-filter bank analysis for computing a set of compensated cepstral features representative of transmitted speech signals that provides that ASR processing in the network 100 by the recognizer 204 is substantially insensitive to acoustic variations caused by the enhancement. The Mel-LPC cepstral features are provided to the recognizer 204, which compares them to the recognition models stored in the memory 206 for determining the best word or sequence of words which matches the phonetic unit represented by the computed features. A suitable recognizer may, for example, utilize well-known hidden Markov model (HMM) based recognition techniques. It is noted that some a priori knowledge of the enhancement introduced, such as TrueVoice sm enhancement, is required for performing the compensation of enhancement using frequency domain representative of speech signals in accordance with the present inventive technique. This information may be generally available to the public. Alternatively, information concerning enhancement may be determined, for example, using standard techniques of empirically sampling speech signals transmitted through a network connection path, which includes speech signal enhancement circuitry. In comparison, prior art techniques of cepstral mean subtraction and signal bias removal did not require prior knowledge of acoustic variations in speech signals and are considered as blind deconvolution based techniques. FIG. 4 shows a method 220 for computing Mel-LPC cepstrum features of enhanced time varying speech signals at the compensated feature extractor 202 of the processor 102 using the exemplary functional blocks shown in FIG. 3. For purposes of illustration, it is assumed that speech signals are digitized, preferably at an 8.0 kHz rate, for emulating a digital telephone network, before being routed to the extractor 202. Referring to FIG. 4, in step 222, the receiver 211 receives the enhanced speech signals from the enhancer 107. Then, in step 224, the preemphasizer 212 processes the digitized speech signal samples to provide for spectral flattening. Preemphasis is suitably performed using, for example, a conventional first order digital network. In step 226, the frame blocker 213 collects or blocks the speech signal samples into frames. A single frame consists of sections of consecutive speech samples, which correspond to n msec of signal, and consecutive frames are spaced m msec apart. In step 228, the Hamming multiplier 214 multiplies each frame of samples by a Hamming window as is conventional in the art. In step 230, the frequency domain converter 215 performs a Fourier transformation on each windowed segment of speech for generating a set of spectral samples corresponding to the enhanced speech signals received at the receiver 211. In step 232, the compensator 216 processes each frame to cause the power spectrum for each frame of spectral samples to pass through a set of M mel-filter banks, may be suitably triangular in shape. The mel-filter banks are suitably weighted to compensate for the effects that enhancement may have on the computation of the cepstral features of the transmitted speech signals. In other words, the effects of enhancement on speech signals are compensated for at the compensator 216, which performs operations on frequency domain representations of the enhanced speech signals. It is noted that the Mel-LPC cepstrum computation provides that the gain of each spectral band may be completely controlled. This allows for compensation of speech signal enhancement simply by imposing a weighting function on the mel-filter banks. For example, compensation for speech signal enhancement may be performed for implementing ideal high pass filtering by de-emphasizing those undesirable mel-filter banks prior to computing cepstral features from the frequency domain representation of the speech signals using LPC techniques. This weighting of data value coefficients in the spectral domain is extremely advantageous and provides for the computation of compensated cepstral features of enhanced speech signals which may be suitably matched to recognition models not trained using enhanced speech signals. It is noted that cepstral features, once computed, cannot be weighted in an analogous manner. It is further noted that such processing of enhanced speech signals may be roughly approximated by filtering of the enhanced speech signals while they are still in their time domain representation. In the exemplary embodiment of the network 100 where TrueVoice sm enhancement would be introduced to speech signals by the enhancer 107, information concerning the characteristics of the TrueVoice sm enhancement would be utilized for determining the weightings applied to the mel-filter banks of the compensator 216. From experimental analysis, it was found that TrueVoice sm enhancement involved filtering of speech signals using a pre-emphasis filter (p-filter) and applying a slowly varying AVC (Automatic Volume Controller) to the signals, where the p-filter introduced a 10-20 dB spectral pole somewhere in the frequency band between 150 Hz and 220 Hz. In this case, the mel-filter banks would implement the desired high pass filtering in the frequency domain, which would result in modification of the computed cepstrum features and thus improve robustness of speech recognition performance towards low frequency power boosting. For example, mel-filter banks with a frequency below 290 Hz may be assigned zero weight and filter banks having frequencies above 290 Hz may be assigned unitary weight. In other words, the effect of TrueVoice sm enhancement on speech signal recognition would be minimized by de-emphasizing the low frequency mel-spectral bands used in the computation of the Mel-LPC cepstrum. Preferably, a threshold of 290 Hz would be implemented, such that mel bank frequency bands below this threshold would not be considered during the computation of the cepstrum. This Mel-LPC cepstrum processing may be considered as ideal high-pass filtering in which the low frequency bands to 290 Hz are heavily attenuated. Referring to FIG. 4, in step 234, the cepstral feature computer 217 applies an IDCT (inverse discrete cosine transform) to the smoothed power spectrum to yield Q autocorrelation coefficients, where Q is suitably set to 12. In step 236, the cepstral feature computer 217 converts each set of autocorrelation coefficients first to LPC coefficients using, for example, Durbin's recursion algorithm, and then to cepstral parameters using the standard LPC-to-cepstrum recursion. The dimension of the cepstral vector is suitably set to 12. Finally, in step 238, the recognizer 204 attempts to match the recognition models stored in the model memory 206 to the computed cepstral features for the compensated enhanced speech signals, and provides data representative of the feature matching operations for further routing to the station S2, as is conventional in the art. It is noted that the Mel-LPC cepstrum feature computation technique may provide for superior robustness towards speech enhancement, and in some cases, not adversely affect ASR performance even when enhancement has not been introduced and the Mel-LPC cepstrum features provide for compensation of a particular enhancement to speech signals. It is to be understood that different filters and cut-off frequencies may be utilized for implementing the present inventive technique of using mel-filter bank processing to compensate for enhancement to speech signals. It is further noted that, for signal enhancement involving fine modifications to the speech spectrum, a robust front-end system, such as one which computes Mel-LPC cepstrum features, provides easy access to the speech recognition system for altering the speech spectrum in any manner required for purposes of computing features of speech signals which are to be matched to recognition models not trained using enhanced speech signals. It is to be understood that the embodiments and variations shown and described above are illustrative of the principles of this invention only and that various modifications may be implemented by those skilled in the art without departing from the scope of the invention.

Claims: A method for generating speech feature signals characterizing speech signals, the method comprising the steps of: generating a set of spectral parameters characterizing speech signals; selectively weighting the set of spectral parameters (232) so as to compensate for the effect of enhancement on the spectral characteristics of the speech signals caused by enhancers (107) in a network; and generating speech characterizing feature signals based on the weighted set of spectral samples. The method according to claim 1, comprising the following steps: receiving the Speech signals; blocking the Speech signals into frames; performing in combination linear predictive coding and cepstral recursion analysis on the blocked speech signals to produce mel-LPC cepstrum feature signals. The method of claim 2, further comprising the step of comparing the mel-LPC cepstrum features to a recognition model. The method of claim 2, further comprising the step of: utilizing a mel-filter bank to filter the blocked speech signals and produce mel-filtered signals which are then analyzed by performing linear predictive coding and cepstral recursion analysis in combination. The method of claim 2, further comprising the step of preemphasizing the speech signals to achieve spectral flattening of said signals. The method of claim 2, further comprising the step of multiplying each of the frames by a Hamming window. The method of claim 6, further comprising the step of transforming each of the Hamming window frames from a time representation to a frequency domain representation. The method of claim 7, further comprising filtering each of the frequency domain representations using mel-filter bank weighting values stored in a memory. The method of claim 8, further comprising the step generating autocorrelation coefficients from the filtered frequency representations. The method of claim 9, wherein the linear predictive coding analysis operates to convert the autocorrelation coefficients to LPC coefficients and the cepstral recursion analysis operates to compute cepstral parameters from the LPC coefficients. The method of claim 1, wherein the steps of selectively weighting the set of spectral parameters is performed by selectively weighted mel-filter banks. An apparatus for generating speech feature signals characterizing speech signals comprising: a frequency domain converter (215) for generating a set of spectral samples representing the speech signals; a weighting unit (216) for selectively weighting the set of spectral samples to compensate for the effect of the enhancement on the spectral characteristics of the speech signals caused by enhancers in a telecommunication network; and a feature computer (217) for generating speech characterizing feature signals based on the weighted set of spectral samples. The apparatus of claim 12, further comprising a receiver, wherein the receiver is connected to the frequency domain converter through a preprocessing circuit. The apparatus of claim 13, wherein the preprocessing circuit comprises a preemphasizer circuit. The apparatus of claim 13, wherein the preprocessing circuit comprises a frame blocker. The apparatus of claim 13, wherein the preprocessing circuit comprises a Hamming multiplier. The apparatus of claim 12, wherein the weighting unit comprises a set of M selectively weighted mel-filter banks. The apparatus of claim 12, wherein the weighing unit further comprises selectively weighted mel-filter banks.

Inventors: RAHIM MAZIN G, WILPON JAY GORDON, RAHIM, MAZIN G., WILPON, JAY GORDON
Applicants: AT & T CORP, AT&T CORP.
Application year: 1996
Main classification: G10L15
IPC codes: G10L15