Patent Grant
7177810
1. Field of the Invention
The present invention generally relates to speech processing techniques and, more particularly, the invention relates to a method and apparatus for performing prosody-based speech processing.
2. Description of the Related Art
Speech processing is used to produce signals for controlling devices or software programs, transcription of speech into written words, extraction of specific information from speech, classification of speech into document categories, archival and late retrieval of such information, and other related tasks. All such speech processing tasks are faced with the problem of locating within the speech signal suitable speech segments for processing. Segmenting the speech signal simplifies the signal processing required to identify words. Since spoken language is not usually produced with explicit indicators of such segments, segmentation within a speech processor may occur with respect to commands, sentences, paragraphs or topic units.
For example, a system that is controlled by voice commands needs to determine when a command uttered by a user is complete, i.e., when the system can stop waiting for further input and begin interpreting the command. The process used to determine whether a speaker has completed an utterance, e.g., a sentence or command, is known as endpointing. Generally, endpointing is performed by measuring the length of a pause in the speech signal. If the pause is sufficiently long, the endpointing process deems the utterance complete. However, endpointing processes that rely on pause duration are fraught with errors. For example, many times a speaker will pause in mid-sentence while thinking about what is to be said next. An endpointing process that is based upon pause sensing will identify such pauses as occurring at the end of a sentence, when that is not the case. Consequently, the speech recognition processing that is relying upon accurate endpointing will erroneously process the speech signal.
Therefore, there is a need in the art for a method and apparatus that accurately identifies an endpoint in a speech signal.
The present invention generally provides a method and apparatus for finding endpoints in speech by utilizing information contained in speech prosody. Prosody denotes the way speakers modulate the timing, pitch and loudness of phones, words, and phrases to convey certain aspects of meaning; informally, prosody includes what is perceived as the “rhythm” and “melody” of speech. Because speakers use prosody to convey units of speech to listeners (e.g., a change in pitch is used to indicate that a speaker has completed a sentence), the invention performs endpoint detection by extracting and interpreting the relevant prosodic properties of speech.
In one embodiment of the invention, referred to as “pre-recognition endpointing”, prosodic properties are extracted prior to word recognition, and are used to infer when a speaker has completed a spoken command or utterance. The use of prosodic cues leads to a faster and more reliable determination that the intended end of an utterance has been reached. This prevents incomplete or overly long stretches of speech from being sent to subsequent speech processing stages. Furthermore, because the prosodic information used to make the endpointing determination only includes speech uttered up to the potential endpoint, endpointing can be performed in real-time while the user is speaking. The endpointing method extracts a series of prosodic parameters relating to the pitch and pause durations within the speech signal. The parameters are analyzed to generate an endpoint signal that represents the occurrence of an endpoint within the speech signal. The endpoint signal may be a posterior probability that represents the likelihood that an endpoint has occurred at any given point in the speech signal or a binary signal indicating that an endpoint has occurred.
So that the manner in which the above recited features of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings.
It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
The present invention is embodied in software that is executed on a computer to perform endpoint identification within a speech signal. The executed software forms a method and apparatus that identifies endpoints in real-time as a speech signal is “streamed” into the system. An endpoint signal that is produced by the invention may be used by other applications such as a speech recognition program to facilitate accurate signal segmentation and word recognition.
The endpoint signal may be a binary signal that identifies the occurrence of an endpoint or the endpoint signal may be a continuously generated signal that indicates a probability that an endpoint has occurred at any moment in time. At step 208, the endpoint signal and the speech signal are coupled to an application program such as a speech recognition program, a speech-to-text translation program and the like. These programs use the endpoint signal to facilitate speech signal segmentation and word recognition.
The dashed line 210 represents the iterative nature of the endpointing process. Each sample of the speech signal is processed to generate the endpoint signal, then the next sample is processed. The new sample will be used to update the endpoint signal. As such, a continuous flow of endpointing information is generated as the speech signal is processed. Thus, endpoint information can be supplied in real-time or near-real-time depending on the computing speed that is available.
The pause analysis module 314 performs a conventional “speech/no-speech” algorithm that detects when a pause in the speech occurs. The output is a binary value that indicates whether the present speech signal sample is a portion of speech or not a portion of speech. This module 314 is considered optional for use in the inventive method to facilitate generation of additional information that can be used to identify an endpoint.
The duration pattern module 312 analyzes whether phones are lengthened with respect to average phone durations for the speaker. The lengthening of phones is indicative of the speaker not being finished speaking. The output of module 312 may be a binary signal (e.g., the phone is longer than average, thus output a one; otherwise output a zero) or a probability that indicates the likelihood that the speaker has completed speaking in view of the phone length.
The pitch processing module 318 is used to extract certain pitch parameters from the speech signal that are indicative of the speaker has completed an utterance. The module 318 extracts a fundamental pitch frequency (f o) from the speech signal and stylizes “pitch movements” of the speech signal (i.e., tracks the variations in pitch over time). Within the module 318, at step 302, a pitch contour is generated as a correlated sequence of pitch values. The speech signal is sampled at an appropriate rate, e.g., 8 kHz, 16 kHz and the like. The pitch parameters are extracted and computed (modeled) as discussed in K. Sonmez et al., “Modeling Dynamic Prosodic Variation for Speaker Verification”, Proc. Intl. Conf. on Spoken Language Processing, Vol. 7, pp 3189–3192 (1998) which is incorporated herein by reference. The sequence can be modeled in a piecewise linear model or in a polynomial of a given degree as a spline. At step 304, a pitch movement model is produced from the pitch contour using a finite state automaton or a stochastic Markov model. The model estimates the sequence of pitch movements. At steps 306 and 308, the module 318 extracts pitch features from the model, where the pitch features signal whether the speaker intended to stop, pause, continue speaking or ask a question. The features include the pitch movement slope (step 306) and the pitch translation from a baseline pitch (step 308). Baseline processing is disclosed in E. Shriberg et al. “Prosody-Based Automatic Segmentation of Speech into Sentences and Topics”, Speech Communication, Vol. 32, Nos. 1–2, pp 127–154 (2000).
More particularly, step 308 produces a unique speaker-normalization value that is estimated using a lognormal tied mixture approach to modeling speaker pitch. Such a model is disclosed by Sonmez et al in the paper cited above. The technique compares the present pitch estimate with a baseline pitch value that is recalled from a database 310. The database contains baseline pitch data for all expected speakers. If a speaker's pitch is near the baseline pitch, they have likely completed the utterance. If, on the other hand, the pitch is above the baseline, the speaker is probably not finished with the utterance. As such, the comparison to the baseline enables the system to identify a possible endpoint, e.g., falling pitch prior to a pause. An utterance that ends in a question generally has a rising pitch movement slope, so that the baseline difference information can be combined with the pitch movement slope feature to identify an endpoint of a question.
The pitch contour generation step (step 302) may include the voicing process that produces a value that represents whether the sample is a portion of a voiced speech sound. In other words, the module identifies whether the sampled sound is speech or some other sound that can be disregarded by the endpointing process. The value is either a binary value or a probability (e.g., a value ranging from 0 to 100 that indicates a likelihood that the sound is speech). In one embodiment the voice process may couple information to the pitch processing module 318 to ensure that the pitch processing is only performed on voice signals. Pitch information is not valid for non-voice signals.
At step 316, the extracted prosodic features are combined in either a data-driven fashion (i.e., estimated from an endpoint-labeled set of utterances, to generate predictors relevant to endpointing or using an a priori rule set that is generated by linguistic reasoning. Combinations of both approaches may be used. The output is a endpoint signal that represents the occurrence of an endpoint in the speech signal. The endpoint signal may take the form of a binary signal that identifies when an endpoint has occurred or the endpoint signal may be a posterior probability that provides a likelihood that the speech signal at any point in time is an endpoint (e.g., a scale of 0 to 100, where 0 is no chance of the speech being at an endpoint and 100 identifying that the speech is certainly at an endpoint). The endpoint signal may contain multiple posterior probabilities such as a probability that the utterance is finished, the probability that a pause is due to hesitation, and the probability that the speaker is talking fluently. The posterior probability or probabilities are produced on a continuous basis and are updated with changes in the detected prosodic features.
The forgoing embodiment extracts the features of the speech signal “on-the-fly” in real-time or near real-time. However, the invention may also be used in a non-real-time word recognition system to enhance the word recognition process. For example, the features may be extracted at a frame level (e.g., with respect to a group of speech samples that are segmented from the continuous speech signal). Additional frame-level features can be extracted that represent duration related features based on the phone level transcription output. Such features include speaker-and utterance-normalized duration of vowels, syllables, and rhymes (the last part of a syllable, or nucleus plus coda). Such features can provide a continuously updated posterior probability of utterance endpoint to enhance the speech recognition accuracy, i.e., information regarding vowels, syllables, and so on is useful for the speech recognition system to identify particular words. Furthermore, the additional information that is extracted because of the availability of segments of speech to analyze can be used to enhance the endpointing posterior probability. In effect, an initial posterior probability that was generated in real-time (pre-recognition processing), could later be updated when a frame-level analysis is performed.
While foregoing is directed to the preferred embodiment of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.
“This invention was made with Government support under Grant No. IRI-9619921 awarded by the DARPA/National Science Foundation. The Government has certain rights to this invention”.
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