Signal noise reduction using magnitude-domain spectral subtraction

[0002] The present invention pertains to techniques for reducing noise in a signal. More particularly, the present invention relates to techniques for reducing noise in a signal representing speech.

[0003] In a speech recognition system, the presence of noise in an input speech signal can degrade recognition accuracy. Noise can be introduced from many different sources and may be introduced through either acoustic coupling or electrical coupling. Acoustic coupling of noise into a speech signal might occur when a speaker is located in a noisy setting, such as a busy office environment. Electrical coupling of noise may result from electromagnetic radiation emitted by electrical devices in proximity to components of the speech recognition system. Various techniques are known for reducing noise in a speech signal. However, these techniques generally are not adaptable, or are not sufficiently adaptable, to the amount of noise in the speech signal at any given time. A typical consequence of this shortcoming is that a given noise reduction technique may perform adequately in a noisy environment but perform poorly in a low-noise environment. Such techniques, therefore, tend not to be very flexible in terms of handling signals under a variety of conditions. In addition, prior art noise reduction techniques may not be capable of operating upon individual frequency components of a signal. Furthermore, many noise reduction techniques used in speech recognition handle the beginning of a sentence very inefficiently, since few samples have been observed at that point in time, and a signal-to-noise ratio is difficult to estimate accurately at that time.

[0004] The present invention includes a method and apparatus for reducing noise in data representing an audio signal, such as a speech signal. For each of multiple frequency components of the audio signal, a spectral magnitude of the data and an estimate of noise in the data are computed. The estimate of noise is scaled by a noise scale factor that is a function of the corresponding frequency component, to produce a scaled noise estimate. The scaled noise estimate is subtracted from the spectral magnitude to produce cleaned audio data. The scale factor may be also be a function of an absolute noise level for each frequency component, a signal-to-noise ratio for each frequency component, or both.

[0005] The noise reduction technique operates well on both noisy input signals and clean input signals. The technique can be implemented as a real-time method of reducing noise in a speech signal, which begins the noise reduction process immediately when the speech signal becomes available, thereby reducing undesirable delay between production of the speech signal and its recognition by a speech recognizer.

[0006] Other features of the present invention will be apparent from the accompanying drawings and from the detailed description which follows.

[0007] The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which:

[0008] FIG. 1 is a block diagram of a speech recognition system;

[0009] FIG. 2 is a block diagram of a computer system;

[0010] FIG. 3 is a flow diagram showing an overall process for extracting feature vectors from an input speech waveform;

[0011] FIG. 4 is a flow diagram showing a process for removing noise from a speech signal;

[0012] FIG. 5 is a flow diagram showing a process for generating a noise magnitude estimate; and

[0013] FIG. 6 is a flow diagram showing a process for computing a scale factor for a noise estimate.

[0014] A method and apparatus for reducing noise in a speech signal are described. The noise reduction technique operates well on either noisy speech or clean speech. As will be described in greater detail below, in the noise reduction technique, a noise magnitude estimate is determined for each of multiple frequency indexes by updating a histogram of spectral magnitude, and determining the noise magnitude estimate as a predetermined percentile of the histogram. A signal-to-noise ratio is also determined for each frequency index. A scale factor is computed for each frequency index as a function of the signal-to-noise ratio and the noise magnitude estimate. The noise magnitude estimate is then scaled by the scale factor, and the scaled noise magnitude estimate is subtracted from the spectral magnitude of the noisy speech, to produce cleaned speech data. The technique can be implemented as a real-time method of reducing noise, which begins the noise reduction process immediately when the speech signal becomes available, thereby reducing undesirable delay between production of the speech signal and its recognition by a speech recognizer.

[0015] Note that in this description, references to “one embodiment” or “an embodiment” mean that the feature being referred to is included in at least one embodiment of the present invention. Further, separate references to “one embodiment” in this description do not necessarily refer to the same embodiment, however, neither are such embodiments mutually exclusive.

[0016] FIG. 1 shows an example of a speech recognition system in which the present noise reduction technique can be implemented, to provide advantages over prior art speech recognition systems. It should be noted, however, that the described noise reduction technique is not limited to use in speech recognition systems, nor is it limited in application to speech signals or to any particular type of signal. The illustrated system includes a dictionary 2, a set of acoustic models 4, and a grammar/language model 6. Each of these elements may be stored in one or more conventional memory devices. The dictionary 2 contains all of the words allowed by the speech application in which the system is used. The acoustic models 4 are statistical representations of all phonetic units and subunits of speech that may be found in a speech waveform. The grammar/language model 6 is a statistical or deterministic representation of all possible combinations of word sequences that are allowed by the speech application. The system further includes an audio front end 7 and a speech decoder 8.

[0017] An input speech waveform is received by the audio front end 7 via a microphone, telephony interface, computer network interface, or other appropriate interface. The audio front end 7 digitizes the speech waveform and extracts feature vectors (also known as features or observations) from digitized speech. The audio front end 7 is essentially responsible for processing the speech waveform and transforming it into a sequence of data points that can be better modeled by the acoustic models 4 than the raw waveform. The extracted feature vectors are provided to the speech decoder 8, which references the feature vectors against the dictionary 2, the acoustic models 4, and the grammar/language model 6, to generate recognized speech data. The recognized speech data may further be provided to a natural language interpreter (not shown), which interprets the meaning of the recognized speech.

[0018] The noise reduction technique described herein may be implemented within the audio front end 7. Note that audio front ends which perform the above functions but without the noise reduction technique described herein are well known in the art. The noise reduction technique may be implemented using software, hardware, or a combination of hardware and software. For example, the noise reduction technique may be implemented by a microprocessor or Digital Signal Processor (DSP) executing sequences of software instructions. Alternatively, the technique may be implemented using only hardwired circuitry, or a combination of hardwired circuitry and executing software instructions. Such hardwired circuitry may include, for example, one or more microcontrollers, Application Specific Integrated Circuits (ASICs), Programmable Logic Devices (PLDs), Field Programmable Gate Arrays (FPGAs), or other suitable components.

[0019] The system of FIG. 1 may be implemented in one or more conventional computer systems, such as a personal computer (PC), workstation, etc. That is, the system may be implemented in a single computer system, or it may be distributed over multiple computer systems, which may be connected on a network. FIG. 2 illustrates an example of such computer system at a high-level. The computer system of FIG. 2 includes a central processing unit (CPU) 10 (e.g., a microprocessor), random access memory (RAM) 11, read-only memory (ROM) 12, and a mass storage device 13, each connected to a bus system 9. Mass storage device 13 may include any suitable device for storing large volumes of data, such as magnetic disk or tape, magneto-optical (MO) storage device, or any of various types of Digital Versatile Disk (DVD) or compact disk ROM (CD-ROM) storage, flash memory etc. The bus system 9 may include one or more buses connected to each other through various bridges, controllers and/or adapters, such as are well-known in the art. For example, the bus system 9 may include a system bus that is connected through an adapter to one or more expansion buses, such as a Peripheral Component Interconnect (PCI) bus.

[0020] Also coupled to the bus system 9 are an audio interface 14, a display device 15, input devices 16 and 17, and a communication device 30. The audio interface 14 allows the computer system to receive an input audio signal 8 that includes the speech waveform. Hence, the audio interface 14 includes circuitry and/or software instructions for receiving an input audio signal which includes the speech signal, which may be received from a microphone, a telephone line, a network interface, etc., and for transferring such signal onto the bus system 9. The display device 15 may be any suitable device for displaying alphanumeric, graphical and/or video data to a user, such as a cathode ray tube (CRT), a liquid crystal display (LCD), or the like, and associated controllers. The input devices 16 and 17 may include, for example, a conventional pointing device, a keyboard, etc. The communication device 18 may be any device suitable for enabling the computer system to communicate data with another processing system over a network via a data link 20, such as a conventional telephone modem, a wireless modem, a cable modem, an Integrated Services Digital Network (ISDN) adapter, a Digital Subscriber Line (xDSL) adapter, an Ethernet adapter, or the like.

[0021] Note that some of these components may not be present certain embodiments, and certain embodiments may include additional or substitute components that are not mentioned here. Such variations will be readily apparent to those skilled in the art. As an example of such a variation, the audio interface 14 and the communication device 18 may be implemented as a single device in certain embodiments. As another example, the peripheral components connected to the bus system 9 might further include audio speakers and associated adapter circuitry.

[0022] FIG. 3 shows an overall process that may be implemented by the audio front-end 7 in the processing of a speech waveform, including the removal of noise, according to one embodiment. The noisy speech waveform S is first divided into a series of partially overlapping portions, or “frames”. The process of FIG. 3 is then performed on each frame of data. First, at block 301, the nth frame of data is extracted from the input speech waveform S. The frame of data is then filtered at block 302 using a conventional filtering technique. Next, a spectral analysis is applied to the filtered nth frame of data. The results of the spectral analysis are a set of values that represent the frequency content of that frame of data in terms of spectral energies. Specifically, at block 303 the spectral energies E_sof the nth frame are determined. A spectral energy value E_scan be computed as the sum of the squares of the real and imaginary components of the Fourier transform of the noisy speech signal S. At block 304, the square roots of the energies E_sare computed to obtain the spectral magnitudes |S| of the noisy speech signal S. A noise magnitude estimate |N| is then determined at block 305 for each of the spectral magnitudes |S|. At block 306, the noise magnitude estimates |N| are scaled and subtracted from the corresponding spectral magnitudes |S| to produce “clean” (reduced-noise) spectral magnitudes |C|. The clean spectral magnitudes |C| are squared at block 307 to produce the clean spectral energies E_c. The clean spectral energies E_care then integrated at block 308 into several (e.g. 20) frequency bands in a filterbank. At block 309, the logarithm of the integrated spectral energies is computed, and at block 310, further processing is performed based on the logarithm of the integrated spectral energies, in a manner well known in the art, to generate a feature vector.

[0023] The noise reduction technique described herein acts upon the spectral amplitudes, but leaves the rest of the audio front end unchanged. More specifically, if the speech waveform is noisy, then the spectral energies are affected by this noise. These noisy spectral energies produce noisy features, which will tend to degrade speech recognition. The technique described herein computes an estimate of the noise that is present in the spectral amplitudes and subtracts this estimate from the noisy spectral magnitudes (blocks 305 and 306 in FIG. 3). The resulting spectral magnitudes are cleaner (less noisy) and more easily decoded by the speech recognition system.

[0024] FIG. 4 illustrates the noise removal process in greater detail, according to one embodiment. At block 401, a frequency index f_indexis initialized to the first frequency bin, f_init. At block 402, the spectral energy E_sof the noisy speech signal S is computed for the current frequency index. The spectral energy E_scan be computed as the sum of the squares of the real and imaginary components of the Fourier transform of the noisy speech signal S. At block 403, the magnitude |S| of the noisy speech signal S is computed for this frequency index as the square root of the spectral energy E_s. At block 404, the magnitude |N| of the noise for this frequency index is estimated. A process for estimating the magnitude |N| of the noise is described in greater detail below. The signal-to-noise ratio (SNR) for this frequency index is estimated as |S|/|N| for the current frequency index at block 405. At block 406, a scale factor K is computed for this frequency index. A process for computing the scale factor K is described in greater detail below. At block 407, the noise magnitude estimate |N| is multiplied by the corresponding scale factor K for this frequency index, and the resulting quantity (the scaled noise magnitude estimate) is subtracted from the spectral magnitude |S| for this frequency index, to produce a clean spectral magnitude |C|. That is, the clean spectral magnitude |C| is computed as |C|=|S|−K·|N|. The clean spectral magnitude |C| is then squared at block 408 to produce a clean spectral energy value E_cfor this frequency index. If the frequency index f_indexdoes not equal the highest frequency bin, f_max, i.e., there are additional frequency indexes f_indexto process for this frame (block 409), then the frequency index f_indexis incremented at block 410, and the process repeats from block 402. Otherwise, the process exits. Note that the total number of frequency indexes f_indexmay be quite large, e.g., on the order of 100 or more f_indexvalues may be desirable.

[0025] FIG. 5 shows in greater detail a process for determining the noise magnitude estimate |N|. The noise magnitude |N| is estimated in the foregoing process by computing a histogram of the observed spectral magnitudes |S| and then taking the value of a predetermined low percentile (e.g. the 18th percentile) of the histogram as the estimate of the noise magnitude |N|. A separate histogram is maintained for each frequency index. Thus, at block 501 the histogram for the current frequency index f_indexis updated based on the current spectral magnitude |S|. At block 502, the noise magnitude estimate |N| is determined as the mth percentile of the histogram, where m is a predetermined number. Note that the computation of the noise estimate |N| is done in the magnitude domain, as is the computation of the SNR. As a result, the spectral subtraction (blocks 306 and 407) is also performed in the magnitude domain, in contrast with certain prior art noise reduction techniques which subtract noise in the energy domain.

[0026] The scale factor K is an important feature of the present noise reduction process, which contributes to its ability to operate well on either noisy speech or clean speech. The scale factor K is a function of the absolute level of the noise spectrum, the SNR, time, and frequency. As described further below, the scale factor K is computed such that the noise magnitude estimate |N| tends to be scaled down to a greater extent for a relatively low-noise speech signal and to a lesser extent for a noisy speech signal. More specifically, the scale factor K is computed as a function of the product of a noise scale factor K_Nand a SNR scale factor K_SNR.

[0027] The noise scale factor K_Nfor a given frequency index is based on a sigmoid function of the noise magnitude estimate |N| for that frequency index. Similarly, the SNR scale factor K_SNRfor a given frequency index is based on a sigmoid function of the estimated SNR for that frequency index. For purposes of this description, a sigmoid function is a smooth, symmetrical non-linear function that asymptotically tends to −1 for very small arguments and asymptotically tends to 1 for very large arguments. In this context, therefore, the sigmoid of an arbitrary variable x may be defined according to the following formula:

[0028] sigmoid(x)=(e^x−e^−x)/(e^x+e^−x)

[0029] The noise scale factor K_Nis defined to have a range of 0 to 1.0, with K_Nincreasing as |N| increases. Thus, the noise scale factor K_Nhas a value of 0 when the noise magnitude estimate |N| (is small and a value of 1.0 when the noise magnitude estimate is large. The SNR scale factor K_SNRis also defined to have a range from 0 to 1.0, but with K_SNRdecreasing as SNR increases. Hence, the SNR scale factor K_SNRis defined to have a value of 0 when the SNR estimate is large and a value of 1.0 when the SNR estimate is small.

[0030] FIG. 6 illustrates a process for computing the overall noise scale factor K. At block 601, the noise scale factor K_Nis computed for the current frequency index based on the corresponding noise magnitude estimate |N|, as K_N=sigmoid(|N|). At block 602, K_Nis modified to have a range of 0 to 1.0 according to the equation K′_N=(1+K_N)/2. At block 603, the SNR scale factor K_SNRis computed for the current frequency index based on the corresponding SNR, as K_SNR=sigmoid(SNR). At block 604, K_SNRis modified to have a range of 0 to 1.0 according to the equation K′_SNR=(1−K_SNR)/2. At block 605, the overall scale factor K is computed as K=(1−C)·K′_N·K′_SNR+C, where C is a constant between 0 and 1.0. The specific value of C depends upon the application in which the process is used and the specific system response that is desired.

[0031] The two sigmoids (for K_Nand K_SNR) can be parameterized to adjust exactly when and how abruptly each function changes from −1 to 1. For example, rather than using a sigmoid function of the noise magnitude estimate |N| in the form of sigmoid(x), one may instead use a parameterized sigmoid function of the form sigmoid(Ax+B). The specific values of the parameters A and B also depend upon the application and the desired response. It may be desirable to select values for these parameters which work over a wide range of acoustic conditions. However, recognition may be improved by restricting application of the present technique to certain types of acoustic data and selecting the values of A and B accordingly.

[0032] When the scale factor K is computed as described above, the following results are observed in the noise reduction process. When the SNR is small and the noise level is high, the overall scale factor K approaches 1.0. The noise estimate is fully subtracted from the spectral magnitudes of the noisy speech if and only if the SNR is small and the noise level is large. This is the result of the noise reduction process when the speech waveform is highly corrupted by noise. In contrast, when the SNR is high or the noise level is low, the overall scale factor K approaches a small value (close to 0). As a result, a very small fraction of the noise estimate is subtracted from the spectral energies. This is the result of the noise reduction process when the speech waveform is relatively uncorrupted by noise. Note that if both the noise scale factor K_Nand the SNR scale factor K_SNRwere set to 1.0 for all frequencies and for all timeframes, the recognition performance would improve in noise but worsen on clean speech. Thus, the present noise reduction technique operates well on both noisy speech and clean speech, in contrast with prior art noise reduction techniques.

[0033] It should also be noted that, because the overall scale factor K is a function of frequency, more noise can be subtracted from those frequency components that have a low SNR than those that have a higher SNR. Further, the fact that the scale factor K is a function of the absolute noise level is advantageous for at least two reasons. First, at the beginning of a speech signal (e.g., the beginning of a sentence), few samples are available and an accurate calibration of the SNR may not yet be available. In the present technique, however, the noise value that is subtracted at the beginning of the speech waveform is determined primarily by the absolute noise level, not the SNR. Consequently, the technique can be implemented as a real-time method of reducing noise, which begins the noise reduction process immediately when the speech signal becomes available, thereby reducing undesirable delay between production of the speech signal and its understanding by a speech recognizer. Second, the noise scale factor K_Nand the SNR scale factor K_SNRcan be set so as to avoid distortion of the spectrum of very clean speech signals.

[0034] Thus, a method and apparatus for reducing noise in a speech signal have been described. Although the present invention has been described with reference to specific exemplary embodiments, it will be evident that various modifications and changes may be made to these embodiments without departing from the broader spirit and scope of the invention as set forth in the claims. Accordingly, the specification and drawings are to be regarded in an illustrative sense rather than a restrictive sense.