Data processing: speech signal processing – linguistics – language – Speech signal processing – For storage or transmission
Reexamination Certificate
1999-05-27
2001-11-06
Korzuch, William (Department: 2741)
Data processing: speech signal processing, linguistics, language
Speech signal processing
For storage or transmission
C381S094200, C704S215000
Reexamination Certificate
active
06314394
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention generally relates to a method of reducing an undesired component from a signal having a desired component and an undesired component.
There are a variety of applications where separation of signal components is useful or required. In some situations, an undesired noise component must be filtered out or reduced compared to a desired sound component to achieve understandable transmissions.
One example is when an individual is speaking on a mobile telephone within an automobile. The presence of ambient noise often interferes with the ability of the person whom the individual is speaking with to hear what the individual is saying while driving. This is especially true when one attempts to use a hands-free, speaker phone within a vehicle while driving. The ability to use a speaker phone within a vehicle is desirable because it enhances safe operation so that a driver can keep both hands on the steering wheel and not be distracted from the task of driving the vehicle as much as when the driver must hold onto a cellular telephone, for example. The inability to communicate effectively, however, limits the usefulness of currently available vehicle speaker phones.
There are a variety of other applications where ambient noise is also detected by a microphone, which interferes with the projection of the desired speech component. Other applications where ease, clarity and accuracy of communication of speech is necessary include voice recognition systems, voice control systems and cabin communication systems.
With the wide variety of applications for noise reduction systems, it is not surprising that several attempts have been made to fulfill this need. One example of a noise reduction method is sometimes referred to as “spectral subtraction.” This approach typically works in the frequency domain and depends upon separating speech, which typically has non-stationary statistics, from noise, which typically has stationary statistics.
While spectral subtraction methods may be useful, they are not without shortcomings or drawbacks. For example, the noise reduction provided by such methods may include musical artifacts in the reproduced speech. Additionally, there typically is a requirement for voice activity detectors in the equipment utilized to perform the method. Moreover, the spectral subtraction methods require considerable computation for Fast Fourier Transforms and may exhibit processing delays that affect the quality of the reduced speech. The large amount of computation time not only affects the quality of the reproduced speech but also can impose relatively high costs on a noise reduction system. In many applications, such as the hands-free cellular telephone within a vehicle mentioned above, the expenses associated with providing sufficient computational capability and computer memory to accomplish a spectral subtraction method typically render it not practical for such an application.
There is a need for a more efficient, more effective and more economical signal separation method that can have wide application. This invention provides such a method and avoids the shortcomings and drawbacks described above.
SUMMARY OF THE INVENTION
In general terms, this invention is a method of reducing an undesirable component from a signal that contains a desired component such as speech. The method of this invention includes several basic steps. First, a signal having a desired component and an undesirable component is captured. A power spectral density approximation of the captured signal is then made and an error component is separated out from that approximation. The desired component is then determined from the error component of the power spectral density approximation.
In the preferred embodiment, the error component of the power spectral density approximation is filtered to separate out portions having a frequency above a preselected maximum. For example, where the desired component is speech, any sound components having a frequency that exceeds the typical high end frequency of human speech (e.g., 1500 Hz) are filtered out using a bandpass filter, for example.
A system designed according to this invention preferably includes a collector, such as a microphone, that collects signals or vibrations that include a desired component. The collector generates a signal indicative of the collected signals. An autoregressive module is in communication with the collector and receives the signal from the collector. The autoregressive module determines a power spectral density approximation, which includes an error component, of the signal from the collector. A filter module filters the error component to remove portions of the error component that have a frequency above a preselected maximum such that the filtered error component includes a reduced amount of undesired components and the desired component is more clearly discernable.
The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiments. The drawings that accompany the detailed description can be briefly described as follows.
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Brooks & Kushman P.C.
Korzuch William
Lear Corporation
Lerner Martin
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