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  Method and system for filtering a signal and for providing echo cancellationUSPTO Application #: 20070036345Title: Method and system for filtering a signal and for providing echo cancellation Abstract: The present invention provides for adaptive filters that have improved computational and memory bandwidth proprieties. When applied to telecommunication applications, the present invention additionally provides for improved methods and systems of canceling echoes. In one embodiment of the adaptive filter of the present invention, a filter, preferably an adaptive finite impulse response (FIR) filter, of an appropriate length, N, is chosen. Once the filter is chosen, convergence is achieved and the filter is converted to an infinite impulse response (IIR) filter. In the course of operation, data is received from an input source and used to adapt the zeroes of the IIR filter using the least means square (LMS) approach, keeping the poles fixed. The adaptation process generates a set of converged filter coefficients that are then applied to the input signal to create a modified signal used to filter the data. The novel adaptive filter method and system presented herein can be used to improve the calculation of the echo impulse response by, among other things, reducing the computational complexity and memory requirements of the coefficient calculation conducted within the adaptive filter. (end of abstract) Agent: Patentmetrix - Irvine, CA, US Inventors: Mohammad Usman, Jon Laurent Pang, Amjad Luna, Imtinan Elahi USPTO Applicaton #: 20070036345 - Class: 379406080 (USPTO) Related Patent Categories: Telephonic Communications, Echo Cancellation Or Suppression, Using Digital Signal Processing, Adaptive Filtering The Patent Description & Claims data below is from USPTO Patent Application 20070036345. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD OF THE INVENTION [0001] The present invention relates generally to an adaptive filter with improved operational characteristics, and, more specifically, to methods and systems of achieving echo cancellation employing the improved adaptive filter. BACKGROUND OF THE INVENTION [0002] Adaptive filters are used in numerous applications to remove undesired frequencies from a signal. In an exemplary application, adaptive filters are used in telecommunication systems, more specifically in echo cancellation systems, to remove from a signal echoes that may arise as a result of the reflection and/or retransmission of modified input signals back to the originator of the input signals. Commonly, echoes occur when signals that were emitted from a loudspeaker are then received and retransmitted through a microphone (acoustic echo) or when reflections of a far end signal are generated in the course of transmission along hybrids wires (line echo). [0003] Although undesirable, echo is tolerable in a telephone system, provided that the time delay in the echo path is relatively short; however, longer echo delays can be distracting or confusing to a far end speaker. Understandably, the telecommunications industry has devoted substantial resources to developing systems that minimize echo, without adversely affecting the ability of two speakers to communicate with one another. [0004] Acoustic echo often occurs in speakerphones that employ one or more microphones together with one or more speakers to enable "hands-free" telephone communication. Line echo originates because telephone facilities usually comprise two-wire circuits within each area connecting individual subscribers with the switching office and four-wire transmission circuits between switching offices in different local exchange areas. A call between subscribers in different exchange areas is carried over a two-wire circuit in each of the areas and a four-wire circuit between the areas, with conversion of speech energy between the two and four-wire circuits being conducted by hybrid circuits. If the hybrid circuit input ports had perfectly matched impedances of the two and four-wire circuits, the signals transmitted from one exchange area to the other would not be reflected or returned to the first area as echo. Unfortunately, due to impedance differences that inherently exist between different two and four-wire circuits, and because impedances must be matched at each frequency in the voice band, it is difficult for a given hybrid circuit to perfectly match the impedances of any particular two and four-wire transmission circuit. [0005] To substantially remove echoes from a communication system, echo cancellation systems and methods employ adaptive filters to generate an estimate of the echo-generating signal (echo estimate) that is then removed from the signal being transmitted back to the originator of the echo-generating signal (far-end source). More specifically, a far-end source transmits a signal (far-end signal) that passes through a connection medium and into an input terminal of a communication unit. The far-end signal received at the input terminal is cross-coupled via a cross-coupling path (either acoustically or in line) and creates a cross-coupling echo component. That echo component combined with a new signal from the near-end (near-end signal) is transmitted back to the far-end source as a composite output signal. In this system, an echo it canceller is conventionally deployed to monitor the far-end signal and generate an estimate of the actual echoes expected to return in the form of a composite signal with the near-end signal. The echo estimates are then applied to a subtractor circuit in the transmit channel to remove or at least reduce the actual echo. [0006] To create an accurate estimate of the actual echoes, various types of adaptation methods are known in the prior art and can be employed in the echo canceller in the form of an adaptive filter. Conventionally, a finite impulse response (FIR) filter is used which carries out the convolution between the far-end signal and the estimated impulse response, N samples in length, of the echo paths. In the most basic model, an adaptive filter (filter vector) operates on a far-end signal vector to produce an estimate of the echo, which is subtracted from the combined near-end and echo signal. The overall output of the adaptive echo canceller is then used to control adjustments made to tap values of the filter vector. [0007] In the aforementioned application, and other applications requiring the use of adaptive filters, a critical design requirement is the ability of the filter system to achieve convergence in a rapid, stable manner and, in the process, use a minimal amount of computational resources. A trade-off traditionally exists between stability, accuracy, and speed of convergence of an adaptive filter. [0008] In that regard, digital filters are commonly categorized into two classes: infinite-length impulse response (IIR) filters and finite-length impulse response (FIR) filters. FIR filters have certain advantages relative to IIR filters, namely that FIR filters are stable and have a linear-phase response. Linear-phase FIR filters are widely used in digital communication systems, image processing, speech processing, spectral analysis and applications where non-linear phase distortion cannot be tolerated. Compared to IIR filters, FIR filters generally require shorter data word length but have much higher orders for the same magnitude specification and, at times, introduce large delays that make them unsuitable for certain applications. For example, when dealing with a system where echo cancellation must be performed for hundreds of channels on the same processor, the use of a conventional linear finite impulse response (FIR) filter to model a long impulse response requires substantial memory and computational resources. [0009] Notwithstanding the above, different types of adaptive filter systems have been employed in echo cancellation systems that attempt to improve the rate of convergence and stability, while still minimizing the computational resources required. U.S. Pat. No. 5,995,620 discloses a method of canceling an echo that, according to the inventors, has an improved convergence time with low complexity. The echo cancellation method includes the step of canceling the echo in a far-end signal with a Kalman filter having a time varying Kalman gain vector K(t) proportion to the vector(p.sub.1(t).times.(t-1) . . . p.sub.n(t).times.(t-n)).sup.T where p.sub.i(t) are the diagonal elements of a diagonal matrix P(t) satisfying a Riccati equation, i denotes the ith diagonal elements of P(t), t denotes discrete time, n denotes the number of filter taps, and T denotes transpose. [0010] Despite the aforementioned prior art, an adaptive filter is still needed that achieves rapid convergence without an increase in computational resources or the introduction of instability. Additionally, a method and system of echo cancellation having an improved computational speed, while still remaining stable and minimizing the computational resources required, is also needed. SUMMARY OF THE INVENTION [0011] The present invention is directed toward a novel adaptive filter and novel methods and systems for conducting echo cancellation in telecommunication systems. In one embodiment of the adaptive filter of the present invention, a filter is chosen, preferably an adaptive finite impulse response (FIR) filter of an appropriate length N. Once the filter is chosen, convergence is achieved using a convergence process. With convergence complete, the filter is converted to an infinite impulse response (IIR) filter using a generalization of the ARMA-Levinson approach. In the course of operation, data is received from an input source and used to adapt the zeroes of the IIR filter using the least-mean-square (LMS) approach, keeping the poles fixed. The adaptation process generates a set of converged filter coefficients that are then applied to the input signal to create a modified signal used to filter the data. The error between the modified signal and actual signal received is monitored and used to further adapt the zeroes of the IIR filter. [0012] In a second embodiment of the adaptive filter of the present invention, a filter is chosen, preferably an adaptive finite impulse response (FIR) filter of an appropriate length N. Once the filter is chosen, convergence is achieved using a convergence process. With convergence complete, the filter is converted to an infinite impulse response (IIR) filter using a generalization of the ARMA-Levinson approach. In the course of operation, data is received from an input source and used to adapt the zeroes of the IIR filter using the LMS approach, keeping the poles fixed. The adaptation process generates a set of converged filter coefficients that are then applied to the input signal to create a modified signal used to filter the data. The error between the modified signal and actual signal received is monitored and used to further adapt the zeroes of the IIR filter. If the measured error is greater than a pre-determined threshold, convergence is re-initiated by reverting back to the FIR convergence step. [0013] The present invention is also directed toward an exemplary use of the novel adaptive filter method and system, namely novel media gateways, echo cancellation, and channel equalization methods and systems. Used in media gateways and echo cancellation systems, adaptive filters are used to generate an echo signal component used to cancel the echo generated by the engagement of a far-end signal with a cross-coupling path. The novel adaptive filter method and system presented herein can be used to improve the calculation of the echo impulse response by, among other things, reducing the computational complexity and memory requirements of the coefficient calculation conducted within the adaptive filter. In one embodiment, the novel filter of the present invention is used to generate the echo signal component. After having achieved convergence on a FIR filter and converted the filter to an IIR filter, in accordance with the previously described methodology, the adaptive filter generates an echo estimate by obtaining individual samples of the far-end signal on a receive path, convolving the samples with the calculated coefficients, and then subtracting, at the appropriate time, the resulting echo estimate from the received signal y on the transmit channel. Ongoing adaptation of the filter occurs by the adjustment of the zeroes of the IIR filter. [0014] In a preferred embodiment of an echo cancellation application, the converged FIR filter is the truncated by taking a first set of taps, K, from the truncated FIR filter, taking the last N-K taps of the truncated FIR filter, referred to as h.sub.iir, and converting h.sub.iir, to an IIR model where K is preferably at or around 10. The truncated FIR filter, together with the IIR filter, is then used in combination to track the system response and filter data. [0015] The present invention provides for adaptive filters that have improved convergence, computational, and memory bandwidth proprieties. When applied to telecommunication applications, the present invention additionally provides for improved methods and systems of canceling echoes. BRIEF DESCRIPTION OF THE DRAWINGS [0016] These and other features and advantages of the present invention will be appreciated as they become better understood by reference to the following Detailed Description when considered in connection with the accompanying drawings, wherein: [0017] FIG. 1 is a flowchart describing the operation of one embodiment of an adaptive filter method of the present invention; [0018] FIG. 2 is a block diagram of one embodiment of the novel adaptive filter system; [0019] FIG. 3 is a flowchart describing the operation of a second embodiment of a novel adaptive filter method; [0020] FIG. 4 is a block diagram of a telecommunication system having a voice over packet gateway; Continue reading... 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