Abstract
This paper presents an acoustic noise cancelling technique using an inverse kepstrum system as an innovations-based whitening application for an adaptive finite impulse response (FIR) filter in beamforming structure. The inverse kepstrum method uses an innovations-whitened form from one acoustic path transfer function between a reference microphone sensor and a noise source so that the rear-end reference signal will then be a whitened sequence to a cascaded adaptive FIR filter in the beamforming structure. By using an inverse kepstrum filter as a whitening filter with the use of a delay filter, the cascaded adaptive FIR filter estimates only the numerator of the polynomial part from the ratio of overall combined transfer functions. The test results have shown that the adaptive FIR filter is more effective in beamforming structure than an adaptive noise cancelling (ANC) structure in terms of signal distortion in the desired signal and noise reduction in noise with nonminimum phase components. In addition, the inverse kepstrum method shows almost the same convergence level in estimate of noise statistics with the use of a smaller amount of adaptive FIR filter weights than the kepstrum method, hence it could provide better computational simplicity in processing. Furthermore, the rear-end inverse kepstrum method in beamforming structure has shown less signal distortion in the desired signal than the front-end kepstrum method and the front-end inverse kepstrum method in beamforming structure.
Highlights
During the last five decades, noise cancelling and signal enhancing techniques have been developed.The techniques are fundamentally based on spectral subtraction, cepstrum and complex cepstrum methods using single-microphone sensor, and adaptive noise cancelling (ANC) and beamforming methods using multiple-microphone array sensors.Since research on echo cancellation using adaptive filters and two-microphone sensors started in1965, the adaptive filtering technique has been used as a solution tool for signal enhancing and noise cancelling schemes [1]
The inverse kepstrum method uses the whitening application from one acoustic path transfer function, H 2 ( z) from the reference microphone during the noise alone period as shown in Figure 6(a), where the inverse kepstrum is to be analyzed as the front-end application to the adaptive finite impulse response (FIR) filter from the ANC structure, and the rear-end application to the sum-and-subtract function from the beamforming structure
For real-time processing in a realistic reverberant environment, the kepstrum method and inverse kepstrum method have been applied to ANC and beamforming structures, its performance on modified application [4,6,7,8] to desired signal and adaptive filter has been investigated in detail in this paper
Summary
During the last five decades, noise cancelling and signal enhancing techniques have been developed. The kepstrum method has been used as a system identification technique for unknown systems from the acoustic path transfer functions between microphone sensors input and the noise source in beamforming structure [14,15], with a phase recovering technique from the causal kepstrum domain, where the front-end kepstrum method has produced an improved SNR performance in a different input SNR for real-time processing in reverberant room environments. By considering: (1) signal distortion in a desired signal on the instant application of noise statistics in reverberant environment, (2) noise reduction on noise characteristics with nonminimum phase components and its consistency on its inverted acoustic path transfer function, and (3) the use of small amount of adaptive FIR filter weights for the real-time processing, for the fast convergence in estimate on noise statistics, it is analyzed in both ANC and beamforming structures. The rear-end inverse kepstrum method is compared with the front-end kepstrum method [14], which uses identification of two paths with the ratio of overall acoustic path transfer function, and with front-end inverse kepstrum method using a whitening application [22]
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