DESIGN OF NORMALIZED SUBBAND ADAPTIVE FILTER FOR ACOUSTIC ECHO CANCELLATION Pavya.S, Graduate Student Member IEEE, [email protected], K.S.Rangasamy college of technology, Tiruchengode,tamilnadu,India P.Babu,Associate professor(ECE), [email protected], K.S. Rangasamy College of Technology, Tiruchengode, Tamilnadu, India ABSTRACT: In hands-free telephones and teleconferencing systems, acoustic echo cancellers are required, which are often implemented by adaptive filters. In these applications, the speech input signal of the adaptive filter is highly correlated and the impulse response of the echo path is very long. These characteristics will slow down the convergence rate of the adaptive filter if the well-known normalized leastmean- square (NLMS) algorithm is used. The normalized subband adaptive filter (NSAF) offers a good solution to this problem because of its decorrelating property. This paper proposes a VHDL implementation of normalized subband adaptive filter structure(NSAF) adaptive algorithm.the experiment result shows that the power consumed by subband is lower than that of full band. Index Terms — Acoustic echo canceller, adaptive combination, normalized subband adaptive filter (NSAF), hands-free telephone, teleconferencing system Cascade intergrator-comb filter (CIC). I.INTRODUCTION A common problem encountered in hands-free telephones and teleconferencing systems is the presence of echoes which are generated acoustically by the coupling between the loudspeaker and the microphone via the impulse response of a room [1]. Removal of these echoes requires the precise knowledge of the impulse response of the acoustic echo path, which may be time varying. In recent years, there has been a great interest in the use of adaptive filters as acoustic echo cancellers to remove echoes [2], [3]. An adaptive filter can be characterized by its structure and adaptive filtering algorithm [4]. The transversal filter with the well-known normalized least-mean-square (NLMS) algorithm is one of the most popular adaptive filters because of its simplicity and robust performance [5], [6]. In acoustic echo cancellation (AEC) applications, however, the speech input signal of the adaptive filter is highly correlated and the impulse response of the acoustic echo path is very long. These two characteristics will slow down the convergence rate of the acoustic echo canceller if the NLMS-based adaptive filter is used to remove echoes. One technique to solve the above problem is subband adaptive filtering. In conventional subband adaptive filters (SAFs), each subband uses an individual adaptive subfilter in its own adaptation loop, which decreases the convergence rate of SAFs because of the aliasing and band-edge effects [7]. To solve these structural problems, a family of SAFs based on a new structure has been proposed [8]–[10]. In [8], Courville and Duhamel derived an SAF using a weighted criterion. Based on the polyphase decomposition, Pradhan and Reddy [9] derived another SAF using a similar weighted criterion. In [10], Lee and Gan presented a normalized SAF (NSAF) from the principle of minimum disturbance, with its complexity close to that of the NLMS-based adaptive filter. The NSAF can be viewed as a subband generalization of the NLMSbased adaptive filter. Its central idea is to use the subband signals, normalized by their respective subband input variances, to update the tap weights of a fullband adaptive filter. This strategy leads to the decorrelating property of the NSAF [11]. Although these SAFs can obtain fast convergence rate when applied to AEC applications, they all require a tradeoff between fast convergence rate and small steady-state mean-square error (MSE) because of the use of a fixed stepsize. Several studies have shown that the set- membership, variable regularization parameter, and variable step-size parameter versions of the NSAF can offer possible schemes to overcome the conflicting requirements of fast convergence rate and small steady- state MSE [12]–[14]. However, they all need to estimate the system noise power in advance, which increases the complexity of the system. Recently, an adaptive combination of fullband adaptive filters has been proposed in [15], and its mean square error has been analyzed in [16]. The merit of this combination is that it can obtain both fast convergence rate and small steady-state MSE without estimate of the system noise power. More recently, a combination of subband adaptive filters for AEC has been proposed [17], which is based on a conventional subband structure. In this paper, we propose a new Pavya S et al ,Int.J.Computer Technology & Applications,Vol 3 (3), 1232-1236 IJCTA | MAY-JUNE 2012 Available [email protected]1232 ISSN:2229-6093
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DESIGN OF NORMALIZED SUBBAND ADAPTIVE FILTER FOR ACOUSTIC ECHO
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