Complexity

Volume 2018, Article ID 9039240, 10 pages

https://doi.org/10.1155/2018/9039240

## Effective Approach to Calculate Analysis Window in Infinite Discrete Gabor Transform

^{1}College of Information and Network Engineering, Anhui Science and Technology University, Bengbu 233030, China^{2}School of Mathematics and Physics, Anhui Jianzhu University, Hefei 230601, China

Correspondence should be addressed to Jia-Bao Liu; moc.361@daoabaijuil

Received 3 December 2017; Revised 28 March 2018; Accepted 10 April 2018; Published 15 May 2018

Academic Editor: Danilo Comminiello

Copyright © 2018 Rui Li et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

The long-periodic/infinite discrete Gabor transform (DGT) is more effective than the periodic/finite one in many applications. In this paper, a fast and effective approach is presented to efficiently compute the Gabor analysis window for arbitrary given synthesis window in DGT of long-periodic/infinite sequences, in which the new orthogonality constraint between analysis window and synthesis window in DGT for long-periodic/infinite sequences is derived and proved to be equivalent to the completeness condition of the long-periodic/infinite DGT. By using the property of delta function, the original orthogonality can be expressed as a certain number of linear equation sets in both the critical sampling case and the oversampling case, which can be fast and efficiently calculated by fast discrete Fourier transform (FFT). The computational complexity of the proposed approach is analyzed and compared with that of the existing canonical algorithms. The numerical results indicate that the proposed approach is efficient and fast for computing Gabor analysis window in both the critical sampling case and the oversampling case in comparison to existing algorithms.

#### 1. Introduction

The discrete Gabor transform (DGT) [1], extended from the short-time Fourier transform (STFT), is an important time-frequency analysis tool for processing and analyzing the nonstationary signal [2–4], which multiplied a signal by a time-shifted and frequency-modulated window function with the aim of representing analyzed signals in a time-frequency localized manner. As a general case of STFT, DGT, sampled version of the STFT, is an efficient tool for analyzing the energy distribution of analyzed signals in the time-frequency domain/plane and more valuable than Fourier transform (FT) in many applications which require to extract temporal frequency information and localize simultaneously. According to the type of time-frequency atom, the DGT can be classified into two categories: the complex-valued discrete Gabor transform (CDGT) and the real-valued Discrete Gabor transform (RDGT) [5–8]. The former method for computing the DGT and its inverse transform all involves complex operators while the latter one can utilize discrete Hartley’s cas transform (DHT) kernel, discrete cosine transform (DCT) kernel, or discrete sine transform (DST) kernel as the Gabor basis function which only involves real operators. More specifically, in some real-time applications, the CDGT is more difficult to be implemented in hardware or software and the RDGT for sampled speech and image has an advantage of the computationally efficient implementation.

Over the past decades, the DGT has become a very valuable and widely used mathematic tool in diverse applications [12–19] such as audio processing, speech processing, image processing, sonar and radar signal processing, seismic signal processing, and transient signal processing. The analyzed signal, analysis window, and synthesis window in the periodic DGT usually have the same periodic length, which may confront difficulties in applying to some real-time applications of long-periodic signal sequences. To bridge these gaps, the long-periodic DGT [9, 10, 19] of complex-valued kernel and real-valued kernel has been presented to utilize the short window to analyze and process the long-periodic (or infinite) signal sequences in practical applications. Because existing canonical algorithms [11, 20–27] are mainly used in periodic/finite DGT and derived from Gabor frame theory, in this paper, a fast and effective algorithm, based on the orthogonal analysis approach and FFT algorithm, is proposed to obtain the analysis window for the long-periodic/infinite DGT in both the critical sampling case and the oversampling case.

The rest of the paper is organized as follows. In Section 2, the long-periodic/infinite DGT will be reviewed. In Section 3, a fast approach, which uses the new orthogonal constraint relationship of the analysis window and the synthesis window to deduce a series of independent linear equation sets, is introduced to calculate analysis window in the long-periodic/infinite DGT under both the critical sampling case and the oversampling case. In Section 4, the detailed comparison of computational complexity of related algorithms and numerical experiments have been given to show that the proposed method can overcome the existing algorithms in computing analysis window of the long-periodic/infinite DGT. Finally, the paper is concluded with Section 5.

#### 2. DGT for Long-Period and Infinite Sequences

Let represent a periodic and discrete-time signal with length and the length of analysis window and synthesis window . In order to use and to analyze and process the signal , the three periodic sequences , , and of length should be constructed as follows:where The DGT for long-periodic/infinite sequences [4, 19] is defined by the following:and the Gabor coefficients can be calculated bywhere is the imaginary unit. The DGT for long-periodic/infinite sequences can be obtained by (5) corresponding to its expansion in (4). In (4) and (5), , let the positive integers , are the sampling points in time and frequency, and let and denote a modulation step in frequency and a translation interval in time. For a numerically stable reconstruction, the constrained condition by (or ) has to be satisfied. By choosing proper parameters and , the Gabor oversampling rate is a positive integer. The biorthogonal relationship [4, 19] of and , which are derived from the completeness condition between (4) and (5), can be rewritten aswhere denotes the discrete form of delta function, , and .

#### 3. FFT-Based Method for Solving the Analysis Sequences

To derive a new type biorthogonality relationship from the completeness between (4) and (5), substituting (5) into (4) yieldsrecallingand putting (8) into (7) leads toAccording to the completeness of the transform, the following constraint relationship has to be satisfied:soEquation (11) can be expressed as a equivalent form in the following:where and . Let , , and ; then (12) can be written asObviously, (13) can be rewritten in matrix form as follows:where is an -long unit vector with first element being one and others being zeros; is a -long vector composed byand is a matrix constructed bywhere , , and . Equation (14) can be split into unrelated linear equation sets:Due to the fact that most of elements in and were zeros according to (2) and (3), let , be the positive integer constrained by : (15) and (16) are rewritten as follows:where, , and . Equation (17) can then be rewritten in the following form:where is a unit vector of length . Obviously, the following formula can be easily obtained by (20):where .

##### 3.1. Fast Approach for Computing Analysis Window in the Critical Sampling Case

In the critical sampling case, the oversampling rate ; (21) can be rewritten in following form:Because is a right circulant matrix by (or ), so (22) can be written as a circular convolution in following form:where is the first column of as the following form: By utilizing the circular convolution theorem [28], the FFT algorithm can then be used to transform (25) into component-wise multiplication by recalling (26) as follows: where is the -point DFT. Thus, can be fast computed byThe computation of (29) is obviously much faster than that using the standard Gaussian elimination method to solve the linear equation set.

*Remark 1. *More similar result to (29), the author in [11] provides a method, based on Gabor frame theory [20] and discrete Zak transform [29], to obtain dual Gabor window in periodic/finite DGT under the critical sampling. However, in present paper, the proposed method uses the orthogonal analysis approach and FFT algorithm to solve the analysis window for long-periodic/infinite signal sequences.

##### 3.2. Fast Approach for Computing Analysis Window in the Oversampling Case

In the oversampling case, the oversampling rate can be a positive integer by setting and properly. Because the rank of is less than the number of rows, (21) is an underdetermined linear equation which could have no solution or have an infinite number of solutions. Equation (21) could be rewritten in the following form in order to use convolution theory and FFT algorithm as that in the critical sampling case.where and is a left circulant matrix composed bywhere and is a vector with length constructed byThen we rewrite (30) aswhereLetThen, (33) also can be rewritten as a circular convolution:where is the first column of as follows:By using the circular convolution theorem and the FFT operator, (36) can be expressed in the following form:so can be solved byDue to the fact that should be close to in the sense of the least square error (LSE) as shown in [30], has to satisfy the least of norm which make a Gaussian-type function of the localization property:Equation (41) can be converted into the following optimum problem and easily solved by the Lagrangian approach:where , is a vector with length , is a Lagrangian parameter, andTaking a derivative of with respect to leads tothen the solution of can be obtained from (44):and putting this solution into constraint condition in (42) to solve leads towhere .

*Remark 2. *In sparse applications, the synthesis window without considering localization property () can be used to reconstruct the original signal, in which a certain well-localized window function will be directly utilized as an analysis window.

*Remark 3. *The proposed algorithm can be easily generalized and applied to other DGT for both the periodic/finite sequences and the long-periodic/infinite sequences including complex-valued (DFT-based) and real-valued (DHT-based, DCT-based) and multiwindow discrete Gabor transforms (M-DGT) because those Gabor basis functions in DGT can lead to the similar form of the biorthogonal relationship equation.

#### 4. Computational Complexity Analysis and Numerical Experiments

When calculating the multiplication times of related algorithms, we assume that the -point FFT requires an order of multiplications. Due to the fact that the original linear equation of biorthogonal relationship can be split into unrelated subequation sets in the critical sampling case and subequation sets in the oversampling case, FFT method can be utilized to compute analysis window both in the critical sampling case and in the oversampling case. The computational complexity of the single subequation set which carries -point FFT and -point inverse FFT (IFFT). The computational complexity of is at order of in the oversampling case while it is zero in the critical sampling case. Therefore, as mentioned above, the computational complexity of proposed approach is equal to that of total subequation set, which is at the order ofin the critical sampling case andin the oversampling case. The comparison of computational complexity and numerical experiments between proposed approach and existing methods have been given in Tables 1 and 2 which clearly shows that the total number of multiplications of proposed approach is less than that of others in both the critical sampling case and the oversampling case. In Tables 1 and 2, the symbols , CS, and OS denote , the critical sampling case, and the oversampling case, respectively. In Table 2, a detailed numeric comparison experiments on the number of multiplications related to computational time between the proposed algorithm and existing methods under different Gabor sampling scheme are given by using the formula of computational complexity of each algorithm in Table 1, which clearly demonstrates the efficiency and the advantage of decreasing the computation of the proposed approach as compared to existing algorithms. Example for computing analysis window: a Gaussian synthesis window with length is given (49) and shown in Figure 1.The analysis windows in Figure 2 are computed by the proposed method, [9, 10] in the critical sampling case, with , and the oversampling rate . The analysis windows in Figures 3 and 4 are computed by the proposed method [9, 10] in the oversampling case, with the oversampling rate (, ) and 8 (, ), respectively. The similarity between the synthesis window and its corresponding analysis window is proportional to the oversampling rate , the reason being similar to that given in [30]. The proposed algorithm for the reconstruction of the original signal is also simulated, which is virtually error-free reconstruction (MSE = ).