Combining Auditory Inspirations and Hierarchical Feature Extraction for Robust Speech Recognition

We present speech features inspired by the processing in the auditory periphery and the receptive fields found in the auditory cortex. They have a hierarchical organization and jointly evaluate variations in the spectrotemporal domain. This is why we termed them Hierarchical Spectro-Temporal (HIST) features. For their calculation we apply a Gammatone filterbank to transform the signal into the spectral domain. In a preprocessing based on local competition mechanisms we enhance the formants in the spectrogram. A set of filters learned via ICA (Independent Component Analysis) captures local variations in the spectrogram and constitutes the first layer of the hierarchy. In the second layer these local variations are combined to form larger receptive fields learned via Non Negative Sparse Coding. The dimensionality of the resulting features is reduced via the application of a Principal Component Analysis (PCA) and then fed into a Hidden Markov Model (HMM). We evaluated the performance of these features in a continuous digit recognition task in a variety of different noise conditions, similar to the Aurora task. Our results show, especially in combination with RASTA features, a significant performance improvement in noise. Introduction Already for a long time the process of human speech perception serves as a role model in the development of machine recognition (e. g. Rasta-Plp [1]). Here, we present features which take their inspiration not from psychoacoustic but neurophysiological data. Shamma showed that the primary auditory cortex of young ferrets has a spectro-temporal organization, i. e. the receptive fields are selective to modulations in the time-frequency domain and, as in the visual cortex, have Gabor-like shapes [2]. However, traditionally speech features rely only on spectral representations. Such spectro-temporal features were already used for speech recognition [3, 4, 5], speech detection [6, 7], and source separation [8]. Justified by the found analogies between the visual and auditory cortex in mammals, we developed speech features in strong similarity to the visual object recognition system described in [9]. Its main features are the hierarchical organization in three layers and the unsupervised learning of the receptive fields on the first and second layer. We termed the speech features we derived thereof as Hierarchical Spectro-Temporal (Hist) features and used them a front-end to Hidden Markov Models (HMMs) [10, 11]. In this paper we report further improvements of these features and tests on a continuous digit recognition task. Figure 1: Overview of the feature extraction process. In the following section, the computation and enhancement of the spectrograms are described. The calculation of the Hist features from the spectrograms is explained in the section after that (see Fig. 1 for an overview of the process). Finally, the performance of the Hist features is evaluated, especially in respect to Rasta-Plp features, in the before last section. Preprocessing The spectrograms of the speech signals were computed using a Gammatone filter-bank. We used an Infinite Impulse Response (IIR) implementation of the Gammatone filter-bank [? ] having 128 channels ranging from 80Hz to 8 kHz at a sampling rate of 16 kHz. The spectrograms are obtained by rectification and low-pass filtering of the filter-bank response. The sampling rate of the spectrograms was then reduced to 400Hz. Formant enhancement The remaining preprocessing steps enhance the formants in the spectrogram. Via a preemphasis of +6 dB/oct. the influence of the speech excitation signal was compensated for. Next, we used a set of Mexican Hat filters along the frequency axis to remove the harmonic structure of the spectrograms and form peaks at the formant locations. The size of the filter kernels was chosen constant on a linear frequency axis. Due to the logarithmic arrangement of the center frequencies in the Gammatone filterbank in the implementation the size of the kernels varied accordingly. Additionally, the shapes of the filters were adapted to the nonlinear frequency spacing, i. e. the lower part of the filter is wider than the higher part. A second Mexican Hat filter with smaller kernel sizes for lower frequencies thinned the resulting formant tracks. Figure 2 shows the original spectrogram and the result of the formant enhancement of the digit ”one” spoken by a male Time [s] F re q u en cy [ k H z] 0 0.1 0.2 0.3 0.4 0.1 0.4 1 2 4 8 (a) Time [s] F re q u en cy [ k H z] 0 0.1 0.2 0.3 0.4 0.1 0.4 1 2 4 8