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A new unsupervised single-channel source separation method is presented. The proposed method does not require training knowledge and the separation system is based on nonuniform time-frequency (TF) analysis and feature extraction. Unlike conventional researches that concentrate on the use of spectrogram or its variants, we develop our separation algorithms using an alternative TF representation based on the gammatone filterbank. In particular, we show that the monaural mixed audio signal is considerably more separable in this nonuniform TF domain. We also provide the analysis of signal separability to verify this finding. In addition, we derive two new algorithms that extend the recently published Itakura-Saito nonnegative matrix factorization to the case of convolutive model for the nonstationary source signals. These formulations are based on the Quasi-EM framework and the multiplicative gradient descent (MGD) rule, respectively. Experimental tests have been conducted which show that the proposed method is efficient in extracting the sources' spectral-temporal features that are characterized by large dynamic range of energy, and thus leading to significant improvement in source separation performance.