Optimizing 2D Input Representations and Sub-phase Fusion Strategies for Differential Diagnosis of Asthma and COPD Using CNN- and GRU-Based Networks
This study addresses the challenge of inconsistent time-frequency representation dimensions in lung sound signals caused by variable respiratory cycle lengths. To resolve this, the authors propose an adaptive-length windowing strategy that standardizes the spatiotemporal dimensions of both MFCCs and log-Mel spectrograms. Building upon this unified representation, they employ CNNs to extract sub-phase features and systematically evaluate fusion approaches, including direct concatenation, GRU, and GRU with attention mechanisms. Experimental results demonstrate that the MFCC-based model achieves the best F1 scores of 0.877 and 0.855 at the respiratory-cycle and subject levels, respectively, significantly outperforming models based on log-Mel spectrograms and VAR. The findings also underscore the critical importance of real-world data, while revealing that more complex fusion strategies and data augmentation techniques such as mixup do not yield further performance gains.