The channel-wise performance of four settings: (a) without perturbation, (b) resizing, (c) adding Gaussian noise, and (d) JPEG compression. The channel 0 is DC (Direct Current) and from the first channel to the 26th channel are corresponding to AC1 to AC26 (Alternating Current). The blue line is the energy percentage of each channel and the red, magenta and green lines are the F1-score of the training, validation and testing dataset. We observe that high-frequency channels without perturbation in 2(a) has a higher performance. After applying resizing, adding Gaussian noise and compression, the performance of high-frequency channels degrades as shown in 2(b), 2(c), and 2(d). The test score and validation score are closely related, indicating that the validation score can be used to select the discriminant channels.