I am watching a tutorial on using mel spectrograms to classify the audio's genre via CNN. My question is why apply local min-max normalization to each individual mel spectrogram? What I mean by local is that the min and max value is calculated from the individual mel spectrogram and then min-max normalization is applied; thus, you have to get min and max for each mel spectrogram and then apply the min-max normalization based on its own min and max.
Why apply this local min-max normalization and not take into consideration of the whole training sets min and max first, then apply the normalization. Also, why not do Standardization (Z-score Normalization)?