Why is the 1st frame of mel spectrum set to 0?

[youngsuenXMLY]

Hello, in train.py, line 52
self.S = tf.concat((tf.zeros_like(self.mels[:, :1, :]), self.mels[:, :-1, :]), 1)

the 1st frame values of mel spectrum are set to 0, could you please explain why to do this?

[hccho2]

For causal cut, the last frame was cut off.
To keep the length, the 1st frame was padded.