the padding for dilated convolution seems to be wrong

[zcyang]

For encoder, the dilated convolution for encoder pads (filter_width - 1) * dilation/2, this means after reshaping there are (filter_width-1) zeros at the beginning. But conv1d uses SAME padding, which again, will pad (filter_width-1) number of zeros, which duplicates the zeros needed.

Assume filter_width=3, dilation=2, the input is
1 2 3 4 5
Ater padding in dilated convolution function, the input becomes
0 0 1 2 3 4 5 0 0
After the reshape,

0 1 3 5 0 
0 2 4 6 0

becomes the input to conv1d, which will again, pad with filter_width-1 zeros with the SAME padding scheme

0 0 1 3 4 0 0  
0 0 2 4 6 0 0

[paarthneekhara]

For filter_width=3, dilation=1 => dilation/2 = 0
=> (filter_width - 1) * dilation/2 = 0 . So the input will not be padded.

[zcyang]

sorry its a typo, dilation = 2

[zcyang]

I just tested
For decoder, if I change the original padding to
padding = [[0, 0], [(filter_width - 1) * dilation/2, (filter_width -1) * dilation/2], [0, 0]]
the result looks normal
But if I change it to
padding = [[0, 0], [0, 0], [0, 0]], the loss is close to 0.

So for the encoder:
padding = [[0, 0], [0, 0], [0, 0]]
and decoder:
padding = [[0, 0], [(filter_width - 1) * dilation/2, (filter_width -1) * dilation/2], [0, 0]]

Another way to correct it is to keep the padding but change the pad in conv1d from SAME to VALID.

[paarthneekhara]

The padding setting for the decoder is correct.

padding = [[0, 0], [(filter_width - 1) * dilation, 0], [0, 0]]

This is done to preserve the causality. Refer to the bytenet decoder diagram for further clarity. Check causal_conv function in :
https://github.com/ibab/tensorflow-wavenet/blob/master/wavenet/ops.py

For the encoder, The current code is fine as well. If you are using an odd filter width (1,3,5..) it shouldn't be a problem. I think having a look at the diagram again might help. The first output of the encoder depends only on the first and the third element, which will be the case if :

0 1 3 5 0 
0 2 4 6 0

is the padded input for conv1d.

Let me know if you still think I may be wrong.

[zcyang]

You are ignoring the padding from '''conv1d''' itself, the input is
''"
0 1 3 5 0
0 2 4 6 0
'''
becomes the input to conv1d, which will again, pad with filter_width-1 zeros with the SAME padding scheme

'''
0 0 1 3 4 0 0
0 0 2 4 6 0 0
''"

[paarthneekhara]

Oh, I see. I just went through the documentation of tf.nn.conv1d. It seems that the input may be paded differently for different input lengths, which is not desirable. Changing it to VALID seems appropriate. I would appreciate if you can make the required change and test machine translation to check if it improves.
Do you think this change might also be required for the decoder? That implementsation is the same as wavenet though.

[paarthneekhara]

@zcyang Updated the ops. The model works correctly now. It was indeed an error in encoder padding. Check Model/ops for the new implementation.