Hi, dear all,

Recently, I ran experiment with CTC loss, which is based on theano-ctc 
(Theano binding of Baidu's warp-ctc library). I use the default SGD 
optimizer. My data timestep is about 3500, the truth length is not longer 
than 639. All the input sequences are zero padded to the timestep, while 
the label are padded with -1. Based on my understanding, the theano-ctc 
will not compute loss for negative value, while the keras masking will 
filter out zero-padding input. My batch size is 32, inupt dimension is 30, 
output dimension is 63.

In the first epoch, everything is fine. However when it goes into second 
epoch, I observe underflow (loss is very very very small displaying as 0). 
Baidu claims that they have optimize the stability by using log value for 
loss. However, I still have this problem. Does anyone know how to solve 
this problem?

Thank you very much!

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