I'm confused about this aspect of RNNs while trying to learn how seq2seq encoder-decoder works at https://machinelearningmastery.com/configure-encoder-decoder-model-neural-machine-translation/.

It seems to me that the number of LSTMs in the encoder would have to be the same as number of words in the text (if word embeddings are being used) or characters in the text (if char embeddings are being used). For char embeddings, each embedding would correspond to 1 LSTM in 1 direction and 1 encoder hidden state.

  1. Is this understanding correct? E.g. If we have another model that uses encoder-decoder for a different application (say text-to-speech synthesis described here https://ai.googleblog.com/2017/12/tacotron-2-generating-human-like-speech.html) tha uses 256 LSTMs in each direction of the bidirectional-encoder, does it mean the input to this encoder is limited to 256 characters of text?

  2. Can the decoder output has to be same length as the encoder input or can it be different? If different what factor describes what the decoder output length should be?


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