6. The method of claim 1 , wherein the first neural network is a pyramid Bidirectional Long Short Term Memory (BLSTM) RNN.
6. The method of claim 1 , wherein the first neural network is a pyramid Bidirectional Long Short Term Memory (BLSTM) RNN. 7. The method of claim 6 , wherein processing the input acoustic sequence using a first neural network to convert the input acoustic sequence into an alternative representation for the input acoustic sequence comprises: processing the input acoustic sequence through a bottom BLSTM layer to generate a BLSTM layer output; and processing the BLSTM layer output through each a plurality of pyramid BLSTM layers, wherein consecutive outputs of each pyramid BLSTM layer are concatenated before being provided to the next pyramid BLSTM layer.