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An Introduction to the Recurrent Neural Networks



Introduction

ANN (Artificial Neural Networks) is composed of multiple nodes which initiate biological neurons of the human brain. The neuron is connected to each other by links and they interact with each other using that link. The node takes the input data through the input layer and performs the operation on the data in the hidden layer. Result of these operations is passed to the other neurons. After computation, the result is passed to the output layer. The output layer delivers the result to the outer world. Modern AI problems are handled by ANN. If the hidden layer is more than two in any neural network than it is known as a deep neural network. It uses a cascade of multiple layers of non-linear processing units for feature extraction. The output of the current layer is fetched to the next layer as input. Deep Neural network consists of:

  • Recurrent Neural Network (RNN)
  • Long Short-Term Memory (LSTM)
  • Convolutional Neural Network (CNN)

Nowadays these three networks are used in almost every field but here we are only focusing on Recurrent Neural Network.

What is RNN?

RNN is a branch of neural network which is mainly used for processing sequential data like time series or Natural Language processing.

The ANN where the connection between nodes does not form a cycle is known as a fully feed-forward neural network. In this, the information flows in only one direction i.e. from i/p layer to hidden layer then from there to o/p layer. Hence, this type of processing creates some problem. Issues with the ANN:

  1. Fixed-size input.
  2. The whole input available simultaneously.

To overcome this problem a special type of feed-forward neural network is introduced which is known as RNN. Since RNN allows variable size input and sequential information, therefore, it can be used for time-series data. This special feature makes it better than all existing other networks.

Recurrent neural networks are similar to Turing Machine. It is invented in the 1980s.

Equation of RNN

ht = fw(ht-1,)    
where ht = new state,
ht-1= previous state,
fw = activation function, 
xt = input vector

Figure 1: Vanilla Architecture

The above structure gives the basic idea behinds the RNN functionality. This structure is very famous and it is known as Vanilla Architecture. This design serves as a base for all other architecture.  All the other RNN architectures all developed based on this idea. In RNN, we generally use the tanh activation function for the non-linearity in the hidden layer. In this architecture, the number of features is fixed. In the above design, x represents the input, RNN represents the hidden layer and y represents the output.

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Types of RNN Architecture

1. One-to-One: It is the most common and traditional architecture of RNN. It is also known as Vanilla Network. This architecture provides a 1 output for 1 input. It is generally used for the purpose of machine learning problems. 

Figure 2: One-to-one

x=1, y=1

2.One-to-Many:  It has 1 input and multiple outputs. It used in scenarios where need multiple outputs for a single input. It can be used in various fields like in the music industry where we produce music from a single input note.

Figure 3: One-to-Many

x=1, y>1

3. Many-to-One: This model is just the opposite of One-to-Many model. On One-to-Many we have a single input and multiple outputs but here we have multiple inputs and a single output for all of them. It can be used for sentimental analysis where we provide a complete sentence (multiple words as multiple inputs) to the model and it produces the sentiment of the sentence (single output). 

Figure 4: Many-to-One

x>1, y=1

4. Many-to-Many: It has multiple inputs and multiple outputs. This architecture is further divided into two subcategories:

x=y 

In this case, the number of inputs to the model is equal to the number of produced outputs. This is used for Named-entity Recognition.

                           Figure 5: Many-to-Many(input=output)

x!=y 

In this case, the number of inputs to the model is not equal to the number of produced outputs. This architecture is famous and is used at a variety of operation, the most common is Machine Translation.

Figure 6:Many-to-Many(input!=output)

Two main obstacles of RNN

training the model, RNN uses a backpropagation algorithm, which is also known Backpropagation-through-time (BPTT) and this algorithm is applied for every timestamp. But using backpropagation also creates some problems which are:

1. Vanishing gradient:

When the value of the gradient is too small and the model stops learning or takes too much time because of that, it is known as the vanishing gradient. Let's see it in detail.

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The main target behind using backpropagation is to calculate the error or loss. We calculate error by squaring the difference of actual output and estimated output.

e=(Estimated output – Actual output)2

After calculating error, we will calculate gradient which is the rate of change in error with respect to the rate of change in weight if this gradient is very very less than 1 then we can say gradient is vanishing. AS time goes this causes loss in the information.                                 

2. Exploring gradient:

When an algorithm assigns high importance to weight without reason then it causes the gradient to be increasing in each iteration at a very high rate which eventually tends to infinity i.e. gradient is very-very large than 1 and may crash the model.

Problems due to these obstacles

  • Difficulty in remembering value due to increasing memory.
  • Training is very difficult
  • Training time is also increased.
  • Performance of the model is decreased
  • Accuracy is not good

Various research proposed many methodologies, some of them are:

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  • Gradient Clipping
  • Input Reversal
  • Identity Initialization
  • Long Short-Term Memory
  • Truncated BTT
  • GRUs,
  • ReLU Activation function, etc.

Here we are not discussing these techniques; we will discuss it in some other blog.

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Applications of RNN

  • Speech Recognition,

  • Language Translation,

  • Video Analysis,

  • Text Mining,

  • Sentimental Analysis,

  • Time Series Prediction,

  • Machine Translation, etc. 

Conclusion

So, guys, this is all about Recurrent Neural Network in a nutshell. In this blog, we understood about Artificial Neural Networks and Deep Learning. Then we saw how Recurrent Neural Network (RNN) is introduced, what it is about. After that, we discussed the architecture of RNN and its types and their applications in detailed. At last, we went through the problems in RNN and their solutions; also, the applications of RNN. Now in the next blog, we will learn about Long Short-Term Memory (LSTM).

Please leave query and comments in the comments section.   


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    A dynamic, highly professional, and a global online training course provider committed to propelling the next generation of technology learners with a whole new way of training experience.


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