# Autoencoder in Machine Learning

An **autoencoder is a specific type of neural network which is designed to be used in unsupervised learning, and specifically in deep learning**. There are two major uses of it which include dimensionality reduction and extraction of features. The autoencoder initially produces a condensed representation of the input data known **as encoding** before the input data is decoded to reveal the original input.

The **two most important components** of an autoencoder's architecture are the encoders and the decoders. The encoder converts the input data into the bottleneck layer, which is also represented as a two-dimensional space and known as **a latent space.**

The decoder then retrieves the original input data from this condensed version. The basic components of an autoencoder are the input layer, the hidden layer which is also known as the bottleneck layer, and the output layer. The **encoder’s main function **is to translate input data into the hidden layers, whereas for the decoder's it is to reverse the mapping such that the hidden layer is translated back into the outer layer. The network is forced to learn a compressed form since the hidden layer typically has fewer neurons than the input and output layers. The autoencoder strives to reduce the gap between the input data and the reconstructed output during training. This is accomplished by measuring the reconstruction error using a loss function, such as mean squared error (MSE). To reduce this reconstruction error, backpropagation, and gradient descent are used to modify the network's parameters (weights and biases).

Data denoising, anomaly detection, dimensionality reduction, and other tasks may all be performed with autoencoders.

## Tutorial on Autoencoders: Its Development

PCA is not favored over autoencoders because:

-With numerous layers and a non-linear activation function, an autoencoder may learn non-linear transformations.

- It is not required to master the complex layers. So in order to determine which is better for video, image, and series data, it can use convolutional layers.

- With an autoencoder, learning several layers is more effective than learning a single, massive change with PCA.

- The output of an autoencoder is a representation of each layer.

- It may employ transfer learning to improve the encoder/decoder by using pre-trained layers from another model.

## Hyperparameters and Properties

Autoencoder characteristics include:

Autoencoders can only compress data that is identical to the data they have been based on.

Lossy: When compared to the original inputs, the decompressed outputs will be inferior.

Automatically picked up from examples: It is simple to train specialized versions of the algorithm that excel with a particular kind of input.

## Autoencoder types

The many autoencoder types are as follows:

1. The most basic sort of autoencoder is the standard version, which consists of an encoder and a decoder. The input data's widths are reduced by the encoder, which the decoder subsequently utilizes to reclaim the original input.

2. Generative models called variational autoencoders (VAEs) are trained on latent spaces with specified structures. With the help of probabilistic encoders and decoders, they may produce new data points by sampling from the latent space. VAEs have applications in activities like picture production and are useful for producing fresh samples that are comparable to the training data.

3. A sparse representation of the input data must be learned using a sparse autoencoder. This indicates that the buried layer's neurons are only sometimes stimulated. For feature selection or identifying significant patterns in the data, sparse autoencoders might be helpful.

4. The denoising autoencoder is taught to recreate the original input from a noisy or distorted version of the data. It aids in the learning of noise-resistant, robust representations.

5. Contractive Autoencoder: By including a penalty term in the loss function, which incentivizes the model to be less sensitive to minute changes in the input data, contractive autoencoders are created to learn robust representations. It makes learning more reliable.

6. The adversarial autoencoder (AAE) combines adversarial networks that are generative (GANs) and autoencoders. A distinction network, a decoder, and an encoder make them up. While the discriminator network attempts to discriminate between samples from the real data distribution and encoded representations, the encoder and decoder try to recreate the input data.

## Uses of Autoencoder

The uses are:

-Autoencoders may be used for dimensionality reduction to learn a reduced representation of high-dimensional data. Dimensionality reduction is accomplished by teaching an autoencoder to encode the input data into a lower-dimensional latent space. Tasks like data visualization, feature extraction, and lowering the computational complexity of following algorithms can all benefit from this.

-Autoencoders can be trained on typical, non-anomalous data and then used to recreate fresh data samples for anomaly detection. The reconstruction error typically increases when abnormal data is supplied, suggesting a departure from the previously learned patterns. As a result, autoencoders may be used to identify abnormalities or outliers in a variety of fields, such as fraud detection, network intrusion detection, and industrial quality control.

-Feature Learning: From the input data, autoencoders may automatically learn useful representations or features. The hidden layers of the network can identify significant patterns and higher-level representations of the data by training an autoencoder on unlabeled input. The performance of these taught features is frequently better than utilizing the raw input features in a variety of supervised learning tasks like classification or regression.

## Architecture of Autoencoder

A bottleneck layer connects the encoder and decoder components to build the architecture of a machine-learning autoencoder. The encoder converts the input data to a lower-dimensional form, while the decoder restores the original input from this compressed representation. The architecture is split down as follows:

**1. Encoder:** The encoder part of the autoencoder is in charge of translating the incoming data into a less complex form. Usually, it is made up of one or more hidden layers that gradually lower the number of dimensions in the data. To introduce non-linearity, each hidden layer often uses activation functions like sigmoid, ReLU, or tanh. The number of neurons in the last hidden layer, is often known as the bottleneck layer which determines the size of the compressed representation.

**2. Bottleneck Layer:** The bottleneck layer, also known as the latent space, is a layer of neurons with smaller dimensions than the input and output layers.

It acts as the input data's compressed representation. The amount of compression or dimensionality reduction that the autoencoder achieves depends on the size of the bottleneck layer.

**3. Decoder:** The decoder component reconstructs the original input data from the bottleneck layer's compressed form. The decoder has one or more hidden layers, much the encoder. To rebuild the original input shape, the decoder's hidden layers gradually increase the dimensionality. The final output layer often uses an activation function that matches the range and distribution of the input data, such as a gradient for binary data or a linear activation function for continuous data.

## Summary

Encoder machine learning can be used to achieve compression of information, reduction of dimensionality, and feature of learning in unsupervised learning. This kind of neural network architecture which is designed to teach an encoder how to map input data to a lower-dimensional latent space and a decoder to retrieve the original input data from the representation of latent space in order to learn efficient representations of input data.

Autoencoders are used in many areas, including anomaly detection, processing natural languages, and visual analysis. When it comes to jobs like image denoising, picture creation, and recommendation systems, they excel.

Autoencoders' capacity to derive meaningful representations from unlabeled data without the need for explicit labels or annotations is one of its main advantages. Autoencoders may efficiently generalize to new, unknown scenarios with the assistance of this unsupervised learning approach by capturing the underlying structure and patterns revealed in the data.

Additionally, by stacking individual autoencoders, deep autoencoder architectures like variational autoencoders (VAEs) and denoising autoencoders (DAEs) may be made. By enabling autoencoders to generate new data samples, handle missing or damaged data, and perform probabilistic modeling, these improvements increase their functionality.

Even though they offer many advantages, autoencoders have certain drawbacks. They are prone to overfitting, particularly when the latent space's dimension or the training dataset are both too high. Furthermore, autoencoder reconstruction quality might not be ideal, particularly for complicated and high-dimensional data.

A flexible machine learning approach that excels in feature extraction, data compression, and unsupervised learning is autoencoders. They continue to be a hub for deep learning research and development, with many uses.