At its heart, an autoencoder is a type of artificial neural network used for unsupervised learning. Its primary goal sounds deceptively simple: to learn how to copy its input to its output. That is, if you feed it some data x, it tries to generate x^ such that x^ is as close to x as possible. While this might seem trivial, the utility of an autoencoder comes from the constraints placed on the network's architecture, specifically a "bottleneck" that forces it to learn a compressed representation of the input.
The basic structure of an autoencoder is typically symmetrical and consists of three main parts, which we'll explore in more detail in subsequent sections:
This structure forces the autoencoder to learn the most salient features of the data that are necessary for reconstruction. Imagine trying to summarize a long document into a few key sentences (the encoding process) and then asking someone else to recreate the original document based only on your summary (the decoding process). To do a good job, your summary must capture the most important information.
Below is a diagram illustrating this fundamental architecture:
An autoencoder processes input data through an encoder to a compressed bottleneck layer, then reconstructs the data using a decoder. The goal is for the reconstructed data to closely match the original input.
Let's briefly describe what happens in each part:
The entire autoencoder is a feed-forward neural network, trained by minimizing a reconstruction loss function. This loss function measures the difference between the original input x and the reconstructed output x^. For example, if your inputs are numerical vectors, the Mean Squared Error (MSE) is a common choice. If your inputs are binary (like black and white images), binary cross-entropy might be more appropriate. We'll discuss loss functions in more detail later in this chapter.
The elegance of this basic structure lies in its ability to learn useful representations without explicit labels, making it a powerful tool for unsupervised feature learning. By training the network to reproduce its input through a constrained bottleneck, we encourage it to discover underlying patterns and structures within the data. These learned patterns, captured in the bottleneck layer, can then be extracted and used as features for various downstream tasks, such as classification, clustering, or anomaly detection. This basic architecture forms the foundation upon which more advanced autoencoder variants, which we'll also cover, are built.
Was this section helpful?
© 2025 ApX Machine Learning