In's and Out's of Video Compression

Did you know that when you watch a stream of your favourite show or film on Netflix, you're experiencing digital video compression firsthand? What is video compression and why does it matter? Read on to discover the in's and out's of video compression in this guide to understanding the basics.

What Is Video Compression?

Whether you want to stream a show or Skype a conference, digital video is always going to involve lots of data. Without quality video compression, you would not be able to view YouTube videos or your favorite shows on Netflix without serious compromises to picture quality. To understand the basics of compression, imagine you want to stream a movie in 1080p resolution. Any given frame of 1080p content has to fill over 2 million pixels of information per frame. Assuming that the content you are watching is viewed at 30 frames per second, this means more than 62 million pixels per second! Each pixel requires its own data, expressed by the term bits per second. If each pixel needs 24 bits of information, this means the movie you are streaming demands well over 100 megabytes per second. If left uncompressed, this would effectively mean a single film's file size could be hundreds of gigabytes in size. This is undesirable for companies like Netflix and their consumers alike. There is simply not enough bandwidth to accommodate such large file sizes for online streaming. Even on sources like Blu-ray and UHD discs — discs that, generally speaking, offer the least amount of compression — some compression still must be used. Otherwise, a single movie could take up a consumer's entire hard drive, which is undesirable and impractical. From faster downloads to manageable hard drive storage, there are plenty of reasons why digital video compression is essential. Compression helps to reduce file sizes, reducing the clog of bandwidth and the burden on hard drives everywhere. In fact, using the example of the hypothetical film that is hundreds of gigabytes in size, compression can reduce the size of such a film to be hundreds of megabytes, rather than hundreds of gigabytes. Digital video compression is what makes this immense file size reduction possible, all while trying to preserve the image quality as much as possible.

How Video Compression Works

Video compression's primary goal is to reduce file sizes from immense to manageable, and this is generally achieved by removing repetitive or superfluous information that can be omitted without major loss to picture quality. There are many different ways to compress video, including various containers and codecs that make compression possible. To keep things simplified, know that there are essentially two forms of video compression: spatial compression and temporal compression. Spatial compression, also known as intraframe compression, is best known for the .jpeg format, and it only applies to individual frames of video. One after another, each frame is compressed until the entire video is completed. Color information is reduced in a process known as chroma subsampling, before the image is split into a set of pixels known as macroblocks before further steps are taken to reduce the file size. These macroblocks will result in what are known as "compression artifacts", visible squares that indicate the file has been heavily compressed. Temporal compression, also known as interframe compression, is best known for the .mpeg format. Unlike the frame by frame method of spatial compression, temporal compression works by removing redundancy. In effect, temporal compression looks to find and compress parts of a video that are identical and do not need to be changed. For example, imagine a video of a professor giving a lecture to a classroom with a whiteboard in the background. That whiteboard will not change throughout the video, making the whiteboard a redundant part of the video that can be compressed. To keep things overly simple for the sake of this beginner guide, temporal compression is greatly efficient, making it ideal for anyone caring about efficiency more than picture quality. The frame by frame nature of intraframe compression, however, makes it ideal for content creation such as your favorite shows on Netflix. If you any of this discussion remains unclear, all of this previous analysis can be broken down into an understanding of bitrate. If you understand bit rate, you fundamentally grasp the basics of video compression and why it matters.

Understanding Bit Rate

Compressed file size is entirely dependent on bit rate. Bit rate is a term meaning the amount of data being used every second. The slower the bit rate, the smaller the compressed file will be in size. Additionally, it will be lower resolution and will have more compression artifacts. Higher bit rates will provide better quality, albeit with larger file sizes as the trade-off. Bit rate is measured with a metric known as bits per second (bps). The more bits you have, regardless of compression method, the better the video will look. Still, there is more to compression than just bi rate. Here are some final tips to keep in mind regarding compression.

Codecs

When it comes to compression, content creators and professionals have plenty of choices. When compressing a video, the codec chosen matters greatly. Codecs are the software or hardware that interprets audio and video signals in order to compress them. A given compression scheme can be implemented in different ways and for different purposes. For example, using the aforementioned MPEG compression scheme, MPEG-2, MPEG-4 and other schemes are all differing codecs of the same MPEG compression scheme. MPEG-2 is widely used by the film industry, for example, whereas MPEG-4 was developed to handle the inefficiency and extremely low bit rates of Internet video. Understand that different codecs will achieve different results. Not all codecs will look great at the same bit rate, which is why many different codecs are needed for different purposes. If you plan on getting into compression yourself, know that H264 is generally thought to be the best current codec for creating the best quality files with the least compression artifacts at manageable file sizes. With this guide, you should now have a better understanding of what video compression is and why it is so important for managing the delicate balance between file size and picture quality.