How video compression works

August 06, 2007 -- dspdesignline.com

Digital video compression and decompression algorithms (codecs) are at the heart of many modern video products, from DVD players to multimedia jukeboxes to video-capable cell phones. Understanding the operation of video compression algorithms is essential for developers of the systems, processors, and tools that target video applications. In this article, we explain the operation and characteristics of video codecs and the demands codecs make on processors. We also explain how codecs differ from one another and the significance of these differences.

Starting with stills
Because video clips are made up of sequences of individual images, or "frames," video compression algorithms share many concepts and techniques with still-image compression algorithms. Therefore, we begin our exploration of video compression by discussing the inner workings of transform-based still image compression algorithms such as JPEG, which are illustrated in Figure 1.

 

The image compression techniques used in JPEG and in most video compression algorithms are "lossy." That is, the original uncompressed image can't be perfectly reconstructed from the compressed data, so some information from the original image is lost. The goal of using lossy compression is to minimize the number of bits that are consumed by the image while making sure that the differences between the original (uncompressed) image and the reconstructed image are not perceptible—or at least not objectionable—to the human eye.

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