HTML5: Up and Running

Video Codecs

When you talk about “watching a video,” you’re probably talking about a combination of one video stream and one audio stream. But you don’t have two different files; you just have “the video.” Maybe it’s an AVI file, or an MP4 file. As described in the preceding section, these are just container formats, like a ZIP file that contains multiple kinds of files within it. The container format defines how to store the video and audio streams in a single file.

When you “watch a video,” your video player is doing several things at once:

• Interpreting the container format to find out which video and audio tracks are available, and how they are stored within the file so that it can find the data it needs to decode next

• Decoding the video stream and displaying a series of images on the screen

• Decoding the audio stream and sending the sound to your speakers

A video codec is an algorithm by which a video stream is encoded. That is, it specifies how to do #2 above. (The word “codec” is a portmanteau, a combination of the words “coder” and “decoder.”) Your video player decodes the video stream according to the video codec, then displays a series of images, or “frames,” on the screen. Most modern video codecs use all sorts of tricks to minimize the amount of information required to display one frame after the next. For example, instead of storing each individual frame (like screenshots), they only store the differences between frames. Most videos don’t actually change all that much from ...