Audio can be inaccessible to people who are deaf or hard of hearing. An audio file in an online course can add information and variety, but may pose a significant barrier for some students.
WCAG:
Priority 1.1 - "Provide a text equivalent for every non-text element (i.e., auditory and visual content.)"
Section 508:
1194.22 - "A text equivalent for every non-text element shall be provided."
If online content contains pure video (no audio), then a blind person or one with severe vision loss will not be able to use the material, unless crucial information is described.
Video with audio (the most common kind) presents a dual problem – both textual and auditory additions may be necessary. In addition, since video and audio and synchronized together, captions and audio descriptions of the video content must also be synchronized together.
WCAG:
Priority 1.4 - For any time-based multimedia presentation (e.g., a movie or animation), synchronize equivalent alternatives (e.g., captions or auditory descriptions of the visual track) with the presentation. [Priority 1] (Checkpoint 1.4)
Section 508:
1194.22 (b) - Equivalent alternatives for any multimedia presentation shall be synchronized with the presentation
Before getting into the details of making video accessible, an understanding of basic video concepts is needed. If you are familiar with compression, bandwidth, and multimedia in general, you can skip this page. However, it is advisable to read the next two pages on Transcripts and Captioning in General.
Most of the time, video starts with a video recorder and is recorded to tape. Video can also originate from other places, such as a television or cable signal or from movies made with 3D modeling software (as was used to create Toy Story and Shrek). The difference is that digital movies already exist on the computer (usually as some kind of video file on the hard drive), whereas the movie on the video recorder is still on tape.
Video capture cards solve this "location" problem. These cards fit inside most desktop computers and are specifically designed to pull the signal from a device outside the computer, such as a television or video camera, and convert the signal into a digital file.
Video capture cards are becoming easier to install and use due to the popularity of home movies. However, the video capture process still takes a considerable amount of time. The hardware needs to be set-up, the video has to be captured correctly in terms of video and audio quality (which can take several attempts), and then delivered on some kind of media. Delivery can be especially difficult at times, due to the large amount of data that needs to be stored and that is usually only deliverable on several CDs or over a very high speed network. For example, the data size of a high-quality video capture usually entails approximately 1 gigabyte of information per minute of video.
Video can be stored in several different formats. Some video may be compressed and some may not. Some video may contain more colors and some fewer colors. The six most common file formats for video are:
Generally, all these formats will contain video with audio; rarely, a format may be used for audio alone. To play or edit each file format, a certain software is usually required.
QuickTime Player
supports MOV files. 
Real Player supports
RM files. 
Windows Media
Player supports ASF and WMV files. You may also hear the term "streaming" video. Back in the mid-90's, when the Internet and video were young, it was necessary to download the entire video file before viewing. Someone then developed the idea of sending only small sections of video that the user was currently viewing. This way, once a video "chunk" was received by the user, it could be immediately viewed; effectively eliminating the long wait before the video could be played.
Cartoons are comprised of individual drawings that, when played in succession at a fast enough rate, give the appearance of movement. The same thing is true of video. Frames per second (fps) affects the size and quality of your video file; the less number of frames that make up a video segment, the smaller the size of the file. So to save file size, why not take a 30 frames per second (fps) video down to 3 fps? Because this will result in a video that is extremely choppy. To put this in time concepts, consider a watch with a second hand; a 1 frames per second video means you will see the video updated once every time the hand moves. For 3 fps, try subdividing the second into thirds by counting to 3 for each second.
What exactly is a desirable number of frames per second (fps) for a video? Anything below 10 fps will be very choppy to the end user. You should have 15 fps or greater, but do not exceed 30 fps because the benefits are usually negligible. A general rule of thumb is that if you half the number of frames per second, you half the file size (depending on the compression); however, this is at the cost of video quality.
The resolution of a video means its size in terms of width and height, which is measured in pixels. A pixel is the smallest light-emitting speck on your screen; this can be seen if you lean in close to your monitor. The smaller the resolution of the video, the smaller the file size, which is beneficial to people with low-bandwidth connections. However, too small of a video size can be frustrating to individuals with vision problems and becomes blocky if stretched.
Another consideration when working with video is the color depth, which means the number of bits that represent each color per pixel on the screen. The higher the number of bits per pixel, the more colors that can be displayed at any one time. One bit per pixel gives 2 colors - monochrome. Two bits per pixel gives 4 colors. Usually, you will have a minimum of 8 bit color (256 colors), but more commonly you will use 16 bits (65,536 colors) or 24 bits (16,777,216 colors). Sometimes you may hear of 32 bit color, where 8 bits are used for the transparency or visibility (sometimes called alpha) of the color. However, more than 24 bits is not really necessary since the human eye cannot see more than 16 million colors (24 bits).
To get a better understanding of a the file size for a video, calculate the figures associated with the resolution, color depth, and frames per second (fps) of a video. For this example, the video has 640 by 480 resolution, 16 bit color depth (or 2 bytes), and is set to play at 30 frames per second.
Bandwidth is a description of how fast information is sent. When computers first came out, they had 300 baud modems, which were slow enough to see each character appear on the screen (a little bit faster than someone typing). Today, in research and large businesses, there are network speeds sending a gigabyte of information per second. The availability of ISDN , cable modems and DSL allows users to have a high bandwidth connection in the home; however, most users still can only connect at a slower speed using a 28.8 bits per second (bps) modem. Video file sizes can require as much as 17 to 18 megabytes (or around 1 billion bits) per second. For video to work over the Internet, its file sizes must be smaller; this technique is called compression.
A piece of software called a Codec, which stands for COder/DECoder, compresses the size of the video or audio files by applying some kind of heuristic. Most of these heuristics degrade the quality of the video or audio to varying degrees. The more compressed, the smaller the file size, but the video or audio is not as clear. Alternately, if less compressed, it usually looks and sounds better, but then the file size is larger. How do you know which is right? The solution depends on the kind of connection your end user has.
A common compression technique that you should understand is frame difference. The basis of this technique is an algorithm in which all the frames of a video are very similar. An example of this is a weather forecast. When a meteorologist stands in front of a map, most of the video remains the same from one frame to another because the map does not change very often. In video, you only have to worry about updating things that change, such as where the speaker is going, and where they came from. However, over time and because of some other compression techniques, the picture can deteriorate; to minimize this, it is important to adjust the screen with a complete redrawing, known as a key frame, as opposed to just performing updates.
The next page will discuss the starting point of accessible media, the transcript.
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