Digital Video

Digital video is audio/visual in a binary format. Information is presented as a sequence of digital data, rather than in a continuous signal as analog information is.

Digital video comprises a series of orthogonal bitmap digital images displayed in rapid succession at a constant rate. In the context of video these images are called frames. We measure the rate at which frames are displayed in frames per second (FPS).

Since every frame is an orthogonal bitmap digital image it comprises a raster of pixels. If it has a width of W pixels and a height of H pixels we say that the frame size is WxH.

Pixels have only one property, their color. The color of a pixel is represented by a fixed number of bits. The more bits the more subtle variations of colors can be reproduced. This is called the color depth (CD) of the video.

An example video can have a duration (T) of 1 hour (3600sec), a frame size of 640x480 (WxH) at a color depth of 24bits and a frame rate of 25fps. This example video has the following properties:

pixels per frame = 640 * 480 = 307,200
bits per frame = 307,200 * 24 = 7,372,800 = 7.37Mbits
bit rate (BR) = 7.37 * 25 = 184.25Mbits/sec
video size (VS) = 184Mbits/sec * 3600sec = 662,400Mbits = 82,800Mbytes = 82.8Gbytes

Types of Video Formats

Windows Media Video (.wmv)

Developed and controlled by Microsoft. WMV is a generic name of Microsoft's video encoding solutions and doesn't necessarily define the technology what it uses -- since version 7 (WMV7) Microsoft has used its own flavour of MPEG-4 video encoding technology. The only difference between ASF files and WMV or WMA files are the file extensions and the MIME types. The basic internal structure of the files is identical. The change in extensions was made to make it easier for an application to identify the content of a media file. A .wma file extension designates a file containing only audio. A .wmv extension designates a file containing both video and audio.

Advanced Streaming Format (.asf)

D.asf Advanced Streaming Format Developed by Microsoft as a multimedia successor to AVI and other individual media file formats, ASF files can contain up to 7 media types including streaming video. ASF is optimized for network distribution and desktop playback. Currently, Microsoft is shifting to the new video file format WMV (Windows Media Video). eveloped by Microsoft as a multimedia successor to AVI and other individual media file formats, ASF files can contain up to 7 media types including streaming video. ASF is optimized for network distribution and desktop playback. Currently, Microsoft is shifting to the new video file format WMV (Windows Media Video). 

QuickTime movie (.mov)

An industry standard developed as part of an entire multimedia architecture by Apple, the QuickTime movie is an open multimedia format which has multi-platform, multi-browser support. Currently (Apr 2003), QuickTime 6 is the newest version and it incorporates streaming and virtual reality capabilities.

Audio-Video Interleave (Video for Windows) (.avi)

The precursor to ASF, AVI files are limited to audio and video only and are not streamable.

RealMedia (.rm)

RealMedia is an industry leader in streaming video technology and is utilized by many Internet real-time video broadcasting sites. Currently, Real One player 6.0 is available.

Various Programs (.mpg)
MPEG-4 is the latest standard for video compression and file format developed by an industry group.