Choosing A Suitable Digital Video Format
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Digital video can have a dramatic impact upon the user. It can reflect information that is difficult to describe in words alone, and can be used within an interactive learning process. This document contains guidelines to best practice when manipulating video. When considering the recording of digital video, the digitiser should be aware of the influence of file format, bit-depth, bit-rate and frame size upon the quality of the resulting video.
Digital video consists of a series of images played in rapid succession to create the illusion of movement. It is commonly accompanied by an audio track. Unlike graphics and sound that are relatively small in size, video data can be hundreds of megabytes, or even gigabytes, in size.
The visual and audio information are individually stored within a digital 'wrapper' an umbrella structure consisting of the video and audio data, as well as information to playback and resynchronise the data.
Digital video remains a complex area that combines the problems of audio and graphic data. When choosing to encode video the designer must consider several issues:
The distribution method will have a significant influence upon the file format, encoding type and compression used in the project.
Removeable media - Video distributed on CD-ROM or DVD are suited to progressive encoding methods that do not conduct extensive error checking. Although file size is not as critical in comparison to Internet streaming, it continues to have some influence.
The compression type is dependent upon the need of the user and the type of removeable media:
| NAME | PURPOSE OF MEDIA | Compression | ||
| Streaming | Progressive | Media | ||
| Advanced Streaming Format (ASF) | Y | Temporal | ||
| Audio Video Interleave (AVI) | Y | Temporal | ||
| MPEG-1 | Y | VideoCD | Temporal | |
| MPEG-2 | Y | DVD | Temporal | |
| QuickTime (QT) | Y | Y | Temporal | |
| QuickTime Pro | Y | Y | Temporal | |
| RealMedia (RM) | Y | Y | Temporal | |
| Windows Media Video (WMV) | Y | Y | Temporal | |
| DivX | Y | Amateur CD distribution | Temporal | |
| MJPEG | Y | Spatial | ||
Table 1: A comparison list of the different file formats, highlighting their intended purpose and compression method.
The provision of video data for an Internet-based audience places specific restrictions upon the content. Quality of the video output is dependent upon three factors:
| Screen Size | Pixels per frame | Bit depth (bits) | Frames per second | Bandwidth required (megabits) |
| 640 x 480 | 307,200 | 24 | 30 | 221.184 |
| 320 x 240 | 76,800 | 16 | 25 | 30.72 |
| 320 x 240 | 76,800 | 8 | 15 | 9.216 |
| 160 x 120 | 19,200 | 8 | 10 | 1.536 |
| 160 x 120 | 19,200 | 8 | 5 | 0.768 |
Table 2: Indication of the influence of screen size, bit-depth and frames per second has upon required bandwidth
When creating video, the designer must balance the video quality with the facilities available to the end user. As an example, an 8-bit screen of 160 x 120 pixels, and 10-15 frames per second is used for the majority of content found on the Internet.
Video presents numerous problems for the designer caused by the complexity of formats and structure. Problems may include:
Temporal Compression - Reduces the amount of data stored over a sequence of frames. Rather than describing every pixel in each frame, temporal compression stores a key frame, followed by descriptive information on changes.
Spatial Compression - Condenses each frame independently by mapping similar pixels within a frame. For example, two shades of red will be merged. This results in a reduction in image quality, but enables the file to be edited in its original form.
Progressive Encoding - Refers to any format where the user is required to download the entire video before they are allowed to watch it.
Internet Streaming - Enables the viewer to watch sections of video without downloading the entire thing, allowing users to evaluate video content after just a few seconds. Quality is significantly lower than progressive formats due to compression being used.
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Last Modified 1-October-2004
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