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Digital Video for Multimedia: Considerations for Capture, Use and Delivery
Section 2: Digital video: issues and choices
Moving video is made up of a series of still frames (images) played at 25 frames per second (30 frames per second in the USA). One may be excused for thinking that the capture and playback of digital video is simply a matter of capturing each frame, or image, and playing them back in a sequence at 25 frames per second. Unfortunately it is not that simple.
A single still image or frame with a window size or screen resolution of 640 x 480 pixels and 24 bit colour (16.8 million colours) occupies approximately 1MB of disc space . Therefore, roughly 25 MB of disc space are needed for every second of video, 1.5 GB for every minute. Digital video files are large! Even if the storage space is available, it is not practical to play back that amount of data per second on today's personal computers.
Therefore, file size must be reduced to within the data transfer rate of the final playback system.
The three basic problems of digital video
There are three basic problems with digital video
- size of video window
- frame rate
- quality of image
It is not difficult to deal with these three problems and, although we will now take a look at these issues one at a time, they are by no means independent of one another. All of these issues can be tackled using compression techniques.
Size of video window
Digital video stores a lot of information about each pixel in each image or frame. It takes time to display those pixels on your computer screen. If the window size is small, then the time taken to draw the pixels is less. If the window size is large, there may not be enough time to display the image or single frame before it's time to start the next one. Remember, we need to draw 25 frames per second.
There are several ways to solve this problem:
- Choose an appropriate window size. It is often not necessary to have full screen video; small window sizes may be perfectly adequate for your needs, especially as with multimedia applications, there are other objects that need to be on screen at the same time. Viewing video on computer screens is not the same as watching video on television. Users will be close to the screen; in some cases a window size of 160 x 120 for a screen set to a 640 x 480 display is adequate (Hamilton et al., 1995).
- The reduction of the window size to anything less than the size or resolution of the original video source (in the case of the European video standard, PAL, this is 576 horizontal lines) will involve some sort of compression. This is because the same information is being represented but in less space. Reducing the window size may not always, therefore, produce desirable results and will depend upon the content of the video.
- Fast hard discs are now available enabling each frame to be read from the disc faster
- Use hardware accelerated playback. The graphic display cards now being supplied with PCs, more often than not,include facilities for Windows acceleration and video playback.
Frame Rates
The issues here are similar to those above - too many pixels and not enough time. There is not enough time to move the data from hard disc or CD to screen. One way to overcome this is to compress the data so that less data is transferred from disc to screen.
Depending on the size of video window chosen, you may also be able to reduce file size by reducing the number of frames per second to, for example, 12 frames per second. At smaller window sizes eg 160 x 120 pixels, video played at reduced frame rates is, in the majority of cases, acceptable. At larger window sizes, the video can sometimes appear jerky.
Image Quality
The image quality will depend on the quality of the original source and the degree of compression used.
During compression you will probably be asked to select a quality setting. This will be represented by an arbitrary scale of, 0-100%, 1-5, etc. A lower setting will result in greater compression and smaller file sizes but the quality of the resulting video sequence will be reduced. The relationship between file sizes and quality varies considerably for different codecs and is illustrated in Section 3: Digital Video on Trial
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