Digital Video
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Digital Video |
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There is one major over-riding concern in digital video:
COMPRESSION
To play one second of uncompressed 8-bit color, 640 X 480 resolution, digital video requires approximately 9 MB of storage. One minute would require about 0.5 GB. A CD-ROM can only hold about 600MB and a single-speed player can only transfer 150KB per second. Data storage and transfer problems increase proportionally with 16-bit and 24-bit color playback. Without compression digital video would not be possible with current storage technology.
The storage required for a video clip can be expressed in a simple relation:
Video Source Data Reduction ==>> Video Compression ==>> Video Storage
The amount of required storage is determined by how much and what type of video data is in the uncompressed signal and how much the data can be compressed. In other words, the orginal video source and the desired playback parameters dramatically affect the final storage needs.
Video or motion video arrives originally through some type of camera, which records what it sees as a sequence of images (measured in frames per second [fps]).
Frame rate (for capture and playback) is
24 fps for movies
30 fps for TV
TV generally uses interleaving, so each frame is actually two (fields), each with half of the lines. Computers could play this back at 60 fps where both fields are combined, for flicker-free rendering (non-interlaced).
The quality of the source data depends on the camera's optics and resolution, which is determined by the number of CCD (charge-coupled device), elements, usually 250- 400K for consumer products.
If the camera is connected to a VTR, then the quality of the videotape recording and playback process limits the quality the capturing system can achieve. Consumer grade recorders used should at least be SVHS, or Hi-8, to give adequate quality of the computer representation. Best would be digital tape, such as D2, or high quality analog tape, like Betacam SP. Alternatively, a laserdisc or broadcast source can be used, but attention should be given to ensure the highest quality.
Broadcast TV, NTSC, generally has about 15 bits/pixel of color depth, and 525 lines of resolution with 4:3 aspect ratio. Scanning practices leave a smaller safe region.
Terran Interactive has a summary of the various video codecs. They also have web-capable comparisons of the various codecs. Also, take a look at their informative tips for Making Movies.
Download the RealPlayer
from Real Networks and install it in your Web browser plugins.
The streaming audio/video technology offered by Real Networks is one
of the best available. There is a freeware and commercial version
available.
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© CS Dept Va Tech, 1998. |
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