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Digital Video

Digital Video. Lecture 07 Razia Nisar Noorani. Basic Digital Video Concepts. Movie length Frame size Frame rate Quality Color bit depth Data rate (bit rate). Movie length. File size is proportional to the movie length. Videos longer than 1 or 2 minutes cause long download times.

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Digital Video

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  1. Digital Video Lecture 07 RaziaNisarNoorani

  2. Basic Digital Video Concepts • Movie length • Frame size • Frame rate • Quality • Color bit depth • Data rate (bit rate)

  3. Movie length • File size is proportional to the movie length. • Videos longer than 1 or 2 minutes cause long download times. • If it is a long video, consider to use streaming video.

  4. Frame size • “Full-screen” video is 640x480 pixels. • The most common frame size for web video is 160x120 pixels. • Not recommend to use a frame size larger than 320x240. • The size depends on the CPU power and the Internet connection bandwidth.

  5. Image and Video?

  6. Frame rate • Frame rate is measured in number of frames per second (fps). • Standard TV-quality video uses 30 fps. • For the web, 15 or even 10 fps is more appropriate and produces fair smooth quality for the user. • Commercial Internet Broadcasts are using 0.5, 0.25 frames per second.

  7. Quality • Many video-editors allow you to set the overall quality of the video. • The degree of compression controls the target quality. • The low or medium setting results a fairly high compression which is appropriate for web delivery. • Frame rate and quality are usually tradeoff in different applications.

  8. Color bit depth • The number of pixel colors in each frame affects the size of the video. • The file size of the video will be greatly reduced by changing the number of colors from 24-bit to 8-bit. • It sacrifices the image quality of the video.

  9. Data rate (bit rate) • This is the rate that the data must be transferred in order to ensure the video can play smoothly without interruption. • It is measured in kilobytes per second (K/sec or Kbps). • It can be calculated by dividing the size of the file (in K) by the movie length (in seconds). • E.g. the video file size is 1.9MB  1900K • Play 40 seconds long, Data rate = 47.5K/sec • Consider the Internet bandwidth!

  10. Calculate space requirements of Video • NTSC video (640 x 480 and 29.97 fps) • Frame size = ([Pixel width x pixel height x bit depth]/8)/1024 • E.g. 200KB/Frame : 6.0 MB/sec • 200KB x 30 fps = 6000KB/s, 6 MB/sec • PAL video (768 x 576 and 25 fps) • E.g. 200KB/Frame : 5.0 MB/sec • 200KB x 25 fps = 5000KB/s, 5 MB/sec

  11. Video CODECs • CODEC is “Compression/Decompression” algorithms. • The sound and frame images of a digital video must be compressed. • The vast amount of data • Compressed in a number of ways • Lossless and Lossy compression • Spatial and Temporal compression

  12. Video Clip Demo • reference

  13. Lossless and Lossy compression • Lossless compression means no information is lost and the final file is the same as the original. • Most compression methods are lossy. • Sacrifices some data from the file in order to achieve higher compression rates. • Use complicated algorithm to toss out some data that is not discernible to the human eye or ear.

  14. Spatial and Temporal compression • Spatial (Intraframe) compression takes place on each individual frame of the video. • Temporal (Interframe) compression applies on a series of frames, it takes advantages of areas of the image remain unchanged from frame to frame. • Relies on the key frames and delta frames. • A key frame is placed once every second. • E.g. 15 fps, a key frame once 15 frames.

  15. Delta frame

  16. Video File Formats • QuickTime Movie (.mov) • Introduced by Apple Computer in 1991. • First developed for Macintosh, now also supports the PCs. • Also supports streaming. • How to create? • Most video editor, QuickTime Pro. • How to play? • QuickTime plug-in or QuickTime player.

  17. http://www.apple.com/quicktime/gallery/cubicvr/times_square.htmlhttp://www.apple.com/quicktime/gallery/cubicvr/times_square.html Quicktime

  18. RealMedia (.rm) • Industry standard streaming format. • RealPlayer for playback. • RealServer for serving streams. • RealProducer for creating .rm files. • Good for • Long-playing video or broadcast to many people. • How to create? • RealSystem Producer • How to play? • RealPlayer (Free), RealPlayer Plus (Commerical)

  19. Windows Media (.wmv/ .asf) • Created by Microsoft, closely integrated with Windows OS. • Support Windows Media Video (.wmv) and Advanced Streaming Format (.asf) and other formats (.avi, .mpeg, …) • Also support streaming. • How to create? • Windows Media Encoder, Windows Media Author • How to play? • Media Player in Windows OS

  20. AVI (.avi) • Stands for Audio/Video Interleaved. • Introduced by Microsoft in 1992. • In a AVI file, the audio and video information are interleaved every frame. • Good for • Short web clips, high-quality video • How to create? • Most video editing tools. • How to play? • Windows Media, QuickTime, etc.

  21. Video Clip Demo • reference

  22. MPEG (.mpg/ .mpeg) • Created by Moving Picture Experts Group (MPEG). • Supports 1) Video, 2) Audio, 3) Streaming. • Extremely high compression rates with small quality degradation (lossy). • MPEG-1 : VHS quality • MPEG-2 : HQ standard for TV broadcast • MPEG-4 : Very HQ for AV compression • MPEG can be compressed by using three schemes: Layer-I, Layer-II, Layer-III.

  23. Official MPEG page • reference

  24. MPEG Comparison

  25. Flash & Shockwave • Advantages: • File sizes are small • Image quality is high • It uses streaming technology • It uses high-quality streaming audio • It is scriptable • Disadvantages: • A plug-in player is required • Expensive authoring software • Problems on printing their content

  26. Summary • Discuss the basic digital audio/video terminology. • Introduction to different formats: WAV, MP3, QuickTime, RealMedia, Windows Media, AVI, MPEG. • To deliver long-playing audio/video or live broadcasts, you should choose one of the streaming media. • Flash and shockwave are popular and appropriate format for the Web.

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