Digital Video

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.