DIGITAL Video Video Creation n Video captures the

DIGITAL Video

Video Creation n Video captures the real world therefore video cannot be created in the same sense that images can be created n video must be captured n n Animation can create a virtual world but this is not video n and we’ve already talked about animation n

Capturing Technique n When light reflected from an object passes through a video camera lens the light is converted into an electronic signal by a special sensor called a charge couple device (CCD). The output of the CCD is processed by the camera into a signal containing through the channels of colors (RGB). There are several video standards for managing CCD output, each dealing with the amount of separation between the component of the signal. n Standard video camera outputs two main elements: luminance, chrominance. In production and post-production recording, the lunimance and chrominance is kept separate (component video) resulting in higher quality. For broadcast and distribution the luminance and chrominance is combined into one signal (composite video). Luminance is the brightness of a pixel-point. Chrominance is colour information. In colour TVs, the chrominance signals are interlaced with the luminance signals. Chrominance is made up of two parts, hue and saturation. Hue describes the actual colour displayed, and saturation is the intensity of the colour.

Camera Operation Beam Splitter G B Camera Tubes Luminance Encoder Zoom Lens Color Filters R Chrominance n n n Camera has 1, 2, or 3 tubes for sampling More tubes (CCD’s) and better lens produce better pictures Video composed of luminance and chrominance signals Composite video combines luminance and chrominance Component video sends signals separately

Scanning Video n The TV receiver is tasked with reconstructing the representation of the scene created by the TV camera, by converting the pixel-point impulses generated by the TV camera back into light. The back of a TV screen is covered with phosphor lines. An electron beam charges the phosphor lines causing them to glow. The TV receiver picks up signals transmitted on a particular frequency which are reproduced on a TV display in a left-to-right, top-to-bottom, scanning sequence. The problem is, however, that phosphor has a "memory span" that is too short: by the time the beam has charged the last line on the screen, the first line's charge will have faded, resulting in flicker. So, to reduce flicker when the video signal is eventually displayed on a TV screen, the TV camera captures each image in alternate odd and even lines. Each sequence of odd (or even) lines is called a field and two fields comprise a frame. This process is called interleaving. The process is continually repeated creating a constant sequence of changing field and frame information.

Scanning Video n n Video is obtained via raster scanning, which transforms a 3 -D signal (function of x, y, and t) into a one-dimensional signal which can be transmitted. Progressive scanning: left-to-right and top-to-bottom -> sampling n Samples in time: frames n Samples along y: lines n Samples along x: pixels (only for digital video) We perceive the images as continuous, not discrete: human Progressive scanning visual system performs the interpolation ! How many frames, lines, and pixels ?

Interlacing n n If the frame rate is too slow - > flickering and jagged movements Tradeoff between spatial and temporal resolution n Slow moving objects with high spatial resolution n Fast moving objects with high frame rate Interlaced scanning: scan all even lines, then scan all odd lines. A frame is divided into 2 fields (sampled at different time) Interlaced scanning (odd) Interlaced scanning (even)

Video Standards n PAL: Phase Alteration Line n n n Used in UK, Europe, Australia and South Africa Frame rate = 25 fps Single video frame is made of 625 horizontal scan lines Interlaced (50 Hz) Resolution: NTSC: National Television Standard Committee used by TV and video industries in US and Japan n n Frame rate = 29. 97 fps Single video frame is made of 525 horizontal scan lines Interlaced (59. 94 Hz) Resolution:

Video Standards n SECAM - Sequential Color and Memory Used in France, Russia and a few other countries n Frame rate - 25 fps n Single frame is 625 horizontal scan lines n Interlaced (50 Hz) n Very different technology than PAL n Resolution: n

Video Standards n HDTV - High Definition Television n Aspect ratio is 16: 9 (1280 X 720 -- SMPTE S 17. 392) (1920 X 1080 -- SMPTE 274 M) n Computer industry pushing 1280 X 720 because of stability and possibility of higher frame rates Can be interlaced or progressive n Frame rates supported (1280 X 720): 23. 976, 24, 29. 97, 30, 59. 94, 60 n Audio sampling rate: 48 k. Hz n Uses MPEG-2 compression standard n

Video Compression n A full-screen 10 -second video clip: n at 30 frames/sec * 10 = 300 frames n at 640 x 480 =. 3072 MB pixels per frame n at “true” color = 3 bytes per pixel n 300 *. 3072 * 3 = 276. 48 MB n Turing has no real quota but 276. 48 MB takes up a lot of space Cannot transfer 276 MB in 10 seconds n n 32 X CD -ROM rate ~ 48 MB in 10 sec. Hard disk rate ~ 330 MB in 10 sec Therefore must compress n n There is video compression hardware Most often, software is used

Video Compression Formats n Motion-JPEG n n n Remember: Joint Photographic Experts Group Applies JPEG compression algorithm to each frame of video Compression rate: 2 -1 to 12 -1 n n lossy: up to 5: 1 is considered broadcast quality Excellent when there are rapid scene changes in the video Easy to edit MPEG n n n Moving Picture Experts Group Family of compression algorithms High compression rate Algorithm stores only changes from one frame to another rather than each entire frame Video info then encoded using DCT (discrete cosine transform--same as JPEG--so what’s difference between motion-JPEG and MPEG? ) Lossy compression but loss of data is generally imperceptible to human eye