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Video Fundamentals September 9, 1999 Lawrence A. Rowe University of California, Berkeley URL: http://www.BMRC.Berkeley.EDU/~larry Copyright @1999, L.A. Rowe Outline • History • Broadcast System Elements • Scanning and Interlace • Color and Gamma • Chroma Subsampling • Representations Multimedia Systems and Applications 2 History • 1839: Daguerreotype Cameras • 1893: Telephone Audio Broadcasting (Puskas) • 1895: Wireless Communication (Marconi, Popov) • 1895: Film Presentation (Lumiere Brothers) • 1919: Radio Broadcasting (Holland, Canada) • 1934: US establishes FCC • 1935: TV Broadcasting (Germany, Britain) • 1941: US B&W TV ADD: telephone invention, founding of ATT, tv inventions, first radio networks/stations Multimedia Systems and Applications 3 History (cont.) • 1951: Videotape Recorder (Bing Crosby Enterprises) • 1953: US Color TV (NTSC) • 1963: Geostationary Satellites • mid ‘70s: Fiber Optic Transmission - cable • 1985: FCC establishes ATSC - standard by 1993? • 1989: Analog HDTV Broadcasting (Japan) • 1996: ATSC Standard Adopted ADD: first ntsc tv broadcasts, 1st cable system, consumer vcr, tivoli/replay device, direct tv, first hdtv broadcasts Multimedia Systems and Applications 4 Standards Groups • ITU-T – ITU Telecommunications Formerly CCITT • ITU-R – ITU Radiocommunications Formerly CCIR • FCC • SMPTE - Society of Motion Picture and Television Engineers … and many more! Multimedia Systems and Applications 5 Broadcast System Transmitter Receiver Goals: 1. Efficient use of bandwidth 2. High viewer perception of quality Multimedia Systems and Applications 6 Scanning and Interlace • Transmission is continuous signal Transmitter captures images and encodes Receiver decodes for display • Image traced out line-by-line Left-to-right top-down scanning vertical/horizontal blanking interval • Signal is interlaced Improves perception of motion (?) Alternative is progressive scanning Multimedia Systems and Applications 7 Video Display Scanning Amplitude Cathode Time • Video composed of luma and chroma signals • Composite video combines luma and chroma • Component video sends signals separately Multimedia Systems and Applications 8 Camera Operation Color Camera Filters Tubes Zoom R Luma Lens Encoder Beam Splitter G Chroma B • Camera has 1, 2, or 3 tubes for sampling • More tubes (CCD’s) and better lens produce better pictures Multimedia Systems and Applications 9 Direct View CRT • Three guns (RGB) energize phosphors Varying energy changes perceived intensity Different energies to different phosphors produces different colors Phosphors decay so you have to refresh • Different technologies Shadow mask (delta-gun dot mask) PIL slot mask Single-gun (3 beams) aperture-grille (Trinitron) Multimedia Systems and Applications 10 Aperture-grille –vs- Shadow Mask R G R B G B Multimedia Systems and Applications 11 Scanning Notation • Lines/”frame rate” specification NTSC 525/59.94 PAL 625/50 • ATSC – everything is variable 1080i 1920x1080 interlaced scanning 720p 1280x720 progressive scanning • Why 59.94? Avoids interference problem between color and sound subcarrier Multimedia Systems and Applications 12 Interlaced Fields Signal Format Line 1 --- vertical blanking Line 21 --- Raster Format Field 1 1 2 3 485 Line 263 --- ... vertical blanking 485 Line 283 --- 2 4 484 ... 485 Field 2 ITU-R Rec. 601: 720x483 484 Line 525 --- Multimedia Systems and Applications 13 Scanning (525/59.94) Total Size Active Area square 780X525 644X483 pixel Active 601 858X525 720X483 Active 4fsc 910X525 757X483 Active Multimedia Systems and Applications 14 Aspect Ratio/Refresh Rate • Aspect ratio Conventional TV is 4:3 (1.33) HDTV is 16:9 (2.11) Cinema uses 1.85:1 or 2.35:1 • Refresh Rate NTSC is 60Hz (59.94Hz) PAL is 50Hz Cinema is 48Hz (but still only 24 fps) Multimedia Systems and Applications 15 NTSC Video (525-lines, 60-fields/sec) • 525 scan lines repeated 29.97 times per second (i.e.33.37 msec/frame) • Interlaced scan lines divide frame into 2 fields each 262.5 lines (i.e.16.68 msec/field) • 20 lines reserved for control information at the beginning of each field Only 483 lines of visible data Laserdisc and S-VHS display around 420 lines (perception) Normal broadcast TV displays around 320 lines (“) • Line lasts 63.6 usec(10.9 usec blanked) Multimedia Systems and Applications 16 PAL Video (625-lines, 60-fields/sec) • 625 scan lines repeated 25 times per second (i.e. 40 msec/frame) • Interlaced scan lines divide frame into 2 fields each 312.5 lines (i.e. 20 msec/field) • Approximately 20% more lines than NTSC • NTSC vs. PAL roughly same bandwidth Multimedia Systems and Applications 17 Color Perception • Color is perceived lightwave 400nm to 700nm received at retina Humans more sensitive to brightness than color • Retina composed of cones and rods Cones respond to different frequencies (RGB) Rods measure brightness at low light levels (i.e., nightvision) • CIE established standards for color CIE XYZ, CIE xyY Linear RGB Multimedia Systems and Applications 18 Gamma • CRT is inherently non-linear Display changes based on voltage driving it • Human vision is also non-linear Lightness sensation is a power function of intensity (y=xw) • Serendipity happens… Non-linear CRT close to inverse human lightness Coding intensity into a gamma corrected signal maximizes perceptual image • Y -vs- Y’ Multimedia Systems and Applications 19 Gamma in Video camera transmission display • Camera performs gamma correction • Display imposes inverse power function y = x 1/0.45 Multimedia Systems and Applications 20 Color Image Coding • Image represented by 24 bit pixel (8 bpp) Each color value between 0 and 255 • Video uses non-linear coding Uniform distribution of colors to codes RGB R’G’B’ (gamma corrected RGB) • Video uses luminance/chromance R’G’B’ Y’CBCR Luminance is Y (technically luma is Y’) Chromance is CBCR Multimedia Systems and Applications 21 Jargon • CBCR actually color difference signals CB is scaled version of (Y’-B’) CR is scaled version of (Y’-R’) • Confusing terms: YUV, YIQ, Y CBCR,… Scaled versions of <Y’, Y’-B’, Y’-R’> Green has highest contribution to luminance • Luminance –vs- Luma (i.e., Y –vs- Y’) Y is linear luminance Y’ is gamma corrected luminance (aka luma) Multimedia Systems and Applications 22 Chroma Subsampling • Chroma subsampling reduces data 2 chroma/luma 16 bpp 1 chroma/luma 12 bpp • Notation is a:b:c a is luma samples b is chroma samples per odd line c is chroma samples per even line • Ex. 4:4:4, 4:2:2, 4:1:1, 4:2:0, … Multimedia Systems and Applications 23 Line Sampling Y Y Y Y Y 4:4:4 CR/CB CR/CB CR/CB CR/CB CR/CB Y Y Y Y Y 4:2:2 CR/CB CR/CB CR/CB Y Y Y Y Y 4:1:1 CR/CB 4:2:2 is referred to as broadcast quality 4:1:1 is referred to as VHS quality 4:2:0 is 2:1 down sampling in horizontal and vertical direction Multimedia Systems and Applications 24 4:2:0 Sampling Luma sample Chroma sample Multimedia Systems and Applications 25 Representations • Composite NTSC - 6MHz (4.2MHz video), 29.97 frames/second PAL - 6-8MHz (4.2-6MHz video), 50 frames/second • Component Separation video (luma, chroma) - svhs, Hi8mm RGB, YUV, YIQ, … YCBCR - used for most compressed representations • Separation video called “s-video” Multimedia Systems and Applications 26 Analog Video Representations • NTSC Y = 0.299R + 0.587G + 0.114B I = 0.596R - 0.275G - 0.321B Q = 0.212R - 0.523G + 0.311B composite = Y + Icos(Fsc t) + Qsin(Fsc t) • PAL Y = 0.299R + 0.587G + 0.114B U = 0.492(B-Y) Q = 0.877(R-Y) composite = Y + Usin(Fsc t) + Vcos(Fsc t) Multimedia Systems and Applications 27 Digitizing • Analog TV is a continuous signal • Digital TV uses discrete numeric values Signal is sampled Samples are quantized Small, discrete regions are digitized • Image represented by pixel array Multimedia Systems and Applications 28 Image Sizes 160 352 720 800 1152 1280 1920 QSIF (19Kp) 120 SIF (82Kp) 240 601 (300Kp) 483 SVGA (500Kp) 600 ATV (1Mp) 720 Workstation (1Mp) 900 HDTV (2Mp) 1080 Multimedia Systems and Applications 29 Workstation Images? • [1 Mega Pixel] 1152 x 900 = 1,036,800 pixels • [xvga] 1024 x 768 = 786,432 pixels • [?] 1280 x 1024 = 1,310,720 pixels Multimedia Systems and Applications 30 Digital Video Representations • Digital Composite Video(D2/D3,SMPTE 244M) 142 Mb/s data rate, either parallel or serial Subsampled color signals 4:2:2 • Digital Component Video(D1/D5,SMPTE RP125) Maintain separate signals for luma and chroma 270 Mb/s data rate, either parallel or serial Subsampled color signals 4:2:2 • Compressed Digital Video MPEG, MJPEG, H.26x, DV, … Multimedia Systems and Applications 31 Digital Video Block Structure macroblock • 4:2:2 YCBCR Y1 Y2 CB1 CR1 16x16 macroblock Y3 Y4 CB2 CR2 8x8 pixel blocks 8 bits/sample = 16 bits/pixel = 4Kbits/macroblock • 4:1:1 YCBCR 3Kbits/macroblock 12 bits/pixel Y1 Y2 CB CR Y3 Y4 Multimedia Systems and Applications 32 What is Video Data Rate? • Digital 720x483 = 347,760 pixels/frame 4:2:2 sampling gives 695,520 bytes/frame 21 MB/sec (167 Mbs) 4:4:4 sampling gives 250 Mbs • ATV (MPEG MP@ML) 1280x720 = 921,600 pixels/frame 4:2:0 sampling gives 1,382,400 bytes/frame 41 MB/sec (328 Mbs) (Note: MPEG coded streams are 1.5-80 Mbs) Multimedia Systems and Applications 33 What is Video Data Rate (cont.)? • ATSC (720P) 720x1280 = 921,600 pixels per frame 4:2:2 sampling = 1,843,200 bytes per frame 24 fps = 44,236,800 bytes per second 44 MB/s = 354 Mbs • ATSC (studio 1080I) 1080x1920 = 2,073,600 pixels per frame 4:4:4 sampling = 6,220,800 bytes per frame 30 fps = 186,624,000 bytes per second 187MB/s = 1.5 Gbs Multimedia Systems and Applications 34 Serial Digital Data Rates • Serial digital is a video transport standard Widely used in broadcast and production studios • Data rates NTSC SDI approximately 200 Mbs ATSC SDI approximately 1.5 Gbs • Routing Switcher Remember typical facility might have a 256x256 routing switcher What is data rate across the switch? Multimedia Systems and Applications 35 Human Perception •What is smooth motion Depends on source material Most action is perceived as smooth at 24 fps •Human most sensitive Low frequencies Changes in luminance and blue-orange axis •Vision emphasizes edge detection Strong bias to horizontal and verticle lines •Visual masking by large luminance changes Multimedia Systems and Applications 36 Producing High Quality Video • Need high quality camera S-Video(SVHS, Hi8mm) better than composite 3 chips better than 1 chip Digital better than analog • Lights, lights, lights… Experiment with filters to change apparent colors • Shoot scene from different angles and cut between them to create visual stimulation • Study film/video techniques Let person exit the scene without moving camera Keep orientation of images correct Change scene/shot to reflect time change Multimedia Systems and Applications 37 Conclusions • NTSC/PAL were excellent standards that lasted over 50 years Technology has changed dramatically during this time • Digital revolution is changing industry Main impact is development of ATSC standard • Internet revolution is also chaning industry Webcasting –vs- mass market broadcasting Wireless –vs- cable –vs- packet transmission Multimedia Systems and Applications 38
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