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Multimedia Systems Video I (Basics of Analog and Digital Video)

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Multimedia Systems Video I (Basics of Analog and Digital Video) Course Presentation Multimedia Systems Video I (Basics of Analog and Digital Video) Mahdi Amiri April 2011 Sharif University of Technology Video Visual Effect of Motion The visual effect of motion is due to biological phenomenons 1. Persistence of vision An object seen by the human eye remains mapped on the eye’s retina for a brief time after viewing (approximately 25 ms) 2. Phi phenomenon When two light sources are close by and they are illuminated in quick succession, what we see is not two lights but a single light moving between the two points (perceiving movement) Due to the above two phenomena of our vision system, a discrete sequence of individual pictures can be perceived as a continuous sequence Page 1 Multimedia Systems, Spring 2011, Mahdi Amiri, Video I Video Definition A sequence of still images representing scenes in motion. Frame Rate Number of still images/pictures per unit time Frame N-1 Frames / Second (fps) Frame rate of video ranges from 6 to 8 fps for old mechanical cameras to 120 or more fps for new professional cameras. The minimum frame rate to achieve Time illusion of a moving image is 15 fps. Frame 0 Page 2 Multimedia Systems, Spring 2011, Mahdi Amiri, Video I Video Representation Video Display In conventional TV sets or monitors, the video signal is displayed using a CRT (Cathode Ray Tube). An electron beam sweeps the screen from top to bottom beam carrying the corresponding pattern information, such as intensity in a viewed scene. Video can be interlaced or progressive ( Are described in the following slides ). Page 3 Multimedia Systems, Spring 2011, Mahdi Amiri, Video I Video Display Progressive Scanning Flicker free at around 50 frames per second (fps) Flicker is a visible fading between cycles displayed on video displays, especially the refresh interval on cathode ray tube (CRT) based computer screens. Flicker and Bandwidth Page 4 Multimedia Systems, Spring 2011, Mahdi Amiri, Video I Video Display Interlaced Scanning Flicker free at around 25 frames per second (fps) First the solid (odd) lines are traced, P to Q, then R to S, etc., ending at T; then the even Field starts at U and ends at V. The jump from Q to R, etc. is called the horizontal retrace, during which the electronic beam in the CRT is blank. The jump from T to U or V to P is called the vertical retrace. Electronic signal for one NTSC scan line. Page 5 Multimedia Systems, Spring 2011, Mahdi Amiri, Video I Video Display Odd and Even Fields Odd Field Even Field A Video Frame Because of interlacing, the odd and even lines are displaced in time from each other - generally not noticeable except when very fast action is taking place on screen, when blurring may occur. Difference Initially the odd-numbered lines are scanned and then the of Fields process is repeated for even-numbered lines - this time starting at the second row. Page 6 Multimedia Systems, Spring 2011, Mahdi Amiri, Video I Video Display Deinterlacing Deinterlacing is the process of converting interlaced video, such as common analog television signals into a non-interlaced form. Method 1: Capturing one field and combining it with the next field Problem: "combing" effect Method 2: Line doubler The most basic and literal way to double lines is to repeat each scanline, though the results of this are generally very crude. Most line doublers use digital interpolation to recreate the missing lines in an interlaced signal, and the resulting quality depends on the technique used. Generally a line doubler will only interpolate within a single field, rather than merging information from adjacent fields, to preserve the smoothness of motion, resulting in a frame rate equal to the field rate. When interlaced video is watched on a progressive monitor with very poor deinterlacing, it exhibits combing when there is movement between two fields of one frame. Serrated image Page 7 Multimedia Systems, Spring 2011, Mahdi Amiri, Video I Analog Broadcast TV Systems NTSC NTSC (National Television System Committee) Mostly used in North America and Japan Aspect Ratio: 4:3 525 scan lines at 30 fps Interlaced scanning (262.5 lines/field) Color Space: YIQ Page 8 Multimedia Systems, Spring 2011, Mahdi Amiri, Video I Analog Broadcast TV Systems YIQ Color Space In the YIQ color system, the I axis runs from cyan to orange, and the Q axis runs from green to violet. Eye is most sensitive to Y, next to I, next to Q. Bandwidth allocation for color components 4 MHz is allocated to Y, 1.5 MHz to I, 0.6 MHz to Q. When compared to PAL in particular, NTSC color accuracy and consistency is sometimes considered inferior, leading to video professionals and television engineers jokingly referring to NTSC as Never The Same Color, Never Twice the Same Color, or No True Skin Colors,[16] while for the more expensive PAL system it was necessary to Pay for Additional Luxury. Page 9 Multimedia Systems, Spring 2011, Mahdi Amiri, Video I Analog Broadcast TV Systems PAL PAL (Phase Alternating Line) Mostly used in Western Europe, China and India Aspect Ratio: 4:3 625 scan lines at 25 fps Interlaced scanning (312.5 lines/field) Color Space: YUV 5.5 MHz is allocated to Y, 1.8 MHz each to U and V. Page 10 Multimedia Systems, Spring 2011, Mahdi Amiri, Video I Analog Broadcast TV Systems Supplementary Materials NTSC Video Signal The horizontal sweep frequency is 525×30 ≈ 15,750 lines/sec, so that each line is swept out in 1/15,750 sec ≈ 63.5μsec. Vertical retrace takes place during 20 lines reserved for control information at the beginning of each field. Hence, the number of active video lines per frame is only 485. Similarly, almost 1/6 of the raster at the left side is blanked for horizontal retrace and sync. The nonblanking pixels are called active pixels. NTSC video is an analog signal with no fixed horizontal resolution. Therefore one must decide how many times to sample the signal for display: each sample corresponds to one pixel output. A typical waveform of a NTSC composite video signal Page 11 Multimedia Systems, Spring 2011, Mahdi Amiri, Video I Analog Broadcast TV Systems Supplementary Materials Modulation of NTSC An NTSC television channel as transmitted occupies a total bandwidth of 6 MHz The actual video signal, which is amplitude-modulated, is transmitted between 500 kHz and 5.45 MHz above the lower bound of the channel. The video carrier is 1.25 MHz above the lower bound of the channel. Like most AM signals, the video carrier generates two sidebands, one above the carrier and one below. The sidebands are each 4.2 MHz wide. The entire upper sideband is transmitted, but only 1.25 MHz of the lower sideband, known as a vestigial sideband, is transmitted. The color subcarrier, as noted above, is 3.579545 MHz above the video carrier, and is quadrature-amplitude- modulated with a suppressed carrier. Spectrum of a System M television channel with NTSC color. The audio signal is frequency-modulated. Page 12 Multimedia Systems, Spring 2011, Mahdi Amiri, Video I Analog Broadcast TV Systems Supplementary Materials Chroma Modulation of NTSC In NTSC, chrominance (C) is encoded using two C t I tcos 2 f t Q t sin 2 f t 3.579545 MHz signals that are 90 degrees out of phase, 00 known as I (in-phase) and Q (quadrature) QAM. These two signals are each amplitude modulated and then f0 is the carrier frequency added together. Chroma Demodulation of NTSC In the ideal case I(t) is demodulated by multiplying the ri t C t 2cos 2 f0 t transmitted signal with a cosine signal. 2 rtIti 2 cos 2 ftQt0 2 sin 2 ft 0 cos 2 ft 0 ri t I t1 cos 4 f00 t Q t sin 4 f t Low-pass filtering ri(t) removes the high frequency terms rtIti It cos 4 ftQt00 sin 4 ft (containing 4πf0t), leaving only the I(t) term. Similarly, Q(t) can be extracted by first multiplying C(t) by 2sin(2πf0t) and then low-pass filtering. Page 13 Multimedia Systems, Spring 2011, Mahdi Amiri, Video I Analog Broadcast TV Systems Supplementary Materials Modulation of PAL The basics of PAL and the NTSC system are very similar; a quadrature amplitude modulated subcarrier carrying the chrominance information is added to the luminance video signal to form a composite video baseband signal. In order to improve picture quality, chroma signals have alternate signs (e.g., +U and -U) in successive scan lines, hence the name “Phase Alternating Line”. This facilitates the use of a (line rate) comb filter at the receiver —the signals in consecutive lines are averaged so as to cancel the chroma signals (that always carry opposite signs) for separating Y and C (chroma) and obtaining high quality Spectrum of a System I television channel with PAL color. Y signals; However, this resulted in a comblike effect known as Hanover bars on larger phase errors. Thus, most receivers now use a chrominance delay line, which stores the received color information on each line of display; an average of the color information from the previous line and the current line is then used to drive the picture tube. Hanover bars Cancellation of Hanover bars through a chroma delay line Page 14 Multimedia Systems, Spring 2011, Mahdi Amiri, Video I Analog Broadcast TV Systems Supplementary Materials SECAM stands for Système Electronique Couleur Avec Mémoire, the third major broadcast TV standard. Aspect Ratio: 4:3 625 scan lines at 25 fps, Interlaced scanning SECAM and PAL are very similar.
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