Multimedia Systems Video I (Basics of Analog and Digital Video) Mahdi Amiri April 2011 Sharif University of Technology

Transcription

1 Course Presentation Multimedia Systems Video I (Basics of Analog and Digital Video) Mahdi Amiri April 2011 Sharif University of Technology

2 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

3 Video Definition A sequence of still images representing scenes in motion. Frame Rate Number of still images/pictures per unit time 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 illusion of a moving image is 15 fps. Frame 0 Frame N-1 Time Page 2

4 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

5 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

6 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

7 Video Display Odd and Even Fields Odd 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. Initially the odd-numbered lines are scanned and then the process is repeated for even-numbered lines - this time starting at the second row. Even Field Difference of Fields Page 6

8 Video Display Page 7 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

9 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

10 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

11 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

12 Analog Broadcast TV Systems NTSC Video Signal The horizontal sweep frequency is ,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. Supplementary Materials A typical waveform of a NTSC composite video signal Page 11

13 Analog Broadcast TV Systems 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. Supplementary Materials The color subcarrier, as noted above, is MHz above the video carrier, and is quadrature-amplitudemodulated with a suppressed carrier. The audio signal is frequency-modulated. Spectrum of a System M television channel with NTSC color. Page 12

14 Analog Broadcast TV Systems Chroma Modulation of NTSC In NTSC, chrominance (C) is encoded using two MHz signals that are 90 degrees out of phase, known as I (in-phase) and Q (quadrature) QAM. These two signals are each amplitude modulated and then added together. Supplementary Materials cos 2 sin 2 C t I t f t Q t f t f 0 is the carrier frequency 0 0 Chroma Demodulation of NTSC In the ideal case I(t) is demodulated by multiplying the transmitted signal with a cosine signal. Low-pass filtering ri(t) removes the high frequency terms (containing 4πf0t), leaving only the I(t) term. i i i 2cos cos sin 2 0 cos cos 4 0 sin 4 0 cos 4 sin 4 r t C t f t r t I t f t Q t f t f t r t I t f t Q t f t ri t I t I t f0t Q t f0t Similarly, Q(t) can be extracted by first multiplying C(t) by 2sin(2πf0t) and then low-pass filtering. Page 13

15 Analog Broadcast TV Systems 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. Supplementary Materials 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 Y signals; Spectrum of a System I television channel with PAL color. 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

16 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. They differ slightly in their color coding scheme: In SECAM, U and V signals are modulated using separate color subcarriers at 4.25 MHz and 4.41 MHz respectively. They are sent in alternate lines, i.e., only one of the U or V signals will be sent on each scan line. Page 15

17 Analog Broadcast TV Systems. Supplementary Materials More at Page 16

18 Analog Color Video Composite video wire Signal Protocols Color (chrominance) and luminance signals are mixed into a single carrier wave. Since color and intensity are wrapped into the same signal, some interference between the luminance and chrominance signals is inevitable. Composite video jacks are often grouped with corresponding stereo audio jacks (the composite video jack is usually yellow) Page 17

19 Analog Color Video Signal Protocols S-Video (Separated video, e.g., in S-VHS) wires, one for luminance and another for composite chrominance signal As a result, there is less crosstalk between the color information and the crucial gray-scale information. A compromise between component analog video and the composite video. A standard 4-pin S-Video cable connector, with each signal pin (3, 4) paired with its own ground pin (1,2) Page 18

20 Analog Color Video Signal Protocols Component video wires Each primary is sent as a separate video signal. The primaries can either be RGB or a luminance-chrominance transformation of them (e.g., YPbPr, YIQ, YUV). Y: Green, Blue: Pb, Red: Pr Best color reproduction YPbPr is analog form of YCbCr Requires more bandwidth and good synchronization of the three components More about signal protocols at Page 19

21 Digital Video Advantages Video can be stored on digital devices or in memory, ready to be processed (noise removal, cut and paste, etc.), and integrated to various multimedia applications. Direct access is possible, which makes nonlinear video editing achievable as a simple, rather than a complex, task. Repeated recording does not degrade image quality. Ease of encryption and better tolerance to channel noise. Page 20

22 Digital Video HDTV vs. Conventional TV HDTV has higher resolution or HDTV has a much wider aspect ratio of 16:9 instead of 4:3. 16:9 is closer to aspect ratio of the human eye sight HDTV moves toward progressive (non-interlaced) scan. The rationale is that interlacing introduces serrated edges to moving objects and flickers along horizontal edges. Page 21

23 Digital Video 720p 1080p 1080i HDTV Display Resolutions Referred to in marketing materials as HD , progressive scan 0.9 megapixels Referred to in marketing materials as Full HD , progressive scan 2.0 megapixels , interlaced scan Aspect Ratio for all is 16:9 (W:H) Page 22

24 Digital Color Video Signal Protocols DVI (Digital Visual Interface), 1999 Analog and Digital fps fps Max Gbit/sec HDMI (High-Definition Multimedia Interface), 2003 Uses a pixel based data stream fps fps Max Gbit/sec Page 23

25 Digital Color Video DisplayPort, 2007 Signal Protocols Uses a packetized data protocol often used in high-speed data communications. This provides a faster data rate over the same number of wires or 2.7 Gbit/sec Page 24

26 Multimedia Systems Video I (Basics of Analog and Digital Video) Thank You Next Session: Critical Reading Review FIND OUT MORE AT Page 25