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Module 1: Digital Video Signal Processing Lecture 4: Introducing Digital Video and Digital Video Standards Objectives_template Module 1: Digital Video Signal Processing Lecture 4: Introducing Digital video and Digital video standards The Lecture Contains: Analog Video Raster Interlaced Scan Characterization of a video Raster Analog Color TV systems Signal Bandwidth Digital Video Parameters of a digital video Pixel Aspect Ratio file:///D|/...rse%20(Ganesh%20Rana)/MY%20COURSE_Ganesh%20Rana/Prof.%20Sumana%20Gupta/FINAL%20DVSP/lecture4/4_1.htm[12/30/2015 4:09:40 PM] Objectives_template Module 1: Digital Video Signal Processing Lecture 4: Introducing Digital video and Digital video standards Analog Color TV systems Many of the concepts that we have talked about are present in analog color TV systems. Major constraints in designing color TV system are that (1) overall BW of color TV signal must fit within the bandwidth of the monochrome TV signal (6MHz for NTSC 8MHz “ PAL) (2) All the color signals must be multiplexed into a single composite signal such that a monochrome TV receiver can extract from it the luminance signal. The main processing steps involved in color TV signal production, transmission and reception are shown in Fig. below. (Figure 7) file:///D|/...rse%20(Ganesh%20Rana)/MY%20COURSE_Ganesh%20Rana/Prof.%20Sumana%20Gupta/FINAL%20DVSP/lecture4/4_2.htm[12/30/2015 4:09:40 PM] Objectives_template Module 1: Digital Video Signal Processing Lecture 4: Introducing Digital video and Digital video standards There are 3 different systems used worldwide: NTSE (Used in North America, Japan & parts of Asia), PAL (Used in Western Europe, Asia, China & Mid East) and SECAM (France, Russia, Europe). All 3 color TV systems use 2:1 interlaced scan techniques for capturing as well as displaying video images. NTSC system uses a field rate of 59.94 Hz and line number of 525 lines/frame. (PAL/SECAM) systems both use a field rate of 50Hz and a line nos. of 625 lines/frame. These frame rates are so chosen as to not interfere with the standard electric power system in the involved countries. They also match with the critical flicker fusion frequency of HVS. All systems have IAR of 4:3. Parameters of NTSC, PAL & SECAM are summarized below: For NTSC: line interval But horizontal retrace takes hence, the actual time for scanning each line is The vertical retrace between adjacent fields is This is equivalent to the time for 21 scan lines/field. Hence number of active lines is 525 5-42=483/frame. The actual vertical retrace takes the time equivalent to scanning 9 horizontal lines. The remaining time (For 12 scan lines) is used by broadcasters to transmit additional data in TV signal (eg closed caption, teletext etc.) Signal Bandwidth The bandwidth (BW) of a video raster can be estimated from its line rate. Let number of active lines/frame be teh vertical resolution defined as number of resolved horizontal lines is known to be lines/frame. Where is known as Kell factor that depends on camera & display operator functions. Typical TV camera has . We define the Kell factor as the ratio of number of perceived vertical lines to the number of active scan lines and is Using the IAR, the horizontal resolution defined as number of resolvable vertical lines is element/line. Bandwidth of the video signal is _____________________ file:///D|/...rse%20(Ganesh%20Rana)/MY%20COURSE_Ganesh%20Rana/Prof.%20Sumana%20Gupta/FINAL%20DVSP/lecture4/4_3.htm[12/30/2015 4:09:40 PM] Objectives_template Module 1: Digital Video Signal Processing Lecture 4: Introducing Digital video and Digital video standards Alternatively, the BW of video raster can also be estimated in the following manner. Firstly, the maximum vertical frequency results when white and black lines alternate in a raster frame. This is equal to cycles/picture height where represents the number of active lines. The maximum frequency that can be rendered properly by the system is usually lower than this theoretical limit. The attenuation factor known as Kell factor (K) depends on the camera r display aperture functions. Typical TV cameras have a Kell factor of ; Hence, the maximum vertical frequency that can be accommodated is: ________ cycles/picture ht. If we assume that the max horizontal frequency is identical to the vertical for the same spatial distance, then ______________ Because each line is scanned in secs, the maximum frequency of the ID-raster signal is, For NTSC video format, we have Consequently maximum frequency of luminance component is 4.2MHz. Although the potential BW of chrominance signal could be just as high, usually it is significantly lower than luminance signal. Also HVS is much less sensitive to chrominance. Typically, the two chrominance signals are band limited to have much lower BW. For NTSC systems, chrominance components I and Q have a BW of 1.5 and 0.5MHz. In PAL system, both U and V chrominance components have a BW of 1.3 MHz each. The luminance BW is 4.2 MHz and 5.5 MHz for NTSC and PAL systems respectively. Digital Video A digital video is obtained either by sampling a raster scan or directly using a digital camera. Presently all digital cameras use CCD sensors. As with analog cameras, a digital camera samples the imaged scene as discrete frames. Each frame consists of output values from a CCD array, which by nature is discrete, both horizontally and vertically. file:///D|/...rse%20(Ganesh%20Rana)/MY%20COURSE_Ganesh%20Rana/Prof.%20Sumana%20Gupta/FINAL%20DVSP/lecture4/4_4.htm[12/30/2015 4:09:40 PM] Objectives_template Module 1: Digital Video Signal Processing Lecture 4: Introducing Digital video and Digital video standards Parameters of a digital video A digital video is typically defined by the parameters, frame rate the line number and number of samples per line Temporal sampling interval or frame interval . Vertical sampling interval, Horizontal sampling interval, Another important parameter of digital video is the number of bits/ pixel. Let the number of bits used per pixel be . Conventionally, the luminance and / or each of the color values are specified with 8 bits or 256 levels. So, bits for monochrome and bit for color video. Data rate R of digital video is given by, In general, the temporal and spatial sampling rates can be different for the luminance and chrominance components of a digital video signal. In such cases, should reflect the equivalent number of bits used for each pixel in the luminance sampling resolution. Example: If horizontal and vertical sampling rates for each chrominance component are both half of that for luminance, then there are two chrominance samples for every four Y samples. The equivalent number of bits per sample in the Y resolution is file:///D|/...rse%20(Ganesh%20Rana)/MY%20COURSE_Ganesh%20Rana/Prof.%20Sumana%20Gupta/FINAL%20DVSP/lecture4/4_5.htm[12/30/2015 4:09:41 PM] Objectives_template Module 1: Digital Video Signal Processing Lecture 4: Introducing Digital video and Digital video standards Pixel Aspect Ratio When displaying digital video on a monitor, each pixel is rendered as a rectangular region with a constant color that is specified for this pixel. The pixel aspect ratio: It is related to IAR of the display area and image dimension. For proper display of digital video, one must specify PAR or IAR along with and . The display device should conform to the PAR specified for the signal. Otherwise object shape will get distorted. For example, the image of a person will become fatter and shorter if PAR of display device is larger than specified PAR. In computer industry, PAR of 1.0 is used. In TV industry non square pixels are used. file:///D|/...rse%20(Ganesh%20Rana)/MY%20COURSE_Ganesh%20Rana/Prof.%20Sumana%20Gupta/FINAL%20DVSP/lecture4/4_6.htm[12/30/2015 4:09:41 PM].
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