Module 1: Digital Video Signal Processing Lecture 3: Characterisation of Video raster, Parameters of Analog TV systems, Signal bandwidth

Transcription

1 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 /...se%20(ganesh%20rana)/my%20course_ganesh%20rana/prof.%20sumana%20gupta/final%20dvsp/lecture3/3_1.htm[12/31/ :05:59 AM]

2 Analog Video Raster Analog video systems of today use raster scan for video capture and display. In a raster scan, a camera captures a video sequence, by sampling it in both temporal and vertical directions. The resulting signal is stored a continuous 1-D waveform. Two most commonly used raster scanning methods are: Progressive and interlaced scans. Progressive Scan: A frame is formed by a single scanning pass i.e. the electronic beam continuously scans the image region from top to bottom and then back to top. This is shown in Figure 3. The resulting raster signal consists of a series of frames separated b a frame interval. Each frame consists of a consecutive set of horizontal scan lines separated by regular vertical spacing. The bottom line is scanned about one frame interval later than the top line of the same frame. For analysis purposes, we assume that all the lines in a frame are sampled at the same time. The intensity values captured along continuous scan lines over consecutive frames form a ID analog waveform called raster scan. For color camera, 3 color raster are formed and converted to a composite raster. (Figure 5) file:///d /...se%20(ganesh%20rana)/my%20course_ganesh%20rana/prof.%20sumana%20gupta/final%20dvsp/lecture3/3_2.htm[12/31/ :05:59 AM]

3 Interlaced Scan Each frame is scanned in two fields and each field contains half the number of lines in a frame. This is called 2:1 interlace. The time interval between two fields, called the field interval is half the frame interval. Following MPEG standard,we call the field containing the first line and following alternate lines as Topfield and the field containing second alternate lines as Bottom field. In some systems the top field is sampled first while in others the bottom is sampled first. The two adjacent lines in a frame are separated in time by the field interval. This fact leads to the infamous Zigzag artifacts in interlaced video images that contain fast moving objects with vertical edges. Motivation for using the interlaced scan is to trade off the vertical resolution for an enhanced temporal resolution (or reduced flickering within a given bandwidth) given the total number of lines that can be recorded within a given time. This will be taken up in our discussion on video sampling. Characterization of a video Raster Raster signal is described by two basic parameters: 1. Frame rate: (frame/sec or fps or Hz) 2. Line number: (lines/frame or lines/picture height), and Line rate:. These two parameters define the temporal and vertical sampling rates of a raster scan. The temporal sampling interval or frame interval. ;and, Vertical sampling interval or line spacing: ; Line interval (time to scan one line):. The line interval, includes the horizontal retrace time,. Actual scanning time for a line is : Similarly, frame interval, includes the vertical retrace time. file:///d /...se%20(ganesh%20rana)/my%20course_ganesh%20rana/prof.%20sumana%20gupta/final%20dvsp/lecture3/3_3.htm[12/31/ :05:59 AM]

4 Therefore, the number of lines that is actually scanned in a frame time, known as Active lines is Normally is chosen as an integer multiple of (See figure 3 for raster scan waveforms) Figure 4 below shows the spectrum of a typical raster signal. It contains peaks at line rate and its harmonics. This is because adjacent scan lines are very similar, so that signal is nearly periodic with period of. The width of each harmonic lobe is determined by the maximum vertical frequency in a frame. The overall bandwidth of the signal is determined by its maximum horizontal spatial frequency. (Figure 6) file:///d /...se%20(ganesh%20rana)/my%20course_ganesh%20rana/prof.%20sumana%20gupta/final%20dvsp/lecture3/3_4.htm[12/31/ :06:00 AM]

5 Quality of a video raster is determined by the frame rate. For example, 1. TV industry uses an interlaced scan with frame rate 25-30Hz; and temporal refresh rate of 50-60Hz and line number of Computer industry has frame rate of 72Hz and line number of 1024 (SVGA display) 2. The frame rates and line numbers are determined based on the visual temporal and spatial thresholds in different viewing environments. 3. Higher frame rates and line numbers are necessary in computer applications to accommodate a significantly shorter viewing distance and higher frequency contents (Line graphics & texts) in the displayed material. The width to height ratio of a video frame is known as image aspect ratio (IAR); IAR of 4:3 used in Std TV and computer display. file:///d /...se%20(ganesh%20rana)/my%20course_ganesh%20rana/prof.%20sumana%20gupta/final%20dvsp/lecture3/3_5.htm[12/31/ :06:00 AM]