Module 4: Video Sampling Rate Conversion Lecture 25: Scan rate doubling, Standards conversion. The Lecture Contains: Algorithm 1: Algorithm 2:

Transcription

1 The Lecture Contains: Algorithm 1: Algorithm 2: STANDARDS CONVERSION file:///d /...0(Ganesh%20Rana)/MY%20COURSE_Ganesh%20Rana/Prof.%20Sumana%20Gupta/FINAL%20DVSP/lecture%2025/25_1.htm[12/31/2015 1:17:06 PM]

2 For scan rate doubling, there exist more than one way to repeat the fields. We provide two algorithms in the following. Algorithm 1: In the first approach, the existing odd frame is repeated to form the next even frame and the existing even frame is repeated for the next odd frame. This can be expressed as This algorithm has reasonably good performance with moving scenes, but poor results in stationary regions is inevitable. (Figure 13) Figure 13: Interpretation of the 3 to 2 pull-down method by linear filtering. file:///d /...0(Ganesh%20Rana)/MY%20COURSE_Ganesh%20Rana/Prof.%20Sumana%20Gupta/FINAL%20DVSP/lecture%2025/25_2.htm[12/31/2015 1:17:07 PM]

3 Algorithm 2: The second approach repeats an even field for the next even field, and an odd field for the next odd field, which can be expressed as This strategy is optimal for stationary scenes but fails for the moving parts. Obviously, neither algorithm alone is sufficient for both stationary and moving regions of the scene, which motivates the need for motion-adaptive or motion-compensated filtering schemes. An interesting application which requires scan rate conversion by a rational factor is conversion from NTSC to PAL and vice versa. The NTSC standard employs 60 fields/sec and lines/field, whereas the PAL standard uses 50 fields/sec and lines/field. The conversion from NTSC to PAL may be achieved by dropping a complete frame (an even and an odd field) every five frames, and spatially interpolating for the missing lines. On the other hand, PAL to NTSC conversion requires dropping some extra lines per frame and replicating a complete frame every five frames. This is discussed in next section. STANDARDS CONVERSION In order for a video signal to be converted to another standard, three aspects of the video signal may need to be changed - field rate, lines/frame and colour encoding. When converting PAL to SECAM, it is only necessary to change the colour encoding of the video signal (since the field rate and the number of lines per frame are the same). When converting from NTSC to PAL, however, it is necessary to change all three parameters. Conversion of the colour encoding method is a fairly simple process and can be relatively easily achieved using linear analog electronics. Unfortunately, the process of changing the field rate and the number of lines per frame is more complicated and is generally performed using digital electronics. There are three main ways in which the number of fields per second and the number of lines per field are converted: Field/Line Omission/Duplication, Field/Line Interpolation and Motion Estimation. file:///d /...0(Ganesh%20Rana)/MY%20COURSE_Ganesh%20Rana/Prof.%20Sumana%20Gupta/FINAL%20DVSP/lecture%2025/25_3.htm[12/31/2015 1:17:07 PM]

4 Field/Line Omission/Duplication This is the simplest process and requires the least complicated electronics. In what can be considered a two step process, the number of lines per field is first converted to the new number and then the number of fields per second is converted. In a PAL to NTSC conversion, firstly the number of lines per field is converted from lines/field to lines per field. This is done by omitting one line from every six. This is illustrated in Figure 14. The field rate is then be converted from 50 fields per second to 60 fields per second. This is done by duplicating or repeating one field in every five. Note that because each field now consists of only lines it is possible to display 60 fields per second. This is illustrated in Figure 1(b). With an NTSC to PAL conversion, it is necessary to repeat one in every five lines and omit one in every six fields to obtain lines per field and 50 fields per second. (Figure 14) PAL to NTSC conversion. a. Omission of lines when converting a line field to a line field b. Duplication of fields when converting from 50Hz to 60Hz. This is the simplest and lowest quality conversion technique. It introduces some conversion artefacts especially when motion is present in the scene. Subjectively the conversion is acceptable. file:///d /...0(Ganesh%20Rana)/MY%20COURSE_Ganesh%20Rana/Prof.%20Sumana%20Gupta/FINAL%20DVSP/lecture%2025/25_4.htm[12/31/2015 1:17:07 PM]

5 Field/Line Interpolation In this method, individual lines and fields in the output standard are a product of several lines or fields of the input standard. This is an extension of the previous scheme where individual lines and fields in the output standard were based on single lines or fields from the input standard. In a simple implementation of such a system, a new line in the output standard is calculated as a linear interpolation between two lines from the input standard. The particular input lines from which the output line is calculated and the weightings used are determined from the position in the scan where the output line must be generated. This is illustrated in Figure 15 which shows a PAL to NTSC conversion. For example, line 5 in the output standard is calculated as 24% of line 5 and 76% of line 6 from the input standard. This calculation continues such that the correct number of output lines is generated from the input lines. The conversion of the number of fields per second is a similar process and is illustrated in Figure 2(b). For example, output field number 3 occurs at t=2/60 seconds. It is calculated from inputs field numbers 2 and 3 (which occur at t=1/50 and t=2/50 seconds) at a weighting of 33% of field 2 and 67% of field 3. a. interpolation of the new line rate and b. interpolation of the new field rate. (Figure 15) PAL to NTSC conversion. Four line/four field converters are also available. They work in a similar way to the process explained above except that each individual output line is based on a weighted average of four input lines and each individual output field is based on the weighted average of four input fields. The performance of this conversion method with standard video is much better than the previous method; however some conversion artifacts are still evident, particularly with scene motion. It should be noted that the details I have provided give only a brief description of the process. A full explanation is contained in Sandbank (1990). file:///d /...0(Ganesh%20Rana)/MY%20COURSE_Ganesh%20Rana/Prof.%20Sumana%20Gupta/FINAL%20DVSP/lecture%2025/25_5.htm[12/31/2015 1:17:07 PM]