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Digital Video Digital video is basically a sequence of digital images Processing of digital video has much in common with digital image processing First.

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Presentation on theme: "Digital Video Digital video is basically a sequence of digital images Processing of digital video has much in common with digital image processing First."— Presentation transcript:

1 Digital Video Digital video is basically a sequence of digital images Processing of digital video has much in common with digital image processing First we review the basic principles of analog television

2 Television Fundamentals Color television cameras and television receivers use the RGB (red, green, blue) color system to create any color We have seen how raster scan devices operate Commercial television systems, however, use interlaced scanning as opposed to the progressive scanning of computer monitors

3 Television Fundamentals In interlaced scanning, one half of the horizontal scan lines (every other line) are transmitted and drawn by the receiver Then the other half of the lines are transmitted and are drawn in between the first scan lines Each half is known as a field, and two fields together are known as a frame

4 Television Fundamentals Since the phosphors retain their values for longer than the time that it takes to transmit two fields, and since rate of transmission of a field is shorter than the human eye can perceive, the viewer does not perceive this interlacing If the frame rate is at least frames per second the viewer does not perceive motion in an image sequence as discrete, but as continuous

5 Interlaced Scanning In the figures, the first field is transmitted at time t = 0 and displayed at time t = f / 2 f is the frame rate The second field is transmitted at time t = f / 2 and displayed at time t = f Note that the display at time t = f consists of information (scan lines) from two distinct points in time

6 The first field of a television transmission

7 The second field of a television transmission

8 A complete frame

9

10 Interlacing Interlaced scanning is used in commercial television systems to decrease the bandwidth of the transmitted signal and to reduce the phenomenon known as large area flicker These problems had been overcome by the time bit-mapped computer monitors were being developed

11 Deinterlacing There are several common operations that you might want to perform on interlaced video Producing stills resizing the video changing the frame etc. Performing these operations on raw, interlaced, video can produce undesirable artifacts

12 Interlacing Artifact

13 Deinterlacing Deinterlacing provides a way around these problems All deinterlacing methods involve turning the field-based image into a frame-based image by modifying one of the fields in the image Popular methods include duplication and interpolation

14 Deinterlacing Duplication Interpolation

15 Television Systems The exact frame rate depends on the system as does the number of scan lines per frame There are currently three conventional commercial television systems in use throughout the world North America, South America and Japan use NTSC The United Kingdom, Western Europe, Africa and Australia use PAL France, Eastern Europe and Russia use the SECAM

16 Television Systems SystemScan lines/ frame Frame rate Pixels / frame Bandwidth NTSC52530 / sec 130, MhZ PAL62525 / sec 210,0006 MhZ SECAM62525 / sec 210,0006 MhZ

17 Digital Television Considering the bandwidth of the NTSC signal, how would digital transmission compare to todays analog? We have: 30 frames/second x 130,000 pixels/frame x 24 bits/pixel = 93.6 Mbits/second To be competitive with analog transmission, a data compression of more than 20:1 is required All digital television standards therefore include some form of compression. The disadvantage of digital television therefore, is the extra bandwidth required

18 Digital Television The real advantage may be seen by examining the signal-to-noise ratio of digital vs. analog television This figure shows the approximate ratio of error rate to signal-to-noise ratio for digital transmission

19 Digital Television An error rate of or one bit in 100 million bits is practically undetectable Channel error rates of still permit acceptable pictures, especially if error correction techniques are used An analog TV signal requires a channel with a signal-to-error ratio (SER) of 55dB

20 Digital Television If we use PCM for a digital television signal, the principal source of error is due to quantization The error is a maximum of + or - 1/2 the least significant bit For a quantization level of 8 bits, this is + or - 0.2% This fine quantization would appear as white noise if viewed as a picture

21 Digital Television Theoretically, the SER with 8 bits is 59 dB and for each 1 bit reduction in quantization, the SER is reduced 6 dB The actual SER of a composite color TV signal is about 4 dB less Thus, 8-bit PCM encoding of a noise-free NTSC composite color signal yields a SER of 55 dB

22 Digital Television A bit error rate of is practically undetectable From the figure above, this requires a SER of only 21 dB If we use the rate with error correction bits added, a SER of 18 dB may be sufficient This requires less than 1 bit/pixel The essential problem in digital TV coding is therefore to reduce the picture bandwidth at the expense of the bit error rate and retain acceptable picture quality

23 Aspect Ratios Each of the systems listed above has an aspect ratio (ratio of width to height) of 4:3 Cinematic films and high-definition television (HDTV) systems have aspect ratios of approximately 16:9

24 Aspect Ratios 4:3 aspect ratio 16:9 aspect ratio

25 Compatible Color TV In order to permit compatibility of color TV transmission with preexisting black and white receivers, the RGB image generated by a television camera is converted to a YIQ image by using the transform

26 Compatible Color TV

27 See book notes for more info on YIQ The bandwidth allocated to a black and white television signal is illustrated on the next slide

28 Compatible Color TV

29 In order to maintain compatibility, the color TV signal has to fit in the same bandwidth This is accomplished by first combining the I and Q signals using a method called quadrature modulation The two signals are multiplied by a sine and cosine function, respectively, added and become a single composite signal The second idea is to choose the color subcarrier to be an odd multiple of one half the line frequency The resulting bandwidth allocation is illustrated on the next slide

30 Compatible Color TV

31 At the receiver, the inverse transformation given on the next slide reforms R, G, B from the received Y, I, Q signals

32 Compatible Color TV

33 Pixel Aspect Ratio When we are displaying a digital video stream on a standard television receiver, we have another parameter to consider - the pixel aspect ratio This is related to the aspect ratio of the television screen and to the sampling rate

34 Pixel Aspect Ratio If we have a screen with an aspect ratio of 4:3 and we have a digital image of size 711x487, then in order to maintain the 4:3 aspect ratio we must have a pixel aspect ratio p, where p can be found as follows. 3/4 = 487/711 * p p = 0.75 * 711/487 = Computer monitors generally have pixel ratios of 1.0 (square pixels)

35 Aspect Ratio Conversion We are given a video sequence with an aspect ratio of 16:9 and we want to display the sequence on a device with an aspect ratio of 4:3 For example, the source image may be 640x360 and the display device may have a resolution of 480x360 We have several alternatives

36 Aspect Ratio Conversion In the letterbox technique, we scale the source image by the same amount in both the vertical and horizontal directions: 480/640=.75 First, we create a 480x270 image: S2[i,j] = S1[INT(4/3*i),INT(4/3*j)] where S1 is the original image and INT is a function which rounds a floating point value to the nearest whole number

37 Aspect Ratio Conversion We now use this image to form the target 480x360 image S3 as follows: S3[i,j] = S2[i,j-45] for 45j314 S3[i,j] = 0 for 0j44, 315j359 The second line gives the black bars of the letterbox format In horizontal compression, we map the source image so that it exactly fills screen of the target device This results in a stretching of the source image in the vertical direction The transformation is: S2[i,j] = S1[INT(3/4*i),j]

38 Aspect Ratio Conversion The crop and pan transformation is different from the previous two in that the transformation varies over time, depending on the contents of the source image We will basically be showing one of the three transformed images: S2[i,j] = S1[i+80,j] S3[i,j] = S1[i,j] S4[i,j] = S1[i+160,j]

39 Aspect Ratio Conversion S2 is the center 3/4 of the source image, S3 is the left 3/4 of the source image and S4 is the right 3/4 of the source image If there is an object of interest in the leftmost 1/4 of the source image, we use the transformation S3 if there is an object of interest in the rightmost 1/4 of the source image, we use S4 otherwise, we use S2 We view the original video sequence to determine when we need to focus on the corners of the source image, and we define the transformation accordingly

40 Aspect Ratio Conversion The use of only these three transformations will lead to jerkiness when we shift from one transform to another We can get the effect of a camera panning by making a time-dependent transformation For example, imagine we want to pan from the center view to the right view over some period We can define the transformation as follows: S5[i,j,t] = S1[i+80+INT(80*(t-t1)/(t2-t1)),j,t], t1tt2 assuming that there is no parallel change of frame rate

41 Sample rate conversion At times, it will be necessary to convert the sampling rate in a source signal to some other sampling rate Consider converting from a CCIR 601 signal (a digital video standard) to an MPEG SIF signal MPEG is a compression standard for video SIF is the source input format for the compression) CCIR 601 is an interlaced signal with a 50 Hz field rate The signal consists of three components: Y, U and V

42 Sample rate conversion The Y component (luminance) is sampled at a resolution of 720x480 The U and V components (chrominance) are sampled at a resolution of 360x480 The sampling pattern is shown on the next slide

43 CCIR 601 Sampling Pattern

44 Sample rate conversion MPEG SIF samples luminance at a resolution of 360x240 and chrominance at a resolution of 180x120 The sampling pattern is different as well, as shown on the next slide

45 MPEG SIF Sampling Pattern

46 Sample rate conversion The conversion to the lower sampling resolution begins with the discarding of one of the interlaced fields This reduces the picture rate to 25 Hz (non-interlaced) and reduces the vertical resolution by one half

47 Sample rate conversion Now, the luminance is decimated by one half in the horizontal direction One possibility is to subsample the values, however better results are obtained by applying an FIR filter before subsampling One filter which has been found to give good results in decimating the luminance is shown on the following slide

48 Luminance subsampling filter weights

49 Sample rate conversion The results of multiplying the filter weights by the original values is divided by 256 Use of a power of two allows a simple hardware implmentation An example of the use of this filter followed by subsampling is shown on the next slide At the ends of lines, some special technique such as renormalizing the filter or replicating the last pixel must be used In the example below, the data in the line is reflected at each end

50 Example of filtering and subsampling of a line of pixels

51 Sample rate conversion The chrominance samples have to be placed at a horizontal position in the middle of the luminance samples A linear filter with a phase shift of half a sample is used for this task e.g.

52 Chrominance subsampling filter weights

53 Sample rate conversion The result is divided by eight The chrominance values are vertically decimated after they have been horizontally decimated The conversion process is pictured on the next slide

54 CCIR to SIF Conversion

55 After this conversion, further compression takes place and the MPEG video is transmitted At the receiver, the signal must be uncompressed and then reconverted After zeroes have been inserted between samples, a linear phase FIR filter is applied for upsampling For reconversion, both luminance and chrominance components are upsampled both horizontally and vertically

56 Upsampling filter for luminance

57 Upsampling filter for chrominance

58 Temporal resampling We may also find it necessary to convert from one picture rate to another Consider film digitized at 24 frames per second which is to be shown on television at 60 fields per second The required conversion can be accomplished by the technique of 3:2 pulldown as shown

59 3:2 Pulldown

60 Temporal resampling Video coded at 25 pictures per second can be converted to 50 fields per second by displaying the original decoded lines in the odd fields and the interpolated lines in the even fields Video coded at 24 pictures per second may be converted to 50 fields per second by speeding it up by about 4% and decoding it as if it had been encoded at 25 pictures per second

61 Temporal resampling The decoded pictures can be displayed in the odd fields, and interpolated pictures in the even field The audio must be maintained in synchronization, either by increasing the pitch, or by speeding it up without a pitch change

62 High Definition Television High-Definition Television (HDTV) is high-resolution digital television (DTV) combined with Dolby Digital surround sound (AC-3) HDTV is the highest DTV resolution in the new set of standards

63 High Definition Television History of HDTV in the United States The Federal Communications Commission (FCC) formed the Advanced Television Systems Committee (ATSC) to examine proposals for a standard for HDTV in the United States In March 1990, the FCC announced that it would consider only simulcast systems for a standard - systems in which one broadcast channel carries an NTSC signal and a second channel carries an HDTV signal

64 High Definition Television There were five proposals Four were from American groups All of these employed digital broadcasting The fifth proposal, from the Japan Broadcasting Corp. (NHK) was an analog system called MUSE (Multiple Sub-Nyquist Encoding) MUSE was used on a test basis in Japan The simulcast signals must make use of the currently unoccupied and unavailable taboo channels These channels are those which would cause interference if they were used for broadcasting NTSC signals

65 High Definition Television With UHF channels, for example, the minimum distance between co-channel transmitters (those with the same channel allocation) varies between 250 and 355 kilometers ( miles) For adjacent channels, the minimum specified distance is 90 km (55 miles)

66 High Definition Television The digital proposals all advocate using the taboo channels due to the fact that digital signals can be transmitted with far less power than can analog signals The required average transmitter power for the digital portion of an HDTV signal can be less than 10 percent of that required by an NTSC transmitter with the same service area

67 High Definition Television All of the proposed digital systems used some form of video bandwidth compression after analog-to-digital conversion All systems also used some form of error correction coding so that the digital signal input to the source decoder portion of HDTV decoders will be accurate

68 High Definition Television HDTV signals require compression because they are much larger than NTSC signals The signals have twice as much luminance definition, both vertically and horizontally (four times as many luminance pixels) Further additional pixels are needed for the wider screen (16:9 aspect ratio instead of 4:3) The increase in luminance detail requires about five times the video bandwidth of conventional television systems

69 High Definition Television When the additional chrominance detail is factored in, the total bandwidth required is 6 to 8 times that of conventional systems Eventually, the 4 competitors joined into a Grand Alliance which combined the best of the proposals into a single proposal This proposal was accepted in 1996

70 High Definition Television Of the 18 DTV formats, six are HDTV formats, five of which use progressive scanning and one interlaced scanning Of the remaining formats, eight are SDTV (four wide-screen formats with 16:9 aspect ratios, and four conventional formats with 4:3 aspect ratios), and the remaining four are video graphics array ( VGA ) formats Stations are free to choose which formats to broadcast

71 High Definition Television The formats used in HDTV are: 720p x720 pixels progressive 1080i x1080 pixels interlaced 1080p x1080 pixels progressive

72 Digital Television DTV uses MPEG-2 compression to fit in the 6 MhZ bandwidth used by analog television MPEG-2 is also used in DVD videos Some satellite TV broadcast systems The FCC has mandated that all stations be capable of broadcasting HDTV by 2006

73 DTV Transition Timeline November Affiliates from the top 30 markets (which reach 50 percent of U.S. households) must have constructed digital facilities. December Stations in some of the major markets, including New York, Los Angeles, Atlanta and Chicago, were broadcasting digital signals May The remaining markets after the top 30 (there are 211 total) will have constructed digital facilities. April All stations must simulcast at least 50 percent of their NTSC programs on their digital TV channel.

74 DTV Transition Timeline April Stations must simulcast 75 percent of their NTSC programs on their digital TV channel. April Stations must simulcast 100 percent of their NTSC programs on their digital TV channel The Federal Communications Commission has targeted this year for the complete conversion from analog to digital broadcasting.


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