7 - digital video

8/13/2019 7 - Digital Video

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Digital Video

Multimedia


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Digital Video A digital camera samples the imaged scene as

discrete frames Each frame consists of sample values that are

discrete both horizontally and vertically. Let m and n be the integer indices in the horizontal

and vertical directions and let k be the frame number. The actual spatial and temporal locations are

x =m x , y =m y , and k =m t . We may use (m , n , k ) to describe a digital video.

25/11/2013 Multimedia Coding and Processing 2


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Digital Video

Each pixel is render as a rectangular region with aconstant colour for the pixel.

The ratio of the width to the height of this area isknown as Pixel Aspect Ratio (PAR). PAR relates to the Image Aspect Ratio by

The display device should conform to this PAR toavoid distortions.

, ,/ .

s y s x PAR IAR f f



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Digital Video

ITU-R developed the BT.601 to standardize thedigital TV format.

To convert a raster scan to a digital video signal,one need only sample the 1D waveform.

If a total number of f s, x samples are taken per line,the equivalent sampling rate is

, , , , . s s x s y s t s x l f f f f f f



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Digital Video The sampling rate in BT.601 standard is chosen to

satisfy two constraints: The horizontal sampling resolution should match the

vertical sampling resolution as close as possible. That is,

x y . The same sampling rate should be used for NTSC andPAL/SECAM and it should be a multiple of respectiveline rates. Thus,

f s, x IAR* f s,y .

Using f s = f s, x * f l and f l = f s,t * f s,y , we have f s IAR* f 2s,y * f l .



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Digital Video

f s 11 and 13 MHz for NTSC and PAL/SECAM. A number that is closest to both numbers and

satisfies the second criterion is then chosen. We

have f s = 858* f l (NTSC) = 864* f l (PAL/SECAM) =13.5 MHz.

This gives the 525/60 and 625/60 signals.



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Digital Video


ActiveArea

720 pels

858 pels

122pels

16pels

480lines

525lines Active

Area

720 pels

864 pels

132pels

12pels

576lines

625lines

NTSC PAL/SECAM


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Digital Video

Note that both formats have the samenumber (720) of active pixels/line.

Note that the pixel width-to-height ratio is not1.

PAR= x /y = IAR* f 's,y / f 's, x PARNTSC= (4/3)*480/720 = 8/9 PARPAL = (4/3)*576/720 = 16/15



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Digital Video - BT.601

BT.601 also uses the YCbCr representation. The YCbCr values (0-255) are related to the RGB

values (0-255) by

0.257 0.504 0.098 16

0.148 0.291 0.439 128

0.439 0.368 0.071 128b

r

Y R

C G

C B



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Digital Video - BT.601

The inverse relation is

1.164 0.000 1.596 16

1.164 0.392 0.813 128

1.164 2.017 0.000 128

b

r

R Y

G C

B C



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Spatial Sampling Rate

Human eyes are less sensitive to colour differenc thanbrightness. Chrominance values are sampled at lower frequency than

the luminance value. 4:2:2 format. Each chrominance component, Cb and Cr,

are usually sampled at half of the sampling rate of theluminance component.

4:1:1 format. Each chrominance component is sampled at of the sampling rate of the luminance component.

4:2:0 format. Each chrominance component is sampled at of the sampling rate both horizontally and vertically.

4:4:4 format. The chrominance values are sampled at thesame rate as the luminance values.



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Chrominance Subsampling


4:2:2 format

Each 2 2 Y pixels2Cb & 2Cr pixels(horizontal subsampling)

4:1:1 format

Each 2 2 Y pixels1Cb & 1Cr pixels(4:1 horizontal subsampling)

Y

Cb

Cr


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Chrominance Subsampling


4:4:4 format

Each 2 2 Y pixels4Cb & 4Cr pixels(no subsampling)

4:2:0 format

Each 2 2 Y pixels1Cb & 1Cr pixels(2:1 subsampling bothhorizontally and vertically)

Y

Cb

Cr


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Computer Video Formats

CGA Colour Graphics Adapter Resolutions: 320 200 pixels with 4 colours (2 bits) (320x200) pixels 2 bits/pixel = 15.625 KB/image

EGA Enhanced Graphics Adapter Resolutions: 640 350 pixels with 16 colours (4 bits) (640x350) pixels 4 bits/pixel = 109.375 KB/image



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Computer Video Formats VGA Video Graphics Array

Resolution: 640 x 480 pixels with 256 colours (8 bits) (640x480) pixels x 8 bits/pixel = 300 KB/image

XGA Extended Graphics Array Resolution: 640 x 480 pixels with 65,536 colours (16 bits) or 1024

x 768 pixels with 256 colours (640x480) pixels x 16 bits/pixel = 600 KB/image (1024x768) pixels x 8 bits/pixel = 768 KB/image

SVGA Super VGA Resolution: 800 x 600 pixels with 16,777,216 colours (24 bits) or

1024 x 768 pixels with 65,536 colours (16 bits) (800x600) pixels x 24 bits/pixel = 1.37 MB/image (1024x768) pixels x 16 bits/pixel = 1.5 MB/image



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Sampling

Digitization of an analog waveform Take samples at different temporal

locations Frequency of taking samples: sampling rate Amplitude of taken samples: quantization Objective : To maintain the information of

the original analog waveform



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Sampling Rate Sampling rate is the no. of samples taken per unit time Affects whether the no. of samples are enough to reproduce

the waveform.


time0 1 2 3 4 5 6 7 8 9 10 11 12

E.g. if only 1 sample per periodReproduced waveform= a straight line


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Nyquist Sampling Theorem For lossless digitization, the sampling rate must be at

least twice of the maximum frequency That is, at least 2 samples must be obtained within

each cycle. Otherwise, either the maximum or theminimum amplitude in a period is gone; leading toloss of information in the digitization process.


time0 1 2 3 4 5 6 7 8 9 10 11 12


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Sampling & Compression

When the sampling rate is reduced, Fewer samples are accessed per unit time Lower temporal resolution Object size is reduced The information of the higher frequency

waveforms is also lost



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Quantization Consider using a decimal number to describe a value. Each digit

has ten different values. One digit specifies 10 different values (0-9) Two digits specifies 100 different values (0-99)

Three digits specifies 1000 different values (0-999). Each additional digit increases the range by 10 times.

Consider using a binary number to describe a sample value. Each additional digit(bit) doubles the range of values.



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Quantization

Quantization of a sample the mapping ofvalues to integral values in describing asample value. Mathematically,

No. of bits, Nb = log2(no. of integral values)


),(sgn/),(),( y xU y xU y xV


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Quantization

Analog: A continuousslope

Digital: steps in thestairs


Step size


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More bits better resolution

More values(v) need more bits(b): v 2b 8 bits: 256 values 16 bits: 65,536 values 24 bits: 16,777,216 values

Affects the step size in the reproducedwaveform

More bits smaller step size bettersample quality



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No. of Bits Affects Quality Values in 7-bit quantization are

0.101101, 1.000, 0.101101, 0.000, -0.101101, -1.000, -0.101101,0.000,

Quantized value in decimal becomes0.703125, 1, 0.703125, 0, -0.703125, -1, - 0.703125, 0,


time0 1 2 3 4 5 6 7 8 9 10 11 12


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No. of Bits Affects Quality

Values in 4-bit quantization are0.101, 1.000, 0.101, 0.000, -0.101, -1.000, - 0.101, 0.000,

Quantized value in decimal becomes0.625, 1, 0.625, 0, -0.625, -1, - 0.625, 0,


time0 1 2 3 4 5 6 7 8 9 10 11 12


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Quantization and Compression

When the no. of integral values to represent asample is reduced,

The number of bits per sample is reduced Lower sample resolution Object size is reduced The quality of each sample value is also reduced



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Summary to Sampling andQuantization

Digitization of the analog waveform involvessampling and quantization.

Sampling rate must be at least twice of the

highest frequency to avoid loss of information No. of bits in quantization affects the quality of

each sample value Object size are reduced with loss of information

by lowering the sampling rate and number ofsample values.



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Data Representations Summary Computer graphics are represented using the coordinates on

the screen. Computer animations are done by updating changes to the

frame buffers and these changes are drawn on the display

Images are represented as 2D pixels. Each pixel can berepresented using RGB, YUV, YCbCr, or CMYK.

A/D converters digitize an analog wave by taking samples ofamplitudes at fixed time intervals.

A video is represented as an array of frames. 24 to 30 framesshould be displayed per second to show full motions.



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Exercise A sine curve of maximum amplitude of 16 is sampled

at 6 times per cycle and the sample values arequantized using 8 bits. Alternatively, the sine curvemay be sampled at 8 times per cycle and the samplevalues are quantized using 6 bits.

Find the Mean Absolute Difference of each quantizedcurves with the original sine curve.

Find the Mean Square Errors of the two quantizedcurves.

Compare the PSNR of the two quantized curves.


7 - digital video

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