Transcript Multimedia Communications

Multimedia Information
Representation
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Analog Signals
Analog signal and
frequency components

Signal bandwidth and
effect of bandlimiting channel
Fourier transform and analysis
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Signal encoder design
Associated wave form
Signal encoder
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Nyquist sampling theorem
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In order to obtain an accurate representation of a time-varying analog
signal, sampling rate >= 2 x highest sinusoidal frequency component
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Nyquist rate
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Bandlimiting Filter
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Undersampling (<
Nyquist rate) results in
additional FC being
generated that are not
present in the original
signal
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Cause the original signal
to become distorted
Need bandlimiting
(antialiasing) filter
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Quantization Intervals
Source of errors
Quantization Interval
q = 2Vmax / 2n
Dynamic range of signal
D = 20log10(Vmax/Vmin)dB
Noise polarity
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Decoder Design
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Text
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Unformatted text: plain text
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ASCII character set
Mosaic character set used in Videotex and Teletext
Formatted text: richtext
Hypertext
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Pages and hyperlinks
HTML, SGML
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Images: Graphics
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Graphics: computer-generated images
Representation of a computer graphics
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High-level version: e.g) PostScript
Bit-map format: GIF, TIFF
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Images: Digitized Documents
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Images: Digitized Pictures
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Color principles
Additive color mixing
Subtractive color mixing
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Raster-scan principles
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Raster-scan Display Architecture
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Pixel depth: # of bits per pixel
CLUT (color look-up table)
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Screen Resolution
Visible lines per frame
Digitization spatial resolution
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Digital Cameras and Scanners
Color image capture
RGB signal generation alternatives
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Audio
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Speech signals: 50Hz ~ 10kHz
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Nyquist rate = 20ksps
Music-quality audio: 15Hz ~ 20kHz
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Nyquist rate = 40ksps
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PCM Speech
Signal encoding and decoding
Compressor
U-law : North America, Japan
A-law: Europe and others
Expander
Obtain 12-bit performance!!
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CD-quality Audio
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CD-DA (Digital Audio)
standard
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Sampling rate: 44.1 ksps
16 bits/sample
Bit rate for stereo music
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2 channel x 44.1 ksps x 16
bits = 1.411 Mbps
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Synthesized audio
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MIDI (Music Instruments
Digital Interface)
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A MIDI message = (status
byte, data bytes, … )
Music: sequece of
message
Play out by the sequencer
program or saved in a file
Need sound card
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Video Applications
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Entertainment
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Interpersonal
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Broadcast TV, VCR/DVD recording
Video telephony, videoconferencing
Interactive
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Windows containing short video clips
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Interlaced Scanning
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TV broadcast: Color Signals
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Requirements
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Color TV broadcast should be used by an existing
monochrome (back-and-white) TV set
Color TV should be able to produce black-and-white
pictures from monochrome broadcast
Properties of a color source
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Brightness: amount of energy that stimulates the eye
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Hue(색조): actual color of the source
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Varies on a grayscale from back through to white (highest)
Independent of the color of the source
Each color has a different frequency/wavelength
Saturation(채도): strength or vividness of the color
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Luminance and Chrominance
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Luminance: related to the brightness
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Amount of white light
Same as the signal used by monochrome TV
Ys = 0.299Rs + 0.587Gs + 0.114Bs
Chrominance: related to the hue and saturation
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Color difference signals i.e no brightness info.
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Cb = Bs – Ys,
Cr = Rs – Ys
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Composite video signal
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Magnitude of the two color differences are both scaled
down
If the two color difference signals are transmitted, amplitude of
luminance signal can become greater than that of the equivalent
monochrome signal
PAL
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Y = 0.299R + 0.587G + 0.114B
U = 0.493(B-Y)
V = 0.877(R-Y)
NTSC
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Y = 0.299R + 0.587G + 0.114B
I = 0.74(R-Y) – 0.27(B-Y)
Q = 0.48 (R-Y) + 0.41(B-Y)
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Signal Bandwidth
NTSC
PAL
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TV Broadcast
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TV
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Analog TV
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Frame rate: # of frames per second produced by a
video signal
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Number of scan lines per frame
scan rate: # of lines scanned per second
= frame rate x # of scan lines
Interlacing: to reduce the flickering
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common rate: 25 - 75Hz
odd field, even field
Aspect ratio: width : height
Digital TV
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convert analog signal to digital signal
sampling, quantization, coding
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Standard Analog TV Systems
Standard
Frame rates Scan lines
Aspect ratio Interlacing
NTSC
29.97
525(480)
4:3
2:1
PAL
25
625(576)
4:3
2:1
SECAM
25
625(576)
4:3
2:1
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Digital Video (I)
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4:2:2 format
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Studio-Quality TV: ITU-R 601
Bandwidth 6MHz
 sampling rate: 13.5MHz
 720 samples / line
Bit rate = 216Mbps (PAL,
NTSC)
Subsampling 4:2:2
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Digital Video (II)
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4:2:0 format
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Bit rate = 162Mpbs
HDTV format
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16/9 aspect ratio: 1920 x 1152 pixel
Refresh rate: 25Hz / 30Hz
Studio application: 4:2:2 format
Broadcast application: 4:2:0 format
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Digital Video (III)
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SIF (source intermediate format)
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CIF (common intermediate format)
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VCR quality
Used in MPEG-1
½ resolution in both horizontal
and vertical directions (by
subsampling): 4:1:1 format
½ refresh rate (temporal
resolution)
Bit rate: 81Mbps
Videoconferencing quality
Used in H.261
4:1:1 format
Y = 360 x 288, Cb = Cr = 180 x
144
30Hz
QCIF (quarter CIF)
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Video Phone Quality
Used in H.261
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PC Video
Digitization
Format
4:2:0
SIF
CIF
QCIF
System
525-line
625-line
525-line
625-line
Spatial resolution
Y
Cb = Cr
640 x 480 320 x 240
768 x 576 384 x 288
320 x 240 160 x 240
384 x 288 192 x 144
384 x 288 192 x 144
192 x 144
96 x 72
Temporal
resolution
60Hz
50Hz
30Hz
25Hz
30Hz
15/7.5 Hz
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