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Lecture 1: Sketch of Multimedia Communications Lecture 2: Introduction to Multimedia

Multimedia Communications 2. Lecture 1: Sketch of Multimedia Communications Lecture 2: Introduction to Multimedia

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Lecture 1: Sketch of Multimedia Communications Lecture 2: Introduction to Multimedia

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  1. Multimedia Communications 2 • Lecture 1: Sketch of Multimedia Communications • Lecture 2: Introduction to Multimedia • Multimedia: representation of mixed modes of information such as text, data, image, audio, and video • Multimedia Communications: technologies to manipulate, transmit, and control audiovisual signals across a networked communications channel

  2. Internet and Multimedia Communications 2 • Experiences of surfing Internet with WWW browser • Web Pages written in HTML (Hypertext Markup Language) include both text and images, and pointers to other web pages • Some involve speech, audio, and video • Deficiencies of Internet for Multimedia Communications • Images require much more data space to depict. Video requires even more data space

  3. Internet and Multimedia Communications 2 • Prior to WWW, most uses of Internet: e-mail or file transfer (FTP) • Downloading of graphics & images: rare • Speech communications over Internet: rare • Real-time communications: not a major concern • WWW has changed the entire story

  4. Internet and Multimedia Communications 2 • Build new information superhighways (ISH) for future Internet: fast and efficient Multimedia Communications • Design of ISH: complicated, not as simple as building wide-band networks alone • Access, control, and monitor issues

  5. Continuous & Discrete Media 2 • Continuous media: time dependent, such as audio & video • Discrete media: time independent, such as text (formatted or unformatted), still images, and graphics • Discrete media communications: relatively straightforward • Continuous media communications: sophisticated, e.g., video & audio data streams transmission in synchronization

  6. Digital Signals 2 • In traditional telecommunications systems: information in analog form • In computer communications: audiovisual signals in digital form • analog signals  digital signals: sampling and quantization + encoding

  7. Digital Signals 2 • Sampling • s(t)= {s(T), s(2T), s(3T), … , s(nT)} • T: sampling interval • f=1/T sampling frequency • Nyquist’s sampling theorem: if s(t) is band-limited to f0, the minimum sampling frequency to represent the signal accurately > 2 f0

  8. Digital Signals 2 • A speech signal band-limited by 3kHz  must be sampled at least 6kHz faster • a more conservative sampling rate of 8kHz adopted due to need for quantization & encoding

  9. Digital Signals 2 • Quantization & encoding: the sampled value of signals quantized and encoded as a string of bits • In telephone speech applications: use 16 bits per sample, thus lead to 216 distinct voice levels • In other speech compression applications: perhaps use only 8 bits per sample, thus lead to 28 distinct voice levels

  10. Digital Signals 2 • Bit rate = sampling rate  #(quantization bits) • bandwidth of speech signal ~ 3kHz • sampling rate = 8kHz • 8-bit quantizer • bit rate needed for telephone speech = 64 kbps

  11. Digital Signals 2 • Compact audio disk (CD): • in high fidelity, bandwidth ~ 20 kHz • sampling rate = 44.1 kHz • 16-bit quantizer • Bit rate for stereo (2 channels) CD = 1,410 kbps

  12. Digital Signals 2 • In date communications, bit rate: an important parameter • Channel capacity of public data networks: measured in kbps or Mbps • In ISDN (Integrated Service Digital Network), the standard bit-rate for speech: 64 kbps

  13. Digital Signals 2 • Teleconferencing: • sampling rate = 16 kHz • bandwidth = 7 kHz • bit rate = 256 kbps • Digital audio tape: • sampling rate = 48 kHz • bandwidth = 20kHz • bit rate = 1,536 kbps

  14. Still Images 2 • Images composed of pixels (picture element) • Pixel: the smallest resolvable unit area of an image, on a screen or in memory • Each pixel in a 8-bit monochrome image has its own brightness: from 0 for black to 255 for white • Each pixel in a color image has its own brightness and color

  15. Still Images 2 • Computer images: bit maps of pixels • A standard computer display: 7681024 pixels • A color display: 24 bits per pixel (bpp) for color & brightness • #(bits of a color image on computer screen) = 768  1024  24 = 18.874 Mbits • Send this color image over a 56 kbps modem: • transmission time = 18874000/56000 ~ 337 secs ~ 5.6 min

  16. Still Images 2 • Send the image over a faster channel, such as T1 line (1.544 Mbps) • Reduce #(bits per pixel) • Reduce resolution of display • Remove redundancy in display • Image compression combines the last three approaches

  17. Text & Line Drawing 2 • Each plain text character: 1 byte • Each formatted text character: 2 bytes • A single page of text: 6480 characters • #(bits of a single full page) = 64  80  2  8 = 82 kbits • For 56 kbps modem, it takes 1.46 secs to transmit

  18. Text & Line Drawing 2 • Line drawing: revisable or editable • A straight line: two end points • A circle: center and radius • Need much less storage space in memory & transmission time over a network than bit-mapped images

  19. Video & 3D Graphics 2 • Video or motion pictures composed of temporal sequences of images or frames • Frame rate: if too low, the motion becomes jerky • Movies: 25~30 fps • A frame rate of 16 or more needed to depict smooth motion • Each camera shot is an individual frame

  20. Video & 3D Graphics 2 • In video displays, the frame rate is that rate at which the temporal sequence is played back: usually, ~25-30 fps • A second of video under the Common Intermediate Format (CIF) operating at 30 fps, frame size 288360 pixels, 24 bits for brightness and color at each pixel: ~74.65 Mbps

  21. Video & 3D Graphics 2 • Most commonly used multimedia device: PC • Most common communications device used by PCs: modem attached to the ordinary telephone lines • For a 56 kbps modem, a single second of CIF video requires 1333 secs • The PC can not receive the video in real-time • The PC needs gigabits of video storage memory

  22. Encoding & Decoding 2 • Human beings can sense only analog audiovisual signals • For digital communications systems, analog signals must be converted into digital form by encoder (A/D conversion, quantization, and compression) • Modem telephone networks such as ISDN completely digital: digital transmission & digital switching • End users: humans (D/A conversion), or machine • Encoder/decoder: also data compression

  23. Encoding & Decoding 2 • Speech compression algorithms distinctly different from video compression techniques • The evaluation of compression methods depends upon human audiovisual perception of signal quality • Many compression techniques use knowledge of mechanisms of human perception (perceptual coding) • Just noticeable distortion (JND) • For high-fidelity audio, the JND variation versus frequency can be measured for a wide range of human listeners

  24. Bandwidth vs. Compression 2 • In today’s telecommunications, considerable progress in both compression technology and high-speed networking • Great availability of broader bandwidths at lower costs of LANs & WANs  the need for signal compression decreases • With increasing number of users sending/receiving multimedia data, compression is still needed

  25. Bandwidth vs. Compression 2 • Households with a high bandwidth network are still in their infancy • Mobile computers communicate over wireless links: lower bandwidths, higher transmission error rates, and more frequent disconnections in comparison to wired networks • Mass storage: digital library, image & video archival, CDs for audio and/or video

  26. Bandwidth vs. Compression 2 • Constant progress in compression technology: • in telephone speech: • 64 kbps in 1972 for network quality speech • 32 kbps in 1984 • 16 kbps in 1992 •  8 kbps • Better and better speech compression algorithms, as well as more and more powerful integrated circuits for speech compression

  27. Bandwidth vs. Compression 2 • Many international compression standards in use today apply to network telephony, audio, image, and video transmission

  28. Bandwidth vs. Compression 2 • CD-quality audio: • bandlimit: 20 kHz • sampling rate: 44 kHz • quantization level: 16 bits • yield 1.412 Mbps stereo • By using Perceptual Audio Coder (PAC) of AT&T Bell Laboratories, the capability of broadcasting CD-quality music at 64 kbps • The CD-quality sound can now be sent over a basic rate ISDN channel

  29. Bandwidth vs. Compression 2 • In Europe and in Japan, use of basic-rate ISDN for business & in-home applications is widespread • In US, it is becoming so • With bandwidths  28.8 kbps, multimedia communications with data compression is certainly feasible • In many commercial environments, LANs offer bandwidths of 10 Mbps or more, and WAN-connectivity of T1 (1.536 Mbps)

  30. Bandwidth vs. Compression 2 • In the near future, asynchronous transfer mode technology (ATM) will offer higher and higher data rates for both LANs and WANs • With rapid deployment of ATM-based networks, B-ISDN (i.e., broadband ISDN) will be increasingly available for multimedia communications

  31. Bandwidth vs. Compression 2 • In the public-switched telephone network (PSTN), the capability of broadband communications: use of fiber optics transmission & switching technologies • Current fastest data rate over commercial fiber optics networks: 2.5 Gbps • At 1.1 Tera bps, Fujitsu Laboratories of Japan as well as AT&T Bell Laboratories: 4 million newspaper pages, 250 years’ worth of newspaper transmitted in one second • 1.1 Tera bps ~ 15 million 64 kbps ISDN circuits

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