lecture 1: sketch of multimedia communications lecture 2: introduction to multimedia

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Multimedia Communications

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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

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Internet and Multimedia Communications

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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

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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

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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

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Continuous & Discrete Media

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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

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Digital Signals

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In traditional telecommunications systems: information in analog form

In computer communications: audiovisual signals in digital form analog signals digital signals: sampling and quantization +

encoding

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Digital Signals

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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

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Digital Signals

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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

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Digital Signals

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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

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Digital Signals

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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

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Digital Signals

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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

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Digital Signals

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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

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Digital Signals

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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

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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

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Still Images 2

Computer images: bit maps of pixels A standard computer display: 7681024 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

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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

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Text & Line Drawing

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Each plain text character: 1 byte Each formatted text character: 2 bytes A single page of text: 6480 characters

#(bits of a single full page) = 64 80 2 8 = 82 kbits For 56 kbps modem, it takes 1.46 secs to transmit

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Text & Line Drawing

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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

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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

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Video & 3D Graphics

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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 288360 pixels, 24 bits for brightness and color at each pixel: ~74.65 Mbps

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Video & 3D Graphics

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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

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Encoding & Decoding

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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

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Encoding & Decoding

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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

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Bandwidth vs. Compression

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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

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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

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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

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Many international compression standards in use today apply to network telephony, audio, image, and video transmissionStandard Bit rate Application

G.721 32 kbps telephony

G.728 16 kbps telephony

G.722 48 - 64 kbps teleconferencing

MPEG-1 (audio) 128 - 384 kbps 2-channel audio

MPEG-2 (audio) 320 kbps 5-channel audio

JBIG 0.05 - 0.10 bpp binary images

JPEG 0.25 - 8.0 bpp still images

MPEG-1,2 (video) 1 – 8 Mbps video

Px64 64 – 1,536 kbps videoconferencing

HDTV 17 Mbps Advanced TV

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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

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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)

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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

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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

lecture 1: sketch of multimedia communications lecture 2: introduction to multimedia

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