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CP2022 - Lecture 9

Media communication standards

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Data types

Static and dynamic data Static data does not have a time

element Usually a spatial relationship Text and Image

Dynamic data has a time element Data is used in time sequence Audio and Video

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Text and image

Static formats - not time-related No special communication needs No particular communication

standards Used on the web as

ASCII (IA5) GIF & JPEG

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Use of text

% of text in document

Usable

Online reading

Browsing

Offline readingdata rate

Text can be used in different ways on-line Download - Browsing - Online reading Depends on many factors

purpose, use, familiarity, application etc.

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Image

No real “standard” for communication Image is usually data-intensive

Quality proportional to size (colour-depth) Useful reductions gained by compression Especially if

image detail is not critical lossy compression is applicable

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Audio and Video

Dynamic communication -Time-related Standards are related to telecoms (ITU)

Digital telephony and video telephony Time relationship increases chances of

compression (especially in video) similar frames are redundant similar waveforms are redundant

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Redundancy example

Audio consists of repetitive waves (sounds) For example

Video consists of repetitive images

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Audio communication

Some “standards” are not standard Most standards based on PCM or derivatives Various efforts to improve compression

Good voice quality audio requires 64 kbps

8 bit sample x 8000 Hz sample rate Mobile telephones use compression to

reduce this Less data - lower quality

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Audio

MPEG compresses video and audio uses filters and psycho-acoustic effects

MPEG audio has three layers of compression

MPEG Audio Layer 3 - MP3MP3 compresses CD quality sound at about 10:1 gives near CD quality in 192 kbps can be used at different compression rates

and with different sample rates

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MPEG Audio Layers

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Audio communication

MP3 is widely used on the Internet/WWW on DCC (digital compact cassettes) DAB (Digital audio broadcasting)

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Video Communication

MPEG is now common in video transmission (Digital TV) but not on the Internet

H.261 is the International standard for video telephony

Based on 64 kbps channel size (ISDN) H.261 compresses video

similar scheme to MPEG but lower bit rates and lower quality

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H.261

CIF frame

Group of blocks

Macroblock

1 23 45 67 89 10

11 12

1 111223

2233

........................

........................

........................

Individual pixels

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QCIF frame

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Brief details of H.261

CIF and QCIF formats used Macroblocks are “transformed” to

reduce data Colour sampling is at a lower rate

than luminance (brightness) Uses frame prediction similar to

MPEG

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MHEG

MHEG is an “umbrella” standard for multimedia objects

The ISO standard for digital television It provides a language for

control of delivery of multimedia objects both local and distributed multimedia uses other standards for components

e.g. MPEG etc.

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Summary

Communication of any data requires standards

Multimedia objects require different standards depending on static or dynamic data amount of redundancy quality issues

Standards are continually developing