1. 10/10/04 www.eej.ulster.ac.uk/~ian/modules/COM342/COM342_L5.ppt L5/1/28 COM342 Networks and Data Communications Ian McCrumRoom 5D03B Tel: 90 366364 voice mail on 6 th ring Email: IJ.McCrum@Ulster.ac.uk Web site: http://www.eej.ulst.ac.uk Lecture 5: Practical applications of Multimedia in Particular

2. 10/10/04 www.eej.ulster.ac.uk/~ian/modules/COM342/COM342_L5.ppt L5/2/28 After this lecture you should be able to: Understand the need for data compression, particularly in relation to video and audio. Realise the difference between lossless and lossy encoding systems. Be aware of the difference between symmetrical and asymmetrical coding systems. Have a good appreciation of the implementation the JPEG standard. Invitation to look at MPEG standard

3. 10/10/04 www.eej.ulster.ac.uk/~ian/modules/COM342/COM342_L5.ppt L5/3/28 That which must be compressed Audio frequencies –20Hz to 20KHz –amplitude variation of million –comment upon power-amplitude p724 Sample amplitude and digitise using ADC –sampling frequency 40,000/sec from Nyquist –number samples 8bits reduce perception of quantisation. telephone 8000x8bit samples per sec and CD (44.1k samples of 16 bits)/sec Music Instrument Digital Interface

4. 10/10/04 www.eej.ulster.ac.uk/~ian/modules/COM342/COM342_L5.ppt L5/4/28 Fig 7.57 (a) AST

5. 10/10/04 www.eej.ulster.ac.uk/~ian/modules/COM342/COM342_L5.ppt L5/5/28 Quantisation

6. 10/10/04 www.eej.ulster.ac.uk/~ian/modules/COM342/COM342_L5.ppt L5/6/28 Telephone systems Compression has been attempted which has a model of the head which generates the human voice. Vocal tract, resonant cavity, tongue, jaw etc. the parameters of the model are determined from sampling a portion of speech. the coefficients are then transmitted representing speech. fine until non-head sound is attempted.

7. 10/10/04 www.eej.ulster.ac.uk/~ian/modules/COM342/COM342_L5.ppt L5/7/28 That which must be compressed II Video –film ~20 frames/sec –TV 25 full frames or 50 1/2 frames/sec –see Fig 7-77 UK TV 625lines * 25frames/sec = line frequency of 15.625kHz USA TV 525lines * 30frames/sec = line frequency of 15.750kHz therefore 640samples * 3colours * 8 bits *625lines *25frames/sec =240*10 6 bits/sec

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9. 10/10/04 www.eej.ulster.ac.uk/~ian/modules/COM342/COM342_L5.ppt L5/9/28 points to ponder nb Teletext, interlaced fields, PAL, NTSC, luminance and chrominance. requirement for colour pictures to viewed on existing B/W TV

10. 10/10/04 www.eej.ulster.ac.uk/~ian/modules/COM342/COM342_L5.ppt L5/10/28 Compression encoding decoding symmetrical asymmetrical lossless lossy think about the process of reducing an A4 page to 50% of its original size and then expanding it up again, in terms of the list above.

11. 10/10/04 www.eej.ulster.ac.uk/~ian/modules/COM342/COM342_L5.ppt L5/11/28 Entropy encoding no account taken of type of bits –run length encoding –Huffman encoding –Colour lookup tables determine actual colours combinations used out of 2 24 possible colours perhaps only 256 actual, determine table and then choose by index. (asymmetrical and lossless)

12. 10/10/04 www.eej.ulster.ac.uk/~ian/modules/COM342/COM342_L5.ppt L5/12/28 Source encoding recognise properties of data. differential encoding(audio) –difference from previous value –possibly cannot track large scale rapid changes in consecutive samples –information lost –symmetrical Transformations –transform signals from one domain to another –time(frequency) as a list of amplitudes –TX incomplete list –lossy

13. 10/10/04 www.eej.ulster.ac.uk/~ian/modules/COM342/COM342_L5.ppt L5/13/28 Transformations continued cf 4 x 4 matrix of pixels 8bit grey scale subtract upper lhs element from all others. if data is slowly varying then encode as 4bits with one 8bit number for ulhs sample lossless symmetrical

14. 10/10/04 www.eej.ulster.ac.uk/~ian/modules/COM342/COM342_L5.ppt L5/14/28 Source encoding continued Vector quantisation for image data –divide image into rectangles –construct a code book –send index to rectangle –perform dynamically –asymmetrical pattern matching when encoding –can be lossy if best match is used –should recognise when no compression is possible

15. 10/10/04 www.eej.ulster.ac.uk/~ian/modules/COM342/COM342_L5.ppt L5/15/28 JPEG leading to MPEG Image of 640 x 480 RGB 24bits/pixel=7,372,800b –Fig 7.80 Compute matrix of luminances and chrominances according to formulae –Fig 7.81 now 640 x 480 x 8bits luminance = 2,457,600b 2 off 320 x 240 x 8bits chrominance = 1,228,800 subtract 128 so that 0 is middle value. divide each matrix into 8x8pixel blocks Y has 4800 and U and V each 1200

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17. 10/10/04 www.eej.ulster.ac.uk/~ian/modules/COM342/COM342_L5.ppt L5/17/28 Luminance

18. 10/10/04 www.eej.ulster.ac.uk/~ian/modules/COM342/COM342_L5.ppt L5/18/28 Chrominances

19. 10/10/04 www.eej.ulster.ac.uk/~ian/modules/COM342/COM342_L5.ppt L5/19/28 next apply discrete cosine transformation to each of the blocks see Fig 7.82 output is an 8x8 matrix of coefficients describes spectral power at each spatial frequency (losses due to rounding) quantisation (further losses) to reduce higher spatial frequencies. encode (0,0) element differentially using previous block’s(0,0) run length encode the zig zag list of elements. Huffman encode resulting numbers. 20:1 lossy and asymmetrical

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26. 10/10/04 www.eej.ulster.ac.uk/~ian/modules/COM342/COM342_L5.ppt L5/26/28 Compression pointers Stalling’s Book can be got from the link below: http://www1.shore.net/~ws/DCC5e.html Tanenbaum’s home page can be got at the url: http://www.cs.vu.nl/~ast/ Compression utilities can be found for all operating systems at: http://www.gromwin.demon.co.uk/ vanessa/compress.htm

27. 10/10/04 www.eej.ulster.ac.uk/~ian/modules/COM342/COM342_L5.ppt L5/27/28 you should now be able to: Understand the need for data compression, particularly in relation to video and audio. Realise the difference between lossless and lossy encoding systems. Be aware of the difference between symmetrical and asymmetrical coding systems. Have a good appreciation of the implementation the JPEG standard you should now be able to investigate the MPEG standard 738-744 A.T.