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CIS679: Multimedia Basics

CIS679: Multimedia Basics. Multimedia data type Basic compression techniques. Multimedia Data Type. Audio Image Video. Audio. Digitization Sampling Quantization Coding Higher sampling rate -> higher quality

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CIS679: Multimedia Basics

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  1. CIS679: Multimedia Basics • Multimedia data type • Basic compression techniques

  2. Multimedia Data Type • Audio • Image • Video

  3. Audio • Digitization • Sampling • Quantization • Coding • Higher sampling rate -> higher quality • Nyquist sampling theorem: for lossless digitization, the sampling rate should be at least twice the maximum frequency responses • Higher bits per sample -> higher quality • Sampling at 8 KHz, 8 bit samples -> 64kbits/sec • CD-quality audio • Sampling at 44.1KHz, 16 bit samples -> 705.6 kbits/sec

  4. Image/Video • Digitization • Scan a picture frame • Digitize every pixel • Color represented by RGB • Normally converted to Y (black and white TV), U and V • Luminance Y = 0.30R + 0.59G + 0.11 R • Chrominance U = (B-Y) * 0.493 V = (R-Y) * 0.877

  5. Video Transmission Standards • NTSC • Y = 0.30R + 0.59G + 0.14B • I = 0.60R + 0.28G + 0.32B • Q = 0.21R + 0.52G + 0.21B • PAL

  6. Studio-quality TV • NTSC • 525 lines at 30 frames/second • Y sampled at 13.5 MHz, Chrominance values at 6.75 MHz • With 8-bit samples, • Data rate = (13.5 + 6.75 + 6.75) * 8 = 216 Mbps

  7. Summary of Multimedia Data Types • Audio data rate = 64kbps, and 705.6kbps • Video date rate = 216 Mbps • Compression is required!

  8. Can Multimedia Data Be Compressed? • Redundancy can be exploited to do compression! • Spatial redundancy • correlation between neighboring pixels in image/video • Spectral redundancy • correlation among colors • Psycho-visual redundancy • Perceptual properties of human visual system

  9. Categories of Compression • Lossless • No distortion of the original content • Used for computer data, medical images, etc. • Lossy • Some distortion • Suited for audio and video

  10. Compression Techniques

  11. Entropy Encoding Techniques • Lossless compression • Run-length encoding • Represent stream as (c1, l1), (c2, l2),…, (ck, lk) • 1111111111333332222444444 = (1, 10) (3, 5) (2,4) (4, 5) • Or ABCCCCCCCCDEFGGG = ABC!8DEFGGG • Pattern Substitution • Substitute smaller symbols for frequently used patterns

  12. Huffman Coding • Use variable length codes • Most frequently used symbols coded with fewest bits • Codes are stored in a codebook • Codebook transferred with the compressed stream

  13. Source Encoding Techniques • Transformation encoding • Transform the bit-stream into another domain • Data in the new domain more amenable to compression • Type of transformation depends on data • Image/video transformed from time domain into frequency domain (DCT)

  14. Differential/Predictive Encoding • Encoding the difference between actual value and a prediction of that value • Number of Techniques • Differential Pulse Code Modulation (DPCM) • Delta Modulation (DM) • Adaptive Pulse Code Modulation (APCM) • How they work? • When consecutive change little • Suited for audio and video

  15. Vector Quantization • Divide the data stream into blocks or vectors • One or two dimensional blocks • Use codebooks • Find the closest symbol in codebook for a given sample • Transmit the reference to that symbol • Codebook present at sender/receiver • When no exact match, could send the error • Lossy or lossless • Useful with known signal characteristics • Construct codebooks that can match a wide range of symbols

  16. Major Steps of Compression • Preparation • Uncompressed analog signal -> sampled digital form • Processing • Source coding • DCT typically used: Transform from time domain -> frequency domain • Quantization • Quantize weights into integer codes • Could use different number of bits per coefficient • Entropy encoding • Lossless encoding for further compression

  17. Conclusion • Multimedia data types • Why multimedia can be compressed? • Categories of compression • Compression techniques • Entropy encoding • Source encoding • Hybrid coding • Major steps of compression • What’s next? • JPEG • MPEG

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