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CIS679: Multimedia Basics r Multimedia data type r Basic compression techniques.

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Presentation on theme: "CIS679: Multimedia Basics r Multimedia data type r Basic compression techniques."— Presentation transcript:

1 CIS679: Multimedia Basics r Multimedia data type r Basic compression techniques

2 Multimedia Data Type r Audio r Image r Video

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

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

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

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

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

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

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

10 Compression Techniques Run-length Coding Entropy Encoding Huflfman Coding Arithmetic Coding DPCM Prediction DM FFT Transformation DCT Source Coding Bit Position Layered CodingSubsampling Sub-band Coding Vector Quantization J PEG MPEG Hybrid Coding H.261 DVI RTV, DVI PLV

11 Entropy Encoding Techniques r Lossless compression r Run-length encoding m Represent stream as (c 1, l 1 ), (c 2, l 2 ),…, (c k, l k ) m 1111111111333332222444444 = (1, 10) (3, 5) (2,4) (4, 5) m Or ABCCCCCCCCDEFGGG = ABC!8DEFGGG r Pattern Substitution m Substitute smaller symbols for frequently used patterns

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

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

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

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

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

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


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