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Multimedia Communications EG 371 and EG 348

Multimedia Communications EG 371 and EG 348. Dr Matthew Roach Dr.matthew.roach@googlemail.com Lecture 2 Digital video signals – acoustic. Acoustic Signal. Digitisation Quantisation Error also known as noise Sampling frequency Nyquist sampling theorem Maximum frequency x 2. Speech

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Multimedia Communications EG 371 and EG 348

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  1. Multimedia CommunicationsEG 371 and EG 348 Dr Matthew Roach Dr.matthew.roach@googlemail.com Lecture 2 Digital video signals – acoustic Multimedia communications EG-371 Dr Matt Roach

  2. Acoustic Signal • Digitisation • Quantisation • Error also known as noise • Sampling frequency • Nyquist sampling theorem • Maximum frequency x 2 Multimedia communications EG-371 Dr Matt Roach

  3. Speech maximum frequency is 10 kHz minimum sampling rate is 20,000 samples per second sampling precision is 12 bits 20,000 samples/sec x 12 bits/sample = 240 kbps Telephony standard 8kHz All mobile phones in the world 8 bit mu/a law non-linear amplitude matched to characteristics of speech wide dynamic range Typical frequency ranges Multimedia communications EG-371 Dr Matt Roach

  4. CD quality sound • Acoustic frequency : 20 ~ 20,000 Hz • Minimum sampling frequency : 40,000 Hz • 10% over sampling • 44,100 Hz • 16 bits sample is used for CD system • each channel of the audio stream is converted into bits at a rate of • 44,100 samples/sec x 16 bits/sample = 705,600 bps • Stereo : left, right channel • bit rate is 1.41 Mbps (stereophonic) Multimedia communications EG-371 Dr Matt Roach

  5. Lowing the bit rate • Without compression • Lower the sampling rate • high frequency components are lost - lower quality signal • Lower the bits per sample • higher levels of quantisation noise • simple to implement • Use compression algorithm • comparable perceptual value to original signal • reduced bandwidth Multimedia communications EG-371 Dr Matt Roach

  6. Pulse code modulation (PCM) • Analogue system • 200Hz – 3.4KHz • Nyquist = 6.8KHz • 8KHz was used due to poor band limiting filters • Bits per sample • 7 and 8 • 56kbs, 64kbs Multimedia communications EG-371 Dr Matt Roach

  7. Differential pulse code modulation (DPCM) • Difference between successive samples • Smaller range • Fewer bits with the same sampling rate • Typical saves one bit compared to PCM • 64 kbps reduced to 56 kbps • Difference signal • Accuracy determined by previous sample • Residue signal Multimedia communications EG-371 Dr Matt Roach

  8. DPCM encoder • Errors • Accumulative • Solution use average of predictors (R) PCM DPCM Band limiting filter Subtractor ADC Register (R) adder Multimedia communications EG-371 Dr Matt Roach

  9. DPCM encoder (robust) • 3rd order predictor • Predictor coefficients • C1, C2, C3 • Bit shifting PCM DPCM Band limiting filter Subtractor ADC adder C1 C2 C3 R1 R2 R3 Multimedia communications EG-371 Dr Matt Roach

  10. Bit shifting C1 = 0.5 C2 = C3 = 0.25 • Contents of R1 shifted right one bit • multiply by 0.5 • Contents of R2 and R3 each shifted right two bits • multiply by 0.25 • Added together to form the new predictive value Multimedia communications EG-371 Dr Matt Roach

  11. Speech, Image & Multimedia communications Dr Matt Roach Lecture 2 History of video formats Multimedia communications EG-371 Dr Matt Roach

  12. Chromatic images • Colour • Represented by vector not scalar • Red, Green, Blue (RGB) • Hue, Saturation, Value (HSV) • luminance, chrominance (Yuv , Luv) S=0 Green Hue degrees: Red, 0 deg Green 120 deg Blue 240 deg Red Green V=0 Multimedia communications EG-371 Dr Matt Roach

  13. Chrominance components • HVS • Sensitive to luminance • RGB (4:4:4) • Luminance • 2 chrominance • Y = 0.299 R + 0.587 G + 0.114 B • Cb = B - Y • Cr = R - Y Multimedia communications EG-371 Dr Matt Roach

  14. 4:2:2 format 1 2 3 4 5 - - - - - - - - - - - - - - - - - - - - - - - - M • Studio quality • History dictates • Luminance • 13.5 MHz • Chrominance • 6.75 MHz 1 2 3 4 5 N Chrominance sample Multimedia communications EG-371 Dr Matt Roach

  15. 4:2:0 Digitised format 1 2 3 4 5 - - - - - - - - - - - - - - - - - - - - - - - - M • Broadcast quality • 525 line system • Y = 640 x 480 • Cb = Cr = 320 x 240 • 60 Hz • 625 line system • Y = 768 x 576 • Cb = Cr = 384 x 288 • 50 Hz 1 2 3 4 5 N Chrominance sample Multimedia communications EG-371 Dr Matt Roach

  16. Source intermediate format SIF 4:1:1 1 2 3 4 5 6 7 - - - - - - - - - - - - - - - - - - - - - M • VCR quality • 525 line system • Y = 320 x 240 • Cb = Cr = 160 x 120 • 30 Hz • 625 line system • Y = 384 x 288 • Cb = Cr = 192 x 144 • 25 Hz 1 2 3 4 5 6 7 N Chrominance sample (Y) Luminance sample (Cb, Cr) Multimedia communications EG-371 Dr Matt Roach

  17. Common intermediate format CIF (4:1:1) • CIF • Spatial resolution SIF 625 line system • Y = 384 x 288 • Cb = Cr = 192 x 144 • Temporal resolution SIF 525 line system • 30 Hz • 4CIF • Y= 720 x 576, Cb = Cr = 360 x 288 • 16CIF • Y= 1440 x 1152, Cb = Cr = 720 x 576 Multimedia communications EG-371 Dr Matt Roach

  18. Quarter CIFQCIF (4:1:1) 1 2 3 4 5 6 7 8 9 10 11 12 13 - - - - - - - - M • 64 Kbps networks • Y = 180 x 144 • Cb = Cr = 90 x 72 • 15/7.5 Hz • Modems • S-QCIF • Y = 128 x 96 • Cb = Cr = 64 x 48 1 2 3 4 5 6 7 8 9 10 11 12 13 N Chrominance sample (Y) Luminance sample (Cb, Cr) Multimedia communications EG-371 Dr Matt Roach

  19. Bit rates Spatial Resolution • Luminance • Chrominance • Temporal resolution • Frames per second • Number of bits per sample • 8 bits i.e. 1 byte Multimedia communications EG-371 Dr Matt Roach

  20. Bit rate example CIF • Y = 384 x 288 • Cb = Cr = 192 x 144 • 30 Hz • 384 x 288 x 8 x 30 + 2(192 x 144 x 8 x 30) • 39,813,120 bps = 39 Mbps Multimedia communications EG-371 Dr Matt Roach

  21. Bit rate example4:2:0 (broadcast quality) • Y = 720 x 480 • Cb = Cr = 480 x 240 • 60 Hz (interlaced fields) • 720 x 240 x 8 x 60 + 2(480 x 240 x 8 x 60) • 124,416,000 bps = 124 Mbps Multimedia communications EG-371 Dr Matt Roach

  22. CIF • CIF • 39 Mbps • QCIF • 4.6 Mbps • S-QCIF • 2.2 Mbps • ISDN • 64 Kbps • 128 Kbps • 2.0 Mbps • Modem • < 56 Kbps Multimedia communications EG-371 Dr Matt Roach

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