Multimedia communications EG-371Dr Matt Roach Multimedia Communications EG 371 and EG 348 Dr Matthew Roach Lecture 2 Digital video signals – acoustic

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

Multimedia communications EG-371Dr Matt Roach Typical frequency ranges 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

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

Multimedia communications EG-371Dr Matt Roach DPCM encoder (robust) 3 rd order predictor Predictor coefficients –C1, C2, C3 –Bit shifting ADC Subtractor adder R1 Band limiting filter PCMDPCM R2R3 C3C2C1

Multimedia communications EG-371Dr Matt Roach Bit shifting C 1 = 0.5 C 2 = C 3 = 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-371Dr Matt Roach Speech, Image & Multimedia communications Dr Matt Roach Lecture 2 History of video formats

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

Multimedia communications EG-371Dr Matt Roach Chrominance components HVS –Sensitive to luminance RGB (4:4:4) –Luminance –2 chrominance Y = R G B C b = B - Y C r = R - Y

Multimedia communications EG-371Dr Matt Roach 4:2:2 format Studio quality History dictates –Luminance 13.5 MHz –Chrominance 6.75 MHz M 12345N12345N Chrominance sample

Multimedia communications EG-371Dr Matt Roach 4:2:0 Digitised format Broadcast quality 525 line system –Y = 640 x 480 –C b = C r = 320 x 240 –60 Hz 625 line system –Y = 768 x 576 –C b = C r = 384 x 288 –50 Hz M 12345N12345N Chrominance sample

Multimedia communications EG-371Dr Matt Roach Source intermediate format SIF 4:1:1 VCR quality 525 line system –Y = 320 x 240 –C b = C r = 160 x 120 –30 Hz 625 line system –Y = 384 x 288 –C b = C r = 192 x 144 –25 Hz M N Chrominance sample (Y) Luminance sample (C b, C r )

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

Multimedia communications EG-371Dr Matt Roach Quarter CIF QCIF (4:1:1) 64 Kbps networks –Y = 180 x 144 –C b = C r = 90 x 72 –15/7.5 Hz Modems –S-QCIF –Y = 128 x 96 –C b = C r = 64 x M N Chrominance sample (Y) Luminance sample (C b, C r )

Multimedia communications EG-371Dr Matt Roach 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-371Dr Matt Roach Bit rate example CIF Y = 384 x 288 C b = C r = 192 x Hz 384 x 288 x 8 x (192 x 144 x 8 x 30) 39,813,120 bps = 39 Mbps

Multimedia communications EG-371Dr Matt Roach Bit rate example 4:2:0 (broadcast quality) Y = 720 x 480 C b = C r = 480 x Hz (interlaced fields) 720 x 240 x 8 x (480 x 240 x 8 x 60) 124,416,000 bps = 124 Mbps

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