1/12 Data Encoding Part IV : Analog to Digital Conversion Surasak Sanguanpong nguan@ku.ac.th http://www.cpe.ku.ac.th/~nguan Last updated: 25 November 2004 Applied Network Research Group Department of Computer Engineering, Kasetsart University 2/12 Analog t0 Digital Conversion 011100011011001100 Analog Analog/Digital Digitized Analog/Digital signal Converter signal Converter (CODEC) (CODEC) Sending voice signal over a long distance digital line � A CODEC (Coder-Decoder) is required for the conversion � Applied Network Research Group Department of Computer Engineering, Kasetsart University 1
3/12 Conversion Process PCM Transmitter A/D Converter Digital Analog Binary Sampling Binary Sampling Quantization Encoder Quantization Encoder Encoding Circuit Encoding Circuit 1 Sampling of analog 3 2 4 signal with Pulse Analog signal is Assign integral � Amplitude Modulation Binary digits are converted to digital values in a specific (PAM) Technique transformed into a signal range to sample digital signal using instance D to D encoding Applied Network Research Group Department of Computer Engineering, Kasetsart University 4/12 PAM Double-polarity PAM Input signal Single-polarity PAM Sampling clock pulse � The analog Signal is sampled at equally spaced intervals in time Applied Network Research Group Department of Computer Engineering, Kasetsart University 2
5/12 Sampling Rate The higher sampling rate, the closer the recovered signal � approaches the original signal. Practically, voice signals are sampled at 8KHz based on the � Nyquist sampling theorem Applied Network Research Group Department of Computer Engineering, Kasetsart University 6/12 Nyquist theorem “ In a perfectly noiseless channel, if f is the maximum frequency the medium can transmit, the receiver can completely reconstruct a signal by sampling it 2*f times per second” Nyquist, 1920 Applied Network Research Group Department of Computer Engineering, Kasetsart University 3
7/12 Quantization 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 16 levels of quantization require 4 bits A/D converter quantization level = 2 (number of A/D converter bits) Applied Network Research Group Department of Computer Engineering, Kasetsart University 8/12 PCM sample Original signal 3.9 4.2 3.4 3.2 2.8 1.2 PAM pulse 4 4 3 3 3 PCM pulse 1 1 with quantized error 011 100 011 011 001 100 PCM output 011100011011001100 Applied Network Research Group Department of Computer Engineering, Kasetsart University 4
9/12 Nonlinear encoding Without nonlinear encoding With nonlinear encoding 15 15 14 14 13 13 12 12 11 Strong signal 11 10 10 9 Weak signal 8 9 8 7 7 6 5 6 4 5 3 4 3 2 2 1 1 0 0 Applied Network Research Group Department of Computer Engineering, Kasetsart University 10/12 Companding process 11 11 10 Implement nonlinear 01 Implement nonlinear � � 00 11 10 10 encoding via companding encoding via companding 01 00 11 process 01 10 process 01 00 11 00 10 Companding = � Companding = � 01 00 11 Compressing Expanding 10 00 Compressing Expanding 01 00 11 01 10 01 00 11 10 10 01 00 11 10 11 01 00 Compressor Linear Linear Expander Compressor Linear Linear Expander v i v o Network circuit A/D D/A circuit circuit A/D D/A circuit v i v’ o Applied Network Research Group Department of Computer Engineering, Kasetsart University 5
11/12 Companding standard A-Law (North America) V out 1 μ=255 A|V i | 0 ฃ |V i | ฃ 1 0.8 |V 0 | = μ=80 A 1+ ln A 0.6 μ=0 1+ ln [ A|V i | ] ฃ |V i | ฃ 1 1 0.4 |V 0 | = 1+ ln A A 0.2 0 V in 0 0.2 0.4 0.6 0.8 1.0 V out 1 μ Law (ITU-T) Α=100 0.8 ln [ 1 + μ |V i | ] 0 ฃ |V i | ฃ 1 Α=80 0.6 |V 0 | = ln [ 1+ μ] Α=1 0.4 0.2 0 1.0 V in 0 0.2 0.4 0.6 0.8 Applied Network Research Group Department of Computer Engineering, Kasetsart University 12/12 PCM System PCM Transmitter Digitized Analog voice signal voice signal A/D Sampling A/D Sampling Encoder Converter Encoder Circuit Converter Circuit PCM Receiver Digitized Analog voice signal voice signal D/A Regeneration D/A Regeneration Decoder Decoder Converter Circuit Converter Circuit Applied Network Research Group Department of Computer Engineering, Kasetsart University 6
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