A/D Conversion No. 1  Seattle Pacific University Analog to Digital Conversion Based on Chapter 5 of William Stallings, Data and Computer Communication Kevin Bolding Electrical Engineering Seattle Pacific University

A/D Conversion No. 2  Seattle Pacific University Analog to Digital Conversion Convert a continuous analog signal to a stream of bits for transmission Why convert? 1. Transmission channel uses digital symbols (e.g. line coding) 2. Storage method is digital (e.g. computer system) 3. Digital methods have some sort of preferred characteristic Overview of the process 1. Sample the analog signal 2. Quantitize (convert analog samples to discrete) 3. Send digital data as a stream of bits

A/D Conversion No. 3  Seattle Pacific University Sampling Sampling theorem: If sample rate > 2x max frequency (f) And samples have infinite precision (analog)  Can reproduce signal exactly after filtering out frequencies >f Pulse-Amplitude Modulation – PAM Samples have analog (infinite precision) values Undersampling If sample rate is < 2f then it is possible to map multiple waveforms to the data (aliasing) Sampling – Take “snapshots” of analog signal at a regular interval

A/D Conversion No. 4  Seattle Pacific University Pulse Code Modulation PCM: Approximate analog samples with a discrete sample n bit sample  2 n levels Errors Not analog, so quantizing error is present Each additional bit halves the quantizing error (in volts) SNR is Power ratio (proportional to V 2 ) Each extra bit used increases SNR by factor of 4 (6.02 dB) For n-bit quantization, the SNR = 6.02(n) dB

A/D Conversion No. 5  Seattle Pacific University Nonlinear coding Linear coding samples using a linear scale Large amplitude  Cross many levels during waveform (good) Nonlinear encoding More levels concentrated near the center level Wider spacing at edges Small amplitude  Cross few levels during waveform (bad)

A/D Conversion No. 6  Seattle Pacific University Delta Modulation Send info on changes to the signal Standardize to a quantization level Signal increase --> ‘1’ Signal decrease --> ‘0’ May have slope overload Increase to a multibit delta 2-bit delta allows +2,+1,-1,-2 No code for “don’t change” Flat signals become square waves 1-bit Delta 2-bit Delta