Chapter 4 Digital Mod_Part 2_1

4.2 Digital TransmissionOutlinesPulse Modulation (Part 2.1)Pulse Code Modulation (Part 2.2)Delta Modulation (Part 2.3)Line Codes (Part 2.4) Sampling analog information signalConverting samples into discrete pulsesTransport the pulsesfrom source to destination over physical transmission medium. Contd...Four (4) Methods1. PAM2. PWM3. PPM4. PCM Analog Pulse ModulationDigital Pulse Modulation Contd...nalog !ulse ModulationCarrier signal is pulse "aveform and the modulated signal is "here one of the carrier signals characteristic (either amplitude, width or position) is changed according to information signal. The amplitude of pulses is varied in accordance with the information signal.Width & position constant.Pulse Amplitude Modulation (PAM) Pulse Width Modulation (PWM)Sometimes called !ulse #uration Modulation (!#M).The "idth of pulses is varied in accordance to information signal.mplitude $ position constant. Contd... % Modulation in "hich the temporal positions of the pulses are varied in accordance "ith some characteristic of the information signal.% mplitude $ "idth constant.Pulse Position Modulation (PPM) dvantages $ #ra"&ac's of !ulse Modulation (oise immunity. )elatively lo" cost digital circuitry. &le to &e time division multiple*ed "ith other pulse modulated signal. Storage of digital streams. +rror detection $ correction )e,uires greater -. to transmit $ receive as compared to its analog counterpart. Special encoding $ decoding methods must &e used to increased transmission rates $ more di/cult to &e recovered. )e,uires precise synchroni0ation of cloc's &et"een T* $ )*. Sampling process of ta'ing samples of information signal at a rate of (y,uists sampling fre,uency.(y,uists Sampling Theorem 1The original information signal can &e reconstructed at the receiver "ith minimal distortion if the sampling rate in the pulse modulation system e,ual to or greater than t"ice the ma*imum information signal fre,uency.fs ! fm (ma") Contd...T"o &asic techni,ues used to perform the sampling function1Natural samplingFlat-top sampling (atural SamplingTops of the sample pulses retain their natural shape during the sample interval.Fre,uency spectrum of the sampled output is di2erent from an ideal sample.mplitude of fre,uency components produced from narro"3 4nite5"idth sample pulses decreases for the higher harmonics )e,uiring the use of fre,uency e,uali0ers (atural Sampling Flat5top SamplingCommon used in !CM systems.ccomplish in a sample-and-hold circuitTo periodically sample the continually changing analog input voltage $ convert to a series of constant5amplitude !M voltage levels.The input voltage is sampled "ith a narro" pulse and then held relatively constant until the ne*t sample is ta'en. Contd6Sampling process alters the fre,uency spectrum $ introduces aperture error.The amplitude of the sampled signal changes during the sample pulse time.dvantages17ntroduces less aperture distortion Can operate "ith a slo"er #C Flat5top Sampling +(# 8F !)T 9.: