02n 176db pcm example pcm pcm block diagram pcm non

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Unformatted text preview: n at each since sample interval sample hence can encode each sample as single bit 1 for up or 0 for down Delta Modulation Example Delta Delta Modulation Operation Delta Delta Modulation - Performance Delta Good voice reproduction Good PCM - 128 levels (7 bit) Voice bandwidth 4khz Should be 8000 x 7 = 56kbps for PCM Data compression can improve on this e.g. Interface coding techniques for video The principal advantage of DM over PCM The is the simplicity of its implementation. is In general, PCM exhibits better SNR In characteristics at the same data rate. characteristics PCM verses Delta Modulation PCM DM has simplicity compared to PCM but has worse SNR issue of bandwidth used eg. for good voice reproduction with PCM • want 128 levels (7 bit) & voice bandwidth 4khz • need 8000 x 7 = 56kbps data compression can improve on this still growing demand for digital signals use of repeaters, TDM, efficient switching PCM preferred to DM for analog signals Analog Data, Analog Signals Analog modulate carrier frequency with analog data why modulate analog signals? higher frequency can give more efficient transmission permits frequency division multiplexing (chapter 8) types of modulation Amplitude Frequency Phase Analog Modulation Modulation Techniques Amplitude Modulation Frequency Modulation Phase Modulation Analog Data, Digital Signals performance Why digital techniques in communication? Repeaters are used instead of amplifiers, Repeaters there is no additive noise. there time-division multiplexing (TDM) is used for time-division digital signals instead of the frequencydigital division multiplexing (FDM) used for analog division signals, with TDM, there is no intersignals, modulation noise, whereas we have seen that modulation this is a concern for FDM. this The conversion to digital signaling allows the The use of the more efficient digital witching techniques. techniques. Analog Data, Analog Signals Analog Why modulate analog signals? Higher frequency can give more efficient Higher transmission transmission Permits frequency division multiplexing Permits (chapter 8) (chapter Types of modulation Amplitude Frequency Phase Analog Modulation Modulation QAM QAM Is the popular signaling techniques used in ADSL. Send two different signals simultaneously on the same carrier one shifted by 90° w.r.t. other Each carrier is ASK modulated. QAM Modulator & demodulator QAM Spread Spectrum Spread Analog or digital data Analog signal Spread data over wide bandwidth Makes jamming and interception harder Frequency hoping Signal broadcast over seemingly random series Signal of frequencies of Direct Sequence Each bit is represented by multiple bits in Each transmitted signal transmitted Chipping code Frequency Hopping Frequency Frequency Hopping Frequency Direct Sequence Direct Summary Summary looked at signal encoding techniques digital data, digital signal analog data, digital signal digital data, analog signal analog data, analog signal...
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