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4.2 Digital Transmission

4.2 Digital Transmission. Outlines Pulse Modulation (Part 2.1) Pulse Code Modulation (Part 2.2) Delta Modulation (Part 2.3) Line Codes (Part 2.4). PULSE MODULATION (PM). Sampling analog information signal Converting samples into discrete pulses

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4.2 Digital Transmission

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  1. 4.2 Digital Transmission Outlines Pulse Modulation (Part 2.1) Pulse Code Modulation (Part 2.2) Delta Modulation (Part 2.3) Line Codes (Part 2.4)

  2. PULSE MODULATION (PM) • Sampling analog information signal • Converting samples into discrete pulses • Transport the pulses from source to destination over physical transmission medium.

  3. Cont’d... • Four (4) Methods 1. PAM 2. PWM 3. PPM 4. PCM Analog Pulse Modulation Digital Pulse Modulation

  4. Cont’d... • Analog Pulse Modulation • Carrier signal is pulse waveform and the modulated signal is where one of the carrier signal’s characteristic (either amplitude, width or position) is changed according to information signal.

  5. Pulse Amplitude Modulation (PAM) • The amplitude of pulses is varied in accordance with the • information signal. • Width & position constant.

  6. Pulse Width Modulation (PWM) • Sometimes called Pulse Duration Modulation (PDM). • The width of pulses is varied in accordance to information signal. • Amplitude & position constant.

  7. Cont’d...

  8. Pulse Position Modulation (PPM) • Modulation in which the temporal positions of the pulses are varied in accordance with some characteristic of the information signal. • Amplitude & width constant.

  9. Noise immunity. Relatively low cost digital circuitry. Able to be time division multiplexed with other pulse modulated signal. Storage of digital streams. Error detection & correction Requires greater BW to transmit & receive as compared to its analog counterpart. Special encoding & decoding methods must be used to increased transmission rates & more difficult to be recovered. Requires precise synchronization of clocks between Tx & Rx. Advantages & Drawbacks of Pulse Modulation

  10. Sampling • A process of taking samples of information signal at a rate of Nyquist’s sampling frequency. • Nyquist’s Sampling Theorem : The original information signal can be reconstructed at the receiver with minimal distortion if the sampling rate in the pulse modulation system equal to or greater than twice the maximum information signal frequency. fs >= 2fm (max)

  11. Cont’d... • Two basic techniques used to perform the sampling function: • Natural sampling • Flat-top sampling

  12. Natural Sampling • Tops of the sample pulses retain their natural shape during the sample interval. • Frequency spectrum of the sampled output is different from an ideal sample. • Amplitude of frequency components produced from narrow, finite-width sample pulses decreases for the higher harmonics • Requiring the use of frequency equalizers

  13. Natural Sampling

  14. Flat-top Sampling • Common used in PCM systems. • Accomplish in a sample-and-hold circuit • To periodically sample the continually changing analog input voltage & convert to a series of constant-amplitude PAM voltage levels. • The input voltage is sampled with a narrow pulse and then held relatively constant until the next sample is taken.

  15. Cont’d… • Sampling process alters the frequency spectrum & introduces aperture error. • The amplitude of the sampled signal changes during the sample pulse time. • Advantages: • Introduces less aperture distortion • Can operate with a slower ADC

  16. Flat-top Sampling

  17. END OF PART 2.1

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