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Lecture 3 Paul Flynn

Modulation. Lecture 3 Paul Flynn. Frequency Spectrum. Long Waves, Medium Waves and Short Waves. Modulation of Analog Signals. Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM). Figure 5.24 Analog-to-analog modulation. Types of analog-to-analog modulation.

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Lecture 3 Paul Flynn

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  1. Modulation • Lecture 3 • Paul Flynn

  2. Frequency Spectrum

  3. Long Waves, Medium Waves and Short Waves

  4. Modulation of Analog Signals Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM)

  5. Figure 5.24Analog-to-analog modulation

  6. Types of analog-to-analog modulation

  7. Amplitude modulation (AM)

  8. Frequency modulation (FM)

  9. Phase Modulation (PM)

  10. Digital modulation • Three basic methods • – Amplitude shift keying (ASK) • – Frequency shift keying (FSK) • – Phase shift keying (PSK

  11. Amplitude shift keying (ASK) • Use different amplitude to represent 0 and 1. • – Simple, low bandwidth – Sensitive to interference. • • Multi-path propagation, noise or path loss heavily influence the amplitude. • – A constant amplitude in wireless environment can not be guaranteed. • • Used in wired optical communication. • – A light pulse =1, no light =0.

  12. Frequency shift keying (FSK) • Binary FSK (BFSK) – One frequency for 0 and one • frequency for 1. – needs larger bandwidth • • Avoid discontinuity – Discontinuity creates high • frequencies as side effects. – Continuous phase modulation (CPM) can be used. • • Demodulation: – Use two bandpass filters for 2 frequencies.

  13. Phase shift keying (PSK) • Use shift in phase to represent data. • • Binary PSK (BPSK) – Shift the phase by 180. • • Synchronization is important • • More resistant to interference • • More complex transmitters and receivers.

  14. Digitizing Analog Signals • Sample • Quantize • Encode

  15. Sampling Analog Signals Original Signal Sample Times Sample Values Reformed Signal

  16. Digitizing Analog Signals Original Signal and Samples Actual Sample Values Quantized Sample Values Reconstructed Signal Original Signal

  17. Linear Quantizing Actual Sample Values Quantized Sample Values Reconstructed Signal Original Signal

  18. Linear Quantizing Linear: quantizing steps for all segments are the same height. SQR 2-42

  19. Quantizing Noise Transmit Receive

  20. Voice Digitization Process

  21. Pulse Code ModulationWaveform-Time Domain-Nondifferential Sample Values Quantized value of each sample is coded

  22. 111 110 101 100 Answer: 3 bits for 8 levels L=2n 011 010 001 000 Coding How many bits are needed to code this many levels?

  23. Companding or Non-linear Encoding • Companding = compressing + expanding • Why companding? • Quantization levels not evenly spaced • Reduces overall signal distortion • Can also be done by companding

  24. Binary Signaling Encoding Schemes EIA 232 0 = 1 0 to 1 1 to 0 1=sq sq 1 & 0 c x on 1 chg on 0 n n n n s s s s n n sq wv 1 inv 2nd 1 alt mark inversion (B8ZS) B8Zs

  25. 2B1Q (ISDN) EACH LEVEL REPRESENTS TWO BITS 00, 01, 10, AND 11

  26. The digital Loop Signal

  27. AMI (Alternate Mark Conversion) Violation NO MORE THAN 15 0s IN A ROW START OF CONTROL INFO

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