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Chapter 5 – Signal Encoding and Modulation Techniques. 1/21. signal. Analog Data, Digital Signal. Digitization is conversion of analog data into digital data which can then: be transmitted using NRZ-L (digital signal) be transmitted using code other than NRZ-L

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  1. Chapter 5 – Signal Encoding and Modulation Techniques 1/21

  2. signal Analog Data, Digital Signal • Digitization is conversion of analog data into digital data which can then: • be transmitted using NRZ-L (digital signal) • be transmitted using code other than NRZ-L • be converted to analog signal by using modulation techniques (ASK, PSK, FSK) 2/21

  3. Codec (Coder-decoder) • Analog to digital conversion done using a codec (coder-decoder). Two techniques: • Pulse Code Modulation (PCM) • Delta Modulation (DM) 3/21

  4. Pulse Code Modulation (PCM) • Sampling Theorem: • “If a signal is sampled at regular intervals at a rate higher than twice the highest signal frequency, the samples contain all information in original signal” • Samples can be used to reconstruct the original signal • e.g., 100-4000Hz voice data, requires 2*4000=8000 sample per sec • These are analog samples, called Pulse Amplitude Modulation (PAM) samples • To convert to digital, each of these analog samples must be assigned a binary code 4/21

  5. Pulse Code Modulation (PCM) Example • The signal is assumed to be band-limited with bandwidth B • The PAM samples are taken at a rate of 2B, or once every Ts=1/(2B) seconds • Each PAM sample is quantized into one of 16 levels • Each sample is then represented by 4 bits. • 8 bits→256 level →better quality • 4000Hz voice→(8000sample/s)*8bits/sample=64Kbps 5/21

  6. Pulse Code Modulation (PCM) Block Diagram • By quantizing the PAM samples, the resulting signal is an approximation of the original one • This effect is known as quantization error or quantization noise • The Signal-to-Noise-Ratio (SNR) for quantizing noise: 6/21

  7. Linear Versus Non-Linear Encoding • Linear Encoding (uniform quantization): • Equally spaced quantization steps • Lower amplitude values are relatively more distorted • Non-Linear Encoding (non-uniform quantization): • Non-equally spaced quantization steps • Large number of quantization steps for signals with low amplitude, and smaller number of quantizing steps for signals with large amplitude 7/21

  8. Compressing Expanding Companding (Compressing-Expanding) • Instead of non-linear encoding, use companding+linear encoding • Companding gives more gain to weak signals than to strong signals on the input. At output, the reverse operation is performed Y X X 8/21

  9. Delta Modulation (DM) • An analog input is approximated by a staircase function that moves up or down by one quantization level () at each sampling interval (Ts). • A 1 is generated if the staircase function is to go up during the next interval; a 0 is generated otherwise. • The staircasefunction tracks the original waveform 9/45

  10. Delta Modulation Operation • For transmission: • the analog input is compared to the most recent value of the approximating staircase function. • If the value of the analog input exceeds that of the staircase function, a 1 is generated; otherwise, a 0 is generated. • Thus, the staircase is always changed in the direction of the input signal. • For reception: • The output of the DM process is therefore a binary sequence that can be used at the receiver to reconstruct the staircase function. Staircase 10/21

  11. Pulse Code Modulation (PCM) Versus Delta Modulation (DM) • DM has simplicity compared to PCM • DM has worse SNR compared to PCM • PCM requires more bandwidth • eg., for good voice reproduction with PCM • want 128 levels (7 bit) & voice bandwidth 4khz • need 8000 sample/s x 7bits/sample = 56kbps • PCM is more preferred than DM for analog signals 11/21

  12. analog data modulated signal Modulator Demodulator carrier signal Analog Data, Analog Signal • Modulate carrier signal with analog data (voice) • Why modulate analog signals? • higher frequency can give more efficient transmission • permits frequency division multiplexing (chapter 8) • Types of modulation • Amplitude Modulation (AM) • Frequency Modulation (FM) • Phase Modulation (PM) 12/21

  13. Amplitude Modulation (AM) • AM is the simplest form of analog modulation • Used in AM radio with carrier • Used also in analog TV broadcasting • Analog data modulates a carrier signal • Mathematically, the AM wave can be expresses as 13/21

  14. Time Domain description of AM Signal • Derive an expression for the AM wave if the input signal: • Envelope of AM signal: 11/45

  15. Carrier Upper Side Band(USB) Lower Side Band(LSB) Frequency Domain description of AM Signal • The Double SideBand Transmitted Carrier (DSBTC): 15/21

  16. Frequency Domain description of AM Signal • Consider a voice signal m(t) with a bandwidth that extend from 300Hz to 3000Hz being modulated on a 60 KHz Carrier 11/45

  17. Variations of AM signal • Double Side Band Transmitted carrier (DSBTC) • wast of power as the carrier is transmitted with the side bands • wast of bandwidth as both upper and lower side bands are transmitted (each side band contains the complete spectrum of the message signal m(t) ): Transmitted bandwidth=BT=2B • Double Side Band Suppressed Carrier (DSBSC) • Less power is required as no carrier is transmitted • wast of bandwidth as both upper and lower side bands are transmitted: Transmitted bandwidth=BT=2B • Single Side Band (SSB) • Less power is required as no carrier is transmitted • Less bandwidth as one side band is transmitted: BT=B 17/21

  18. Angle Modulation • Frequency Modulation (FM) and Phase Modulation (PM) are special cases of angle modulation • Used in FM radio with carrier • The angle modulated signal is expressed as: • Phase Modulation (PM): - Example: find s(t) if 18/21

  19. Frequency Modulation (FM) • The angle modulated signal is expressed as: • FM when: - Example: find s(t) if 19/21

  20. Transmitted Bandwidth for AM, PM and FM • Transmitted bandwidth for AM: • Transmitted bandwidth for PM and FM: • Thus, both PM and FM require greater bandwidth than AM 20/21

  21. AM, PM, FM 21/21

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