1. 4.2 Digital Transmission Pulse Modulation Pulse Code Modulation
Outlines
Pulse Modulation
Pulse Code Modulation
Delta Modulation
Line Codes
EKT 231 : COMMUNICATION SYSTEM
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2. Conversions Between Signal Types
Review
Conversions Between Signal Types
Sampling
Quantizing
Encoding

3. PULSE MODULATION (PM) Sampling analog information signal
Converting samples into discrete pulses
Transport the pulses from source to destination over physical transmission medium.
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4. Cont’d... Four (4) Methods of PM 1. PAM 2. PWM 3. PPM 4. PCM
PAM–Pulse Amplitude Modulation, PWM-Pulse Width Modulation
PPM-Pulse Position Modulation, PCM-Pulse Code Modulation
Analog Pulse Modulation
Digital Pulse Modulation
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5. 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.
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6. Pulse Amplitude Modulation (PAM)
The amplitude of pulses (carrier) is varied in accordance with the information signal.
Width & position constant.
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7. Pulse Width Modulation (PWM)
Sometimes called Pulse Duration Modulation (PDM).
The width of pulses is varied in accordance to information signal.
Amplitude & position constant.
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8. Cont’d... EKT 231 : COMMUNICATION SYSTEM
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9. 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.
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10. Advantages & Drawbacks of Pulse Modulation
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.
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
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11. 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)
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12. Example 1
A CD audio laser disk system has a frequency bandwidth of 20Hz to 20kHz.
What is the minimum sample rate required to satisfy the Nyquist sample rate?
Ans: fs=2fm(max)=40kHz.
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13. Cont’d... Two basic techniques used to perform the sampling function:
Natural sampling
Flat-top sampling
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14. 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
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15. Natural Sampling EKT 231 : COMMUNICATION SYSTEM
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16. Flat-top Sampling Most commonly 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.
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17. 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
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18. Flat-top Sampling EKT 231 : COMMUNICATION SYSTEM
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19. PULSE CODE MODULATION (PCM)
Basic scheme of PCM system
Quantization
Quantization Error
Companding
Block diagram & function of TDM-PCM communication system
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20. Basic scheme of PCM system
The most common technique for using digital signals to encode analog data is PCM.
Example: To transfer analog voice signals off a local loop to digital end office within the phone system, one uses a codec.
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21. codec Coder-decoder EKT 231 : COMMUNICATION SYSTEM
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22. Cont’d...
Because voice data limited to frequencies below 4000 Hz, a codec makes 8000 samples/sec. (i.e., 125 microsecond/sample).
(Nyquist)
If a signal is sampled at regular intervals at a rate higher than twice the highest signal frequency, the samples contain all the information of the original signal.
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23. PCM Block Diagram Four step process
Most common form of analog to digital modulation (better noise and interference immunity)
Four step process
Signal is sampled using PAM (Sample)
Integer values assigned to signal (PAM)
Values converted to binary (Quantized)
Signal is digitally encoded for transmission (Encoded)
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24. 4 Steps Process EKT 231 : COMMUNICATION SYSTEM
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25. Quantization
Quantization-the process of converting an infinite number of possibilities to a finite number of conditions.
The process of rounding off the amplitudes of flat top sample to a manageable number of levels.
The process of segmenting a sampled signal in a PCM system into different voltage levels, each level corresponding to different binary number.
The quantization levels determine the resolution of the digitizing system.
Analog signals are quantized to the closest binary value.
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26. Cont’d… Analog signal is sampled.
Converted to discrete-time continuous-amplitude signal
(Pulse Amplitude Modulation)
Pulses are quantized and assigned a digital value.
A 7-bit sample allows 128 quantizing levels.
PCM uses non-linear encoding, i.e., amplitude spacing of levels is non-linear
There is a greater number of quantizing steps for low amplitude
This reduces overall signal distortion.
This introduces quantizing error (or noise).
PCM pulses are then encoded into a digital bit stream.
8000 samples/sec x 7 bits/sample = 56 Kbps for a single voice channel.
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27. Quantized waveform Coded waveform 3 bit PCM pulse train
Sampling pulses
Quantization level (V)
Quantized waveform
Coded waveform
3 bit PCM pulse train
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28. PCM Example EKT 231 : COMMUNICATION SYSTEM
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29. Quantization
A process of converting an infinite number of possibilities to a finite number of conditions (rounding off the amplitudes of flat-top samples to a manageable number of levels).
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30. Cont’d… The quantization interval or quantum
= the magnitude difference between adjacent steps.
The resolution = the magnitude of a quantum
= the voltage of the minimum step size.
The quantization error (Qe) = the quantization noise (Qn)
= ½ quantum
= (orig. sample voltage – quantized level)
Folded PCM code = (sample voltage/resolution)
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31. Cont’d... Analog input signal Qe Exactly 2V No Qe Approximately +2.6 V
Sample pulse
PAM signal
PCM code
Exactly -1V
No Qe Qe
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32. Example 2
From the information given in the previous slide, determine the following;
(i) Resolution
(ii) Sample voltage at t3
(iii) Qe at t3
(iv) Explain the quality of the generated PCM
Ans: 1V,2.6V,0.4V,
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33. QUANTIZATION ERROR
A difference between the exact value of the analog signal & the nearest quantization level.
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34. Types of Quantization Midtread Midrise EKT 231 : COMMUNICATION SYSTEM
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35. Example 3
Using the same information, for the PCM code for analog sample voltage of +1.07V, determine;
(i) Quantized voltage
(ii) Qe
(iii) PCM code
Ans : 1,0.07,101
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36. Types of Quantizer
1. Uniform type : The levels of the quantized amplitude are uniformly spaced. 2. Non-uniform type : The levels are not uniform.
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37. Dynamic Range (DR)
Ratio of the largest possible magnitude/smallest possible magnitude.
Where
DR = absolute value of dynamic range
Vmax = the maximum voltage magnitude
Vmin = the quantum value (resolution)
n = number of bits in the PCM code
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38. Example 4
Calculate the dynamic range for a linear PCM system using 16-bit quantizing.
Calculate the number of bits in PCM code if the DR = dB
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39. Coding Efficiency = Minimum number of bits x 100
A numerical indication of how efficiently a PCM code is utilized.
The ratio of the minimum number of bits required to achieve a certain dynamic range to the actual number of PCM bits used.
Coding Efficiency = Minimum number of bits x 100
Actual number of bits
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40. Example 5 Given A PCM system with the following parameters
Maximum analog input frequency = 4kHz.
Maximum decoded voltage at Rx = ±2.55V.
Minimum dynamic range = 46 dB
Determine :
Minimum sample rate. (ans: 8kHz)
Minimum number of bits used (ans :n=7.63≈8)
Resolution (ans : 0.01V )
Quantization error (ans: 0.01V/2=0.005V)
Coding efficiency (ans: (8.63/9)x100%=95.89%)
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