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Digital Signal FormatsDigital Signal FormatsSyed A. Rizvi
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Why Digital Signaling? Why Digital Signaling? � Low cost of digital circuitry� Better resilience to noise� Flexibility: digitized data derived from
analog sources (voice, video signals etc.) can be combined with pure digital data (computer data) to form a general purpose communication system
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Analog to Digital ConversionAnalog to Digital Conversion� Pulse Amplitude Modulation� Pulse Code Modulation� Delta Modulation� Differential Pulse Code Modulation
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Pulse Amplitude Modulation Pulse Amplitude Modulation
� Pulse amplitude modulation (PAM) is a simple technique to convert an analog signal to a pulse-type signal where the amplitude of the pulse denotes the analog information
� PAM can easily be generated using an analog switch, as shown in figure below
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PAM (Continued)PAM (Continued)
Baseband Analog Waveform
Switching Waveform
Resulting PAM Signal
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Pulse Code ModulationPulse Code Modulation
� In this technique, a digital word (series of bits) is generated representing an instantaneous sample of an analog waveform (signal)
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PCM (Continued)PCM (Continued)
OutputVoltage
Input Voltage
Quantizer Output-Input Characteristics
Error Signal
PCM Signal
Analog, PAM, and Quantized PAM Signals
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PCM (Continued)PCM (Continued)
*B is the bandwidth of Analog Signal
Performance of a PCM System with Uniform Quantization (Noiseless Channel)
Number of Quantizer Levels Used
Length of the PCM Word Used
Bandwidth of PCM Signal
S/N (dB)
2 1 2B* 6.0 4 2 4B 12.0 8 3 6B 18.1 16 4 8B 24.1 32 5 10B 30.1 64 6 12B 36.1 128 7 14B 42.1 256 8 16B 48.2
65,536 16 32B 96.3
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PCM (Continued)PCM (Continued)
Advantages:� Relatively inexpensive digital circuitry� PCM data can be merged with pure digital data and can be
transmitted over common high-speed digital communication system� Regenerative repeaters can be used to reconstruct a clean PCM signal
from noisy/distorted PCM signal at appropriate intermediate points along the transmission path
� Superior S/N performance than that of an analog system
Disadvantage:� Higher bandwidth required than is required by an analog system
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Delta Modulation (DM)Delta Modulation (DM)� Very simple technique� Inexpensive to implement� Poor S/N performance when compared to PCM
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DM (Continued)DM (Continued)Operation:1. Generate a flat-topped PAM signal from the analog
input signal2. Compare the PAM signal with the accumulated output
of the Integrator (AO/P)3. If PAM > AO/P, Comparator’s output is 1; otherwise
it’s 04. Comparator’s output is fed to the Integrator to generate
accumulated output 5. Output of the Comparator form the DM signal
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DM (Continued)DM (Continued)
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DM (Continued)DM (Continued)S/N performance of a DMsystem as a Function of stepsize
� A small step size causesslop-overload distortion
� A larger step size causesgranular noise
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Amplitude Modulation (AM)Amplitude Modulation (AM)� In amplitude modulation, the amplitude of a high frequency
carrier is changed with the amplitude variations in the signal to be transmitted (baseband signal)
Baseband Signal
Resulting AM Signal
100max ×−=c
c
AAA
100min ×−=c
c
AAA
1002
minmax ×−=cAAA
% Positive Modulation
% Negative Modulation
% Modulation
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AM (Continued)AM (Continued)AM Broadcast Station Technical Standards
Item FCC Technical Standard
Assigned frequency 540-1700 KHz (in 10 KHz increments)
Channel Bandwidth 10 KHz
Carrier Frequency Stability +/- 20 Hz of the assigned frequency
% Modulation Maintain 85-95%; max: 100% negative, 125% positive
Maximum power licensed 50 kW
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Frequency Modulation (FM)Frequency Modulation (FM)
� In frequency (phase) modulation, the instantaneous frequency (phase) of the carrier is changed with the change in the baseband signal.
Baseband Signal
Instantaneous Frequency of the Corresponding FM Signal
Corresponding FM Signal
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FM (Continued)FM (Continued)
Item FCC Technical Standard
Assigned frequency 88.1MHz to 107.9MHz (in 200 KHz increments)
Channel Bandwidth 200 KHz
Carrier Frequency Stability
+/- 2 kHz of the assigned frequency
Noncommercial Stations
88.1 MHz to 91.9 MHz
Commercial Stations 92.1 MHz to 107.9 MHz
FCC FM Broadcasting Standard
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FM (Continued)FM (Continued)
FCC Two-way FM Mobile Radio Standard
Item FCC Technical Standard
Assigned Frequency
30-50 MHz (low VHF band) 144-148 MHz (2-m Amateur band) 148-174 MHz (high VHF band) 420-450 MHz (3/4 –m Amateur band) 475-470 MHz (UHF band) 470-512 MHz (UHF, T band) 806-928 MHz (900-MHz band)
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Time Division Multiplexing (TDM)Time Division Multiplexing (TDM)� Time division multiplexing is a technique for transmitting
information from several different sources serially over a communication channel by time interleaving of samples from these sources.
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TDM (Continued)TDM (Continued)� In an n-channel TDM system, the bandwidth of
the TDM system is n times the highest bandwidth of the source.
� TDM receiver stores the multiplexed data and directs it to appropriate output channel using frame synchronization.
� Cross talk is referred to the phenomenon when PCM samples from one channel appear in another channel. This is caused by poor filtering of PCM samples.
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Frequency Division Multiplexing (FDM)Frequency Division Multiplexing (FDM)
� Frequency division multiplexing is a technique for transmitting information from different sources simultaneously over a wideband channel.
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FDM (Continued)FDM (Continued)� Each signal is modulated on a separate carrier.� A composite baseband signal is formed by summing the
modulated subcarriers.� The composite signal is then modulated on the main
carrier.
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Amplitude Shift Keying (ASK)Amplitude Shift Keying (ASK)� In this technique, a carrier sinusoid is switched (keyed)
on and off with a binary signal.
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Phase Shift Keying (PSK)Phase Shift Keying (PSK)� In this technique, the phase of a sinusoid carrier is shifted
0o or 180o with a binary signal.
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Frequency Shift Keying (FSK)Frequency Shift Keying (FSK)� In this technique, the frequency of a sinusoidal carrier is
shifted from one frequency to another. In this way, one frequency of the carrier represents binary “1” and the other represents a binary “0.”