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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.”