TRANSMISSIONOF

DIGITAL SIGNALS

Pranshu PathakSachin ArtaniIT-B

DIGITAL SIGNALS

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A digital signal refers to an electrical signal that is converted into a pattern of bits. Unlike an analog signal, which is a continuous signal that contains time-varying quantities, a digital signal has a discrete value at each sampling point.

CLASSIFICATION OF DIGITAL SIGNALS TRANSMISSION

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Baseband data transmission

Pulse analog Pulse digital1. PAM 1. PCM2. PWM 2. DPCM3. PPM 3. DM

4. ADM

Band Pass or Broadband data transmission

1. ASK2. FSK3. PSK

The fundamental difference between baseband and band pass data transmission is with respect to channel. For baseband the channel is coaxial cable or fibre optic cable or twisted pair cable and for, band pass, the channel is free space i.e. air.

BASEBAND DATA TRANSMISSION

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m(t)

c(t)

PAM

Pulse Amplitude ModulationThe carrier frequency is high periodic rectangular pulses. In PAM, the amplitude of the pulses is changed according to the sampled value.

Applications:1. Ethernet2. Photo biology3. Electronic devices for LED

lighting4. Digital television

GENERATION OF PAM SIGNAL

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Multiplierm(t) PAMSignal

c(t)

When a continuous message signal and a high frequency carrier signal are passed through the multiplier, PAM signal is generated.

PULSE WIDTH MODULATION

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m(t)

c(t)

PWM

This is also called as pulse duration modulation or pulse length modulation.

Here, The width of the pulse changed according to the sampled value of message signal.

Pulse width modulation is used in a variety of applications including sophisticated control circuitry. A common way we use them is to control dimming of RGB LEDs or to control the direction of a servo motor.

GENERATION OF PWM SIGNAL

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m(t) PAMSignal

c(t)

Multiplier

Slicer

It is generated by multiplying the message signal with a high frequency periodic rectangular carrier signal and passing the resultant signal from slicer, which slice the pulse into appropriate width.

PULSE POSITION MODULATION

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This is also called Pulse time modulation.Here the position of the pulse is changed depending on the sampled value of message signal.

All three modulation technics, PAM, PWM and PPM are used in telemetry applications. Telemetry is an automated communications process by which measurements and other data are collected at remote locations for recording and analysis.PPM is also used in satellite communication.

m(t)

c(t)

PWM

PPM

PULSE CODE MODULATION

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• In PCM, a message signal is represented by a sequence of coded pulses, which is accomplished by representing the signal in discrete form in both time and amplitude.

• The LPF is included to prevent aliasing of the message signal.

• The basic operations performed in the transmitter of a PCM signal are sampling, quantizing and encoding.

• The quantizing and encoding operations are usually performed in the same circuit, which is called an analog-to-digital converter.Sampler converts a continuous varying time signal

into discrete time signal. While Quantizer converts a continuously varying amplitude signal to a discrete amplitude signals with limited levels.

BLOCK DIAGRAM OF PCM SIGNAL

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Source of continuous time signal

LPF Quantizer

Sampler

Decoder

Encoder

Reconstruction filter

Destination

PCMSignal

PCM Signal

Transmitter

Receiver

DIFFERENTIAL PULSE CODE MODULATION

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Differential PCM can reduce the quantization error of the message signal but not its Bandwidth. The difference between an input value and its quantized value (such as round-off error) is referred to as quantization error.

Transmitter Receiver

Delay

Sampler

Quantizer

Encoder∑

∑∑

m(t) +- DPC

M

Decoder

LPF

Delay

DELTA MODULATION

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It is used to decrease the bandwidth of transmitted signal. The block schematic is similar to DPCM but the encode is 1 bit encoder.

Transmitter Receiver

Delay

Sampler

Quantizer

Encoder∑

∑∑

m(t) +- 0 Decoder

LPF

Delay

1

QUANTIZATION ERROR IN DELTA MODULATION

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When step size is large, the staircase approximation may vary over flat m(t). This is called Granular noise.

When step size is too small for the stair case approximation, it is possible that the approximation may not follow step increase in m(t). This is called Slope overload distortion.

Slope overhead distortion Granular distortion

ADAPTIVE DELTA MODULATION

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In ADM additional hardware is designed to provide variable step size, thereby reducing slope-overload effects without increasing the granular noise.

BAND PASS DATA TRANSMISSION

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In Band pass or broadband data transmission there are three technics:

1. Amplitude Shift Keying (ASK)2. Frequency Shift Keying (FSK)3. Phase Shift Keying (PSK)

LINE CODES : (ELECTRICAL REPRESENTATION OF 1’S AND 0’S)

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1. Unipolar/on-off signaling: 1 is represented by a pulse and 0 is not represented.

2. Polar/NRZ signaling: 1 and 0’s are represented by +ve and –ve pulses.

3. RZ signaling: Pulse of half symbol wide is sent for 1, nothing for 0.

4. Bipolar signaling: +ve and –ve pulses are used for 1. nothing for 0.

5. Manchester code (Split-phase): For ‘1’, +ve to –ve and for a ‘0’, -ve to +ve pulses.

1 1 0 1 0 1 1

AMPLITUDE SHIFT KEYING

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In ASK, the amplitude of the carrier is modulated according to the message signal.

When an on-off signaling binary message signal is multiplied with a high frequency sinusoidal carrier signal, then ASK signal is formed.

BLOCK DIAGRAM OF ASK

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Binary Data

Multiplier

ASK Signal

Carrier Wave

PHASE SHIFT KEYING

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• Phase-shift keying (PSK) is a digital modulation scheme that conveys data by changing (modulating) the phase of a reference signal (the carrier wave).

• It is widely used for wireless LANs and Bluetooth communication.

• Types of PSK modulation include BPSK, QPSK and 8PSK. BPSK stands for Binary Phase Shift Keying. QPSK stands for Quadrature Phase Shift Keying. 

FREQUENCY SHIFT KEYING

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Frequency-shift keying (FSK) is a method of transmitting digital signals. The two binary states, logic 0 (low) and 1 (high), are each represented by an analog waveform. Logic 0 is represented by a wave at a specific frequency, and logic 1 is represented by a wave at a different frequency. A modem converts the binary data from a computer to FSK for transmission over telephone lines, cables, optical fibre, or wireless media. The modem also converts incoming FSK signals to digital low and high states, which the computer can "understand."

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