A digital signal is a sequence of discrete discontinuous voltage pulses. Each pulse is a signal element (symbol). Binary data are transmitted by encoding each data bit into signal elements.

Digital-to-digital Conversion

Basic Ideas of conversion:

BINARY DATA BINARY DATA to to

DIGITAL SIGNAL.DIGITAL SIGNAL.

There is a variety of language that can be used to describe the process of transforming binary data into a digital signal. Line coding is another term used to describe digital encoding

Line coding and decoding

Digital Transmission

Ratio of Data/Signal

r = d / s

Data element – The actual information/data/message to be deliveredSignal element – The right format of signals or suitable signal types

used to carry the data element (actual data)

d = data elements = signal element

The higher the r the better the transmission efficiency

Data element (Actual information/message)

Signal element (how the information is ‘carried’)

Both elements can be specified in terms of:

Time interval.

Both measured in secs

Rate.

Data rate: Bits per second (bps).

Signal rate: Symbols per second or Baud.

The relationship between the date rate and the signal rate will indicate how efficiently the bandwidth is being used.

Requirements for digital signalingAdequate noise immunity. (The receiver can determine a level above the noise. This will either be above a threshold, in relation to previous symbols or relative to 0V.)Synchronization. (The receiver knows when to sample the signal.)Security. (Only the receiver is able to decode the signal)

Effect of lack of synchronization

Adequate Noise immunity

This effectively means maintaining an adequate signal to noise ratio. As the ability to control noise is limited the mechanisms focus on signal viability. Two broad approaches are considered:

Minimize attenuation mechanisms. The significant factor here is to remove any DC bias because coupling will severely attenuate this component of the signal.

Use transitions and changes as mechanisms to encode the data.

Line coding schemes

UNIPOLAR

NRZ scheme (Non Return to Zero)

Main problem of NRZ: No timing information is carried to provide correct synchronisation at

the receiver. (Not knowing when one bit ended and next bit starts) DC Component – when voltage level in a digital signal is constant for a

while. The spectrum creates a very low frequencies which cannot allow to be passed by certain devices/systems (e.g. transfomer/ telephone)

Baseline Wandering: Running average of received signal power.

(BI)POLAR

RZ scheme: (Return to Zero) at the middle of the bit

No DC component problem

Main problem of RZ: Requires two signal changes to

encode a data bit (low r), which needs extra bandwidth

3 signal levels results in more complexity

Baseline Wandering can still exists.

POLAR Bi-phase: Manchester and differential Manchester

Duration of signal bit divided into two halves. (two signal levels)The transition at the middle of the bit is used for synchronization.

Only drawback: r = 0.5 (or 1/2): The minimum bandwidth of Manchester and differential Manchester is 2 times that of NRZ (1/2bit/baud).

No DC component problem

No Baseline Wandering

Timing information is provided

BIPOLAR schemes: AMI* and pseudoternary

In bipolar encoding, we use three levels: positive, zero, and negative.

AMI - Alternate Mark Inversion

No DC component problem No Baseline Wandering Signal rate ‘r’ can be

improved Need to provide Timing

information via: Start/stop bits Control bits prior to actual

data

http://upload.wikimedia.org/wikipedia/commons/b/b6/Ami_encoding.svg

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Analog to Digital Conversion/Encoding

A-D conversion requires Sampling method; where a series of pulse-trains is applied to the analog signal. This process is called Pulse Amplitude Modulation (PAM).

Hence, it is require to apply sufficient sample of pulse-trains or sampling frequency to the analog signal:

fs = 2 X Signal_Bandwidth

(according to Nyquist Sampling Theorem).

After suitable sampling, each pulse will be represented a binary number: the process of converting this pulse-level to binary level in called Quantisation. Number of M bits used for Quantisation is a crucial factor.

The final resultant is the digital representation of the analog signal in a binary

coding representation via the line-coding or binary encoding. The entire A-D

process is called Pulse Code Modulation (PCM)

Analog-to-Digital Conversion

Nyquist’s Sampling Theorem

To convert an analogue signal to a digital signal, the sampling frequency involved (frequency of pulse-trains) needs to be:

fs = 2 × WSampling frequency

Bandwidth of signal

A band-limited signal of finite energy, which has no frequency components higher than W Hertz, may be completely described by specifying the values of the signal at instants of time separated by (1/2W) seconds or can be recovered from a knowledge of its samples taken at a rate of 2W samples per second.

Message signal Frequency Content

Frequency Content of the sampled message signal

Sampling frequencyMessage bandwidth

fs = 2 × W

Nyquist’s Sampling Theorem

Sampling frequency = message bandwidth

Message signal cannot be recovered from the sampled signal !!

Nyquist’s Sampling Theorem

Components of PCM encoder

Three different sampling methods for PCM

Pulse Amplitude Modulation (PAM) - Sampling

Pulse Amplitude Modulation (PAM) - Sampling

Duty cycle =

T / Ts

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Quantized PAM Signal

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

Quantizing Using Sign and Magnitude

Quantized PAM Signal

The McGraw-Hill Companies, Inc., 1998

PCM

The McGraw-Hill Companies, Inc., 1998

From Analog to PCM

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

From Analog to PCM

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

From Analog to PCM

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

From Analog to PCM

Extra Notes on

Line coding:

MULTILEVEL. In mBnL schemes, a pattern of m data elements is encoded as a pattern of n signal elements.

Multilevel: 2B1Q scheme(2bits/baud)

Multilevel: 8B6T scheme (8/6 bits/baud).

Converts bytes of data into 6 digits of ternary code (36 = 729 possible states)

Multitransition: MLT-3 scheme

Summary of line coding schemes