EET260 Introduction to digital communication

Digital signals

 Binary digital signals use two discrete voltage levels to represent binary 1 or 0.

 Combining multiple bits into words permits us to represent larger values.

 Digital circuits operate on digital signals performing logic and arithmetic functions.

1 0 1 1 0 1 5 V 0 V time

Analog systems

 Analog systems use electrical signals that vary continuously, not having discrete values  Analog signals are electrical representations of signals from nature (pressure, light, sound, etc.)  Examples of analog systems: AM/FM radio, cassettes, telephone, VCR, standard television time Time (sec)

Analog examples

Digital systems

 Digital systems use electrical signals that represent discrete, binary values.  Digital signals are not representative of signals that occur in nature (pressure, light, sound, etc.).

 Natural signals must be converted into digital format.

 Historically, signals in communications systems have been analog but a migration to digital systems has been underway for the last 25 years.

Digital examples

Advantages of digital signals

 The most important advantage of digital communications is noise immunity.

 Receiver circuitry can distinguish between a binary 0 and 1 with a significant amount of noise.

1 0 1 1 0 1 Time (sec) analog signal with noise digital signal with noise

Advantages of digital signals

 Digital signals can be stripped of any noise in a process called signal regeneration.

 Consider a network of relay stations.

microwave relay stations

Advantages of digital signals

 An analog signal is received, amplified and retransmitted at each station.

 However, the noise is also amplified each link.

original analog signal signal at repeater 1 signal at repeater 2 signal at repeater 3 microwave relay stations

Advantages of digital signals

 An digital signal is received, regenerated, then retransmitted at each station.

 The noise can be eliminated at each repeater.

original analog signal signal at repeater 1 signal at repeater 2 signal at repeater 3 microwave relay stations

Advantages of digital signals

 Even if a digital signal does contain bit errors, many of these errors can be fixed at the receiver through the use of error correcting codes.

 Error correcting codes allows CDs with minor scratches to be played without errors.

 We will discuss such codes later.

scratched CD

Advantages of digital signals

 Digital signals are easier to multiplex.

 Multiplexing is the process of allowing multiple signals to share the same transmission channel.

Advantages of digital signals

 Digital is the native format for computers.

 Computers permit the digital signal processing (DSP) and digital storage of communication signals.

 DSP allows operations such as filtering, equalization and mixing to be done numerically without the use of analog circuits.

 DSP also permits data compression.

Transmission of digital data

 There are two ways to move bits from one place to another:  Transmit all bits of a word simultaneously (parallel transfer).  Send only 1 bit at a time (serial transfer).

Serial transmission

 In serial transmission, each bit of a word is transmitted sequentially, one after another.  The least significant bit (LSB) is transmitted first, and the most significant bit (MSB) last.  Each bit is transmitted for a fixed interval of time.

 Serial data can be transmitted faster and over longer distances than parallel data.

 Serial buses are now replacing parallel buses in equipment where very high speeds are required.

Parallel transmission

 Parallel data transmission is extremely fast because all the bits of the data word are transferred simultaneously.

 There must be one wire for each bit of information to be transmitted. Multi-wire cable must be used.

Parallel transmission

   Parallel data transmission is impractical for long-distance communication because of:  Cost of laying multi-wire cables.  Signal attenuation over long distances.

 At high speeds, capacitance and inductance of multiple wires distorts the pulse signal. To reduce this, line lengths must be severely shortened.

 To achieve clock speeds up to 400 MHz, line lengths must limited to a few inches Parallel data transmission by radio would be complex and expensive. One transmitter and one receiver for each bit.

Serial-parallel conversion

 Because both parallel and serial transmission occur in computers and other equipment, there must be techniques for converting between parallel and serial and vice versa.  Such data conversions are usually taken care of by

shift registers,

sequential logic circuits made up of a number of flip-flops connected in cascade.

Parallel outputs Data input

D

0

C Q

0

Q

0

D

1

C Q

1

Q

1

D

2

C Q

2

Q

2

D

4

C Q

4 Clock input

Q

3

Serial-parallel conversion

Conversion from analog to digital

 Before we can use digital transmission, we must convert the signal of interest into a digital format.  Translating an analog signal into a digital signal is called analog-to-digital (A/D) conversion, digitizing a signal, or encoding.

 The device used to perform this translation is known as an analog-to-digital converter or ADC.

Conversion from analog to digital

 An analog signal is a smooth or continuous voltage or current variation.  Through A/D conversion these continuously variable signals are changed into a series of binary numbers.

01101010100111001101010101111

Time (sec)

A/D conversion

 The first step in A/D conversion is a process of sampling the analog signal at regular time intervals.

sample points sampling frequency

f

 1

T

Time (sec) sampling period (

T

)

A/D conversion

 How often do we need to sample the signal?

 How large does our sampling frequency

f

need to be in order to accurately represent the signal?

high sampling rate Time (sec) Time (sec) Time (sec) low sampling rate Time (sec)

Minimum sampling frequency

 The minimum sampling rate required in order to accurately reconstruct the analog input is given by the Nyquist sampling rate

f N

given

f N

 2

f m

where

f m

input.

is the highest frequency of the analog  The Nyquist rate is a theoretical minimum.

 In practice, sampling rates are typically 2.5 to 3 times the Nyquist rate

f N

.

Example Problem 1

Consider the signal from the oboe depicted below in time and frequency domain representations.

a.

What is the maximum frequency present in the oboe signal?

b.

c.

Based upon this, what would be the minimum sampling rate according to Nyquist?

What would be practical sampling rate?

1 0.5

0 -0.5

-1 1 1.0005 1.001 1.0015 1.002 1.0025 1.003 1.0035 1.004 1.0045 1.005

Time (sec) 0.25

0.2

0.15

0.1

0.05

0 0 1000 2000 3000 Frequency (Hz) 4000 5000 6000

A/D conversion

 The actual analog signal is smooth and continuous and represents an infinite number of actual voltage values.  It is not possible to convert all analog samples to a precise binary number.

 Therefore, samples are converted to a binary number whose value is close to the actual sample value.

A/D conversion

 The A/D converter can represent only a finite number of voltage values over a specific range.  An A/D converter divides a voltage range into discrete increments, each of which is represented by a binary number.

A/D conversion

 The process of mapping the sampled analog voltage levels to these discrete, binary values is called

quantization

.

 Quantizers are characterized by their number of output levels.

 An

N

-bit quantizer has 2

N

levels and outputs binary numbers of length

N

.

 Telephones use 8-bit encoding   CD audio use 16-bit encoding  2 2 16 8 = 256 levels = 65,536 levels