Physics

Unit 10: ANALOG AND DIGITAL SIGNALS

Topic Area: DIGITAL TECHNOLOGY

Sub-Topic Area: ANALOG AND DIGITAL SIGNALS

Key unit competence: By the end of this unit I should be able to differentiate analog from digital signals.

Unit Objectives: 

By the end of this unit learners will be able to;

     ◊   explain the transmission of information in a communication system.     

     ◊   explain with examples the use of digital and analog signals in everyday applications

10.1 INTRODUCTION

A signal is any kind of physical quantity that conveys information. Audible speech is certainly a kind of signal, as it conveys the thoughts (information) of one person to another through the physical medium of sound. Hand gestures are signals too. This text is another kind of signal, interpreted by your English-trained mind as information about electric circuits. In this unit, the word signal will be used primarily in reference to an electrical quantity of voltage or current that is used to represent or signify some other physical quantity.

A communication system is made up of devices that employ one of two communication methods (wireless or wired), different types of equipment (portable radios, mobile radios, base/fixed station radios and repeaters) accessories (examples include speaker microphones, battery eliminators and carrying cases) and/or enhancements (encryption, digital communications, security measures, and networking) to meet the user needs.

The most common processing of a signal in a communication system consists of passing the signal through a linear time-invariant system.  In this context, such a system is often spoken of as a “filter”.  These systems are usually applied to reduce some undesirable components in the signal, to compensate for some undesirable distortion of the signal, or to accentuate some characteristic of a signal. This unit discusses digital and analog signals and their use in modern communication.

10.2  INFORMATION TRANSMISSION  IN A COMMUNICATION SYSTEM

A communication system comprises of three sections or parts; transmitting end, propagation medium and receiving end. This is shown on Fig. 10.1 below.

 The modulated signal is sent along a channel in the propagating medium by a transmitter. The propagation medium is a channel through which information is transmitted. This may be a cable or free space. At the receiving end, the receiver may have to select and perhaps amplify the modulated signal before the demodulator extracts from it the information signal for delivery to the receptor of information.

A propagation or transmission medium can be classified as;

Linear medium: if different waves at any particular point in medium can be superposed.

Bounded medium: if it is finite in extent, otherwise unbound.

Uniform medium or homogeneous medium: if its physical properties are unchanged at different points.

Isotropic medium: if its physical properties are the same in different directions.

10.3 COMMUNICATION TERMS AND CONCEPTS 

1. Communication is the process of sharing the messages through continuous flow of symbols.

2. Communicators (Sender/receiver) are the participants in communication. Typically the roles reverse regularly.

3. Message is a single uninterrupted verbal or nonverbal utterance.

4. Code means a system suitable for creating/carrying messages through a specific medium. 

    •    encode (put into code) and  

    •    decode (take out of code)

5. Channels (verbal, nonverbal, etc.) means the specific mechanism (“pipeline”) used to transmit the message.

6. Mode of communication (face-to-face, television, web, phone, etc.) - form or technology of transmission— determines kind of code used.

7. Noise - interference with message — external (physical), internal (mental) or semantic (misunderstanding/reaction).

8. Environment (part of context) - is that which surrounds and provides a basis for the meaning of a message:   

    •    Physical  (surroundings) 

    •    Temporal (point in time) 

    •    Relational (the existing relationship between communicators - friends, strangers, etc.)  

    •    Cultural  (language and behaviour of community and the communicator(s) come from)

9. Feedback - checks effects of messages 

    •    positive feedback eg. “keep doing what you’re doing”  

    •    negative feedback eg. “change what you’re doing”.

10. Levels (contexts) of Communication 

    •    Intrapersonal  

    •    Interpersonal  

    •    Public Communication  

    •    Mass Communication (non-interactive)  

    •    Computer Mediated Communication (interactive)

10.4 ELEMENTS OF COMMUNICATION

Sender

The sender is a party that plays the specific role of initiating communication. To communicate effectively, the sender must use effective verbal as well as nonverbal techniques. Such as:-

    •    Speaking or writing clearly.

    •    Organizing your points to make them easy to follow and understand.

    •    Maintaining eye contact.

    •    Using proper grammar.

    •    Giving accurate information.

All the above components are essential in the effectiveness of your message. One will lose the audience if it becomes aware of obvious oversights on ones part. The sender should have some understanding of who the receiver is, in order to modify the message to make it more relevant.

Receiver

The receiver means the party to whom the sender transmits the message. A receiver can be one person or an entire audience of people. In the basic communication model, the receiver is directly connected with the speaker. The receiver can also communicate verbally and nonverbally. The best way to receive a message is:

      •    To listen carefully.

      •    Sitting up straight.

      •    Making eye contact.

      •    Don’t get distracted or try to do something else while you’re listening.

      •    Nodding and smiling as you listen.

      •    Demonstrate that you understand the message.

Message

The message is the most crucial element of effective communication which includes the content a sender conveys to the receiver. A message can come in many different forms, such as an oral presentation, a written document, an advertisement or just a comment. In the basic communication model, the way from one point to another represents the sender’s message travelling to the receiver. The message isn’t necessarily what the receiver perceive it to be. Rather, the message is what the sender intends the message to be. The sender must not only compose the message carefully, but also evaluate the ways in which the message can be interpreted.

Channel

The channel is a medium through which a message travels from the sender to the receiver. The message travels from one point to another via a channel of communication. The channel is a physical medium stands between the sender and receiver.

Many channels or types of communication exist, such as

    •    The spoken word,

    •    Radio or television,

    •    An Internet site or

    •    Something written, like a book, letter or magazine.

Every channel of communication has its advantages and disadvantages. For example, one disadvantage of the written word, on a computer screen or in a book, is that the receiver cannot evaluate the tone of the message. For this reason, effective communicators should make written word communications clear so receivers don’t rely on a specific tone of voice to convey the message accurately. The advantages of television as a channel for communication include its expansive reach to a wide audience and the sender’s ability to further manipulate the message using editing and special effects.

Feedback

This describes the receiver’s response or reaction to the sender’s message. The receiver can transmit feedback through asking questions, making comments or just supporting the message that was delivered. Feedback helps the sender to determine how the receiver interpreted the message and how it can be improved.

10.5 TYPES OF INFORMATION AND REQUIREMENTS

Constructional/creative information: This includes all information that is used for the purpose of producing something. Before anything can be made, the originator mobilizes his intelligence, his supply of ideas, his know-how, and his inventiveness to encode his concept in a suitable way.

Operational information: All concepts having the purpose of maintaining some “industry” in the widest sense of the word are included under this kind of information. Many systems require operational information in the form of programs for proper functioning. Examples of operational information include:

•    the operating system of a computer (eg. DOS programs),

•    the program controlling a robot or a process computer,

•    warning systems for airplanes and ships,

• the hormonal system of the body

Communication information: This is composed of all other kinds of information, eg. letters, books, phone calls, radio transmissions, bird songs and also the message of the Bible. Aspect of such information does not include the construction of a product, neither it is involved in maintaining some process. The goals are transmission of a message, spreading joy, amusement, instruction and personal confidences.

10.6 SIMPLEX TRANSMISSION

Simplex transmission is a single one-way base band transmission. Simplex transmission, as the name implies, is simple. It is also called unidirectional transmission because the signal travels in only one direction. An example of simplex transmission is the signal sent from the TV station to the home television.

Data in a simplex channel is always one way. Simplex channels are not often used because it is not possible to send back error or control signals to the transmit end.

10.7 HALF-DUPLEX COMMUNICATIONS

Half-duplex transmission is an improvement over simplex transmission because the traffic can travel in both directions. Unfortunately, the road is not wide enough to accommodate bidirectional signals simultaneously. This means that only one side can transmit at a time. Two-way radios, such as police or emergency communications mobile radios, work with half-duplex transmissions. If people at both ends try to talk at the same time, none of the transmissions get through.

10.8 FULL-DUPLEX COMMUNICATIONS

Full-duplex transmission operates like a two-way, two-lane street. Traffic can travel in both directions at the same time.

A land-based telephone conversation is an example of full-duplex communication. Both parties can talk at the same time, and the person talking on the other end can still be heard by the other party while they are talking. Although when both parties are talking at the same time, it might be difficult to understand what is being said.

Full-duplex networking technology increases performance because data can be sent and received at the same time. Digital subscriber line (DSL), two-way cable modem, and other broadband technologies operate in full-duplex mode. With DSL, for example, users can download data to their computer at the same time they are sending a voice message over the line.

10.9 BANDWIDTH AND SIGNAL FREQUENCY

Frequency is a parameter that determines how often the sinusoidal signal goes through a cycle.  It is usually represented with the symbol f, and it has the unit hertz .

Where T is a periodic time and is measured in seconds.

The bandwidth of a composite signal is the difference between the highest and the lowest frequencies contained in that signal. It is typically measured in hertz, and may sometimes refer to passband bandwidth or baseband bandwidth, depending on context.

Mathematically, the bandwidth is given by;

Where f_USB and f_LSB stand for upper side band and lower side band respectively.

10.10 ANALOG SIGNAL SYSTEM

A system is a physical set of components that take a signal and produces a signal. In terms of engineering, the input is generally some electrical signal and the output is another electrical signal. Analog systems operate with values that vary continuously and have no abrupt transitions between levels. For a long time, almost all electronic systems were analog, as most things we measure in nature are analog.  For example, your voice is analogous; it contains an infinite number of levels and frequencies.  Therefore, if you wanted a circuit to amplify your voice, an analog circuit seems a likely choice.

In Rwanda recently analog systems were replaced by digital systems that provide greater capacity of data transfer and increased reliability and security.

10.9.1 Example of an analog electronic system

A public address system

A public address system (PA system) is an electronic sound amplification and distribution system with a microphone, amplifier and loudspeakers, used to allow a person to address a large public, for example for announcements of movements at large and noisy air and rail terminals or a sports stadium.

10.11 ANALOG SIGNALS

Analog signal is a continuous signal that contains time varying quantities. An analog signal is a continuous wave denoted by a sine wave and may vary in signal strength (amplitude) or frequency (time). The sine wave’s amplitude value can be seen as the higher and lower points of the wave, while the frequency (time) value is measured in the sine wave’s physical length from left to right.

Analog signal can be used to measure changes in physical phenomenon such as light, sound, pressure, or temperature. For instance, microphone can convert sound waves into analog signal. Even in digital devices, there is typically some analog component that is used to take in information from the external world which will then get translated into digital form –using analog to digital converter.

10.12  ADVANTAGES AND DISADVANTAGES OF ANALOG SIGNALS

Advantages

 •    Uses less bandwidth than digital sounds.

 •    More accurate representation of sound.

 •    It is the natural form of sound.

 •    Because of editing limitations, there is little someone can do to tinker with the sound, so what you are hearing is the original sound.

Disadvantages

  •    There are limitations in editing.

  •    Recording analog sound on tape is expensive.

  •    It is harder to synchronize analogous sound.

  •    Quality is easily lost if the tape becomes ruined.

  •    A tape must always be wound and rewound in order to listen to specific part of sound which can damage it.

   •    Analog is susceptible to clipping where the highest and lowest notes of a sound are cut out during recording.

10.13 DIGITAL SIGNALS

In electronic signal and information processing and transmission, digital technology is increasingly being used because, in various applications, digital signal transmission has many advantages over analog signal transmission. Numerous and very successful applications of digital technology include the continuously growing number of PC’s, the communication network ISDN as well as the increasing use of digital control stations (Direct Digital Control: DDC).

Unlike analog technology which uses continuous signals, digital technology encodes the information into discrete signal states. When only two states are assigned per digital signal, these signals are termed binary signals. One single binary digit is termed a bit - a contraction for binary digit.

10.14. ADVANTAGES OF DIGITAL TECHNOLOGY

•    More capacity from the same number of frequencies; that is, they provide superior Spectral Efficiency. This is a result of the modulation methods used, and the fact that, in many cases more than one ‘conversation’ can be accommodated within a single radio channel.

•    Consistent voice clarity at low received signal levels near the edge of coverage. The general consensus is that digital radios provide better audio quality than analog ones. With analog FM radios, the audio quality steadily declines as the received signal strength gets weaker. Digital radios however, will have a consistent audio quality throughout the full service area. The edges of the coverage area in a digital radio system are similar to those experienced with cellular telephones.

•    Data is defined in the standard. This means data implementations are no longer proprietary, there are a wide variety of data mechanisms and inter operability can extend into the data domain. With the accepted increase of efficiency by using data communications over voice, this will further increase the usability and effectiveness of digital radio systems.

•    Secure transmissions: In digital technologies, data and voice can be secured using encryption without impacting voice quality using industry standard encryption techniques.

10.15 COMPARING DIGITAL AND ANALOG SIGNALS

10.16 LOGIC GATES

There are three basic logic gates each of which performs a basic logic function. They are called NOT, AND and OR. All other logic functions can ultimately be derived from combinations of these three. For each of the three basic logic gates a summary is given including the logic symbol, the corresponding truth table and the Boolean expression.

The AND gate is an electronic circuit that gives a high output (1) only if all its inputs are high.  A dot (.) is used to show the AND operation i.e. A.B. It can also be written as AB.

The OR gate is an electronic circuit that gives a high output (1) if one or more of its inputs are high. A plus (+) is used to show the OR operation.

The NOT gate is an electronic circuit that produces an inverted version of the input at its output. It is also known as an inverter. If the input variable is A, the inverted output is known as NOT A. This is also shown as . as shown at the outputs.

The ‘Exclusive-OR’ gate is a circuit which will give a high output if either, but not both, of its two inputs are high.  An encircled plus sign (⊕) is used to show the EX–OR operation.

                                                           EXERCISE 10.1

1. Produce a truth table from the following logic circuit (network)

2. For the logic circuits below produce the truth tables. Rember, if there are 2 inputs then there will be 4 outputs; if there are 3 inputs then there will be 8 possible outputs. Use the idea shown in the logic circuits discussed in section 10.6.

Information transmission in a communication system

The signals from information source are added to the carrier in the modulator. The modulated signal is sent along a channel in the propagating medium by a transmitter. The propagation medium is a channel through which information is transmitted. This may be a cable or a free space.

Communication Terms and Concepts

     •    Communication

     •    Communicator

     •    Message

     •    Medium

     •    Noise

     •    Environment

     •    Feedback

     •    Levels Elements of communication

     •    Sender

     •    Receiver

     •    Message

     •    Channel

     •    Feedback

Types of information and requirements

     •    Constructional/creative information

     •    Operational information

     •    Communicational information

Simplex transmission

Simplex transmission is a single one-way base band transmission. Simplex channels are not often used because it is not possible to send back error or control signals to the transmit end.

Half-duplex communications

Half-duplex transmission is an improvement over simplex because the traffic can travel in both directions. Full-duplex networking technology increases performance because data can be sent and received at the same time.

Bandwidth and signal Frequency

The bandwidth of a composite signal is the difference between the highest and the lowest frequencies contained in that signal.

Mathematically, the bandwidth is given by;

BW = f_USB – f_LSB

Where F_USB and f_LSB stand for upper side band and lower side band respectively.

•    Medium

•    Noise

•    Environment

•    Feedback

•    Levels

Elements of communication

Analogue signal system

Analogue systems operate with values that vary continuously and have no abrupt transitions between levels.

Analog signals

Analog signal is a continuous signal that contains time varying quantities. An analog signal is a continuous wave denoted by a sine wave and may vary in signal strength (amplitude) or frequency (time).

Digital signals

Unlike analog technology which uses continuous signals, digital technology encodes the information into discrete signal states. Numerous and very successful applications of digital technology include the continuously growing number of PC’s, the communication net work ISDN as well as the increasing use of digital control stations (Direct Digital Control: DDC).

Advantages of digital technology

•    More capacity from the same number of frequencies.

•    Consistent voice clarity at low received signal levels near the edge of coverage.

•    Data is defined in the standard.

•    Secure transmissions.

Logic gates

There are three basic logic gates each of which performs a basic logic function, they are called NOT, AND and OR. All other logic functions can ultimately be derived from combinations of these three.