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8.1 Key terms

You should be able to define the following terms in your own words.

  • agent

  • amplitude

  • analogue

  • ASCII

  • avatar

  • base 10

  • base 2

  • binary system

  • bit (binary digit)

  • bitmap

  • byte

  • Cartesian coordinates

  • CCD

  • character

  • charge-coupled device

  • chromin
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6.7 Summary

This section has looked at simulations, in which digital models of key aspects of the real world can be manipulated by programs. The examples included models of the world's climate, the early cosmos, stock markets, biological evolution and fantasy worlds and personalities. I've offered the view that simulation has far reaching implications for science, politics and society and will invite you to question that view in the final section.


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5.5.5 Summary

In this section I've briefly considered the very contentious question of what digital representations mean, but this debate must be left to another course. I have also described some of the devices that take digital information back into the analogue world of sight and sound, presenting it in a form that is meaningful to human eyes and ears.


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5.5 Types of output devices

We can make a start by appealing to your own general knowledge.

Exercise 16

You have a computer; list the output devices that it uses.

Dis
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4.3 Graphics and video: images

Vision is far and away humankind's most dominant sense. Every sighted person lives their entire waking (and dreaming) life at the centre of a visual field, a sphere of light, shade, colour, form and movement. Painters down the ages have tried to capture the essence of our visual life, as in the beautiful painting in Author(s): The Open University

3.8 How computers work with numbers

Today's mass media wrap computers in a damaging myth. The message of TV thrillers seems to be that computers are inscrutable, subtle devices, far beyond the ordinary person's comprehension. Only spectacularly gifted, young, ‘cool’ people seem to be capable of working with them successfully. For such individuals, a few clicks on the keyboard work miracles. The myth suggests that when we less gifted mortals use a computer and something goes wrong, it's our fault. We're simply too stupid for
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3.7 How we work with numbers

Most civilisations have had to face the problem of counting and recording numbers. Our own culture has adopted the so-called Arabic system of numbers. This system is now used more or less worldwide. In this section I will look very briefly at some of its key features.

We have an infinity of numbers at our disposal. If we start counting from 1, we can in theory go on for ever. But although there is an infinity of numbers, we only have a very small, fixed number of digits to
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3.5 Digital things

The terms ‘discrete’ and ‘digital’ are often used interchangeably. For example, The New Penguin Dictionary of Computing contains the following definition.

Digital. Any communication or computing technology whose data may only have a finite number of discrete values.

However, I want you to be clear about the strong association between a digital thing and a number
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2.5: Crossing the boundary

So computers are used to acquire, store and present, exchange, and manipulate interesting characteristics of the world. But this raises a serious problem: the world we inhabit and know so well and the world inside the computer are very different in kind. We live in an analogue world. The world of the computer is digital. The exact meaning of these terms may not be very clear to you at the moment. I will define them both in the next section. For the moment, the only point
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2.4 The pervasive computer

We can start with a simple proposition:

The computer's job is to acquire, store, present, control, exchange and manipulate interesting characteristics of the natural world.

So what are ‘interesting characteristics of the natural world‘? Obviously that depends on your point of view. An ordinary tourist might want to capture a memory of some scene she was experiencing – moonlight on the
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Learning outcomes

After studying this course, you should be able to:

  • give examples of quantities that are intrinsically analogue, and quantities that are intrinsically discrete/digital

  • define the terms ‘bit’, ‘byte’ and ‘word’

  • outline how visual information, such as pictures, diagrams and moving images can be expressed numerically inside a computer

  • describe how sounds such as speech and music can be represented inside a computer in terms of
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8.6 Summary

The logic operations introduced here are summarised in Table 1, which is an example of what is known as a ‘truth table’. It shows what the result (‘output’) of each logic operation is for all possible combinations of ‘input’ values. You may find this format a useful one for remembering t
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8.5 The exclusive-OR operation

The exclusive-OR operation (usually abbreviated to XOR, pronounced ‘ex-or’) combines two binary words, bit by bit, according to the rules:

  • 0 XOR 0 = 0

  • 0 XOR 1 = 1

  • 1 XOR 0 = 1

  • 1 XOR 1 = 0

In other words, the result is 1 when either bit is 1 but not when both bits are 1 or both bits are 0, or the result is 1 when the two bits are different and 0 when they are the sam
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8.4 The OR operation

The OR operation (occasionally called the inclusive-OR operation to distinguish it more clearly from the exclusive-OR operation which I shall be introducing shortly) combines binary words bit by bit according to the rules:

  • 0 OR 0 = 0

  • 0 OR 1 = 1

  • 1 OR 0 = 1

  • 1 OR 1 = 1

In other words, the result is 1 when either bit is 1 or when both bits are 1; alternativel
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7.5 Dividing 2's complement integers

Just as multiplication can be turned into repeated additions, so division can be turned into repeated subtractions. And just as shifting a binary integer one place to the left equates to multiplying by two, so shifting a binary integer one place to the right equates to dividing by two.

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7.3.2 Subtraction

As I indicated at the start of this section, subtraction is converted to addition by replacing the number to be subtracted by its additive inverse, which in the case of binary arithmetic is its 2's complement. An example should make this clear.

Example 9

Subtract the signe
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4.4 Representing sound

Sound, such as speech or music, is an analogue physical quantity that varies with time, and so the ideas you have already met in Section 2.5 about converting analogue weights to digital form are relevant here too. In particular, samples of the sound will have to be taken, and each sample will have to be quantised to the nearest binary code in the digital representation.

It's important to appreciate that sound such as speech or music varies rapidly with time, and so samples of it will ha
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3.2 Representing still images

There are two basic methods of representing still images in a computer: bit maps (also sometimes called raster graphics or raster images) and vector graphics (also sometimes called geometrical-shape graphics or graphics metafiles). Bit maps are usually used when there is a great deal of detail, as in photographs, or when there are irregular shapes, such as in drawings of natural objects. Vector graphics are usually reserved for line and blocked-colour drawings consisting of regu
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2.6 Representing true/false quantities

Sometimes a quantity that is to be represented in a computer has only two possible values, either true or false. An example of such a true/false quantity in the kitchen scales is the one that represents whether the scales are to weigh in metric or in imperial measure. The value of this true/false quantity is given by the true/false response to the statement ‘the most recent push of the input button made the measuring system metric’.

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2.2.3 Positive integers: converting denary numbers to binary

If computers encode the denary numbers of the everyday world as binary numbers, then clearly there needs to be conversion from denary to binary and vice versa. You have just seen how to convert binary numbers to denary, because I did a couple of examples to show you how binary numbers ‘work’. But how can denary numbers be converted to binary? I'll show you by means of an example.

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