Assessment

• New lesson editor
• Computing
• Upper Secondary (Key Stage 4)
• GCSE

Assessment

Learning objective

End of unit assessment

Y10 Data representation end of unit assesment – Fill in form

https://www.101computing.net/

https://www.bbc.co.uk/bitesize/subjects/z34k7ty

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Images

Describe bitmap image representation

Light reflecting off the object is captured through the lens of the device.

The colour of light detected at each part of the image is stored as single points known as pixels, (picture elements).

Describe bitmap image representation

This ‘map of pixels’ is referred to as a
bitmap image.

As with any data in a computer, this must be represented as a sequence of bits.

Each pixel is represented by a binary sequence.

Fill in the blanks

The colour of light detected at each part of the image is stored as single points known as pixels

A bitmap image is a grid of pixels

Each of these pixels in the image need to be represented by a binary sequence

colour

pixels

bitmap

grid

represented

binary

bit depth

The number of bits per pixel used in a bitmap image
is referred to as its colour depth.

A large colour depth allows an image to display
more colours.

However, this results in an image requiring a larger number of bits for its representation.

Describe bitmap image representation

Describe bitmap image representation

The shortest possible binary sequence for a pixel is 1 bit.

Each state can be used to represent a different colour.

1 = black

0 = white

As the binary sequence for a pixel is increased, more colours can be represented.

Describe bitmap image representation

11 = green

10 = red

01 = black

00 = white

The number of sequences, and therefore the number of colours, can be worked out as 2n where n is the number of bits.

Describe bitmap image representation

11 = green

10 = red

01 = black

00 = white

2ⁿ = 2 x 2 = 4

How many colours can be represented by a sequence
of 4 bits?

2ⁿ

A

3

B

6

C

8

D

16

Describe image resolution

Image resolution is the size of an image measured
in pixels.

It is measured by the number of pixels across the width and height of the image.

width

height

Describe image resolution

Image resolution can be expressed as its pixel dimensions.

Image resolution = width x height

180 pixels

300 pixels

Image resolution = 180 x 300

Describe image resolution

Image resolution can also be expressed by the total number of pixels in the image.

Image resolution = width x height

180 pixels

300 pixels

Image resolution = 180 x 300

Image resolution = 54,000 pixels

Which statement is correct?

A

Resolution is the number of colours an image can represent.

B

Resolution is the total number of pixels in an image.

C

Resolution is the number of bits used for each pixel.

D

Resolution is the file size of an image.

These properties are used to find the file size.

Calculate bitmap file size

colour depth

1111111100000000

16 bits used to represent the colour of each pixel

The image has 240 000 pixels of 16 bits.

Calculate bitmap file size

1111111100000000

file size = resolution ✕ colour depth

The image has 240 000 pixels of 16 bits.

Calculate bitmap file size

1111111100000000

file size = 240 000 ✕ 16

Calculate bitmap file size

1111111100000000

file size = 3 840 000 bits

Calculate bitmap file size

1111111100000000

file size (bytes) = 3 840 000 ÷ 8

Calculate bitmap file size

1111111100000000

file size (bytes) = 480 000

Text Representation

The Problem

Computers only store: 0 and 1

But we need them to store: A B C ? ! £ 😊

Big Question:
How can a computer store text if it only understands binary?

If you had to invent a system for storing letters using numbers, what would you do?

Encoding messages

Imagine you are designing a computer system.

You need to give each character a number:

If we encode the message

CAB!

it becomes

3125

Character sets

This is exactly the basic idea behind character sets. The computer does not “understand” the letter. It stores a code that represents the letter.

Why would it be a problem if every computer used a different code for A?

Files and messages would not display correctly on different computers.

But computers need binary

Our made-up code used denary numbers:

A = 1
B = 2
C = 3

But computers store numbers in binary.

So a real computer might store:

A = 01000001
B = 01000010
C = 01000011

The letter itself is not stored. The computer stores a binary number, and the character set tells the computer which character that number represents.

How many keys on a standard QWERTY Keyboard

A

104

B

87

C

105

D

91

Representing text characters

Shaping the learning

• 104 keys on a keyboard

• 95 distinct printable characters

A computer understands only 1s and 0s, each key is given a unique 8 bit binary number. When a key is pressed the computer is sent that binary code.

Printable characters include letters, numbers, punctuation, symbols and even spaces

one key does not equal 1 character

one key can create many characters

upper case + lower case, numbers + symbols

Character sets

Shaping the learning

A character set is a set of letters, symbols and digits that can be represented by a computer

There are 2 major types we use

• ASCII (standard and extended)

• Unicode (16 bit and 32 bit)

ASCII

Shaping the learning

Extended ASCII (American Standard Code for Information Interchange) has become the standard code, used worldwide.

It uses 8 bits to store all possible combinations with 256 possible options.

The problem with ASCII

Shaping the learning

ASCII was designed mainly for English characters.

It can represent basic letters, numbers and symbols.

But the world uses many more characters:

é ñ ü أ Ж 你 £ € 😊

ASCII does not have enough space for every language and symbol.

ASCII

Could Standard ASCII store this message?

Hello! 😊 ¿Cómo estás?

1. Which characters could ASCII probably store?

2. Which characters might need Unicode and why?

Unicode

Shaping the learning

We invented UNICODE to solve this problem.

UNICODE uses 32 bits (2 sets of 16 bits) to represent every character in various languages around the world.

Unicode has space for up to 149,878 characters

https://symbl.cc/en/unicode-table/

Calculate and compare text file sizes

Consider the text “ASCII and Unicode encode text.”

There are 30 characters to represent including the spaces and the full stop.

If ASCII encoding is used, each character requires 7 bits for
its representation.

Therefore, the total
file size to store this text would be 7 ✕ 30 = 210 bits

210 bits / 8 = 26.25 bytes

In bytes, what is the file size of an ASCII text file with
the text “password” in it?

A

7 bytes

B

56 bytes

C

8 bytes

D

120 bytes

Sound representation

Describe sound

Sound is a pressure wave that causes air particles to vibrate.

Izzy

A microphone detects these vibrations and creates a changing electric signal that matches the vibrations.

Describe sound

Describe sound

A higher amplitude makes a sound …

A

Higher pitched

B

Lower pitched

C

Quieter

D

Louder

Drag and Drop

1

2

3

4

5

Describe sound

Representing sound

Create a binary sequence from a sound wave

Create a binary sequence from a sound wave

Create a binary sequence from a sound wave

Create a binary sequence from a sound wave

Fill in the blanks

When digitising sound, a sample is taken of the sound wave.

A measurement of the wave is taken from a reference point to be turned into a binary sequence.

digitising

sample

measurement

reference point

binary

Create a binary sequence from a sound wave

Create a binary sequence from a sound wave

What is measured in hertz when digitising sound?

A

Amplitude

B

Sample rate

C

Resolution

D

Frequency

The number of bits used in digitisation is reffered to as...

A

Audio depth

B

Resolution

C

Sample rate

D

Frequency

Calculate sound file size

Sample rates are often expressed in kilohertz (kHz)
so need converting.

Duration is often expressed in minutes so will also need to be converted.

1 kilohertz = 1000 Hz

1 minute = 60 seconds

A 4-minute sound recording has been made using
a sample rate of 10 kHz and a sample resolution of 6 bits.

Calculate sound file size - We do

file size

=

sample resolution

sample rate

✕

duration

✕

The file size is calculated as:

file size

=

file size

=

Calculate sound file size

file size

=

sample resolution

sample rate

✕

duration

✕

file size

=

file size

=

A 2-minute sound recording has been made using
a sample rate of 20 kHz and a sample resolution of 4 bits.