Computer Science - 3 Data Representation

1 bit

a single 1 or 0

1 byte

8 bits

1 nibble

4 bits / 1/2 byte

Order of prefixes [4]:

- kilo
- mega
- giga
- tera

One character of text:

1 byte

A full page of text:

30 kB

One small digital colour photograph:

3 MB

Music CD capacity:

650 MB

DVD capacity:

4.5 GB

Hard disc capacity:

1 TB

Most significant bit (MSB)

the leftmost bit in a binary number - which has the highest value

Least significant bit (LSB)

the rightmost bit in a binary number - which has the lowest value

The number of combinations from n bits =

2ⁿ

Hexadecimal

a number system that uses base 16

0 - 15 in hexadecimal =

0 - 9 and A - F

Hexadecimal to decimal conversion [2]:

- multiply value of left digit by 16
- add value of right digit

Decimal to hexadecimal conversion [2]:

- divide using integer division by 16 (value = left digit)
- remainder = right digit

Binary to hexadecimal conversion [3]:

- divide 1 byte into two nibbles (left and right)
- convert the value of each nibble to hexadecimal digits
- combine the two

Hexadecimal to binary conversion [2]:

- represent each digit in turn as 4-bit numbers
- combine them

Advantages of hexadecimal [4]:

- simpler to remember
- quicker to write or type
- less likely to make error
- easy to convert between hex and binary

Rules of binary addition:

- 0 + 0 = 1
- 0 + 1 = 1
- 1 + 0 = 1
- 1 + 1 = 0 (carry 1)
- 1 + 1 + 1 = 1 (carry 1)

1 + 1 + 1 + 1 = ? [3]

- 4
- 100
- 1 carried over by 2

Overflow error

error from attempting to represent a number that is too large.

A right logical shift causes:

a division by 2 (if float, round down)

A left logical shift causes:

a multiplication by 2

Number of unique characters on a keyboard

98

What are characters on a keyboard represented by?

a binary value

Character set

the group of symbols used to render text on a page

What are the two major character sets? [2]

- ASCII
- Unicode

ASCII is usually ...-bit but can be ...-bit

7, 8

Unicode is ...-bit and extended Unicode is ...-bit

16, 32

What does ASCII stand for?

American Standard Code for Information Interchange

In ASCII, 'A' begins at ... and ends at ...

65, 90

In ASCII, 'a' begins at ... and ends at ...

97, 122

In ASCII, 0-9 are ...

48-57

In ASCII, a space is ...

32

Concatenate

chain together

Concatenate strings in Python ("Help ", "me!": string1, string2):

string1 = "Help "
string2 = "me!"
concatenated = string1 + string2
print(concatenated)

#output: "Help me!"

Convert character to ASCII in pseudocode [2]:

- CHAR_TO_ASCII
- CHAR_TO_CODE

Unicode, as well as English characters, is used to store... [2]:

- characters from other languages
- emojis

Bitmap/raster image

digital image divided into a grid of dots (pixels)

Bitmap images are made of ... or ...

PICture ELements, pixels

Pixel

the smallest identifiable area of an image

A pixel is [2]:

- one solid colour
- given a binary code to represent that colour

Bitmap image file formats [5]:

- BMP
- JPG
- GIF
- PNG
- TIFF

Vector image file format:

SVG

Image resolution

the concentration of pixels in a specific area

PPI

pixels per inch (screen)

DPI

dots per inch (printing)

Colour/bit depth

the number of bits used for each pixel in an image file

File size (bits) =

height x width x bit depth

If you increase the file size but not the number of pixels...

the pixel concentration decreases as well as the resolution

Portable bitmap image (PBM) [3]

- uses 1s and 0s to store pixels
- image dimensions given in 1st row
- image details given after

Each pixel has a proportion of ..., ... and ..., called its ... value

red, green, blue, RGB

RGB

uses varying intensities of red, green, and blue light are added together to reproduce a broad array of colors

Why are hexadecimal and decimal values used for RGB?

colour pickers are used by designers and developers and these forms are easier to remember than binary

Each colour value (R, G, B) requires:

8 bits

Each pixel requires:

24-32 bits

In 32-bis per pixel 8 bits are given to a ...

transparency channel

Metadata

data about data

What metadata may be present in an image file? [4]

- colour depth in bits per pixel
- resolution (height and width in pixels)
- date created
- author

Why might file sizes not add up?

metadata

Analogue sound signal

continuous wave

Digital sound signal

discrete - wave sampled at regular intervals

How is sound digitised?

by repeatedly measuring and recording the sound wave

ADC

analogue to digital converter

What is an ADC used for?

convert inputs to digital sounds

DAC

digital to analogue converter

What is a DAC used for?

convert digital sounds to outputs

Sound sample

a measure of amplitude at a point in time

Sampling resolution

number of bits used to represent sound amplitude

Sampling resolution is also called...

bit depth

Sampling rate

the number of samples taken per second

What is sampling rate measured in?

Hertz (Hz)

The higher the bit depth/sampling resolution...

the more accurate the amplitude value

What is the sampling rate for a CD?

44.1 kHz

File size (bits) =

sampling rate x resolution x duration

What improves recording quality? [2]

- sound sampled more frequently
- wave height recorded more accurately

Lossy compression

removes sound that we can't easily hear or that least affect playback quality - this is irreversible

What is lossy compression useful for in music?

downloading and streaming music

Examples of lossy compression sound file formats [2]:

- MP3
- AAC

MP3

MPEG-1 Audio layer 3

AAC

Advanced Audio Coding

Lossless compression

data compression technique where no data is lost

Examples of lossless compression sound file formats [3]:

- FLAC
- ALAC
- WMA Lossless

FLAC

Free Lossless Audio Codec

ALAC

Apple Lossless Audio Codec

WMA Lossless

Windows Media Audio

Examples of uncompressed sound file formats [2]:

- WAV
- AIFF

WAV

Waveform Audio File Format

AIFF

Audio Interchange File Format

Compression algorithm

an algorithm that compresses file size

Decompression

returning compressed data to its original format

With lossy compression, data is...

lost permanently

With lossless compression, data is...

not lost, and the file can be decompressed

Digital artefact

small mistakes in image and video as a result of lossy compression

Which type of compression reduces file size more?

lossy compression

Run length encoding

replacing a long series of a repeated character with a count of the repetition

(RLE) 1111000 =

4 1 3 0

Examples of lossy compression image file formats [1]:

- JPG