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Computer Science > Unit 3 - Data representation > Flashcards

Unit 3 - Data representation Flashcards

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1
Q

What are whole numbers?

A

Any positive or negative whole numbers are called integers.

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2
Q

What are rational numbers?

A

Rational numbers (ℚ) include values that can be expressed as fractions or ratios.

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3
Q

What are irrational numbers?

A

Some numbers cannot be expressed exactly as a fraction.
The decimal values of these numbers are endless.
Examples of these irrational numbers are √2 or pi.

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4
Q

What are real numbers?

A

If a number is either rational, (an integer or a fraction) or irrational, it is considered to be areal number ℝ.

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5
Q

What are ordinal numbers?

A

Numbers can be used to describe the position in which values appear.
For example, in an ordered list of names.

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6
Q

What digits does the binary number system use?

A

The binary number system uses only two digits: 0 and 1.

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7
Q

What is the decimal number system?

A

The decimal number system uses a combination of just ten symbols to represent any number.
Number systems are referred to by their base; that is, the number of symbols used to construct values.
Decimal is base 10 and the base may be referred to as a subscript.

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8
Q

What base do binary numbers use?

A

Base 2

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9
Q

What base is hexadecimal?

A

Base 16

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10
Q

Why do we use hexadecimal?

A

A hexadecimal value is much easier to read and remember than a string of binary digits.
It is quicker to write or type, since a hex digit takes up only one character, not four.
There is less chance of making an error when typing hex characters than a string of 1s and 0s.
It is used to define colours, in MAC addresses, in assembly languages and machine code.
It is very easy to convert to and from binary.

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11
Q

What is a positive and negative of using decimal numbers?

A

Decimal numbers are easy to understand but they are not easy to use in electronics.

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12
Q

What is a positive and negative of using binary?

A

Binary is simple to use with circuitry but is difficult to read by humans.

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13
Q

What are binary digits?

A

Each individual digit in a binary value is referred to as a bit, (from the term binary digit).
In a computer we can represent binary values by using ON and OFF voltage signals for each individual bit.
For n bits a computer can produce 2n different combinations of values.

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14
Q

What are number prefixes?

A

When dealing with large quantities it is more convenient to summarise this using number prefixes
A common example of this is the kilogram k which is equivalent to 1000 grams.

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15
Q

What is a kibi?

A

KiB - 2^10 = 1,024 bytes.

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16
Q

What is a mebi?

A

MiB - 2^20 = 1,048,576

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17
Q

What is a gibi?

A

GiB - 2^30 = 1,073,741,824

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18
Q

What is a tebi?

A

TiB - 2^40 = 1,099,511,627,776

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19
Q

What is ASCII?

A

In 1963 the American Standard Code for Information Interchange, (ASCII,) was established to encode symbols found in the English alphabet
It was composed of a 7 bit character set giving 27 (128) possible binary codes.

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20
Q

What is unicode?

A

The Unicode system was introduced to standardise the encoding of characters from all languages
Unicode can apply a variable length encoding of either 16 bits or 32 bits
In order to improve adoption of this new standard the first 128 Unicode characters were set to be the same as the ASCII character set.

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21
Q

What can cause transmission errors?

A

Electrical interference
Power surges
Synchronisation issues
Wear and tear on the cable or connectors

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22
Q

What can transmission errors do to bits?

A

These errors can cause bits to flip from 1s to 0s and 0s to 1s.

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23
Q

What are techniques that can be used to check for errors in transmission?

A

Parity bits
Majority voting
Check digits
Checksums

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24
Q

What happens when a transmission error is detected?

A

The data cannot be corrected
A request to resend the data is invoked.

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25
Q

What is a parity bit?

A

Computers use either odd or even parity
When sending a byte of data, one bit is used as a parity bit
This bit is set to a 1 or 0 to make the total number of 1s or 0s in the byte (including the parity bit) odd or even depending on the machine.
For example:
0101010 odd parity would be 0 because then there’s 3 1s. even parity would be 1 because then there’s 4 1s.

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26
Q

How can parity be used for error detection?

A

When data is transmitted, the parity bit is set at the transmitting end and parity is checked at the receiving end
If the wrong number of bits are ‘on’, an error has occurred
The receiving computer notifies the transmitting end and the data is resent

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27
Q

What is majority voting?

A

Each bit of a message is sent three times.
If a bit value is flipped erroneously the recipient computer uses the majority rule and assumes that the two bits that have not changed were therefore correct.

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28
Q

What is a check digit?

A

A check digit is an additional digit at the end of a string of other numbers designed to check for mistakes in input or transmission.

The first 12 digits of the barcode are the unique item number, the 13th is the check digit calculated by an algorithm based on the other 12 digits

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29
Q

What is a checksum?

A

A checksum works in a similar way to a check digit
A total sum of all bytes in a transmission of data is calculated using an algorithm
The sum is often sent with the data
The receiving computer recalculates the checksum based on the data it received and compares it with the checksum received or a known, expected value
If it does not match, the data may have been altered or corrupted during transmission and the data is resent.

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30
Q

What is an example of when checksums are used?

A

Checksums are used with credit card numbers.

31
Q

What is an overflow error?

A

When the result of an addition is too large for the number of bits the computer works with there will be an overflow error.
For example if there are continuous 1s or 0s.

32
Q

What are the binary multiplication rules?

A

0x0=0
0x1=0
1x0=0
1x1=1
As with decimal any value multiplied by 0 is 0.

33
Q

How are negative numbers represented?

A

Negative values are most commonly represented using a method called two’s complement.
A binary number starting with 1 is odd and 0 is even.

34
Q

How is the twos complement of signed binary value found?

A

By flipping all the bits and adding one.

35
Q

Find -77 in twos complement.

A

Find the positive value first:
01001101 = 77
Flip the bits and add one:
10110011 = -77

36
Q

What is twos complement range?

A

Maximum for 8 bits is 127 which is 01111111 in binary
Minimum is -128 which is 10000000 in binary.

-(2(n-1)) … 2(n-1) - 1

37
Q

How do you subtract using twos complement?

A

Two’s complement can be used to ensure subtraction will occur when adding negative values.
The calculation 65 - 43 should produce the same result as 65 + -43.
This way we can always use addition when we want to subtract.

38
Q

What is twos complement overflow?

A

Where the result of the addition is too large to fit in the allocated number of bits, overflow will occur
This is the same as standard binary addition
If the result requires more bits than are available, the number will not be represented correctly
A ‘1’ in the left most bit indicates a negative number, therefore overflow occurs above 127 with 8 bits.

39
Q

What is a fixed-point binary value?

A

A fixed-point binary value uses a specified number of bits where the placement of the binary point is fixed.
For example, in an 8 bit fixed-point binary value, the binary point could be set between the fourth and fifth bits.
Fixed point binary is one way of representing binary fractions.

40
Q

What is digital computer data?

A

Data that represents other forms of data using specific machine language systems that can be interpreted by various technologies. It is represented using the binary system of ones and zeros.

41
Q

What is analogue computer data?

A

A continuous representation of physical measurements.
Analogue data can in theory be measured to any degree of accuracy.

42
Q

How is sound or light converted into binary code?

A

Sound is captured by a microphone or light is detected by a digital camera.
Each measurement is converted into a binary code using an electronic component called an analogue-to-digital convertor.
The digital signal can then be processed by computers or other devices.

43
Q

How is resolution calculated?

A

Width x Height

44
Q

What is bit depth?

A

A pixel is attributed a number of n bits
The number of combinations (2^n) dictates the bit depth and therefore the number of colours that can be represented
A higher bit depth gives a greater range of colour and a better quality of image.
For example a bit depth of 2 (2^2) can represent 4 colours.

45
Q

How is image file size calculated?

A

Resolution (width x height) x colour depth.

46
Q

What is meta data?

A

Metadata is data about data and is stored in the same file as the image data
This includes:
the date it was created
the width and height in pixels of the image
the colour depth and
the GPS coordinates of where a photo was taken

47
Q

Are analogue signals continuous or discrete?

A

Continuous

48
Q

Are digital signals continuous or discrete?

A

Discrete

49
Q

What are some examples of analogue to digital converters?

A

Microphones and speakers

50
Q

What are analogue sound samples recorded with?

A

Analogue sound samples are recorded via an amplifier.

51
Q

How does an amplifier work?

A

Each sample is quantised to measure its wave height and translate this into an integer value
The integer value is then converted and stored digitally as a binary value
To output a sound, the reverse happens.

52
Q

What is sample resolution?

A

The number of bits (audio bit depth) used to record each measurement is known as the resolution.
More bits used per sample enables the height of the wave to be more accurately measured but increases file size.

53
Q

What is sampling rate measured in?

A

Hertz

54
Q

What is sampling rate?

A

The frequency or sample rate per second.
The greater the frequency, the greater the accuracy, and file size.

55
Q

How is file size calculated?

A

sample rate x resolution x length in seconds

Where sampling rate indicates number of samples per second (Hertz) and resolution is number of bits used per sample.

56
Q

What is Nyquist theorem?

A

There is a limit on the lowest sampling rate that can be used for an accurate recording.
Because sound is made up of many components each at different frequencies, samples must be twice the highest frequency in order to replicate the original sound wave

57
Q

What frequency can the human ear hear?

A

Frequencies between 20Hz and 22kHz

58
Q

What is the MIDI standard?

A

It is essentially an alternative system for producing sound.
The MIDI standard (Musical Instrument Digital Interface) creates sounds as requested either from an instrument of piece of software.
It is not a live recording, but a synthesised sound.

59
Q

What are benefits of MIDI?

A

With this system the full sound signal does not need to be transmitted, only the instructions to play the sound need to be transmitted as ‘event messages’.
It is up to the instrument to create the sound, so the instrument can be changed using the same messages for a different sound.
MIDI messages are usually only 2 or 3 bytes long which significantly reduces the amount of data transferred.
As sounds are synthesised, they may be less realistic but since no data is lost about musical notes, the quality can be higher.

60
Q

What are event messages?

A

A sound processor can be linked to several instruments or computers
This can send a timed sequence of event messages to:
Synchronise tempo
Control pitch
Change volume
Introduce and silence other instruments in a digital orchestra

61
Q

What does reducing the amount of data to send or store result in?

A

Reducing the amount of data to send or store ensures that:
Data is sent more quickly
Less bandwidth is used as transfer limits may apply
Buffering on audio and video streams is less likely to occur
Less storage is required

62
Q

What are the two types of data compression?

A

Lossy and Lossless

63
Q

What is Lossy compression?

A

A form of compression that reduces digital file sizes by removing data.
Non-essential data is permanently removed.
JPG

64
Q

What is Lossless compression?

A

Files are reduced in size without the loss of data.
Patterns in the data are spotted and summarised in a shorter format without permanently removing any information.
MP3

65
Q

What is Run Length Encoding (RLE)?

A

A basic method of compression that summarises consecutive patterns of the same data.

66
Q

What is dictionary compression?

A

Spots regularly occurring data and stores it separately in a dictionary
The reference to the entry in the dictionary is stored in the main file thereby reducing the original data stored
Even though the dictionary produces additional overheads the space saving negates this problem

67
Q

What is encryption and what are the two methods?

A

A way of making sure data cannot be understood if you don’t possess the means to decrypt it
Plaintext of a message sent is encrypted using a cipher algorithm and key into equivalent ciphertext
When received, the ciphertext is decrypted back to plaintext using the same or different key
Two methods at the opposite end of the security spectrum are the Caesar cipher and the Vernam cipher.

68
Q

What is Caeser Cipher?

A

The Caesar cipher is most basic type of encryption and the most insecure
Letters of the alphabet are shifted by a consistent amount.

69
Q

What is a Brute force attack?

A

A brute force attack attempts to apply every possible key to decrypt ciphertext until one works.

70
Q

What is frequency analysis?

A

It counts how many of each letter are in the encrypted text. It can be useful to encrypt texts that have no pattern. Since E is the most commonly used letter in the alphabet, the most frequent letter in the encrypted text will most likely be E.

71
Q

What is Vernam Cipher?

A

The encryption key, also known as the one-time pad, is the only cipher proven to be unbreakable
The key must be:
a truly random sequence greater or equal in length than the plaintext and only ever used once
Shared with the recipient by hand, independently of the message and destroyed immediately after use.

72
Q

How is decoding done with binary sequences?

A

Encryption and decryption of the message is performed bit by bit using an exclusive or (XOR) operation with the shared key.

73
Q

What is the one-time pad?

A

The one-time pad must be truly random, generated from a physical and unpredictable phenomenon.
Sources may include: atmospheric noise, radioactive decay, the movements of a mouse or snapshots of a lava lamp
A truly random key will render any frequency analysis useless as it would have a uniform distribution.
Computer generated ‘random’ sequences are not actually random.

74
Q

What is ciphertext?

A

Ciphertext is the result of encryption performed on plaintext using an algorithm, called a cipher.

Computer Science (3 decks)

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