Data Representation

Question 1

How do you convert Digital Signals to Analogue signals?

Answer 1

A device called a digital to analogue converter (DAC) is used - it reads a bit pattern representing an digital signal and outputs an analogue, electrical current

Question 2

How do you convert Analogue Signals to Digital Signals?

Answer 2

1) A sensor such as a microphone outputs an analogue signal
2) Computer uses an Analogue to Digital Convertor
3) ADC takes samples of the analogue signal at regular intervals
4) The amplitude of each sample is approximated to an integer value
5) Each sample is assigned a binary value - so it can be stored digitally

Question 3

How do you calculate the size of a sound file?

Answer 3

Size (Bits) = Sample Rate x Sample Resolution x Length of Sample

Question 4

What is the range that humans can hear at?

Answer 4

Between 20Hz and 20,000Hz

Question 5

What is Nyquist Theorem?

Answer 5

To produce an accurate recording, the sampling rate must be at least double that of the highest frequency in the original signal

Question 6

What is MIDI?

Answer 6

Electronically generated sounds that mimic musical instruments

Question 7

What are the advantages of MIDI?

Answer 7

MIDI allows easy manipulation of music without a loss in quality
MIDI are smaller in size and are lossless - so no information is lost when using MIDI

Question 8

What are the disadvantages of MIDI?

Answer 8

Can’t be used for storing speech
Can result in less realistic sounds

Question 9

What is colour depth?

Answer 9

The number of bits assigned to a pixel in an image - if one bit, then 2^1 colours are available, if 2 then 2^2 bits, etc

Question 10

Give some examples of metadata

Answer 10

Metadata is extrainformation - e.g. Width, Height, Date Created

Question 11

What are vector graphics?

Answer 11

Vector Graphics represent images using geometric objects and shapes

Question 12

Vector Graphics vs Bitmapped Graphics?

Answer 12

1) Vector Graphics can be scaled without losing quality, whereas bitmap will lose quality
2) Vector graphics are suited to simpler images, bitmap to more complex - such as photographs
3) Vector Graphics use less storage space than bitmapped, as they only store information about the shape instead of each pixel

Question 13

What is Lossy Compression?

Answer 13

The process of reducing a file size, by deleting information within the file - this could be lowering image resolution (deleting certain pixels)
This can’t be undone

Question 14

What is Lossless Compression?

Answer 14

The process of reducing a file size without deleting any information, this reduces a file size without lowering qualitty

Question 15

What is Run Length Encoding?

Answer 15

A type of lossless compression
Reduces the size of a file by removing repeated information and replacing it with a singoe occurance, followed by the number of times it is repeated

e.g. In a bitmap, a line of pixels could be
1 White 5 Yellow 1 white

Question 16

What is Computational Security?

Answer 16

Something is computationally secure if it cannot be cracked within reasonable time

Question 17

What are the conditions for the Vernam Cipher to offer perfect security?

Answer 17

1) The encryption key is equal to or longer than the plaintext message
2) The key is random
3) The key is only used once and is then destroyed

Question 18

How do you peform the Vernam Cipher?

Answer 18

1) Separate your word into its letters (e.g. D, A, V, E)
2) Separate your key into its separate characters (e.g. #9[G)
3) Get the ASCII values of each character
4) Do an XOR calculation, comparing first letter to first character, second letter to second character, etc
5) Convert the new ASCII value back

Question 19

What is a Caesar Cipher?

Answer 19

1) Choose a letter and then shift that letter and every letter following it by x number of times

Very easy to crack or brute force as limited number of permutations

Question 20

How does dictionary based compression work?

Answer 20

Take a sentence and give a number to every unique word
(e.g. Walk to the park to say hi)
Walk - 1
To - 2
The - 3
Park - 4
Say - 5
Hi - 6
These new code are stored in unicode
This means that 3 bits are required as (2^3) = 8 and we have 6 codes (Repeated words count for one)
Lossless Compression

Question 21

How does Truncation/Rounding work?

Answer 21

Truncation - Cut the bits off that don’t fit
Rounding - Cut the bits off, but if the first bit to cut off is a 1, then add a 1 to the least significant bit

Question 22

What are absolute/relative errors?

Answer 22

Sometimes, some numbers cannot be represented using fixed/floating point binary (e.g. 1/3). So we work out the closest approximation and calculate the error
Absolute Error - The difference between the desired value and the actual value
Relative Error - Absolute Error / Actual Value * 100

Question 23

Floating Point vs Fixed Point

Answer 23

Floating Point binary allows representation for a greater range of numbers with a given number of bits.
Large exponent, smaller mantissa allows for a large range but little precision
Large Mantissa, Small Exponent allows for small range but great precision

Moving the fixed point to the left increased precision, to the right increases range

Question 24

What is Underflow?

Answer 24

When a number is too small to be represented with the amount of bits available

Question 25

What is Overflow?

Answer 25

When a number is too big to be represented with the amount of bits available

Question 26

What are the benefits of ASCII over Unicode?

Answer 26

1) Takes up less memory as only stores 128 characters
2) Uses 7 bits - so 8th bit can be used as an error check

Question 27

What are the benefits of Unicode over ASCII?

Answer 27

1) Allows a wider range of characters
2) Can be used internationally as allows multiple languages, better for communicating online

Question 28

What are the advantages of Normalisation?

Answer 28

1) Maxmises precision based on the number of bits
2) Gives a unique representation of each number