Unit 3 - Data representation Flashcards

Question

What is a parity bit?

Answer 1

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.

Answer 2

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

Answer 3

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.

Answer 4

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

Answer 5

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.

Answer 6

Checksums are used with credit card numbers.

Answer 7

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.

Answer 8

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

Answer 9

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.

Answer 10

By flipping all the bits and adding one.

Answer 11

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

Answer 12

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

Answer 13

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.

Answer 14

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.

Answer 15

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.

Answer 16

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.

Answer 17

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

Answer 18

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.

Answer 19

Width x Height

Answer 20

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.

Answer 21

Resolution (width x height) x colour depth.

Answer 22

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

Answer 23

Continuous

Answer 24

Microphones and speakers

Answer 25

Analogue sound samples are recorded via an amplifier.

Answer 26

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.

Answer 27

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.

Answer 28

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

Answer 29

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.

Answer 30

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

Answer 31

Frequencies between 20Hz and 22kHz

Answer 32

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.

Answer 33

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.

Answer 34

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

Answer 35

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

Answer 36

Lossy and Lossless

Answer 37

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

Answer 38

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

Answer 39

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

Answer 40

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

Answer 41

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.

Answer 42

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

Answer 43

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

Answer 44

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.

Answer 45

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.

Answer 46

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

Answer 47

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.

Answer 48

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