Coding systems

Question 1

What is ASCII?

Answer 1

A binary coding system for storing characters

Question 2

Describe ASCII

Answer 2

Seven (or eight, for the more recent extended ASCII) bits are used, with each permutation representing a different character. As a result, it can only represent 128 or 256 characters

Question 3

What is unicode?

Answer 3

A binary coding system that has superseded ASCII

Question 4

Describe unicode

Answer 4

It has backwards compatibility with ASCII. There are two common encodings, UTF-8 and UTF-16, which use 8 and 16 bits respectively. It can represent every character in use in the world

Question 5

Describe the parity bit method of error checking

Answer 5

A bit appended to a binary data transmission to check for errors. If the transmissions contains an even number of 1s then the parity bit will be 0, else it will be 1, i.e. it makes the number of 1s even. If the number of 1s is the data received is odd, then it is therefore incorrect

Question 6

Describe the majority voting method of error checking

Answer 6

Each bit is transmitted three times, i.e. 101 would be transmitted as 111000111. Therefore, if there are not three consecutive 1s or 0s, there is an error. In this case, the bit with the most occurrences is chosen, i.e. 001 would be assumed to be 0

Question 7

Describe the check digit method of error checking

Answer 7

Some operation is performed on all digits to obtain a one digit number, i.e. repeatedly adding up the digits until a one digit number is reached. This digit is then appended to the number. When the transmission is received, the number is checked to see if it produces the same check digit. To overcome the problem of two digits being swapped around not having an effect on the check digit, each digit may first be multiplied by a different weight, starting at 2, i.e. 23045 would become 26+35+04+43+5*2

Question 8

What is the resolution of an image?

Answer 8

width*height (in pixels)

Question 9

Describe bitmapped graphics

Answer 9

Each pixel is stored individually using one or more bits. They may also contain metadata, normally found at the beginning of the file and including information such as the image’s width, height and colour depth

Question 10

What is the colour depth of an image?

Answer 10

The number of bits allocated to the colour of each pixel in a bitmapped image

Question 11

How many different colours may an image with a colour depth of n contain?

Answer 11

2ⁿ

Question 12

What is the relationship between the file size of an image, its resolution, and its colour depth?

Answer 12

file size=resolution*colour depth

Question 13

Describe vector graphics

Answer 13

They are created using objects and coordinates. For example, a line could be stored with just two sets of coordinates, a triangle with three, etc. Other information may include thickness, colour, etc.

Question 14

Give two advantages of vector graphics compared to bitmapped graphics

Answer 14

• Smaller file size due to not having to store information about every pixel
• No pixellation occurs when enlarged

Question 15

What is sampling/analogue to digital conversion?

Answer 15

The process of converting analogue sound waves into digital form

Question 16

Explain how analogue to digital conversion works

Answer 16

A series of readings at fixed intervals are taken from the wave, recording the amplitude

Question 17

What is the sampling resolution in analogue to digital conversion?

Answer 17

The number of bits allocated to each sample

Question 18

What is the relationship the file size of an audio file produced through analogue to digital conversion, the sampling rate, the length of recording, and the sampling resolution?

Answer 18

file size=sampling ratelengthsampling resolution

Question 19

What does Nyquist’s theorem state?

Answer 19

To faithfully recreate an analogue signal in digital form, the sample rate should be at least twice the value of the highest frequency in the sampled signal

Question 20

What is compression?

Answer 20

The process of reducing the number of bits required to represent data

Question 21

What is lossless compression?

Answer 21

Compression where the compressed file has the same quality as the uncompressed file

Question 22

What is lossy compression?

Answer 22

Compression where some data is discarded and so the resultant file is of worse quality

Question 23

Explain how run length encoding can compress data

Answer 23

It eliminates repeated data, i.e. aaaaa may be turned into 5a

Question 24

Explain how dictionary based encoding can compress text files

Answer 24

Common strings are coded in some way. For example, “tion” may be replaced with a “1”. It can then be decoded again by looking up what string each numeric token corresponds to

Question 25

Explain how JPEG compression works

Answer 25

By breaking the image up into blocks of 8×8 pixels. It then analyses each block individually, and determines whether any change to the block will be noticeable to the human eye. For example, if all pixels in a block were approximately the same colour, then it could make them all the same colour, and then use run length encoding to decrease the size