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Computer science—Fundamentals of data representation > Coding systems > Flashcards

Flashcards in Coding systems Deck (25):

What is ASCII?

A binary coding system for storing characters


Describe ASCII

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


What is unicode?

A binary coding system that has superseded ASCII


Describe unicode

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


Describe the parity bit method of error checking

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


Describe the majority voting method of error checking

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


Describe the check digit method of error checking

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 2*6+3*5+0*4+4*3+5*2


What is the resolution of an image?

width*height (in pixels)


Describe bitmapped graphics

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


What is the colour depth of an image?

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


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



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

file size=resolution*colour depth


Describe vector graphics

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.


Give two advantages of vector graphics compared to bitmapped graphics

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


What is sampling/analogue to digital conversion?

The process of converting analogue sound waves into digital form


Explain how analogue to digital conversion works

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


What is the sampling resolution in analogue to digital conversion?

The number of bits allocated to each sample


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?

file size=sampling rate*length*sampling resolution


What does Nyquist's theorem state?

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


What is compression?

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


What is lossless compression?

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


What is lossy compression?

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


Explain how run length encoding can compress data

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


Explain how dictionary based encoding can compress text files

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


Explain how JPEG compression works

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