Data Representation

Bit

A digit in the binary number system written as 0 or 1.

Overflow

When the result of a calculation is too large to fit into the number of bits defined for storage

BCD

Binary coded decimal. One binary value representing one digit in a nibble

Packed BCD

2 BCD nibbles in one byte

Applications of BCD

• Displaying digits

• Representing currency values (bcs cannot be represented accurately otherwise)

ASCII code

American Standard Code for Information Interchange

• Limited number of codes represent non-printing characters, these were introduced to assist in data transmission and data handling at a computer terminal

• Majority codes are for characters in English text which are available on a standard keyboard.

ASCII vs Extended ASCII bits

7 and 8

what does the inclusion of 8 in UTF-8 represent?

This version of the standard includes codes defined by one byte in addition to codes using 2, 3 and 4 bytes

Code point

Character in unicode

Vector graphic

a graphic consisting of drawing objects defined in a drawing list

Drawing object

a component defined by geometric formulae and associated properties

Drawing list

contains one set of values for each drawing object

Property

defines one aspect of the appearance of the drawing objecT

Properties of vector graphics

• The dimensions of the objects are not defined explicitly but instead are defined relative to an imaginary drawing canvas (The image is scalable)

• Image pixelation is avoided

How is image distortion avoided in vector images?

Whenever the image is to be displayed the file is read, the appropriate calculations are made and the objects are drawn to a suitable scale. If the user then requests that the image is redrawn at a larger scale the file is read again and another set of calculations are made before the image is displayed. This avoids image distortion, such as the image appearing squashed or stretched.

Pixel

Picture Element, the smallest identifiable component of a bitmap image, defined by just two properties: its position in the bitmap matrix and its colour

Colour depth

the number of bits used to represent one pixel

bit depth

the number of bits used to represent each of the red, green and blue colours

Image resolution

the number of pixels in the bitmap file defined as the product of the width and the height values

Screen resolution

the product of width and height values for the number of pixels that the screen can display

File header

a set of bytes at the beginning of a bitmap file which identifies the file and contains information about the coding used

Analogue data

data obtained by measurement of a physical property which can have any value from a continuous range of values

Digital data

data that has been stored as a binary value which can have one of a discrete range of values

Sampling

taking measurements at regular intervals and storing the value

Sampling resolution

the number of bits used to store each sample

Sampling rate

the number of samples taken per second

Lossless compression

coding techniques that allow subsequent decoding to recreate exactly the original file

Lossy compression

coding techniques that cause some information to be lost so that the exact original file cannot be recovered in subsequent decoding

Huffman coding

lossless. instead of having each character coded in one byte, the text is analysed to find the most often used characters. These are then given shorter codes. The original stream of bytes becomes a bit stream. Can be used for sound

What is done if a vector graphic file needs to be compressed?

Converted to SVG (scalable vector graphics) format

Lossy compression for sound