ALEVEL CS总复习：第一单元背诵要点

Number Representation

Binary

base two number system based on the values 0 and 1 only.

Bit

A digit in the binary number system wirtten using either of the symbols 0 and 1

Kibi: a prefix representing the factor 210 (1024) written as the symbol Ki

Mebi: a prefix rep resenting the factor 220 (1048576) written as the symbol Mi

Gibi: a prefix representing the factor 230 written as the symbol Gi

One’s complement

each binary digit in a number is reversed to allow both negative and positive numbers to be represented.

Two’s complement

each binary digit is reversed and 1 is added in right-most position to produce another method of representing positive and negative numbers.

Binary-coded decimal (BCD)

number system that uses 4 bits to represent each denary digit.

Overflow

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

Character set

a list of characters that have been defined by computer hardware and software.

It is necessary to have a method of coding, so that the computer can understand human characters.

The differences between the ASCII and Unicode character sets.

• UNICODE has greater range of characters than ASCII

• UNICODE represents most written languages in the world while ASCII

does not ASCII used for English only

• ASCII uses 7 or 8 bits or one byte whereas UNICODE uses up to 4

bytes per character

• UNICODE is standardised while ASCII is not

Lossless Compression

Type of compression that allows original data to be perfectly reconstructed from compression

Lossy Compression

type of compression in which file accuracy is low, but file size is smaller than lossless

Run-length encoding 也是一种无损压缩方式

compression in which sequences with same data value in many consecutive

values are stored as a single data value and count

Bit-map image

system that uses pixels to make up an image.

与位图相关的概念：

Pixel – smallest picture element that makes up an image.

Colour depth – number of bits used to represent the colours in a pixel, e.g. 8 bit colour depth can represent 28 = 256 colours.

Bit depth – number of bits used to represent the smallest unit in, for example, a sound or image file – the larger the bit depth, the better the quality of the sound or colour image.

Image resolution图像分辨率 – number of pixels that make up an image, for example, an image could contain 4096 × 3192 pixels (12738656 pixels in total).

File header

Specifies the image size, number of colours, and other data needed to display an image

Screen resolution

number of horizontal and vertical pixels that make up a screen display. If the screenresolution is smaller than the image resolution, the whole image cannot be shown on the screen, or the original image will become lower quality.

Vector graphics

images that use 2D points to describe lines and curves and their properties that are grouped to

form geometric shapes.

• Objects and properties are stored mathematically.

• Drawing list: set of commands used to define a vector

• Vectors are scalable and do not pixelate like a bitmap image therefore are used by corporations to create logos as they can be resized without losing quality

位图和矢量图的对比

Bitmaps may use compression techniques to reduce the file size.

Explain the difference between ‘lossless’ and ‘lossy’ techniques for achieving this compression.

•A bitmap may contain the same sequence of pixels (i.e. a pattern) repeated many times.

•A lossless technique is designed to lose none of the original detail.

•Lossy may result in a loss of detail.

•One lossless technique is ‘run-length encoding/store the colour and the number of consecutive pixels of that colour’. JPEG and GIF file formats use RLE (i.e. a lossless technique).

•Lossy techniques make a decision about what parts of the image are important and then discard certain information.

声音（sound）专业术语

Sampling resolution

number of bits used to represent sound amplitude (also known as bit depth).

Sampling rate

number of sound samples taken per second.

A student has recorded a sound track for a short film.

Explain how an analogue sound wave is sampled to convert it into digital format.

• Amplitude (of the sound wave) measured

• At set / regular time intervals / per time unit / time period

• Value of the sample is recorded as a binary number