Representing Data

Data

raw facts and figures with no context or meaning

Information

data entered into a computer and processed, giving it meaning

Binary

number system in bits used by computers

Bit

smallest unit of digital data represented by a 1 or 0

Nibble

4 bits

Byte (B)

8 bits, used to represent 1 character

Kilobyte (Kb)

1024 bytes

Megabyte (Mb)

1024 kilobytes

Gigabyte (Gb)

1024 megabytes

Terabyte (Tb)

1024 gigabytes

What are units of data used for?

to describe memory capacity

Numeric (integer)

positive or negative whole numbers

Numeric (real)

positive or negative numbers with decimal/ fractional parts

Data/time

represents date and time in multiple formats

Character

single letter, number or symbol which takes up 1 byte of storage

ASCII

character set with each character represented uniquely by a single byte

String (text)

textual data in the form of a sequence of characters

Pixel

smallest unit of an image that can be changed, displayed with its own colour

Resolution

number of pixels per unit measurement which determines the quality/ clarity of the image

Effect of high resolution on file size

will fit large number of pixels into small space, but increases file size as every pixel’s colour code has to be stored

Storing images

needs to have its colour code stored and metadata e.g dimensions so it can be reconstructed and displayed on screen

Colour depth

N bits per pixel can have 2^N different colours

Typical bits per pixel (JPEG)

24 bits or 2²⁴ (16.7million colours)

Bitmap graphics

Stored as a grid of pixels, each with a specific colour and can be compressed

Vector based graphics

mathematical instructions about individual components which create shapes and lines

Bitmap vs vector based graphics

• Bitmap loses quality when resized while vector scales to retain detail

• Vectors more efficient, they don’t store each individual pixel so smaller file size

• Bitmaps can be compressed to reduce their file size

• Bitmaps take long to load due to processing

Formats for bitmap graphics

• .bmp

• .dib

• jpeg

• gif

• tiff

• .png

Formats for vector based graphics

• .svg

• .cgm

• .odg

• .eps

• .xml

Streaming

allows videos to be viewed on a website without downloading or a time delay

How does streaming work?

downloading successive segments in ‘real time’ to prevent waiting which are discarded after

Advantages of streaming

• video is unlikely to be replicated as user doesn’t save full copy

• allows video to be watched while it downloads

• computer doesn’t need to be capable of storing entire file

Buffer

intermediate storage area used to hold downloaded segments of video not played yet

Why use buffering?

prevents possible interruptions if there are time delays in streaming

Analogue signal

continuous varying signal to represent physical quantity e.g sound waves

Analogue to digital converter (ADC)

converts analogue signal (detected by microphone) into voltage variations so computer can store it digitally

Sound sampling

taking regular measurements of sound waves to process digitally

Need for sound sampling

infinitely minor variations in sound waves require too much storage and some we are incapable of hearing

Sample rate

quantity of audio samples captured per second, measured in hertz/ kilohertz

How does sample rate affect file sizes?

higher gives more accurate reproduction but larger file sizes as more measurements are stored

Bit depth

number of bits available for each sample, higher depth is greater quality

Bit rate

data (bits) required to store 1 second of sound, measured in kbps

Calculating bit rate

bit depth × sample rate

Units for frequency

Hertz (Hz)