CP2: Data representation

Why is compression used? (3 reasons)

reduce the amount of storage needed to save files, allow large files to be transmitted as an attachment, allow files to be transmitted in less time

Lossy compression

data encoding method where files are compressed by removing some detail

Lossless compression

data encoding method where files are compressed but no data is lost

Where is lossy compression used?

images, audio files and video files

Lossy compression formats

JPEG, Windows Media Player, MP3, MPEG-1, MPEG-2

Where is lossless compression used?

text and data files

Lossless compression formats

graphic interchange format, portable network graphic, FLAC, ZIP

Vector graphic

graphics that use lines and shapes

Pixel

short for picture element, smallest unit of a digital image or graphic with an individual colour set

Resolution

number of pixels that make up an image, measured in PPI (pixels per inch)

Bitmap image

an image made up of pixels

Colour depth

number of bits per pixel

Equation to calculate file size

size in bytes = (image width * image height * colour depth) / 8

Analogue signals

continuously varying signals

Digital signals

signals with discrete values

Analogue-to-Digital Converter (ADC)

device that takes real-world analogue signals and converts them to a digital representation

How are analogue signals converted in sound waves? (5 steps)

analogue sound received by microphone, converted into electrical analogue signal, signal amplitude (height of wave) measured at regular intervals (sampled), values rounded to a level (quantisation), values stored as series of binary numbers

Amplitude

the height of a wave (from the origin to a crest, or from the origin to a trough)

Sample

a measure of amplitude at a point in time

What two things does the accuracy with which an analogue sound wave is converted to a digital format depend on?

bit depth and sample rate

Sample rate

frequency with which amplitude of sound is record, measured in hertz (1hz = 1 sample per second)

Sample interval

amount of time between two samples, measured in seconds

Bit depth

number of bits used to store each sample

Sample size

number of seconds over which the sample is taken

Equation to calculate size of a sound file

file size in bytes = (sample rate * bit depth * sample size) / 8