3.4 Images/ 3.5 Sound/ 3.6 Compression

Examples of bitmap image file formats (pixels)

PNG, JPG, GIF

Example of vector image file format (algorithms)

SVG

Pixel

A single point in an image

Image resolution

Teh concentration of pixels within a specific area

How to calculate image resolution

Width x height

PPI

Points per inch

DPI

Dots per inch

Colour/bit depth

Number of combinations

Bit depth to represent an image with 3 colours

2

Image file size in bits

image resolution x colour depth

Metadata

A set of data that describes and gives information about other data

ADC

Analogue to digital converter

DAC

Digital to audio converter

Amplitude

Height of a wave

How are soundwaves converted (5 bp)

Analogue sound is received by a microphone

Converted into an electrical analogue signal

The amplitude is measured at regular intervals (samples)

Values are rounded to a level (quantisation)

Values are stored as a series of binary numbers

Why does sound need to be converted to a digital format before it can be stored on a computer system

Sound is analogue and computers use discrete values

Sample resolution

Number of bits per sample

Advantage of a high sample resolution

Better sound quality

Disadvantages of a high sample resolution

Takes up more storage space, will take longer to download

Sampling rate (unit)

Frequency with which you record the amplitude of a sound (Hz)

Hz

One sample per second

Sound file size in bits

Sampling rate x resolution x duration in seconds

Why is compression used

Reduces the amount of storage to save files, uses less bandwidth (saves money), makes it faster to transmit

Lossy compression

Where files are compressed by permanently removing some of the detail

Disadvantages of lossy compression

Can decrease quality

Uses lossy compression

JPEG, MP3

Lossless compression

Reduces file sie without losing nay info

Why is lossless compression important

Essential for text and data files

Uses lossless compression

PNG, GIF

Feature needed for Run Length Encoding (RLE)

Lot of runs

Important things for huffman encoding

Characters with the lowest frequencies should be combined, table sorted by frequency