Images, Sound & Compression

Computer Science

Define Analogue Data

Data that can be measured to any amount of accuracy

What is a bitmapped graphic

An image created using a grid of pixels

Name a common bitmap file type

• PNG

• JPG

• GIF

What is resolution in the context of bitmap images

The amount of pixels used in an image

How many pixels is one megapixel?

1000000

What is colour depth?

The amount of colours used in an image

Image file size =

Number of pixels x Colour depth

Define Metadata and give one example.

Data about data

The date the photo was taken

What is a vector image?

An image made up of shapes which are created by giving instructions to the computer to ‘ draw ‘ them.

Give two advantages of using a vector image over bitmap

• Generally smaller file size

• Image will not be pixelated as you zoom in

What is one advantage of bitmaps over vector graphics

Bitmaps can store photorealistic images

Describe the process of converting from analoge to digital and vice versa

• The waves amplitude is measured at set intervals to create samples

• Each sample is quantised to measure its wave height and transfer into an discrete value

• The integer value is then converted and stored

Define Sample resolution

Number of bits per sample

Define sampling rate and give a unit

The number of samples taken per second (Hertz)

Give a pro and con of a higher sample resolution for sound

More bits used per sample means the wave is measured more accurately, but the file size increases

How do you calculate audio file size?

File size = sample rate x resolution x length(s)

Define the Nyquist Theorem

The sample rate must be at least twice the highest frequency

What is the human range of human hearing?

20Hz - 20KHz

What is MIDI

Musical Instrument Digital Interfaces create sounds as requested by an instrument

Give 4 examples of MIDI Event Messages

• sync tempo

• Control pitch

• Change volume

• Silence instruments

Why is compression used?

- Decrease Download Times

- Use less storage space

Define lossy compression

The original data cannot be recreated from the compressed data as data is lost during compression

Define lossless compression

The original data can be recreated from the compressed data

What is a lossless file type?

• RAW

• PNG

• ZIP

• WAV

Describe the process of RLE

Consecutive patterns of data are summaries with the amount of times it is repeated and then the data

How does dictionary compression work?

Frequently repeating phrases are stored in a dictionary and given an index to be used again

What is plaintext?

The actual text that needs to be received with no cipher on it

How does a Caesar cipher work?

Letters are shifted by a consistent amount

How does a Vernam cipher work?

A key (also known as a one time pad) is generated form a random event (such and nuclear decay) this is used on the text and then destroyed after use

How is a Vernam One Time Pad used?

An XOR is used on the key and plaintext

What does it mean for a cipher to be computationally secure?

It cannot be cracked in a reasonable amount of time