A Level Computer Science AQA, Binary Representation of Images and Sound

Describe how bit patterns may represent other forms of data, including graphics and sound.

Bit patterns can be used to store information about other forms of data, for example images can be stored using the images height and width in pixels with the colour depth and sound can be stored as digital samples of the original sound.

Understand the difference between analogue and digital data.

Analogue data is a continuous wave recorded in its original form, when analogue waveforms are sampled at intervals and turned into a discrete set of numbers it produces digital data.

Understand the difference between analogue and digital signals..

Analogue: a continuous signal which represents physical measurements. Digital: discrete time signals generated by digital modulation.

Describe the principles of operation of an analogue to digital converter (ADC)

A microphone converts the sound energy into electrical energy. The ADC samples the analogue data at a given frequency, measures the amplitude of the wave at each point and converts it into a binary value using the resolution of each sample.

Describe the principles of operation of a digital to analogue converter (DAC).

A digital signal is converted into an electrical signal which is then passes through an amplifier, creating an analogue signal. Commonly used to convert a digital audio signal to an analogue signal.

Explain the following for bitmaps: resolution

Resolution is expressed as number of dots per inch where a dot is a pixel.

Explain the following for bitmaps: colour depth

Colour depth = number of bits stored for each pixel.

Explain the following for bitmaps: size in pixels

Size of an image in pixels is width of image in pixels x height of image in pixels.

How to calculate the storage requirements of a bitmap

Ignoring metadata, storage requirements = size in pixels x colour depth

Examples of metadata.

Width, height, colour depth

Explain how vector graphics represents images using lists of objects.

Vector graphics represent images using geometric objects and shapes such as rectangles, circles and lines. The properties (such as fill colour, fill style and dimensions) of each geometric object or shape in the image are stored in a list.

Give examples of typical properties of objects.

Properties for a rectangle:
fill-colour: green
fill-style: solid
height: 2
width: 10
start-position: (0, 0)

Vector graphics versus bitmapped graphics

Because vector graphics use shapes rather than pixels, they can be scaled without losing quality. Enlarging a bitmap image results in a blurry or even pixelated image whereas enlarging a vector graphic results in no loss of clarity.

Vector graphics are well suited to simple images which use shapes, like company logos, but they're no use for photographs. Bitmapped graphics are used for storing photographs.

Vector graphics frequently use less storage space than bitmapped graphics, as information is stored for each shape, rather than for every single pixel in an image.