COMP135/COMP535 Digital Multimedia, 2nd edition Nigel Chapman & Jenny Chapman Chapter 2 Lecture 2 – Digital Representations

COMP135/COMP535 Devices can only be in one of two states –0 or 1, yes or no, on or off, … Bit: a unit of data that can only have one of two values Byte: an ordered sequence of eight bits Word: a short sequence of bytes, usually four (32 bits) Bits and Bytes

COMP135/COMP535 Numbers to base 2 (binary) – = 97 decimal Characters – associate bit patterns (numbers) with characters via a character set – = a in ASCII Brightness of an image at a point, instantaneous amplitude of a sound wave, etc Interpretation of Bits

COMP135/COMP535 Each byte can be identified by its position in the sequence of all bytes in memory – its address Collections of bytes can be combined into data structures using addresses –e.g. store an image as a sequence of brightness values, use address of the first to access the image data –store a video sequence as series of images, add address of next and previous to each frame Addresses

COMP135/COMP535 Bit patterns that cause the processor to carry out operations on values stored in memory –Hardware is constructed so that the desired effect is achieved Programs stored in memory as a sequence of instructions to be executed in order A computer is a stored program machine –It can therefore be used for many different tasks, depending on the programs it runs Instructions

COMP135/COMP535 Converting a signal from analogue to digital form –Analogue signal can vary continuously, digital is restricted to discrete values Two-stage process –Sampling – measure the value at discrete intervals –Quantization – restrict the value to a fixed set of quantization levels Digitization

COMP135/COMP535 Sampling and Quantization

COMP135/COMP535 Only certain signal values are valid –Relatively immune to corruption by noise –Do not degrade when copied or transmitted over network Some information lost Undersampling –Samples 'too far apart' so cannot accurately reconstruct original signal Digital Signals

COMP135/COMP535 Any periodic waveform can be decomposed into a collection of frequency components –Each one is a pure sine wave The collection of frequencies and their amplitudes represent the waveform in the frequency domain –Compute the frequency domain representation (frequency spectrum) using the Fourier Transform –Higher frequency components are associated with abrupt transitions Frequency Domain

COMP135/COMP535 Compression Definition : An operation performed on data to reduce the amount of storage required to represent it, for reasons of economy or efficiency. Decompression : An operation performed on previously compressed data to restore it as far as possible to its original. Software that performs compression and decompression is often called a codec (short for compressor / decompressor).

COMP135/COMP535 Compression -Compression algorithms can be divided into two classes :  Lossless.  Lossy. -Compression must be often applied to media data. -Different compression algorithms are applicable to different types of media data. Their effictiveness depends on the characteristics of the data itself.

COMP135/COMP535

There are established ways of representing images, video, animation, sound and text in bits. Images are displayed as arrays of pixels and represented using an internal model. Generating the pixels from the model is called rendering. Images may be modeled as bitmaps or vector graphics. Digital Representation of Media

COMP135/COMP535 A bitmap is an array of logical pixels(stored colour values) that can be mapped directly to the physical pixels on the display. Digital Representation of Media

COMP135/COMP535 In vector graphics, the image is stored as a mathematical description of a collection of individual lines, curves and shapes making up the image, which requires computation to render it. Vector graphics are often smaller than bitmaps, are resolution-independent and can be scaled without loss of quality, but they are suitable for certain sorts of synthetic image, not photographs. Digital Representation of Media

COMP135/COMP535 Digital Representation of Media

COMP135/COMP535 Video and animation : - Moving pictures can be created as live-action or animation. - Live-action must be stored as video. - Animation may be represented in other more flexible or efficient ways. Digital Representation of Media

COMP135/COMP535 Digital Representation of Media Sound : - Can be represented as a sequence of samples after digitization. - CD audio is sampled at 44.1 kHz, higher sampling rates are sometimes used. - Audio delivered over the Internet is compressed, often using the MP3 codec.

COMP135/COMP535 Text : - A character set is a mapping from characters to character codes. - Unicode is a character set capable of representing text in all known languages. - A font is a set of character shapes, called glyphs. - Many aspects of layout must be controlled when text is displayed. Digital Representation of Media

COMP135/COMP535 Digital Representation of Media Interactivity is produced by executing a program in response to user input. In multimedia, programs are often written in a scripting language, such as JavaScript or ActionScript. Metadata is structured data about data, which may be attached to media files to help with searching and classifying them.