Multimedia: Digitised Sound Data Section 3

Sound in Multimedia Types: Voice Overs Special Effects Sound can make multimedia presentations dynamic and interesting. They can also be used to assist user. Types: Voice Overs Special Effects Musical Backdrops

Sound Capture Microphone: CD/DVD: Analogue Can be used for voice-overs Digital Capturing music files Copyright laws may apply If it comes from the real world how do we convert into digital? ADC

Sampling Frequency Once an analogue signal is received it is chopped up and a sample is taken The sampling frequency is the number of samples taken per second. Samples of Sound Wave Sound Wave

Sampling Frequency Measurements (or samples) will be taken many times each second The sampling rate for audio CD is 44.1kHZ means 44100 thousand samples per second.

Sampling Depth This describes the number of bits which are taken in each sample This will determine the dynamic range of the recording An audio CD will have a sampling depth of 16bits (65536 levels)

Multimedia: Digitised Sound Data Storage of Sound Data

RAW Uncompressed audio data Sampled digital data which has not been processed further The process of sampling he analogue signal and quantising depending on the strength of the signal (amplitude) is called Pulse Amplitude Modulation (PAM) Quant – turning it into numbers

RAW When data from PAM (Pulse Amplitude Modulation) is stored sequentially it is called Pulse Code Modulation (PCM) PCM allows digital data to be transmitted over a network or to a sound card as it is easily decoded and used to construct the original sound as it is being received.

RIFF Resource Interchange File Format It is a container file It can contain various types of data (e.g. video, bitmaps, sound, etc) RIFF is a multimedia file format. The type of information stored is stated by the header. Header – wave first part of the file

WAV File Wave files are RIFF files containing digitised sound data They are commonly encoded using Adaptive Differential Pulse Code Modulation (ADPCM) Often used in places such as digital telephone systems and minidisc players.

MP3 MPEG Audio Layer-3 (MP3) Uses lossy compression Based on what humans hear best Perceptual Audio Compression Reduces RAW formats to around 10% of original size 3min of CD quality song = 30Mb 3min of MP3 = 3Mb Quality is adjustable

File Size Vs Quality Sampling depth: Sampling frequency: Sound time: increased sound depth Greater range of values Better sound quality and greater file size. Sampling frequency: The higher the sampling frequency, the better the sound quality, the greater the file size. Sound time: Longer track lasts the larger the file size Affects file size but not quality.

Comparing Bitmap image files and digitised sound files Each sample records data for a pixel Bitmap: Resolution Sound: Frequency Bitmap: Number of bits for each sample/number of colours used Colour depth Sound: Number of bits determines number of signal levels that can be recorded Sampling Depth

Multimedia: Digitised Sound Data Storage of Sound Data