Multimedia Technology and Applications Chapter 2. Digital Audio

Basic of digital Audio Sound: a travelling wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing. t Amplitude Period A Amplitude：the magnitude of change during one oscillation. Period：the time interval between two successive occurrences of a recurrent event. Frequency：the number of occurrences of a repeating event per unit time. Essential Properties

Sound Sound Frequency: Speech Signal Frequency：300Hz-3kHz 20Hz 20kHz Infrasonic wave Audible sound Supersonic wave 20Hz 20kHz f(Hz) Speech Signal Frequency：300Hz-3kHz 10 20 50 200 3.4k 7k 15k 20k CD-DA FM Radio AM Radio Tel. f(Hz) Frequency Band

Sound Waveform Sound Speech Music

Analog Signal Digital Signal Digitization of Sound Digitization: conversion to a stream of numbers, and preferably these numbers should be integers for efficiency. The procedure of sound digitization Sampling Quantization Coding Analog Signal Digital Signal Analog Signal Digital Signal A/D ADC D/A DAC (Nature World) (Computer)

Digitization of Sound Continuous Analog Waveform Sampling Discretized Audio Illustration

Affects of Audio Quality Sample Rate 11.025 KHz – Speech (Telphone 8kHz) 22.05 KHz – Low Grade Audio (WWW Audio, AM Radio) 44.1 KHz – CD Quality Bit Size 8 bit = 256 16 bit = 65536 Channel Mono Stereo

Affects of Data Size Data Size =Sample Rate×Bit Size ×#Channel/8(Byte/s) Sample Rate (kHz) Bit Size (bit) Data Size(KB/s) Mono Stereo 11.025 8 10.77 21.53 16 43.07 22.05 86.13 44.1 172.27

Typical Audio Format WAV standard audio file container format used mainly in Windows PCs. Commonly used for storing uncompressed (PCM), CD-quality sound files (large in size) MP3 MPEG Layer-3 format. By eliminating portions of the audio file that are less audible, mp3 files are compressed to roughly one-tenth the size of an equivalent PCM file sacrificing quality. WMA the popular Windows Media Audio format owned by Microsoft. Designed with Digital Rights Management (DRM) abilities for copy protection. MIDI Storing MIDI messages (along with timing information).

MIDI: Musical Instrument Digital Interface A scripting language --- it codes “events” that stand for the production of sounds. A standard adopted by the electronic music industry for controlling devices (synthesizers and sound cards). Supported by most synthesizers, i.e., sounds created on one synthesizer can be played and manipulated on another synthesizer. Computers must have a special MIDI interface (incorporated into most sound cards with both D/A and A/D converters).

Hardware Aspects of MIDI MIDI connectors: - three 5-pin ports found on the back of every MIDI unit MIDI IN: the connector via which the device receives all MIDI data. MIDI OUT: the connector through which the device transmits all the MIDI data it generates itself. MIDI THROUGH: the connector by which the device echoes the data receives from MIDI IN.

MIDI Concepts MIDI Messages: MIDI messages are used by MIDI devices to communicate with each other. MIDI Messages Channel Messages Voice Messages Mode Messages System Messages Common Messages Real-Time Messages Executive Messages

Channel messages: Messages that are transmitted on individual channels rather that globally to all devices in the MIDI network. Voice messages: controls a voice, i.e., sends information specifying which note to play or to turn o, and encodes key pressure. Also used to specify controller effects such as sustain, vibrato, tremolo, and the pitch wheel. Mode messages: determine how an instrument processes MIDI voice messages: respond to all messages, respond just to the correct channel, don't respond at all, or go over to local control of the instrument.

System Messages Commands that are not channel specic, such as timing signals for synchronization, positioning information in pre-recorded MIDI sequences, and detailed setup information for the destination device. System common messages: relate to timing or positioning. System real-time messages: related to synchronization. System exclusive message: included so that the MIDI standard can be extended by manufacturers.

General MIDI MIDI + Instrument Patch Map + Percussion Key Map -> a piece of MIDI music sounds the same anywhere it is played Instrument patch map is a standard program list consisting of 128 patch types. Percussion map specifies 47 percussion sounds. Key-based percussion is always transmitted on MIDI channel 10. Requirements for General MIDI Compatibility: Support all 16 channels. Each channel can play a different instrument/program (multitimbral). Each channel can play many voices (polyphony). Minimum of 24 fully dynamically allocated voices.

Audio Processing Software Adobe Audition

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