Transcript 计算机交叉学科
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. A Period Amplitude t Essential Properties 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. SOUND Sound Frequency: Infrasonic wave Supersonic wave Audible sound f(Hz) 20Hz 20kHz Speech Signal Frequency：300Hz-3kHz CD-DA FM Radio AM Radio Tel. Frequency Band 10 20 50 200 3.4k 7k 15k 20k f(Hz) SOUND DIGITIZATION OF SOUND Digitization: conversion to a stream of numbers, and preferably these numbers should be integers for efficiency. The procedure of sound digitization Analog Signal Sampling Analog Signal (Nature World) Quantization A/D ADC D/A DAC Coding Digital Signal (Computer) Digital Signal DIGITIZATION OF SOUND Continuous Analog Waveform Sampling Illustration Discretized Audio AFFECTS OF AUDIO QUALITY • 11.025 KHz – Speech (Telphone 8kHz) • 22.05 KHz – Low Grade Audio (WWW Audio, AM Radio) • 44.1 KHz – CD Quality • 8 bit = 256 • 16 bit = 65536 • Mono • Stereo AFFECTS OF DATA SIZE Data Size =Sample Rate×Bit Size ×#Channel/8(Byte/s) Sample Rate (kHz) 11.025 22.05 44.1 Data Size(KB/s) Bit Size (bit) Mono Stereo 8 10.77 21.53 16 21.53 43.07 8 21.53 43.07 16 43.07 86.13 8 43.07 86.13 16 86.13 172.27 TYPICAL AUDIO FORMAT WAV • standard audio file container format used mainly in Windows PCs. Commonly used for storing uncompressed (PCM), CDquality 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 onetenth 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. 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 ©Some of the pictures and text are from internet online free resource and wikipedia