1. Multimedia Technology and Applications Chapter 2. Digital Audio

2. 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

3. 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
k 7k 15k 20k
CD-DA
FM Radio
AM Radio
Tel.
f(Hz)
Frequency Band

4. Sound
Waveform Sound
Speech
Music

5. 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)

6. Digitization of Sound Continuous Analog Waveform Sampling
Discretized Audio
Illustration

7. Affects of Audio Quality
Sample Rate
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

8. 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

9. 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).

10. 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).

11. 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.

12. 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

13. 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.

14. 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.

15. 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.

16. Audio Processing Software
Adobe Audition

17. ©Some of the pictures and text are from internet online free resource and wikipedia