1. Digital Audio Application of Digital Audio - Selected Examples
Music Production     
Hard Disk Recording
Sound Synthesis
Samplers
Effects Processing
Video
- Audio Important Element: Music and Effects
Web
- Many uses on Web
Spice up Web Pages
Listen to Cds
Listen to Web Radio
Many More Uses
- try and think of some?

2. Digitization of Sound
Sound is a continuous wave that travels through the air
The wave is made up of pressure differences. Sound is detected by measuring the pressure level at a particular location.
Sound waves have normal wave properties (reflection, refraction, diffraction, etc.).

3. Source Destination Generates Sound Receives Sound Air Pressure changes
Electrical - Loud Speaker
Acoustic - Direct Pressure Variations
Destination
Receives Sound
Electrical - Microphone produces electric signal
Ears - Responds to pressure hear sound

4. Continuous Analogue Waveform
Sound is required input into a computer: it needs to sampled or digitised:
Microphones, video cameras produce analogue signals (continuous-valued voltages).
Continuous Analogue Waveform
To get audio or video into a computer, we have to digitize it (convert it into a stream of numbers). Need to convert Analogue-to-Digital - Specialised Hardware
So, we have to understand discrete sampling (both time and voltage)
Sampling - divide the horizontal axis (the time dimension) into discrete pieces. Uniform sampling is ubiquitous.
Quantization - divide the vertical axis (signal strength) into pieces. Sometimes, a non-linear function is applied.
8 bit quantization divides the vertical axis into 256 levels; 16 bit gives you levels.

5. Digitizing Audio
That is the basic idea of digitizing a sound unfortunately things are (practically speaking) not so simple.
Questions for producing digital audio (Analogue-to-Digital Conversion):
1. How often do you need to sample the signal?
2. How good is the signal?
3. How is audio data formatted?

6. Computer Manipulation of Sound
Once Digitised processing the digital sound is essentially straightforward although it depends on the processing you wish to do (e.g. volume is easier to code than accurate reverb)
Sample Rates and Bit Size
How do we store each sample value (Quantisation)?
8 Bit Value
(0-255)
16 Bit Value
(Integer) ( )
How many Samples to take?
KHz Speech (Telephone 8KHz)
22.05 KHz Low Grade Audio (WWW Audio, AM Radio)
44.1 KHz CD Quality

7. Implications of Sample Rate and Bit Size
Affects Quality of Audio
Ears do not respond to sound in a linear fashion
Decibel (dB) a logarithmic measurement of sound
16-Bit has a signal-to-noise ratio of 98 dB - virtually inaudible
8-bit has a signal-to-noise ratio of 50 dB
Therefore, 8-bit is roughly 8 times as noisy
6 dB increment is twice as loud

8. Typical Audio Formats
Popular audio file formats include .au (Unix workstations), .aiff (MAC, SGI), .wav (PC, DEC workstations)
A simple and widely used audio compression method is Adaptive Delta Pulse Code Modulation (ADPCM). Based on past samples, it predicts the next sample and encodes the difference between the actual value and the predicted value.

9. Delivering Audio over a Network
Trade off between desired fidelity and file size
Bandwidth Considerations for Web and other media.
Compress Files:
Could affect live transmission on Web

10. Streaming Audio Buffered Data:
Trick get data to destination before it's needed
Temporarily store in memory (Buffer)
Server keeps feeding the buffer
Client Application reads buffer
Needs Reliable Connection, moderately fast too.
Specialised client, Steaming Audio Protocol (PNM for real audio).

11. Introduction to MIDI (Musical Instrument Digital Interface)
Definition of MIDI: a protocol that enables computer, synthesizers, keyboards, and other musical device to communicate with each other
Components of a MIDI System:
Synthesizer (a sound generator)
Sequencer (a music editor)
Track (to organize the recordings)
Channel (to separate information in a MIDI system)
Timbre (the quality of the sound)
Pitch (musical note that the instrument plays)
Voice (the portion of the synthesizer that produces sound)
Patch (the control settings that define a particular timbre)

12. Digital Audio and MIDI
There are many application of Digital Audio and MIDI being used together:
Modern Recording Studio - Hard Disk Recording and MIDI
Analogue Sounds (Live Vocals, Guitar, Sax etc) - DISK
Keyboards, Drums, Samples, Loops Effects - MIDI
Sound Generators: use a mix of
Synthesis
Samples
Samplers - Digitise (Sample) Sound then
Playback
Loop (beats)
Simulate Musical Instruments

13. Digital Audio, Synthesis, Midi and Compression - MPEG 4 Structured Audio
We have seen the need for compression already in Digital Audio - Large Data Files
Basic Ideas of compression (see following Chapters) used as integral part of audio format - MP3, real audio etc.
Mpeg-4 audio - actually combines compression synthesis and MIDI to have a massive impact on compression.
MIDI, Synthesis encode what note to play and how to play it with a small number of parameters - Much greater reduction than simply having some encoded bits of audio.
Responsibility to create audio delegated to generation side.

14. MPEG 4 Structured Audio
MPEG-4 covers the whole range of digital audio:
from very low bit rate speech
to full bandwidth high quality audio
built in anti-piracy measures
Structured Audio
Structured Audio Tools
MPEG-4 comprises of 6 Structured Audio tools