Audio Representation

CSCI N351

Andy Harris

Indiana University / Purdue University - Indianapolis

Analog audio

Sound is waves
differences in air pressure
Ear is sensitive to minute pressure changes
Replicate the pressure wave, replicate the sound
Rubens' tube video

Edison's phonograph

edison phonograph

How the Edison phonograph works

Wave pattern produces sound
Needle on membrane inscribes groove on cylinder
Print copies of cylinder
Needle on hardened cylinder moves membrane
Membrane causes wave pattern reproducing original sound

Creating a digital sound

Move a speaker's membrane
Use an oscillator
Need a pattern for motion
Square pattern simplest
Push out for a while
Pull in for a while
The synth applet

Making a better sound

Use another pattern
Sawtooth - start silent - gradually louder
Triangle - gradually louder then softer
Sin wave - 'smooth' curve

Problems with digital oscillator

All patterns are blocky
Limited in X
Limited in Y
Even sine is ultimately blocky
'pure' audio (analog) is smooth

sampling rate

What time does one sample represent?
Measured in Hz (cycles / second)
Or kHz (1000 cycles / second)
Faster sampling rate = better sound
... but larger file
represented by distance on time axis

Common sampling rates (approx. quality)

8 kHz	bad cell phone
16 kHz	phone
22kHz	audio tape
44kHz	CD

Nyquist's Law

optimal sampling rate is double desired output frequency
backwards wagon wheels
20 kHz is a high frequency in human range
Sampling rate is (roughly) 44 kHz

bit depth

How many possible values for each sample?
square wave only requires 1 bit / sample
8 bits = 256 possible values / sample
16 bit = 65,537 possible values / sample
higher bit depth = better sound
... but larger file

Standard bit depths

8 bit
16 bit
32 bit

Quantizing an audio sample

Use bit depth and sample rate to break signal into digits
number of values depends on sample rate
range of each value depends on bit depth

Envelope

Modifies amplitude over lifespan of tone
Greatly changes character of tone
Often measured in ADSR

ADSR Envelope attributes

A - Attack
D - Delay
S - Sustain
R - Release

Improving the timbre

Most sounds are more complex than a simple frequency
You can add multiple oscillators to produce complex sounds

The need for Audio Formats

Raw audio data is expensive
Compression
Reconstruction

The MIDI format

Musical Instrument Digital Interface
NOT technically a file type!
A form of notation
MIDI file is stored by an instrument
Can be replayed by another MIDI Instrument

MIDI's bad rap

MIDI is looked down on in computer audio
Quality is dependant on hardware, skill of sequencer
Most typical computers have poor MIDI capabilities
MIDI can be most efficient format
MIDI can sound very good on higher-end equipment
MIDI accurately handles musical information
MIDI can be modified musically

Examining a MIDI file

Using Anvil Studio
Examining Event list
Events as a programming language

Modifying a MIDI file

Examining a file in the mixer view
Changing an instrument
The midi Instrument bank
Original "Blue Suede Shoes"
Brass version

Using MIDI to study Music

Examining Dvorak's 9th, 4th Movement
Looking at the bassoon part
Finding bassoon solo
Playing notes back

Composing MIDI Music

Using a keyboard or other instrument
Using computer keyboard
Drag and drop

Generating a song

The 'mayan song'
Creating the flute part
Creating the instruments

Sampling audio

using audacity to edit a sound
Choosing a sampling rate
Choosing a bit depth
Recording a sound

Modifying a sound

Using layers of sound
Applying filters echo, noise removal
Trimming a sound

Doing more with a segment

Duplicating a segment
Avoiding clipping
Exporting as an MP3 File
Audio Compression
using db powerAmp

Introducing Ogg Vorbis

Open source alternative to mp3
better compression than mp3
No DRM, legal encumbrances
Currently supported in HTML 5 audio tag
Audio Demo

Streaming Audio

File is compressed
a portion (maybe 10 seconds) is sent to a buffer
user plays file from buffer
New material flows in from server as user plays
As long as input is >= output, all is well
Streaming media is about throughput