Digital Media
Lecture 12: Additional Audio
Georgia Gwinnett College
School of Science and Technology
Dr. Jim Rowan

Refer to Supplemental text:

Audio & Illusions
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Can you hear this?
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“mosquito ring tone” http://www.freemosquitoringtones.org/hear ing_test/
Audio illusion: “Creep”
 http://www.youtube.com/watch?v=ugriWS mRxcM

The nature of sound
First, a video from ted.com
http://www.wimp.com/howsound/

Other related video #1
How to use visualizations of human speech and music to explain computation: http://www.youtube.com/watch?v=mGc6clf_Wt4&feature=bf_prev&list=PL278ECD
A0705DAF3DS

Other related video #2
David Byrne on how the venue shapes the form of the music performed: http://www.ted.com/talks/lang/en/david_byrne_how_architecture_helped_music_e volve.html

The nature of sound
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Three classes of audio that we will discuss
– 1) Environmental sound
(sounds found in the environment)
– 2) Music
– 3) Speech

The nature of sound
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Environmental sounds
– Provides information about the surroundings that the human is currently in
Music and Speech
– Functionally and uniquely different than other sounds
– Music
• Carries a cultural status
• Can be represented by non-sound: MIDI
• Can be represented by a musical score
– Speech
• Linquistic content
• Lends itself to special compression

And it’s complicated…
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Converting energy to vibrations and back
Transported through some medium
– Either air or some other compressible medium
Consider speech
– Starts as an electrical signal (brain & nerves)
– Ends as an electrical signal (brain & nerves)
– But…

No… it’s REALLY complicated..
http://en.wikipedia.org/wiki/Ear
– Starts as an electrical signal (brain & nerves) ==>
– Muscle movement (vocal chords)
• Vibrates a column of air sending out a series of compression waves in the air
– Compression waves cause ear membrane to vibrate
==>
– Moves 3 tiny bones ==>
– Causes waves in the liquid in the inner ear ==>
– Bends tiny hair cells immersed in the liquid ==>
– When bent they fire ==>
– Sends electrical signals to the cerebral cortex
– Processed by the temporal cortex

Audio Illusions
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Audio creep…
Play a 200 Hz pure tone
– Softly at first
– Gradually increase the volume
– Most listeners will report that the tone
drops in pitch as the volume increases
Play a 2000 Hz pure tone
– Softly at first
– Gradually increase the volume
– Most listeners will report that the tone rises in pitch as the volume increases

Why do you think…
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You can’t tell where some sounds come from
(like some alarms for instance)
You only need one sub woofer when you need at least two for everything else
You can’t tell where sound is coming from underwater
Two things running at the same speed make a
“beating” sound

Why do you think… (cont)
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With your eyes closed you can’t tell whether a sound is in front of you or behind you
You hear sound that isn’t there (tinnitis)
Phantom sounds
– Heard… but not there
Masking sounds
– Not simply drowning them out
– Can mask a sound that occurs before the masking sound actually starts

Why do you think… (cont)
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You can hear your name in a noisy room
– Cocktail party effect
– http://en.wikipedia.org/wiki/Cocktail_ party_effect
– Still very much a subject of research

Why? It’s complicated!
 http://en.wikipedia.org/wiki/Psychoaco ustics
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Psychoacoustics
– The study of human sound perception
– The study of the psychological and
physiological affects of sound

Why?
It’s complicated!
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Sound is physical phenomenon that is interpreted through the human perceptual system
– Wavelength affects stereo hearing
• The distance between your ears related to the wavelength
– Speed of sound affects stereo hearing
• The faster the sound travels, the wider apart your ears need to be
– You can tell where a sound comes from if
• the wavelength is long enough and
• the speed that sound travels is slow enough to allow the waves arrive at your ears at different times

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Processing Audio

Processing audio
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How can we characterize sound?
– Amplitude
– Frequency
– Time
Waveform displays
– Summed amplitude of all frequencies & time
– Amplitude & frequency components at one point in time
– Amplitude & frequency & time

Summed energy & time

Croak!
Play Croak!

The sonogram, a snapshot of frequency
Croak!
Play Croak!

Another way to show audio, frequency density across time
Slim Pickens from Dr. Strangelove

Croak!
Play Croak!

More examples…
Pure sine wave G , E , C
Bassoon playing the same notes

G
C
E
Waveform & time

G
C
E
Sonogram

Frequency snapshot

Frequency over time

Digitized audio
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As we have seen earlier this semester
– Sample rate & quantization level
– Reduction in sample rate is less noticeable than reducing the quantization level
Jitter is a problem
– Slight changes in timing causes problems
20k+ frequencies?
– Though they can’t be heard they manifest themselves as aliases when reconstructed

Audio Dithering is Weird… add noise… get better sounding result?!?
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Add random noise to the original signal
This noise causes rapid transitioning between the few quantized levels
Makes audio with few quantization levels seem more acceptable

Audio dithering

Audio processing terms to know
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Clipping
– …but you don’t know how high the amplitude will be before the performance is recorded
Noise gate
– has an amplitude threshold
Notch filter
– remove 60 cycle hum
Low pass filter
High pass filter
Time stretching (or shrinking… Limbaugh)
Pitch alteration
Envelope shaping (modifying attack)

What these filters look like:
High pass filter

What these filters look like:
Low pass filter

What these filters look like:
Notch filter

Audio clipping

One thing about humans…
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We can actively “filter out” what we don’t want to hear
– remember the cocktail party effect?
Over time we don’t hear the pops and snaps of a vinyl record
– Have you ever recorded something that you thought would be good only to play it back and hear the air conditioner or traffic roaring in the background?
A piece of software can’t do this…
– …not yet anyway!

Compressing sound: Voice
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Remove silence
– Similar to RLE
Non-linear quantization
• “companding”
– Quiet sounds are represented in greater detail than loud ones

Compressing sound: Voice
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Differential Pulse Code Modulation (DPCM)
– Related to temporal (inter-frame) video compression
• It predicts what the next sample will be
• It sends that difference rather than the absolute value
• Not as effective for sound as it is for images
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Adaptive DCPM
– Dynamically varies the sample step size
• Large differences were encoded using large steps
• Small differences were encoded using small steps

Sound compression that is based on perception
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The idea is to remove what doesn’t matter
Based on the psycho-acoustic model
– Threshold of hearing
• Remove sounds too low to be heard
– High and low frequencies not as important
(for voice)