Computer Audio

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Computer Audio

• CSCI 1100 Applied Computing

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Overview

• What is sound?
• Hertz
• File Formats
• Digitization
• Sampling rate
• Bit rate
• Compressions

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What is Sound?

• Compressions of air or other media (such as water
  or metal) from something vibrating
• Sounds are made up of high frequency and low
  frequency sounds
• Dont confuse pitch (frequency) with volume!
• Volume is measured in decibels (dB)
• Frequency in Hertz (Hz) cycles per second
• Humans only hear from 20Hz to 20KHz

Frequency
Strange Fact! Speed of sound (Air 340m/s
Water1,230 m/s Gold 3,240m/s)
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Spatial Sound

• 1 Channel mono. Can be split to several
  speakers still no direction
• 2 Channels stereo. Fades from left to right.
  Can determine direction
• 5.1 Audio Common for home theaters
• 3D Sound? Video games (PC). Still has time to
  develop

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The Human Side
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The Equal Loudness ContourKilla Hurts
Ouch!
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Human Perception(InterAural Time Difference)

• Sound hits both ears
• Difference in time

Hasntgotten toleft yet
HitsRightEarFirst
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How Computers Perceive Sound

• Digitization (DAC and ADC)
• Computers listen to the sound a certain number
  of times per second (sample rate)
• 44K is CD
• 22K is good
• 8K is lame
• Computers have to approximate what they heard and
  assign it a number
• 4 bits 16 level to approximate to
• 16 bits 2 million levels to approximate to

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Original Sound
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Low Sampling Rate
TIME
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Low Sampling Rate
TIME
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High Sampling Rate
TIME
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High Sampling Rate
TIME
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2 bits per sample4 Approximations
TIME
15
StairStep Effect
TIME
16
3 bits per sample8 Approximations
TIME
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Less StairStep
TIME
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ABBA vs Mozart(When to use what)
2-8-1
2-8-1
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General Idea

• Different sounds require different consideration
• Extreme frequencies (pop, rock, RB) require
  higher sampling rates
• Extreme changes in volume require more bits per
  sample (because of quantization)
• Speech compresses well, because it doesnt do
  either

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File Formats

• .aiff Audio Interchange File Format
• .au (Sun) usually 8-bit old web format
• .mov this is rare, but still possible
• .mid MIDI file cover this in another lecture
• .ram Streaming technology from Real
• .asf Windows can be streamed (new)
• .wav windows common
• .mp3 from the MPEG3 compression good for music

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Capturing Sounds

• Usually done with
• a microphone (such as voice)
• Line in
• CD
• Computer has sound card
• Input types (RCA, MIDI, mini, ¼, XLR)
• Card has quality (plays 16-bit sound)
• Need some kind of software
• SoundForge
• Windows SoundRecorder (gag)

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Reproducing Sounds

• Numbers translate directly into how far in or out
  the cone of the speaker is supposed to move
• Sine wave
• -50, -45, -35, -15, 0, 15, 35, 45, 50, 49, .

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Common Audio Software

• Recording
• ProTools (MotU)
• SoundForge
• Audacity
• Sound Recorder (Windows)
• Garage Band
• Making Music
• ACID
• Frooty Loops
• Rebirth