sound and digital audio.ppt
TRANSCRIPT
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From the air to the iPod
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Minute disturbances in the air, caused bya vibrating object
Air molecules bunch together, thenspread out
Changes in density (air pressure, orsound pressure)
Causes a chain reaction; sound pressurewave propagates
What is Sound?
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Sound Wave Propagation
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Compression / Rarefaction
High
Low
Normal
Sou
ndPressure
Time
Time domain plot of a waveform
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Periodic Waveforms
1 Cycle
Period:
Frequency:
How long does one cycle last?
How many cycles per second?
Expressed in Hertz (Hz)Ex: 440 Hz (the A above middle C)
Period = 1 / Frequency
(for A440: 0.0023 sec.)
Amplitude
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Frequency and Pitch
Frequency is an acoustic fact.Pitch is a human perception.
Our sense of pitch has a logarithmic relation tofrequency its based on ratios.
Our ear is like a microphone. It changes thephysical wave (acoustic energy) into an electricalsignal.
Range of human hearing
20Hz to 20,000 Hz (20 kHz), approximately(for young folks; old folks cant hear as high)
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The Harmonic Series
Arithmetic series of frequencies givesever-shrinking intervals.
Frequencies in Hz:
Double frequency: octave higher
(flat)
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Analog Representations of Sound
Magnified phonograph grooves, viewed from above:
When viewed from the side, channel 1 goes upand down, and channel 2 goes side to side.
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Analog versus Digital
Analog
Continuous signal that mimics shapeof acoustic sound pressure wave
DigitalStream of discrete numbers thatrepresent instantaneous amplitudesof analog signal, measured at equally
spaced points in time.
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Analog to Digital Recording Chain
ADC
Continuously varying electrical energy isan analog of the sound pressure wave.
Microphone converts acoustic toelectrical energy. Its a transducer.
ADC (Analog to Digital Converter)converts analog to digital electrical signal.
Digital signal transmits binary numbers.
DAC (Digital to Analog Converter) converts digitalsignal in computer to analog for your headphones.
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Analog to Digital Conversion
Instantaneous amplitudes ofcontinuous analog signal, measuredat equally spaced points in time.
A series of snapshots
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[a.k.a. sample word length, bit depth]Precision of numbers used formeasurement: the more bits, the higherthe resolution.
Example: 16 bit
Analog to Digital Overview
Sampling Rate
How often analog signal is measured
Sampling Resolution
[samples per second, Hz]
Example: 44,100 Hz
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Sampling Rate
Nyquist Theorem:
Sampling rate must be at least twice as high as
the highest frequency you want to represent.
Determines the highest frequency that youcan represent with a digital signal.
Capturing just the crest and trough of a sinewave will represent the wave exactly.
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Aliasing
What happens if sampling rate not high enough?
A high frequency signal
sampled at too low a rate
looks like
a lower frequency signal.
Thats called aliasing.
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Common Sampling Rates
Sampling Rate Uses
44.1 kHz (44100) CD, DAT
48 kHz (48000) DAT, DV, DVD-Video
96 kHz (96000) DVD-Audio
22.05 kHz (22050) Old samplers
Most software can handle all these rates.
Which rates can represent the range offrequencies audible by (fresh) ears?
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3-bit Quantization
A 3-bit binary (base 2) number has 23 = 8 values.
0
1
2
3
4
5
6
7
A rough approximation
Amplitud
e
Time measure amp. at each tick of sample clock
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4-bit Quantization
A 4-bit binary number has 24 = 16 values.
0
2
4
6
8
10
12
14
Amplitud
e
A better approximation
Time measure amp. at each tick of sample clock
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16-bit Sample Word Length
A 16-bit integer can represent 216, or
65,536, values (amplitude points).
We typically use signed 16-bit integers,and center the 65,536 values around 0.
32,767
-32,768
0
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Quantization Noise
Round-off error: difference between actual
signal and quantization to integer values
Random errors: soundslike low-amplitude noise
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Common Sampling Resolutions
Word length Uses8-bit integer Low-res web audio
16-bit integer CD, DAT, DV, sound files
24-bit integer DVD-Video, DVD-Audio
32-bit floating point Software (usually only forinternal representation)
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The Digital Audio Stream
Its just a series of sample numbers, to be
interpreted as instantaneous amplitudes:one for every tick of the sample clock.
Previous example:
11 13 15 13 10 9 6 1 4 9 15 11 13 9
This is what appears in a sound file, along
with a header that indicates the samplingrate, bit depth and other things.
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Audio File Size
CD characteristics
- Sampling rate:
44,100 samples per second (44.1 kHz)
How big is a 5-minute CD-quality sound file?
- Sample word length:
16 bits (i.e., 2 bytes) per sample
- Number of channels:
2 (stereo)
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Audio File Size
5 minutes * 60 seconds per minute= 300 seconds
How big is a 5-minute CD-quality sound file?
44,100 samples * 2 bytes per sample * 2 channels
= 176,400 bytes per second
300 seconds * 176,400 bytes per second
= 52,920,000 bytes = c. 50.5 megabytes (MB)
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MPEG Compression
MPEG 1-Layer 3 (.mp3) Motion Picture Experts Group
Different levels for different purposes
E.g. MPEG 2 used for DVDs
Takes out parts of the sound signal thathumans cant hear
How does the size change? Lossy compression
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MPEG Compression
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What is an iPod?
Its a computer! Look at the von Neumann architecture
I/O
2-4GB microdrive, click wheel, LCD panel,USB connection, FireWire connection, audiooutput
Memory 32MB RAM, 32MB ROM
CPU Special digital to analog converter chips Well-designed UI Stores music in various digital formats
AAC (16 to 320 Kbps), MP3 (32 to 320Kbps), MP3 VBR, Audible, AIFF, Apple
iP d S ifi i
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iPod Specifications
CPU: ARM7TDMI Playtime: up to 8 hours Scroll wheel: solid state touch Buttons: mechanical CPU Speed: dual 80 Mhz
embedded Data Path: 32 bit ROM: 32 MB Onboard RAM: 32 MB Screen: 1.67" LCD w/ LED
Maximum Resolution: 1-bit 138x110 Hard Disk: 1" 4 GB 4200 RPM holds 1000 songs. USB: via Dock Connector FireWire: via Dock Connector Audio Out: stereo 16 bit mini Weight: 0.225 lbs. Dimensions: 3.6" H x 2.0" W x 0.5" D
OS: iPod OS 1.1 I t d d J 2004