Today’s topics
Sound
Upcoming
 Intellectual property
 Network analysis
 Security
Reading
CompSci 001
8.1
How does Hearing Work?



The outer ear “catches” sounds
The eardrum vibrates
The inner ear translates the vibrations to nerve impulses for
the brain to interpret
Georgia Institute of Technology
CompSci 001
8.2
Acoustics, the physics of sound

Sounds are waves of air
pressure


Sound comes in cycles
The frequency of a wave is
the number of cycles per
second (cps), or Hertz
• (Complex sounds have
more than one frequency
in them.)

The amplitude is the
maximum height of the
wave
Georgia Institute of Technology
CompSci 001
8.3
Volume and Pitch

Our perception of volume is related (logarithmically)
to changes in amplitude



If the amplitude doubles, it’s about a 3 decibel (dB) change.
A decibel is a ratio between two intensities: 10 * log10(I1/I2)
As an absolute measure, it’s in comparison to threshold of
audibility
• 0 dB can’t be heard.
• Normal speech is 60 dB.
• A shout is about 80 dB

Our perception of pitch is related (logarithmically) to
changes in frequency



Higher frequencies are perceived as higher pitches
We can hear between 5 Hz and 20,000 Hz (20 kHz)
A above middle C is 440 Hz
Georgia Institute of Technology
CompSci 001
8.4
Digitizing Sound


In calculus you learn to
estimate a curve by
creating rectangles
We can do the same to
estimate the sound curve


Analog-to-digital
conversion (ADC) will
give us the amplitude at
an instant as a number: a
sample
How many samples do we
need?
Georgia Institute of Technology
CompSci 001
8.5
What is digital?

What’s the difference between


Phonograph and CD?
VCR tape and DVD?

Why digitize sound?

How is ripping to a mp3
different from recording to a tape?

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

Reproduction: immediate and future
Distribution
Modification
Why do digital media present new challenges from analog
media?

Is copyright infringement new?
CompSci 001
8.6
Nyquist Theorem


We need twice as many samples as the maximum frequency in
order to represent (and recreate, later) the original sound.
The number of samples recorded per second is the sampling
rate
 If we capture 8000 samples per second, the highest
frequency we can capture is 4000 Hz
• That’s how phones work

If we capture more than 44,000 samples per second, we
capture everything that we can hear (max 22,000 Hz)
• CD quality is 44,100 samples per second

Call a friend on a phone and play some music over the phone
 How does it sound?
 Why does it work for voice but not for music?
CompSci 001
8.7
Playing a Sound

We can create a Sound object just as we created a Picture
object
 Get a file name and save a reference to it

– Pick a file that ends in .wav
Create the sound object by asking the class to create a new
Sound object and initialize it by reading data from the
given file name
• sound1 = makeSound(fileName)

Play the Sound
play(sound1)
Georgia Institute of Technology
CompSci 001
8.8
Play Sound Exercise

Try creating a Sound object and playing it by



Specifying it in steps
Specifying it all at once
How would you play the same sound twice?
CompSci 001
Georgia Institute
of Technology
8.9
Digitizing Sound in the Computer


Each sample is stored as a number (two
bytes)
What’s the range of available combinations?


16 bits, 216 = 65,536
But we want both positive and negative values
• To indicate compressions and rarefactions.


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
What if we use one bit to indicate positive (0) or
negative (1)?
That leaves us with 15 bits
15 bits, 215 = 32,768
One of those combinations will stand for zero
• We’ll use a “positive” one, so that’s one less pattern for
positives so the range is from -32,768 to 32,767
CompSci 001
Georgia Institute
of Technology
8.10
The Sound Tool

Not all of the sound
is shown when you
explore a sound


You can zoom in


Skips values to fit in
the window
To see all sample
values
You can zoom out

To fit the sound in the
window again
CompSci 001
Georgia Institute
of Technology
8.11
Getting the Sound Sample Values


A Sound has many values in it
 Numbers that represent the sound at that time in the
sample
You can get an list of SoundSample objects
 samples = getSamples(sound1)
CompSci 001
Georgia Institute
of Technology
8.12
Print the Sound Sample Value



You can get the SoundSample object from the list at an index
 sample = samples[0];
And then get the value from that
 print sample
What are the first 10 values of the Sound created from the file
croak.wav?
CompSci 001
Georgia Institute
of Technology
8.13
Changing the Value of a Sound Sample

You can set the value of a SoundSample


setSample(sample, value);
This will change the value in the Sound object as
well

So how would you change the value of the
first sample to the original value * 2?

How could you double all of the samples?

By hand?
• How many samples are there?

Using a loop?
Georgia Institute of Technology
CompSci 001
8.14
Tracing Execution





The index is set to 0
The value is set to the
value in the array at that
index (59)
The sample value at the
current index is set to 2 *
value
The index changes to the
next index (1)
We check if the index is
less than the length of
the array and


If so do the loop again
Else jump to the first
statement after the loop
Georgia Institute of Technology
CompSci 001
8.15
Memory versus Disk

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When we read from a file we read from disk into memory
 Computers only do calculations on memory
We change the values in memory
The file on the disk hasn’t changed
To save our new sound we need to write a file to the disk
 writeSoundTo(sound1, fileName);
CompSci 001
Georgia Institute
of Technology
8.16
Copyright

US Constitution (Article I, Section 8, Clause 8):
“To promote the Progress of Science and useful Arts”

What can you copyright?
• Fixed, tangible medium of expression with a modicum of originality

How do you copyright?
• Don’t need anything. Registration necessary for copyright
infringement suits


Authors given limited monopoly so they will disclose to public
Concessions
1. Fair use
2. First sale
3. Limited Time


CompSci 001
Evolving Bargain: Copyright holder may profit from works and
public has access and can build upon them
What would happen if information could only be shared if the
owner provided permission?
Georgia Institute
of Technology
8.17
Fair use



Use copyrighted works without permission if the use does not
unduly interfere with the copyright owner’s market for a
work
Include personal, noncommercial uses
4 prong test
1.
2.
3.
4.


Purpose and character of use (commercial vs. non-profit or
educational)
Nature of copyrighted work
Amount and substantiality of the portion used
Effect of the copying upon market
Example: using a VCR to time-shift a broadcast program
Reverse engineering


CompSci 001
OK when extracting unprotected elements
Connectix Virtual PlayStation
Georgia Institute
of Technology
8.18
Digital rights management

Idea: copying is hard to control, so make the
copying process itself difficult

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Digital watermarking


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Restrict the use of digital files in order to protect interest
of copyright holders
Control file access
Implemented in operating system, program software, or in
the actual hardware of a device
Make information so that unauthorized copying can be
detected
Serial Copy Management System (Audio Home
Recording Act 92)
Dystopian and utopian results?
Privacy issues?
CompSci 001
Georgia Institute
of Technology
8.19
Test Case
Artist Def Jeff creates a new song that
includes a four-bar percussion sample from
a Rolling Stones song. He uses the sample
without receiving permission from the
copyright holder, His DJ overlays the track
containing the sample with 40 tracks of
original music and puts the song on his
latest CD.



Were Def Jeff’s actions within the limits of
fair use?
Would he lose a lawsuit?
Georgiaethical?
Institute
Were his actions
CompSci 001
of Technology
8.20
Normalize Sounds

Make the whole sound as loud as possible

How loud can it be?
• The max positive value is 32767
• The max negative value is -32768

First we need to find the largest value (positive
or negative) in the current sound
• Create a variable to hold the max
– What should it be when we start looping?
• And loop through the list and if the absolute value of
the current value is greater
– Store that one instead
Georgia Institute of Technology
CompSci 001
8.21
Creating a Sound Clip

To clip the “This” out of “This is a test”.
 Determine where it starts and stops
 How?
CompSci 001
Georgia Institute
of Technology
8.22
Splicing Sounds Together



Originally meant cutting the sound tape into segments and
then assembling them in the right order
Easy to do digitally
Copy more then one sound into a target sound
 Track the source index and target index
CompSci 001
8.23
Recipe for halving the frequency of a
sound
def half(filename):
source = makeSound(filename)
target = makeSound(filename)
This is how a sampling
synthesizer works!
sourceIndex = 1
for targetIndex in range(1, getLength( target)+1):
setSampleValueAt( target, targetIndex,
getSampleValueAt( source, int(sourceIndex)))
sourceIndex = sourceIndex + 0.5
return target
What about doubling the frequency?
Georgia Institute of Technology
CompSci 001
8.24