Digital Media

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Digital Media

• Lecture 2 SemesterOverview
• Georgia Gwinnett College
• School of Science and Technology
• Dr. Jim Rowan

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The Big Question

• How do you take stuff found in the real world
• Store it as numbers on a computer
• So that it can be manipulated and shared?

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The answer

• It depends on what it is you are trying to
  capture
• We will have to know about the nature of the real
  world
• Some things can be counted
• Some things need to be measured
• The details of this will come a bit later

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As previously seen using hexFiend

• Text, audio, images and videos are all stored in
  a file as numbers on the computer
• Some of this is meant for human consumption
  (ASCII)
• Some of this is meant for program consumption
  (the header)
• Some is used by the program to save and represent
  the world
• http//wiki.ggc.edu/wiki/It27sAllJustBitsITEC2110
  WikiTextJrowan1
• Lets get started Its all just bits!

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But first Numbering systems!

• Which is correct?
• 5 5 10
• 1 1 10
• 1 7 10
• 1 F 10

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But first Numbering systems!

• Which is correct?
• The answer is It depends!
• 5 5 10 (in decimal)
• 1 1 10 (in binary)
• 1 7 10 (in octal)
• 1 F 10 (in hexadecimal)

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But first Numbering systems!

• In this class we deal with
• Decimal
• Binary
• Hexadecimal
• We will not be converting
• We will not do math (a small lie)
• We will learn how to count
• http//wiki.ggc.edu/wiki/HowToCountLikeAnAlien
• Lets get started Counting like an alien!

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The process of counting is simple

• No matter which numbering system
• You count starting with the first digit
• You continue to count through all the digits
  available to you
• When you run out of digits, you go back to the
  first digit
• Add 1 to the column to the left

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How many things can you count if you have 4 your
numbering system here positions?

• In decimal 0000 -gt 9999
• You can count 1000 things
• In binary 0000 -gt 1111
• You can count 16 things
• In hexadecimal 0000 -gt FFFF
• You can count 65,536 things

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How many things can you count if you have 4 your
numbering system here positions?

• The formula is
• the number of digits in the numbering system
  raised to the power of the number of positions
  you are using
• digits in the system positions used
  together
• In decimal 0000 -gt 9999
• 10 4 1000 things
• In binary 0000 -gt 1111
• 2 4 16 things
• In hexadecimal 0000 -gt FFFF
• 16 4 65,536 things

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How do you convert stuff in the real world into
numbers that can be placed on a computer?

• It depends
• It depends on whether the thing is a discrete
  thing or a continuous thing

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Stuff (phenomena) in the real world

• Can be discrete
• These either ARE or ARE NOT
• These can be counted
• The number of cars in a parking lot
• The number of beans in a jar
• Can be continuous
• These have no breaks
• These must be measured
• The height of a wave
• The atmospheric pressure

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Stuff (phenomena) in the real world

• Discrete can be counted
• 5 cars
• 11,223 beans
• Continuous must be MEASURED
• The height of a wave
• 3.76 feet from crest to trough
• The atmospheric pressure
• 30.02 inches of mercury

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The problem is

• Most of the interesting stuff is continuous!
• Sound is continuous compression waves
• Light is continuous electromagnetic waves
• To store continuous phenomena on a computer you
  must measure it and store the measurement

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Sampling

• The process of converting continuous phenomena
  into discrete so that you can store it as a
  number on the computer

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But before we talk about samplingWhat this
stuff means

• Bit binary digit
• Byte 8 Bits
• KB kilo byte (1,000 bytes)
• MB mega byte (1,000,000 bytes)
• GB giga byte (1,000,000,000 bytes)
• TB tera byte (1,000,000,000,000 bytes)
• KBPS kilo (1,000) bits per second
• MBPS mega (1,000,000) bits per second

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What this stuff meansStrictly speaking

• In computing the meanings of K, M, G are powers
  of 2
• K 2 10 1,024 not 1,000
• M 2 20 1,048,576
• not 1,000,000
• G 2 30 1,073,710,825
• not 1,000,000,000
• But in this class, either will do

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What this stuff meansAnd finally

• In some classes B and b when used in
  abbreviations mean Bytes and bits respectively
• This can be confusing
• For this class
• When abbreviating communication speeds the b (or
  B) means bits
• When abbreviating file size the b (or B) means
  bytes

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What this stuff meansSo

• Since kbps (or KBPS) is a communication speed the
  b (or B) means bits
• Since mb (or MB) is a file size the b (or B)
  means bytes

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Network access

• Changing all the time
• Is getting faster and faster
• Is available in a variety of forms
• In this class we will discuss a few of them
• http//wiki.ggc.edu/wiki/NetworkingIssues
• Lets get started Networking issues

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Network access

• Can be symmetric
• The speed into the network is the same as the
  speed out
• But now asymmetric is fairly common in the home
• The speed out of the network is faster than the
  speed into the network
• Unless you are running a server
• Servers usually have very high speed, symmetric
  connections to the network

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Network access

• ADSL example
• Asymmetric Digital Subscriber Line
• Speed in can be 640 kbps
• Speed out can be 6.1 mbps
• Prehistoric example dial up modem
• Asymmetric
• Speed in is 36,000 bps
• Speed out as high as 56,000 bps

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Network access

• If you are running a commercial server (like you
  would have if you were running an online
  business) you may want faster service
• T1 and T3 are faster and symmetric
• T1 can be 1.544 mbps
• T3 can be 44.7 mbps

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And now Sampling

• How many samples do you need to faithfully
  capture a continuous phenomena?
• The answer
• It depends! (of course!)
• What does it depend on?
• It depends on the frequency of the continuous
  phenomena you are trying to capture
• http//wiki.ggc.edu/wiki/SoundInTheRealWorldAndSam
  pling
• Capturing sound in the real world? Sampling!
  Sound and Sampling

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Sampling sound

• Sound radiates out from the source like the waves
  created when you toss a stone into a pond
• In the air it travels at 760 mph

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How many samples are needed?

• Now, an example to show how and why the Nyquist
  rate works
• Below is a note played on a violin and captured
  with an oscilloscope

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A note played on a violin
Sampled at 625 samples per second
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A note played on a violin
Sampled at 1250 samples per second
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A note played on a violin
Sampled at 2500 samples per second
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A note played on a violin
Sampled at 5000 samples per second
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A note played on a violin
Sampled at 10,000 samples per second
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A note played on a violin
Sampled at 20,000 samples per second
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How many samples are needed?

• If you take too few samples
• the sound quality will degrade
• but the file size will be small
• If you take too many samples
• the sound quality will be excellent
• but the file size can get HUGE!
• So
• Wheres the sweet spot?

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How many samples are needed?

• Nyquist states that you need to sample at twice
  the frequency of the highest frequency you want
  to capture and faithfully reproduce
• With humans
• Since some of us can hear 20,000 cps
• You would need to sample at
• 40,000 cps
• CD quality? (with a little wiggle room)
• 44,100 samples per second

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An example Fields of Gold

• We played Fields of Gold in class
• CD quality is
• 44,100 samples per second
• 16 bits (2 bytes) per sample
• with 16 bits you can capture
• 216 65,636 different levels
• Looking at the file
• It is 4 minutes and 59 seconds
• The file size is 1,201,173 bytes long
• Does this make sense?

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An example Fields of Gold

• 4 minutes and 59 seconds
• 4 x 60 59 299 seconds
• 299 seconds x 44,100 sps
• 13,156,000 samples
• 13,156,000 samples x 2 bytes per sample
• 26,371,800 bytes
• But this is stereo (two channels) so
• 26,371,800 bytes x 2 channels
• 52,743,600 bytes
• Thats 52 MB but we said that the music was 1.2
  MB
• How is this possible?
• HMMMMMmmmm

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An example Fields of Gold

• Fields of Gold is an MP3
• Its compressed!
• If we had the original CD it would be 52 MB in
  length

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Types of compressed files

• MP3 is lossy
• What you get back after compressing the file is
  NOT exactly the same as the original
• But its close enough
• Images and sound can use lossy compression
  techniques (more later)
• Zip is lossless
• What you get back is EXACTLY what you started
  with
• Applications must be losslessly compressed
• All the 0s and 1s have to be exactly the same or
  the program will not run

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Further reading

• http//en.wikipedia.org/wiki/Nyquist_rate
• http//en.wikipedia.org/wiki/Sampling_28signal_pr
  ocessing29
• http//en.wikipedia.org/wiki/Mp3

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