Digital Media

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Digital Media

• Dr. Jim Rowan
• Chapter 2

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The Question

• How do you put stuff in a computer
• so that you can manipulate it
• so that you can send it
• so that someone else can see and use it?
• How do you represent the real world in a digital
  world?

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The answer

• Represent the real world as numbers
• Store the numbers
• Transmit the numbers
• Retrieve the numbers
• Display them in a form humans understand

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Today

• Chapter 2 is a first cut of nearly all the
  material that will be covered in greater detail
  this semester
• About the real world
• About digital representation

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File formats and extensions

• Indication to us (the humans) what kind of file
  this is
• Some software looks at the extension
• so... some software will try to open files with
  improper extensions
• results in file corrupted error message
• try it... change the extension from .doc to .jpg

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File formats and extensions

• Some software looks at the data in the file for
  more definitive answer
• important file-related information is encoded in
  the data of the file
• for example some image formats have color tables
  to reduce the size of the file
• some video just saves the changes from one frame
  to the next

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But its all just numbers, and binary numbers at
that!
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Note on paper
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Picture
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Song fieldsOfGold.mp3
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Video
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Numbering systemsDecimalBinary Hexadecimal
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Numbering systems

• Humans decimal
• Humans 10 fingers, 10 digits
• 0, 1, 2, 3, 4, 5, 6, 7, 8 9
• Computers binary
• Computers 1 finger, 2 digits
• 0 1

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Hexadecimal

• Humans and Computers hexadecimal
• Hexadecimal 16 fingers, 16 digits
• Humans organize 0s and 1s into groups of 4
• These groups of 4 are can be represented by a
  single hexadecimal digit (24 16)
• 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F

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How to count using a different number of fingers

• 10 fingers Counting in decimal
• 0, 1, 2, 3, 4, 5, 6, 7, 8, 9,
• start over with 0 and increment the digit to the
  left
• 1 finger Counting in binary
• 0, 1
• start over with 0 but increment the digit to the
  left
• 16 fingers Counting in hexadecimal
• 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F
• start over with 0 but increment the digit to the
  left

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Binary Coding

• Data for a computer... binary
• zeros and ones,
• off and on
• false and true
• Data for humans... ASCII, Hex... others
• Coding schemes are used by humans to reduce the
  volume of binary digits
• Two coding schemes used
• Hexadecimal 4 bits gt 1 Hex
• ASCII
• All end up as 0s and 1s

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ASCII

• Humans and Computers ASCII
• Made of two hexadecimal codes
• One ASCII character - two hex codes
• ASCII code for R (from text pg 317)
• hexadecimal 52
• binary 0101 0010

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From the Real WorldtoStuff on a computer

• A note
• Paper and pen -gt bits (0s and 1s)
• A picture
• Reflected light -gt bits (0s and 1s)
• A song
• Pressure waves in air -gt bits (0s and 1s)
• A video
• Pressure waves in air and Reflected light -gt
• bits (0s and 1s)

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First, the real worldDiscreteContinuous
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Phenomena in the Real world discrete vs
continuous

• Things in the real world can be discrete
• They either ARE or ARE NOT there
• These things can be counted
• Examples
• The number of cars in the parking lot
• The number of beans in a jar

21
Phenomena in the Real world discrete vs
continuous

• Things in the real world can be continuous
• Continuous cant be counted, it must be measured
• Examples
• Atmospheric pressure
• Height of an ocean wave
• Frequency of a sound wave

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But... computers can only count

• Discrete data is easy for a computer
• count it and store it as a number
• Continuous data... easy? not so much
• music
• measure the frequency amplitude
• encode as a collection of numbers
• pictures
• measure the amount of light and its color at each
  spot
• encode as a collection of numbers

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Question...

• If computers only store 0s and 1s...
• How does all this continuous stuff end up in a
  computer so that we can save it and play it back?
• Answer
• Continuous data must be converted to discrete data

24
From the Real World and Back!

• Continuous phenomenon to digital data
• -Do sampling
• Requires two processes
• sampling - equally spaced
• quantization - measuring at each sample
• Digital data back to continuous phenomenon
• Display samples using sample and hold
• Play the sample for the duration of the sample
  time

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But... How many samples?
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single sample
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single sample
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single sample(sample and hold)
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two samples
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two samples
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two samples (sample and hold)
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three samples
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three samples
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three samples (sample and hold)
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four samples
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four samples
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four samples (sample and hold)
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five samples
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five samples
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five samples(sample and hold)
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How frequently should I sample?

• too few
• small file size (good)
• not a faithful representation when replayed
• too many
• large file size (bad)
• excellent representation when replayed
• The Nyquist rate
• twice as many samples as the frequency
• ok file size
• faithful representation when replayed

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CD quality is44,000 samples per second

• Why?
• Human hearing response is in the range of 20 to
  22,000 cycles per second
• Nyquist sample rate
• highest frequency to be captured 22,000 CPS
• 2 x 22,000 44,000 samples per second

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Looking at FieldsOfGold.mp3

• 4 minutes and 59 seconds long
• 1,201,173 bytes in length
• Is this right?
• CD quality
• 44,000 samples per second (sample rate)
• 16 bit samples (quantity stored for each sample)
  (216 65,536 individual levels)

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FieldsOfGold.mp3

• 459 299 seconds long
• 299 x 44,000 samples per second
• 13,156,000 samples
• 13,156,000 x 2 bytes/sample
• 26,312,000 bytes
• Should be 26.3 megabytes!
• Why only 1.2 megabytes?
• HMMMmmm...

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FieldsOfGold.mp3

• Why 26.3 megabytes not 1.2 megabytes?
• This is an MP3!
• Data COMPRESSION!

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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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Project 1 preliminaryDownload AudacityPlay
with itRecord your voiceAdd some effectsEdit
out some stuffSave it as a wav filePlay it back
using Quicktime
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The side effects of samplingsampling artifacts
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Sampling Artifacts

• Under-sampling (too few samples) of continuous
  data can produce undesired artifacts
• audio distortion
• jagged edges on images
• Moire patterns on images
• retrograde motion on video

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4 samples/cycle, 2 cycles
Sampling Artifacts Retrograde Motion
2 samples/cycle, 2 cycles
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Sampling Artifacts (cont.)

• Not enough quantization levels when sampling
  continuous data can produce undesired artifacts
• Images
• too few color colors look artificial
• loss of fine distinction
• too few grey levels gradients become steps
• too few brightness levels posterization

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Sampling Artifacts (cont.)

• Not enough quantization levels when sampling
  continuous data can produce undesired artifacts
• Audio
• too few amplitude levels, quantization noise -
  hiss
• 8 bits (256 amplitude levels) produces
  discernable noise
• 16 bits (65536 amplitude levels) CD quality, no
  discernable hiss
• general sound fuzziness

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Multimedia Hardware Requirements
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Multimedia Hardware RequirementsProduction vs
Consumption

• Multimedia consumption?
• requires only a lower powered machine
• Multimedia production?
• requires a more powerful computer
• consider fields of gold.mp3
• 26megabytes of data uncompressed
• 1.2 megabytes of data compressed
• images are produced in layers
• then flattened for consumption

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Hardware requirements

• Video capture requires large areas of contiguous
  disk space
• Frequent disk defragmentation is required
• http//en.wikipedia.org/wiki/Defragmentation

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defragmentation
black is occupied space white is available space
memory before










largest contiguous space is 5
memory after










largest contiguous space is 11 and there are 6 of
these
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Hardware requirements Form factor...

• screen real estate makes a difference
• size is smaller?
• can/should affect the format of the display
• cannot simply display the same page on
• a desktop computer
• a cell phone
• a pda

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Hardware requirements Form factor...Displayed
unmodified
LG VX3400
Treo
laptop display of my GGCwiki site
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Hardware... RAID

• Redundant Array of Inexpensive Disks
• Designed as a hardware failsafe
• multiple copies of the same data
• Can be used to speed data transfer
• (you may need this in multimedia production)
• http//en.wikipedia.org/wiki/RAID

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RAIDredundant
94731990
disk 1
94731990
disk 2
94731990
disk 3
94731990
disk 4
94731990
94731990
disk 5
94731990
disk 6
94731990
disk 7
94731990
disk 8
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RAIDoverlapped(fast)
9
disk 1
4
disk 2
7
disk 3
3
disk 4
94731990
1
disk 5
9
disk 6
9
disk 7
0
disk 8
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Networks
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Networks

• Local Area Network (LAN)
• local routers, bridges, switches...
• Internet
• Uses TCP/IP protocol (the rules your
  communication must follow)
• http//en.wikipedia.org/wiki/TCP/IP
• you get access through an ISP

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Network access...

• dial up connection
• phone modem
• limited to 56,000 bps (bits, not bytes) max
  downstream (internet to modem)
• 33.6 kbps upstream (modem to internet)
• rarely get these speeds

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Network access...

• ADSL
• asymmetric digital subscriber line
• over copper phone wires
• limited to short distance from phone switch
• 6.1 mbps (million bps) downstream
• 640 kbps upstream

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Network access...

• Other options
• Cable modem (also asynchronous)
• satellite with phone (also asynchronous)
• satellite alone (expensive but available in the
  boonies)
• local wireless networks
• high altitude tethered balloons
• transmission over power lines

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Commercial internet users

• Provide web servers for others to put websites on
• Large commercial enterprises will have their own
  web server
• T1 connection 1.544 mbps
• T3 connection 44.7 mbps

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Time-To-Load calculations

• The Speeds
• Dial-Up
• 56,000 bps internet to modem (downstream)
• 33,600 bps modem to internet (upstream)
• ADSL
• 6.1 mbps (million bps) downstream
• 640 kbps (thousand bps) upstream
• T1
• 1.544 mbps
• T3
• 44.7 mbps

NOTE! bps is bits per second while filesize is
stated in bytes
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Time-To-Load calculations

• For this 1.2 megabyte video
• http//wiki.ggc.usg.edu/mediawiki/images/a/a4/Anan
  d1new.mov
• How long would it take to load it to youTube over
• -fastest dialup
• -adsl
• -T1
• -T3
• How long would it take to download it from
  youTube over
• -fastest dialup
• -adsl
• -T1
• -T3

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Servers and Clients
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Servers Clients...

• Clients consume internet content
• Your browser is a client
• Clients request content from servers
• by sending a server an HTTP//URL message which
  is a request for a web page
• Servers respond to requests for internet content
• send requested web pages to Clients
• The content is sent in HTML code
• HTML is interpreted by the client (browser) and
  displayed on your machine

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Servers Clients...

• URL is a human-readable name
• uniform resource locator
• takes the form www.amazon.com/newStuff/index.html
  
• The domain name www.amazon.com
• The file you want to see is newStuff.index.html
• the name maps to a number called an IP address
• http//en.wikipedia.org/wiki/IP_address

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Servers Clients...

• servers have fixed IPs so they are easy to find
• your computer probably uses DHCP which is a
  dynamic (changing) IP
• An example my IP right now (assigned through
  dhcp) is 10.0.106.91
• my IPv6 address (new addressing scheme) is
  fe80000000000000021124fffe8fabb6

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you at home running a browser (client) DHCP
yahoo.com (server) 235.01.30.564
The Internet
ggc.usg.edu (server) 145.67.33.73
walmart.com (server) 100.43.153.07
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you at home running a browser (client) DHCP
10.0.91.35
yahoo.com (server) 235.01.30.564
The Internet
ggc.usg.edu (server) 145.67.33.73
ISP
walmart.com (server) 100.43.153.07
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you at home running a browser (client) http//www
.yahoo.com
yahoo.com (server) 235.01.30.564
The Internet www.yahoo.com 235.01.30.564
ggc.usg.edu (server) 145.67.33.73
walmart.com (server) 100.43.153.07
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you at GGC running a browser (client) DHCP
yahoo.com (server) 235.01.30.564
The Internet
ggc.usg.edu (server) 145.67.33.73
walmart.com (server) 100.43.153.07
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you at GGC running a browser (client) DHCP
322.21.5.36
yahoo.com (server) 235.01.30.564
ISP
The Internet
ggc.usg.edu (server) 145.67.33.73
walmart.com (server) 100.43.153.07
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you at starbucks running a browser
(client) HTTP//www.walmart.com
yahoo.com (server) 235.01.30.564
The Internet www.walmart.com 100.43.153.07
ggc.usg.edu (server) 145.67.33.73
walmart.com (server) 100.43.153.07
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MIME types

• Multipurpose Internet Mail Extension
• Allows the transmission of more than just ASCII
  text (like youd expect in an email)
• MIME types are specified in the header
• Huge variety of MIME types are allowed
• audio, images, video
• compressed files

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A word about standards

• Standards allow cooperation
• But standards require agreement
• Works well during slow growth
• But in a rapidly changing environment...
• frequently obsolete before adopted
• One company may dominate the market becoming the
  de-facto standard

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Questions?
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