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									Digital Media

 Dr. Jim Rowan

   Chapter 2
        Today’s Question!
• Phenomenon in the real world can be
  described as having two different
  modalities. Discrete phenomenon are
  easy to convert to digital because you
  can simply count them. Continuous
  phenomenon are a bit more
  complicated because you can’t simply
  count them, instead you must do what?
       Before we start…
other software you will be using
      (all found in the application folder)
•   Grab - used to do a screen capture
•   TextEdit - used as a simple word
•   PhotoBooth - used to take your picture
    with the built in camera and take video
    with the built in camera
•   Quicktime Pro - to convert videos to a
    number of different formats and check
    the sample rate and sample size
           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?
              The answer:
•   Represent the real world as numbers
•   Store the numbers
•   Transmit the numbers
•   Retrieve the numbers
•   Display them in a form humans
• 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
  File formats and extensions
         (like .au, .doc, .ppt, .mov)
• 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
  File formats and extensions
• Some software looks at the data in the file for
  more definitive answer (the header)
      • 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
      • we’ve seen the header before when we used hexFiend
        to look at images… image size is stored in the header
But it’s all just numbers, and
  binary numbers at that!
Note on paper
Song: fieldsOfGold.mp3
Numbering systems:

       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
• Humans and Computers: hexadecimal
   – Hexadecimal: 16 fingers, 16 digits
   – 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F
   – Each of these can be represented as binary by
     using 4 binary digits for each hex digit
      • while this seems complicated it is actually easier for
        humans to deal with 16 different digits than 4 0s and 1s
• with 4 binary digits you can represent 16 different
• [number of digits in the numbering system]**[number
  of digits used]…
• 4 binary digits ===> 2**4 = 16
• 4 hexadecimal digits ===> 16**4 = 65,536
 How to count using a different
      number of fingers
(it’s the same process but different number sets)

 • 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
              Binary Coding
• Data for a computer... binary
  – zeros and ones,
  – off and on
  – false and true

• Data for humans... ASCII, Hex... (others)
  – Two of the coding schemes used are:
     • Hexadecimal represented by 1 Hex => 4 bits
     • ASCII represented by 2 Hex codes => 8 bits

• BUT…
  – they all end up as 0’s and 1’s
        Example: ASCII Code
• Humans and Computers: ASCII
  – ASCII code for C
    • decimal: 67
    • hexadecimal: 43
    • binary: 0100 0011
       From the Real World
       Stuff on a computer
• A note
  – Paper and pen -> bits (0s and 1s)
• A picture
  – Reflected light -> bits (0s and 1s)
• A song
  – Pressure waves in air -> bits (0s and 1s)
• A video
  – Pressure waves in air and Reflected light ->
  bits (0s and 1s)
First, the real world:

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
Phenomena in the Real world:
   discrete vs continuous
• Things in the real world can be continuous
• Continuous can’t be counted, it must be
• Examples:
  – Atmospheric pressure
  – Height of an ocean wave
  – Frequency of a sound wave
    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 (intensity) and frequency of light
       (color) in a number of regular places (pixels)
     • encode the frequency and the intensity as a collection of
• 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
           From the Real World
              …and Back!
Continuous phenomenon to digital data:
  Sampling consists of two processes
             1) stop to take a measurement
                     the number per time period is called the
    sample rate
             2) take the measurement
                     the number of different values each sample can
    take                        on is called the quantization level
Digital data back to continuous phenomenon:
  – Display samples using “sample and hold”
     • Play the sample for the duration of the sample time
How many samples do you

      It depends…
single sample
single sample
  single sample
(sample and hold)
two samples
two samples
  two samples
(sample and hold)
three samples
three samples
  three samples
(sample and hold)
four samples
four samples
  four samples
(sample and hold)
five samples
five samples
   five samples
(sample and hold)
     How frequently should I
• 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
  – Results in an ok file size
  – faithful representation when replayed
        CD quality is
 44,100 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 = 22,500 CPS
  2 x 22,500 = 44,100 samples per second
Looking at FieldsOfGold.mp3…
• 4 minutes and 59 seconds long
• 1,201,173 bytes in length
Can this be right?
• CD quality
  – 44,100 samples per second (sample rate)
  – 16 bit samples (quantization level is 16 bit)
  – 16 bits can store 65,536 different levels
     • (2**16 = 65,536 individual levels)
• 4’59 = 299 seconds long
• 299 x 44,100 samples per second
    = 13,156,000 samples
• 13,185,900 x 2 bytes/sample (2 bytes = 16 bits)
    – 26,371,800 bytes
•   Stereo: 2 channels => 52,743,600 bytes
•   Should be 52+ megabytes!
•   Why does it show only 1.2 megabytes?
•   HMMMmmm...

• Why 52+ megabytes not 1.2 megabytes?
  – wait for it…

• Why 52+ megabytes not 1.2 megabytes?

• This is an MP3!
• The data is COMPRESSED!
• If you had the song on a CD it would be 52+
  megabytes long and in .aiff format
  Two types of compression
• Lossy
• Lossless

         run length encoding   .mp3 audio
         table compression     .jpeg images
          Further reading
The side effects of sampling:
     sampling artifacts

    Sampling Artifacts
are the negative side effects
caused by having to sample
      continuous data
        Sampling Artifacts
• Under-sampling: not enough samples
  being taken of continuous data can
  produce undesired artifacts
• Examples might be:
  – Moire’ patterns on images
  – retrograde motion on video
   Sampling Artifacts (cont.)
• Not enough quantization levels when
  sampling continuous data can produce
  undesired artifacts
• Examples might be:
  – too few grey levels: gradients become steps
  – too few brightness levels: posterization
       Sampling Artifacts
       Retrograde Motion

4 samples/cycle, 2 cycles

2 samples/cycle, 2 cycles
   Sampling Artifacts (cont.)
• Audio
  – too few amplitude levels, quantization
     • 8 bits (256 amplitude levels) produces
       discernable noise
     • 16 bits (65,536 amplitude levels) CD
       quality, no discernable hiss
  – general sound “fuzziness” or a “flat”
    Data Representations
How stuff is stored in a computer
 • Images
    • Bitmapped
    • Vector
 • Audio
 • Animation
 • Video
 • Text
        Images, bitmapped
• Are stored as arrays of pixels
• Can be stored directly
  – TIFF for example
• Can have an associated color map
  – JPEG for example
• Generating these pixels from the stored
  model is called rendering
     Images, vector graphic
• Are stored as mathematical descriptions
• Often smaller than bitmapped
• Size is independent of resolution or
  image size
• Not suitable for some type of images
      Example & Comparison
• Bitmapped graphics
   – Defined as spots (pixels) of color
   – Problems scaling
   – File size unaffected by image complexity
   – File size affected by the image size
• Vector graphics
   – Defined by their parts
   – File size affected by image complexity
   – File size unaffected by the image size
      (scaling is easy)
           Moving images
• Captured live with camera
  – iMovie
  – Stored as video
• Generated from animation
  – Blender
  – Similar to 2D vector graphics… but in 3D and
    with a means of creating motion
Network communication
         But first…
   what this stuff means:
Bit: binary digit
Byte: 8 Bits
KB: kilo byte (1000 bytes)
MB: mega byte (1,000,000 bytes)
GB: giga byte (1,000,000,000 bytes)
KBPS: kilo (1,000) bits per second
MBPS: mega (1,000,000) bits per second
         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
         Network access...
 – asymmetric digital subscriber line
 – over copper phone wires
 – limited to short distance from phone switch
 – 6.1 mbps downstream
 – 640 kbps upstream
              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
     Network access...
  Commercial internet users
• T1 connection 1.544 mbps
• T3 connection 44.7 mbps
• Provide web servers for others to put
  websites on
• Large commercial enterprises will have
  their own web server
• 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                                     NOTE!
     • 1.544 mbps                          bps is bits per
  – T3                                     second while filesize
     • 44.7 mbps                           is stated in   bytes
    Time-To-Load calculations

For a 1.2 megabyte video:

How long would it take to load it to youTube over
  -fastest dialup
How long would it take to download it from youTube over
  -fastest dialup
               Note this!
• Communications are usually stated in
  bps (bits per second)
• Filesize is usually stated in bytes

• AND: 8 bits per byte
  – you will have to convert from one to the
    other when you do download/upload
Servers and Clients
              Servers & Clients...
• Clients consume and display 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 sent by the server is interpreted by the client (browser) and
     displayed on your display
• Look at and view source
       URL (uniform resource locator)…
          a human-readable name

• URL takes the form:
• URL has 3 parts
  – the protocol that you are using (http://)
  – The domain name: (
  – The directory and file you want to see:
  – the URL maps to a number called an IP
         Servers & Clients...
• servers have fixed IPs so they are easy to find
• your computer probably uses DHCP which is a
  dynamic (changing from connection to connection) IP

• An example: my IP right now (assigned through
  dhcp) is: (look it up in system preferences)
• IPv4 vs IPv6
      your browser                       webpages
your computer                

                    The Internet

           Domain Name System (DNS)
     your browser                       webpages
your computer               
             ISP                        (server)

                   The Internet

          Domain Name System (DNS)
      your browser                               webpages
your computer                        

                  The Internet      

          Domain Name System (DNS)

      your browser /index.html                   webpages
your computer                        

                  The Internet      

          Domain Name System (DNS)


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