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
processor
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
understand
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
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
.jpg
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
next
• 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
Picture
Song: fieldsOfGold.mp3
Video
Numbering systems:
Decimal
Binary
Hexadecimal
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
Hexadecimal
• 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
things
• [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
to
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:
Discrete
&
Continuous
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
measured
• Examples:
– Atmospheric pressure
– Height of an ocean wave
– Frequency of a sound wave
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 (intensity) and frequency of light
(color) in a number of regular places (pixels)
• encode the frequency and the intensity as a collection of
numbers
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
From the Real World
…and Back!
Continuous phenomenon to digital data:
-SAMPLE!
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
take
the number of different values each sample can
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
But...
How many samples do you
need?
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
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
– 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)
FieldsOfGold.mp3
• 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...
FieldsOfGold.mp3
• Why 52+ megabytes not 1.2 megabytes?
– wait for it…
FieldsOfGold.mp3
• 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
table compression
.mp3 audio
.jpeg images
Further reading
• http://en.wikipedia.org/wiki/Nyquist_rate
• http://en.wikipedia.org/wiki/Sampling_%
28signal_processing%29
• http://en.wikipedia.org/wiki/Mp3
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
noise
• 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”
sound
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...
• ADSL
– 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
Time-to-download/upload
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
Time-To-Load calculations
For a 1.2 megabyte video:
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
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
calculations
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 http://www.pondliner.com/ and view source
URL (uniform resource locator)…
a human-readable name
• URL takes the form:
http://www.amazon.com/newStuff/index.html
• URL has 3 parts
– the protocol that you are using (http://)
– The domain name: (www.amazon.com)
– The directory and file you want to see:
(newStuff.index.html)
– the URL maps to a number called an IP
address
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
requested
webpages
your browser
(Safari)(client)
DHCP:
yahoo.com
(server)
235.01.30.164
your computer
The Internet
Domain Name System (DNS)
walmart.com
(server)
100.43.153.07
your browser
(Safari)(client)
DHCP: 135.10.34.143
requested
webpages
yahoo.com
(server)
235.01.30.164
your computer
ISP
The Internet
Domain Name System (DNS)
walmart.com
(server)
100.43.153.07
your browser
(Safari)(client) http://www.yahoo.com
DHCP: 135.10.34.143
requested
webpages
yahoo.com
(server)
235.01.30.164
your computer
The Internet
Domain Name System (DNS)
http://www.yahoo.com =
235.01.30.164
walmart.com
(server)
100.43.153.07
your browser /index.html
(Safari)(client) http://www.yahoo.com
DHCP: 135.10.34.143
your computer
requested
webpages
yahoo.com
(server)
235.01.30.164
The Internet
Domain Name System (DNS)
http://www.yahoo.com =
235.01.30.164
walmart.com
(server)
100.43.153.07
Questions?