Digital Media

Dr. Jim Rowan

Chapter 2

Roll Call using Banner

Project 1 preliminary

• Details to be posted on the class projects page

• Podcast using Audacity• Import music• Do a voice-over• Post the result on your wiki site

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

• 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 representationa

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

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

Numbering systems

• Humans: decimal– Humans: 10 fingers, 10 digits:– 0, 1, 2, 3, 4, 5, 6, 7, 8 & 9

• Computers: binary– Computers: 2 fingers, 2 digits– 0 & 1

Binary Coding

• Data for a computer– zeros and ones, – off and on– false and true

• Data for humans– Coding schemes are used by humans to reduce

the volume of digits– Two coding schemes used

• Hexadecimal• ASCII

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– 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F

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

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 but put a 1 in the higher position

• 2 fingers: Counting in binary– 0, 1– start over but put a 1 in the higher position

• 16 fingers: Counting in hexadecimal – 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F– start over but put a 1 in the 1 higher position

From the Real Worldto

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)

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... not so much– music:

• measure the frequency & amplitude• encode as discrete

– pictures: • measure the amount of light and its color• encode as discrete

[Switch to Mac] Play/show some stuff

Text (using Text Edit)Audio (using Quicktime)Image (using Preview)Video (using Quicktime)

Open same stuff (using HexFiend) TextAudioImageVideo

(open and crop jayley and manOfScience)

Note on paper

Picture

Song: fieldsOfGold.mp3

Video

Question...

• Computers only store 0s and 1s– They only store digits...

• So... • How does all this continuous stuff end up in a

computer so that we can save it and play it back?

• Continuous data must be converted to discrete data

Converting Continuous (analog) data

to Discrete data• Requires two processes

– sampling - equally spaced– quantization - measuring at each sample

• Usually handled by – analog to digital converter– AKA A to D converter or ADC

Digital back to the real world:– Display samples using “sample and hold”– Play the sample for the duration of the

sample time

Converting Discrete data back to

Continuous (analog) data

But... How many samples?

single sample

single sample

two samples

two samples

three samples

three samples

four samples

four samples

five samples

five samples

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

Nyquist rate

• Why is the sample size used for audio CDs 44,000 samples per second?– Human hearing response is in the range of 0 to

22,000 cycles per second

• Why is the sample size used for audio CDs 44,000 samples per second?– Human hearing response is in the range of 0 to

22,000 cycles per second

FieldsOfGold.mp3

• 4 minutes and 59 seconds long• 1,201,173 bytes in length

Does this make sense?• 4 minutes and 59 seconds long

– 299 seconds

• 44,000 samples per second (sample rate)• 16 bit samples (quantity stored for each

sample)

FieldsOfGold.mp3

• 4’59 = 299 seconds long• 299 x 44,000 samples per second

= 13,156,000 bytes

• 13,156,000 x 2 bytes/sample– 26,312,000 bytes

• Should be 26.3 megabytes!• Why only 1.2 megabytes?• HMMMmmm...

FieldsOfGold.mp3

• Why 26.3 megabytes not 1.2 megabytes?

• This is an MP3!

• Data COMPRESSION!

Undersampling & VideoRetrograde Motion

Further reading

• http://en.wikipedia.org/wiki/Nyquist_rate

• http://en.wikipedia.org/wiki/Sampling_%28signal_processing%29

• http://en.wikipedia.org/wiki/Mp3

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

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

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”

Multimedia Hardware Requirements

• Multimedia consumption?– requires only a lower powered machine

• Multimedia production?– requires a more powerful computer– consider “fields of gold.mp3”

• 26+megabytes of data uncompressed• 1.2 megabytes of data compressed

– images are produced in layers• then flattened for consumption

Hardware requirements

• Video capture requires large areas of contiguous disk space

• Frequent disk defragmentation is required

• http://en.wikipedia.org/wiki/Defragmentation

defragmentation black is occupied spacewhite is available space

memory before

memory after

largest contiguousspace is 5

largest contiguousspace is 11 and thereare 6 of these

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

Hardware requirements Form factor...

Displayed unmodified

laptop display of my GGCwiki site

Treo

LGVX3400

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

RAIDredundant

94731990

94731990

94731990

94731990

94731990

94731990

94731990

94731990

94731990

disk #1

disk #2

disk #3

disk #4

disk #5

disk #6

disk #7

disk #8

RAIDoverlapped(fast)

7

3

1

9

9

4

0

9

94731990

disk #1

disk #2

disk #3

disk #4

disk #5

disk #6

disk #7

disk #8

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

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 (million bps) 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

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

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

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

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

fe80:0000:0000:0000:0211:24ff:fe8f:abb6

yahoo.com(server)

235.01.30.564

The Internet

you at home running a browser

(client)DHCP:

walmart.com(server)

100.43.153.07

ggc.usg.edu(server)

145.67.33.73

yahoo.com(server)

235.01.30.564

The Internet

you at homerunning a browser

(client)DHCP: 10.0.91.35

walmart.com(server)

100.43.153.07

ggc.usg.edu(server)

145.67.33.73

ISP

yahoo.com(server)

235.01.30.564

The Internet

www.yahoo.com=

235.01.30.564

you at homerunning a browser

(client)http://www.yahoo.com

walmart.com(server)

100.43.153.07

ggc.usg.edu(server)

145.67.33.73

yahoo.com(server)

235.01.30.564

The Internet

you at GGCrunning a browser

(client)DHCP:

walmart.com(server)

100.43.153.07

ggc.usg.edu(server)

145.67.33.73

yahoo.com(server)

235.01.30.564

The Internet

you at GGCrunning a browser

(client)DHCP: 322.21.5.36

walmart.com(server)

100.43.153.07

ggc.usg.edu(server)

145.67.33.73

ISP

yahoo.com(server)

235.01.30.564

The Internet

www.walmart.com=

100.43.153.07

you at starbucksrunning a browser

(client)HTTP://www.walmart.com

walmart.com(server)

100.43.153.07

ggc.usg.edu(server)

145.67.33.73

MIME types

• Multipurpose Internet Mail Extension• Allows the transmission of more than

just ASCII text (like you’d expect in an email)

• MIME types are specified in the header• Huge variety of MIME types are allowed

– audio, images, video– compressed files

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

Questions?