Digital Media

Dr. Jim Rowan

Chapter 2

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

• 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

But it’s all just numbers, and binary numbers at that!

Note on paper

Picture

Song: fieldsOfGold.mp3

Video

Numbering systems:

DecimalBinary

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– Humans organize 0s and 1s into groups of 4– These groups of 4 are can be represented by a

single hexadecimal digit (2**4 = 16)– 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F

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

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 => 1 Hex• ASCII

– All end up as 0’s and 1’s

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

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)

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 of light and its color at each spot• encode 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

Continuous phenomenon to digital data:-Do sampling

Requires two processessampling - equally spacedquantization - 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

From the Real World and Back!

But... How many samples?

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– ok file size– faithful representation when replayed

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

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) (2**16 = 65,536 individual levels)

FieldsOfGold.mp3

• 4’59 = 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...

FieldsOfGold.mp3

• Why 26.3 megabytes not 1.2 megabytes?

• This is an MP3!

• Data COMPRESSION!

Further reading

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

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

• http://en.wikipedia.org/wiki/Mp3

Project 1 preliminary

Download AudacityPlay with it

Record your voiceAdd some effectsEdit out some stuff

Save it as a wav filePlay it back using Quicktime

The side effects of sampling:sampling artifacts

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 ArtifactsRetrograde Motion

4 samples/cycle, 2 cycles

2 samples/cycle, 2 cycles

Sampling Artifacts (cont.)

• Not enough quantization levels when sampling continuous data can produce undesired artifacts

• Images– too few colors: 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 Hardware Requirements

Production vs Consumption• 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

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

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

To Repeat…NOTE!

bps is bits per second while filesize is stated in bytes

Time-To-Load calculationsWork this out in class

For this 1.2 megabyte video:http://wiki.ggc.usg.edu/mediawiki/images/a/a4/Anand1new.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

Servers and Clients

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?