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:

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 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)

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) Text Audio Image Video (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

two samples

three samples

four 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 & Video Retrograde Motion

Further reading signal_processing%29

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 n

defragmentation black is occupied space white is available space memory before memory after largest contiguous space is 5 largest contiguous space is 11 and there are 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 LG VX3400

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)

RAID redundant disk #1 disk #2 disk #3 disk #4 disk #5 disk #6 disk #7 disk #8

RAID overlapped (fast) 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) – –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 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 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 The domain name: The file you want to see is: newStuff.index.html the name 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) IP An example: my IP right now (assigned through dhcp) is: my IPv6 address (new addressing scheme) is fe80:0000:0000:0000:0211:24ff:fe8f:abb6

yahoo.com (server) The Internet you at home running a browser (client) DHCP: walmart.com (server) ggc.usg.edu (server)

yahoo.com (server) The Internet you at home running a browser (client) DHCP: walmart.com (server) ggc.usg.edu (server) ISP

yahoo.com (server) The Internet = you at home running a browser (client) walmart.com (server) ggc.usg.edu (server)

yahoo.com (server) The Internet you at GGC running a browser (client) DHCP: walmart.com (server) ggc.usg.edu (server)

yahoo.com (server) The Internet you at GGC running a browser (client) DHCP: walmart.com (server) ggc.usg.edu (server) ISP

yahoo.com (server) The Internet = you at starbucks running a browser (client) walmart.com (server) ggc.usg.edu (server)

MIME types Multipurpose Internet Mail Extension Allows the transmission of more than just ASCII text (like you’d expect in an ) 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?