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 we’ve seen this 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… 2**4=16

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 (from text pg 329) decimal: 67 hexadecimal: 43 binary:

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 and frequency of light (its color) in a number of regular places (pixels) encode the frequency and the amount 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? -SAMPLE! 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 From the Real World …and Back!

But... How many samples do you need? It depends…

single sample

single sample (sample and hold)

two samples

two samples (sample and hold)

three samples

three samples (sample and hold)

four samples

four samples (sample and hold)

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 is 44,000 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 Can this be right? CD quality –44,000 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 44,000 samples per second = 13,156,000 samples 13,156,000 x 2 bytes/sample –26,312,000 bytes Stereo: 2 channels => 52,624,000 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! Data COMPRESSION!

Further reading signal_processing%29

Project 1 preliminary Download Audacity Play with it Record your voice Add some effects Edit out some stuff Save it as a wav file Play 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 Artifacts Retrograde 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 –moire’ patterns

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” –Not enough quantization levels when sampling continuous data can produce undesired artifacts Image artifacts

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

Examples Bitmapped Vector

Moving images Captured live with camera –iMovie –Stored as video Generated from animation –Blender –Similar to vector graphics… but with a means of creating motion

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

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 –T mbps –T 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 calculations Work this out in class For this 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

Servers and Clients

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)

Questions?