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## Digital Media

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**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**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 WorldtoStuff 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... 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**From the Real World and Back!**Continuous phenomenon to digital data: -Do sampling Requires two processes sampling - equally spaced quantization - 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**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 preliminaryDownload AudacityPlay with itRecord**your voiceAdd some effectsEdit out some stuffSave it as a wav filePlay it back using Quicktime