Digital Media Dr. Jim Rowan ITEC Monday, August 27

Roll Call using Banner

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

Binary Coding Binary is all zeros and ones Data is stored on a computer in zeros and ones, off and on, false and true But it is looked at by humans using coding schemes to reduce the volume One way to look at binary is using the coding scheme called Hexa decimal

Hexadecimal Humans: decimal –Humans: 10 fingers, 10 digits: –0, 1, 2, 3, 4, 5, 6, 7, 8 & 9 Computers: hexadecimal –Computers: 2 fingers, 2 digits –0 & 1 –Humans organize these 0s and 1s in groups of 4 –These groups of 4 are called hexadecimal –16 digits –0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F

Counting with a different # 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

Binary Coding Groups of 4 –can be looked at as hexadecimal code Groups of 8 –can be looked at as 2 hexadecimal numbers –can be looked at as ASCII

[Write on board ] decimal: binary: hexadecimal: 0 - F ascii: R hexadecimal: 52 binary:

The real world to Stuff on a computer A note –Paper and pen -> bits (0’s and 1’s) A picture –Reflected light -> bits (0’s and 1’s) A song –Pressure waves in air -> bits (0’s and 1’s) A video –Pressure waves in air and Reflected light -> bits (0’s and 1’s)

Stuff in the Real world: discrete Things in the real world can be discrete –They either ARE or ARE NOT there –These things can be counted –The number of cars in the parking lot –The number of beans in a jar

Stuff in the Real world: continuous Things in the real world can be continuous They can’t be counted, they must be measured –Atmospheric pressure –Height of an ocean wave –Frequency of a sound wave

But... computers can only count Discrete data is easy for a computer 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

Continuous to Discrete Requires two processes –sampling- equally spaced –quantization Usually handled by –analog to digital converter –AKA A to D converter or ADC

[draw sine wave on board] Real world to digital: –show sampling –show quantity Digital to the real world: –draw sampled data –show “sample and hold” The real world (continuous) -> to the digital world (discrete) -> and back to the real world (continuous)

Undersampling [draw sine wave on the board] sample it once & recreate it using sample sample it twice & recreate it using the 2 samples sample it 3 times & recreate it using the 3 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

Questions?