Transcript Digital Media - University System of Georgia

Digital Media
Dr. Jim Rowan
ITEC 2110-01
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: 0 - 16
binary: 0000 - 1111
hexadecimal: 0 - F
ascii: R
hexadecimal: 52
binary: 0101 0010
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
The real world (continuous)
-> to the digital world (discrete)
-> and back to the real world (continuous)
[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”
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
• http://en.wikipedia.org/wiki/Nyquist_rate
• http://en.wikipedia.org/wiki/Sampling_%
28signal_processing%29
• http://en.wikipedia.org/wiki/Mp3
Questions?