Digital Media. Lecture 2: SemesterOverview. Georgia Gwinnett College School of Science and Technology Dr. Mark Iken

Transcription

1 Digital Media Lecture 2: SemesterOverview Georgia Gwinnett College School of Science and Technology Dr. Mark Iken

2 Refer to Supplemental text: Counting like an alien Networking issues Sound and Sampling

3 The Big Question: How do you take stuff found in the real world Store it as numbers on a computer So that it can be manipulated and shared?

4 The answer: It depends on what it is you are trying to capture We will have to know about the nature of the real world Some things can be counted Some things need to be measured The details of this will come a bit later

5 As previously seen using hexfiend: Text, audio, images and videos are all stored in a file as numbers on the computer Some of this is meant for human consumption (ASCII) Some of this is meant for program consumption (the header) Some is used by the program to save and represent the world

6 But first: Numbering systems! Which is correct? = = = F = 10

7 But first: Numbering systems! Which is correct? The answer is: It depends! = 10 (in decimal) 1 = 10 (in binary) 7 = 10 (in octal) F = 10 (in hexadecimal)

8 But first: Numbering systems! In this class we deal with Decimal Binary Hexadecimal We We will not be converting will not do math (a small lie) We will learn how to count

9 The process of counting is simple No matter which numbering system You count starting with the first digit You continue to count through all the digits available to you When you run out of digits, you go back to the first digit Add 1 to the column to the left

10 How many things can you count if you have 4 [ ] positions? your numbering system here In decimal: > 9999 In binary: > 1111 In hexadecimal: > FFFF You can count 10,000 things You can count 16 things You can count 65,536 things

11 How many things can you count if you have 4 [ The your numbering system here ] positions? formula is: the number of digits in the numbering system raised to the power of the number of positions you are using [#digits in the system] ** [# positions used together] In decimal: > ** 4 = 10,000 things In binary: > ** 4 = 16 things In hexadecimal: > FFFF 16 ** 4 = 65,536 things

12 How do you convert stuff in the real world into numbers that can be placed on a computer? It depends It depends on whether the thing is a discrete thing or a continuous thing

13 Stuff (phenomena) in the real world Can be discrete These either ARE or ARE NOT These can be counted The number of cars in a parking lot The number of beans in a jar Can be continuous These have no breaks These must be measured The height of a wave The atmospheric pressure

14 Stuff (phenomena) in the real world Discrete can be counted 5 cars 11,223 beans Continuous must be MEASURED The height of a wave 3.76 feet from crest to trough The atmospheric pressure inches of mercury

15 The problem is Most of the interesting stuff is continuous! Sound is continuous compression waves Light is continuous electromagnetic waves To store continuous phenomena on a computer you must measure it and store the measurement

16 Sampling: The process of converting continuous phenomena into discrete so that you can store it as a number on the computer

17 But before we talk about sampling What this stuff means: Bit: binary digit Byte: 8 Bits KB: kilo byte (1,000 bytes) MB: mega byte (1,000,000 bytes) GB: giga byte (1,000,000,000 bytes) TB: tera byte (1,000,000,000,000 bytes) KBPS: kilo (1,000) bits per second MBPS: mega (1,000,000) bits per second

18 What this stuff means: Strictly speaking In computing the meanings of K, M, G are powers of 2 K = 2 ** 10 = 1,024 not 1,000 M = 2 ** 20 = 1,048,576 not 1,000,000 G = 2 ** 30 = 1,073,710,825 not 1,000,000,000 But in this class, either will do

19 What this stuff means: And finally In some classes B and b when used in abbreviations mean Bytes and bits respectively This can be confusing For this class When abbreviating communication speeds the b (or B) means bits When abbreviating file size the b (or B) means bytes

20 What this stuff means: So Since kbps (or KBPS) is a communication speed the b (or B) means bits Since mb (or MB) is a file size the b (or B) means bytes

21 Network access Changing all the time Is getting faster and faster Is available in a variety of forms In this class we will discuss a few of them Let s get started: Networking issues

22 Network access Can be symmetric The speed into the network is the same as the speed out But now asymmetric is fairly common in the home The speed out of the network is faster than the speed into the network Unless you are running a server Servers usually have very high speed, symmetric connections to the network

23 Network access ADSL example Asymmetric Digital Subscriber Line Speed in can be 640 kbps Speed out can be 6.1 mbps Prehistoric example: dial up modem Asymmetric Speed in is 36,000 bps Speed out as high as 56,000 bps

24 Network access If you are running a commercial server (like you would have if you were running an online business) you may want faster service T1 and T3 are faster and symmetric T1 can be mbps T3 can be 44.7 mbps

25 And now Sampling How many samples do you need to faithfully capture a continuous phenomena? The answer: It depends! (of course!) What does it depend on? It depends on the frequency of the continuous phenomena you are trying to capture Capturing sound in the real world? Sampling!: Sound and Sampling

26 How many samples are needed? If you take too few samples If you take too many samples the sound quality will degrade but the file size will be small the sound quality will be excellent but the file size can get HUGE! So Where s the sweet spot?

27 How many samples are needed? Nyquist states that you need to sample at twice the frequency of the highest frequency you want to capture and faithfully reproduce With humans Since some of us can hear 20,000 cps You would need to sample at 40,000 cps CD quality? (with a little wiggle room) 44,100 samples per second

28 Sampling sound Sound radiates out from the source like the waves created when you toss a stone into a pond In the air it travels at ~760 mph

29 How many samples are needed? Now, an example to show how and why the Nyquist rate works Below is a note played on a violin and captured with an oscilloscope

30 A note played on a violin Sampled at 625 samples per second

31 A note played on a violin Sampled at 1250 samples per second

32 A note played on a violin Sampled at 2500 samples per second

33 A note played on a violin Sampled at 5000 samples per second

34 A note played on a violin Sampled at 10,000 samples per second

35 A note played on a violin Sampled at 20,000 samples per second

36 An example: Fields of Gold We played Fields of Gold in class CD quality is: 44,100 samples per second 16 bits (2 bytes) per sample with 16 bits you can capture 2**16 = 65,536 different levels Looking at the file: It is 4 minutes and 59 seconds The file size is 1,201,173 bytes long Does this make sense?

37 An example: Fields of Gold 4 minutes and 59 seconds = 4 x = 299 seconds 299 seconds x 44,100 sps = 13,156,000 samples 13,156,000 samples x 2 bytes per sample = 26,371,800 bytes But this is stereo (two channels) so 26,371,800 bytes x 2 channels = 52,743,600 bytes That s ~52 MB but we said that the music was 1.2 MB How is this possible? HMMMMMmmmm

38 An example: Fields of Gold Fields of Gold is an MP3 It s compressed! If we had the original CD it would be ~52 MB in length

39 Types of compressed files MP3 is lossy What you get back after compressing the file is NOT exactly the same as the original But it s close enough Images and sound can use lossy compression techniques (more later) Zip is lossless What you get back is EXACTLY what you started with Applications must be losslessly compressed All the 0s and 1s have to be exactly the same or the program will not run

40 Sampling The process used to convert continuous phenomena in the real world to discrete numbers stored on a computer But this process makes an alias of the original; it s an approximation of the original that does have faults These negative side effects are called artifacts

41 Sampling artifact examples: With audio, one artifact is quantization hiss With video, one artifact is retrograde motion; wheels look to turn backwards Moving images: Film With images one artifact is posterization Images as piles of numbers: Bitmapped images: Dithering and posterization

42 Another sampling artifact: Interference

43 A brief look at the Internet Let s get started: Networking issues

44 A brief look at the Internet: How to request stuff Uniform Resource Locator (URL) Has 3 parts Protocol FTP:// SMTP:// Domain name Ends in.edu.com.gov.org Directory structure and the requested page /newstuff/index.html Let s get started: Networking issues

45 A brief look at the Internet: How to request stuff Uniform Resource Locator (URL) Has 3 parts Protocol FTP:// SMTP:// Domain name Ends in.edu.com.gov.org Directory structure and the requested page /newstuff/index.html Let s get started: Networking issues

46 A brief look at the Internet: How to request stuff Uniform Resource Locator (URL) Has 3 parts Protocol FTP:// SMTP:// Domain name Ends in.edu.com.gov.org Directory structure and the requested page /newstuff/index.html Let s get started: Networking issues

47 A brief look at the Internet Let s get started: Networking issues

48 A brief look at the Internet Let s get started: Networking issues

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