Digital Media

Lecture 2: SemesterOverview

Georgia Gwinnett CollegeSchool of Science and Technology

Dr. Jim Rowan

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?

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

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

http://wiki.ggc.edu/wiki/It%27sAllJustBitsITEC2110WikiTextJrowan1 Let’s get started: It’s all just bits!

But first: Numbering systems!

Which is correct?

5 + 5 = 10

1 + 1 = 10

1 + 7 = 10

1 + F = 10

But first: Numbering systems!

Which is correct? The answer is: It depends!

5 + 5 = 10 (in decimal)

1 + 1 = 10 (in binary)

1 + 7 = 10 (in octal)

1 + F = 10 (in hexadecimal)

But first: Numbering systems!

In this class we deal with– Decimal

– Binary– Hexadecimal

We will not be converting We will not do math (a small lie) We will learn how to count http://wiki.ggc.edu/wiki/HowToCountLikeAnAlien

Let’s get started: Counting like an alien!

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

How many things can you count if you have 4 [your numbering system here] positions? In decimal: 0000 -> 9999

– You can count 1000 things

In binary: 0000 -> 1111– You can count 16 things

In hexadecimal: 0000 -> FFFF– You can count 65,536 things

How many things can you count if you have 4 [your numbering system here] positions?

The 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: 0000 -> 999910 ** 4 = 1000 things

In binary: 0000 -> 11112 ** 4 = 16 things

In hexadecimal: 0000 -> FFFF16 ** 4 = 65,536 things

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

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

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• 30.02 inches of mercury

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

Sampling:

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

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

What this stuff means:Strictly speaking…

In computing the meanings of K, M, G are powers of 2

K = 2 ** 10 = 1,024 not 1,000M = 2 ** 20 = 1,048,576

not 1,000,000G = 2 ** 30 = 1,073,710,825

not 1,000,000,000

But in this class, either will do

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

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

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

http://wiki.ggc.edu/wiki/NetworkingIssuesLet’s get started: Networking issues

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

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

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 1.544 mbps– T3 can be 44.7 mbps

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 capturehttp://wiki.ggc.edu/wiki/SoundInTheRealWorldAndSamplingCapturing sound in the real world? Sampling!: Sound and Sampling

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

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

A note played on a violin

Sampled at 625 samples per second

A note played on a violin

Sampled at 1250 samples per second

A note played on a violin

Sampled at 2500 samples per second

A note played on a violin

Sampled at 5000 samples per second

A note played on a violin

Sampled at 10,000 samples per second

A note played on a violin

Sampled at 20,000 samples per second

How many samples are needed?

If you take too few samples– the sound quality will degrade– but the file size will be small

If you take too many samples– the sound quality will be excellent– but the file size can get HUGE!

So… – Where’s the sweet spot?

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

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 = 65,636 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?

An example: Fields of Gold 4 minutes and 59 seconds

= 4 x 60 + 59 = 299 seconds299 seconds x 44,100 sps

= 13,156,000 samples 13,156,000 samples x 2 bytes per sample

= 26,371,800 bytesBut 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…

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

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

Further reading

http://en.wikipedia.org/wiki/Nyquist_rate

http://en.wikipedia.org/wiki/Sampling_%28signal_processing%29

http://en.wikipedia.org/wiki/Mp3

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