42 CHAPTER 3 IMAGE FORMATION AND DIGITAL VIDEO

Vil.ti Bible sell'! ning 1:vi,1ems Some cameras that are used primarily for digital

cinema have a variable scanning system. Their normal.frame rate is 24 frames per second (fps), which is identical to film, but it can be lowered to 15 fps or boosted to

60 fps or even bighet This variable refresh rate allows for smooth accelerated or

slow-motion effects.

f.cum1t col\vl!l'llon To make digital scanning systems even more flexible, you c~

use a frame-rate converter to change the scanning system , say, from 720p at 24 fp

to 1080i at 30 fps. This conversion takes place in the postproductlon phase.

flat -panet Display, The need for larger monitors has spurred the development oftlat-panel d~splays. One of the great advantages offlat-panel displays ls that the screen can get bigger-from th familiar small video display on a laptop computer to a large home theater-type s:een-without getting thicker. Other advantages of flat-panel displays over the standard television cirr are Improved color rendition (colors that are more subtle),

greater contrast ratio and more shades in between, and g~ne~y ~ higher resolu­tion (more pixels per square inch) . One of the major quahty cntena for flat-panel displays is the black level, that Is, how black the black looks . 'Ihe darker the blacks

are, the more vivid the rest of the colors. . You may find, however , that sometimes video image~ on flat-p~nel displays

take on a pasty, almost posterized look , which despite the high resolu~on smacks of low-quality VHS pictures. This usually happens when standard-defim~on or highly compressed video ls shown on the high -deftn1tion flat-pan el television set. But if you don 't get too close to the screen or view it from too sharp an angle , flat-panel

displays look simply stunning. . The two mos t popular albeit incompatible flat-panel systems are the ltquzd

crystal display (LCD) panels and the plasma panels.

I.Cl> pane'I& Toe LCD panel comprises two transparent sheets that contain, s~d­wi hlik a liquid whose crystal molecules change when a video signal is applied.

Th~ m;:y tiny transistors that compose the basic dots of the picture orien t the liquid

crystals in certain ways to allow back light to shine through.

Pbsma pan h Instead of sandwiching a liquid, the two glass panels of the plasma system contain a thin layer of gas. When the gas receives the varying charges of the

video signal, it activates the myriad tiny red, green, and blue dots that are ananged

much like the ones on a standard television receiver.

D ·te the technical differences between the two flat-panel systems, you would

esp> th . I tureS probably be hard-pressed to tell the difference when looking

1at err P c · .

Before we continue praising the virtues of digital video, let s fµ\d out what digital

actually means.

DIGITAi. PROCESS

WHAT IS DIGITAL? All digital video and the way computers process information are based on a binary code that uses on/off , either /o r values for all their operations. The on state is repre­sented by a 1, and the off state is represented by a 0. These binary dtgtts, or bits for short , operate on the light-bulb principle: i.fyou have a 1, the light bulb is on; if you have a 0, the light bulb is off. In the digital world there is noth.ing between the 1 and the O; the light bulb cannot burn at half-intensity.

DIGITAL PROCESS, Digital usually refers to the binary system in which data are represented in the form of on/ off pulses. At first glance this either I or system of binary digits may seem clumsy, but the overwhelming advantage of the digital process is that it has great resistance to data distortion and error . It also permits any number of combinations and shuffling around-an extremely important feature when manipulating pictures and sound.

Analog and DJ9lral Signals An analog signal is an electrical copy of the original stimulus, such as somebody's singing into a microphone. The technical definition is that the analog signal fluctu­ates exactly like the original stimulus. The analog signal is also continuous, which means that it never intentionally skips any part of the signal , however small the skip maybe.

The digUal signal, on the other hand, is purposely discontinuous . It takes the analog signal and selects points (instances) at equal intervals. These successive points represent the original signal-a process called sampltng ( explained below).

Dlgjtal SyJte,m

In the digitizing process, the analog signal is continuously samp led at fixed intervals; the samples are then quantized ( assigned a concrete value) and coded into O's and l's.

Samptlng In this process a number of samples (voltages) are taken of the analog video or audio signal at equally spaced intervals. When you take and measure a rela­tively large number of instances at shorter intervals from the original analog signal, you have a high sampling rate. When you take a reading of relatively few instances at larger .intervals , you have a lower sampling rate. A higher sampling rate produces better signals. The sampling rate of a video signal ls usually expressed in megahertz.

Qu.antlzin.g The quantizing step in the digitizing process changes the sampling points (instances) into discrete numerical values (O's and l 's) by giving each point a specific number.

C'o.nrpntsslon 1hls process of compreuion includes the temporary rearrange­ment or elimination of all data that are not absolutely necessary for preserving the

M .ISl&c , All digital S)

based on tf princip le of t

An electrtc ch, present or It

on state is rei: al , the offs

44

CH

AP

TE

R3

IMA

GE

FO

RM

AT

ION

AN

DD

IGIT

AL

VID

EO

DIG

ITA

LP

RO

CE

SS

RE

STA

TED

maneuvers

t~chang~

ananalog

signaltoa

digitalone.N

otethatthis

metaphor

isnot

necessaril~t~chmcallYaccurate

bu

tism

erelyintended

tosim

plifyan

dillustrate

thecom

plexdigttalprocess.

Dig

italSignalan

dQ

uantizingS

omebody

verysm~

inthe

packingdivision

comes

upw

itha

radicalidea:

why

notcu~~p~eh~ses

mto

smaller

piecesan

dnum

bereach

piecebefore

shipping(quantizm

g).T

hisway

thehoses

canbe

more

easilypacked

andshipped

insm

allercrates

(packets)and

insm

allertrucks(bandw

idth).Because

theshipm

entcont.

aplan

ofthehoses'originalcurves

(computer

software),the

numbered

pieces=b

ereassem

bledto

looklike

theoriginalhoses

(analogsignals).

3.5

ANALOGSINE

WAVE

This

figu

reshow

sa

visualrepresentation

ofa

simp

leanalog

electricalwave.

..............;_

~~nme

..

f·································~·············

Analog

Signal

Let's

lookata

graphicrepresentation

ofasim

pleanalog

electricalwave

Such

wh'h

all'

.w

aves,IC

arenorm

yvanously

shaped,make

upaudio

andvideo

signals.SEE

3.5

Now

ass~e

thatsuchw

avesare

quitelong,have

slightlydifferentshapes

(dif­ferentfre.quencles

andam

plitudes),andare

made

fromgarden

hoses.These

hoses(~alog

SIgnalS)mustnow

beshipped

bytruck

todifferentlocations

(videoand

audiosIgnal

transp~rt

andrecording).

The

requirementis

thattheoriginalshape

ofthe~enthos~s

(SIgnalS)cannotbe

disturbedeven

toa

slightdegree(no

signaldistor­tion~

durmg

shipping.Buteven

them

ostexpensive

shippingcom

pany(high-end

eqmpm

ent)cannotpreventthe

longhoses

fromgetting

some

kinks(.

al.

)d

u.

ki'

SIgnnO

Isen

ng

pac

.ngm

tothe

longan

dcum

bersome

crates(analogrecording)and

duringtransport

(Sl~al~ansport).W

henthe

hoseswith

thekinks

areth

enused

asm

odelsforw

aveduplication

(dubbing),thevarious

distortionsfrom

theoriainal

notonl.

.d

b0

--

curvesare

ym

amtam

eu

toftenexacerbated

byadditionalkinks

(ad

ded

'al

.and

artifacts).SIgn

nOIse

DIG

ITA

LP

RO

CE

SS

RE

STA

TED

Dow

nloadingand

Stream

ingW

hendow

nloading,the

filesare

sentas

datapackets.

Because

thesepackets

areoften

transferredoutoforder,you

mustw

aitforthe

downloading

processto

finish

beforeyou

canw

atcha

videoorlisten

toan

audiofile.

When

streaming

data,thedigitalfiles

aresentas

acontinuous

dataflow

thatcanb

eview

edand

listenedto

while

thedelivery

isunderw

ay.You

canlisten

tothe

first

partofasong

while

therestofitis

stillbeingdelivered.

You

areprobably

tiredby

nowof

allthe

technicaltalk,

solet's

restatethe

digitalprocess in

am

oreuser-friendly

way,using

am

etaphorthathelpsexplain

theim

por­tan

tdifferencebetw

eenthe

two

signaltypesand

why

we

gothrough

suchtechnical

originalqualityofthe

videoan

daudio

signalsfor

storagean

dsignaltransport.T

heprocess

ofrearrangingratherthan

throwing

away

iscalled

losslesscom

pression;the

compression

thatactuallydeletes

redundantdatais

thelossy

kind.T

headvantage

oflosslesscom

pressionis

thatitmaintains

theoriginalm

akeupofthe

digitalvideoand

audiosignals.T

hedisadvantage

isthatthe

systemstillhas

tom

anageoverly

largefiles.S

uchfiles

areusually

toobulky

forstreaming

yourfavoritem

usicforuninterrupted

listeningorforw

atchinga

movie

traileronyourcom

puter

withouthaving

tow

aitfora

prolongeddow

nload.T

headvantage

oflossycom

pressionis

thatitcanm

akea

largefile

considerablysm

aller,which

willthen

takeu

plessspace

when

storedon

acom

puterdisk.The

filescan

alsobe

transportedfaster,w

hichallow

sstream

ing.With

thehelp

ofcompression,

youcan

listento

yourfavoritesong

frombeginning

toen

dw

ithoutoccasionalinter­ruptions

tow

aitforthe

restofthedata

tocatch

up,an

dyou

cansqueeze

anentire

motion

pictureonto

asingle

digitaldisc.The

disadvantageisthathighercom

pression

inevitablyreduces

pictureand

soundquality.

Recallthat H

DV

hashigher

compression

thanH

DT\T,w

hichshow

sup

mainly

asreduced

coloran

dcontrastfidelity.T

hereare

lossycom

pressionsystem

s,such

asM

PEG

-2,how

ever,that

produceam

azinglyhigh-fidelity

pictures.They

arede­

signedto

ignorecertain

picturedetail

thatdoes

notchangefrom

frame

tofram

e.F

orexam

ple,MPE

G-2

mightnotbotherw

ithrepeating

thegreen

ofthegrass

fromfram

eto

frame

when

showing

aclose-up

ofa

golfballrolling

toward

thecup

bu

tw

illsimply

addthe

digitaldatathatshow

therolling

golfball.Whenever

necessary,the

compression

systemborrow

sthe

green-grassinform

ationfrom

thefram

ethat

initiallyshow

edthe

grassthe

firsttime.

To

com

plicate

matters,

there

arem

any

differentco

decs-co

mp

ression

-decom

pressionsy

stems-u

sedfor

avariety

ofcompression

purposes.Apple

Quick­

Tim

e,for

example,

hasseveral

codecs,such

asa

high-quality,less

lossyone

forscreening

yourmasterpiece

anda

fairlylossy

onefor

sendingitoverthe

Internetto

sharew

ithfriends.

Com

pressionrearranges

dig

italdata

(pictu

reand

soundin

form

atio

n)

or

elim

ina

tes

red

un

da

nt

datato

increasestorage

capacityand

speedu

psignaltra

nsp

ort.

Compression rearranges digita l data (picture and sound information) or eliminates redundant

data to increase storage capac ity and speed up signal transport.

CHAPTER 3 IMAGE FORMATION AND DIGITAi. VI DEO

original quality of the video and audto sjgnals for storage and signal transport. The process of rearranging rather than throwing away ls called lossless compreS!ion; the compression that actually deletes redundant data Js the lossy kind .

The advantage oflossless compression is that It maintains the original makeup of the digital video and audio signals . Toe dJsadvaorage is that the system still has to manage overly large files. Such files are usually too bulky for streaming your favorite music for uninterrupted Ustenlng or for watching a movie trailer on your computer without having to wait for a prolonged download .

The advantage of lossy compre .ssion is that it can make a large file consJderably smaller, which will then take up less space when stored on a computer dJsk. The files can also be transported faster, which allows streaming. Wlth the help of compression, you can listen to your favorite song from beginning to end without occas ional inter­ruptions to wait for the rest of the data to cat ch up , and you can squeeze an entire motion picture onto a single digital dtsc. Toe disadvantage ls that hJgher compressJon inevitably reduces picture and SOW1d quality.

Recall that HOV has higher compression than HDTY, which shows up mainly as reduced color and contrast fidelity . There are lossy compression systems , such as MPEG-2, however, that produce amazingly high-fidelity pictures . They are de­signed to ignore certain picture detail that doe.s not change from frame to frame . For example, MPEG-2 might not bother with repeating the green of the grass from frame to frame when showing a close-up of a golf bail rolling toward the cup but will simply add the digital data that show the rolling golf ball . Whenever necessary , the compression system borrows the green-grass information from the frame that initially showed the grass the first time.

To complicate matters, there are many dtfferent codecs-compression ­derompressJon systems-used for a variety of compression purposes . Apple Quick­Time, for example, has several codecs, such as a hlgh -quality, less lossy one for screening your masterpiece and a fairly lossy one for sending it over the Internet to share with friends .

O.ownloadlng and Stre-amlng When downloadtn8, the files are sent as data packets. Because these packets are often transferred out of order , you must wait for the downloadlng process to finish before you ca.o watch a video or listen to an audio file.

When streaming data , the dJgital files are sent as a continuous data flow that can be viewed and listened to while the delivery ls under way. You can listen to the first part of a song while the rest of it is still belng delivered .

D1GITA1L PROCESS RESTATED You are probably tired by now of all the technical talk. so let's restate the digital process in a more user -friendly way, using a metaphor that helps explain the impor ­tant difference between the two signal types and why we go through such technical

DI GI TAL PROCESS RESTATED

.......... -r········• ..................... ~ ........ ..

: : . . : , . .. . . ..... .... ........ . . HHOOO : •UO O oo O

•••••• ••••; •, ,,,,., .. ,,,,.,,,, , , ,_ ,,, 1

3,5 ANALOG SINE WAVE This figure shows a vlsual representation of a simple analog electrica l wave.

maneuvers to change an analog signal to a digital one . Note that this metaphor is not necessarily t~chnically accurate but ls merely intended to simplify and illustrate the complex digital process .

Analog Signal

Let's look at a graphic representation of a simple analog electrical wave. Such waves whJch are normally variously shaped , make up audio and video sJgnals . iu u '

Now ass~e that such waves are quite long, have slightly different shapes (clif­ferent frequencies and amplitudes), and are made from garden hoses . These hoses (analog signals) must now be shipped by truck to different locations (video and audJo sJgnal transp~rt and recording) . The requirement is that the original shape of the ::~t hoses (signals) cannot be dJsturbed even to a slight degree (no signal distor-

) during shipping. But even the most expensive shipplng company (high -end equipment) c~not prevent the long hoses from getting some kinks (signal noise) during packing into the long and cumbersome cra tes ( analog recording) and during transport (signal transport) . When the hoses with the kinks are then used as models for wave duplication (dubbing) , the various dJstortions from the orlgi.nal curves are not only maintained but often exacerbated by additional kinks (added signal · and artifacts). nolse

Digital Slgnol and Quantizing

Somebody very smart in the packing dtvision comes up with a radical idea· why not cu~ ~p the hoses into smaller pieces and number each piece befo re shi~plng ( quanUzU'lg)? Th.is way the hoses can be more easily packed and shipped ln small crates (packets) and in smaller trucks (bandwidth). Because the shipment contai:; a plan of the hoses ' original curves {computer software), the numbered pieces can be reassemb led to look like the original hoses (analog signals).

Returning

fromtransporting

gardenhoses

tothe

digitalprocess,youm

ayw

onderw

hyw

ebother

with

sucha

complicated

systemw

henwe

alreadyhave

ananalog

signalthatperfectlyrepresents

theoriginalstim

ulus.Your

computer

and

howyou

useitcan

giveyou

alltheansw

ers.T

hisseem

inglycrude

either/or,on/offprinciple

letsyou

dubvideo

andaudio

with

virtuallyno

deterioration,compress

thesignals

tosave

storagespace

andfacili­

tatesignaltransport, an

dm

anipulatepictures

andsound

with

relativeease.

WH

YD

IGIT

AL

?

Picture

and

Sound

Qu

alityin

Dubs

Because

eachsam

plepointis

numbered

throughquantizing,

therobusteither/or

digitalsystem

identifiesonly

two

typesofdata:

thegood

typethat

contributesto

thecreation

ofpicturesand

soundan

dthe

badtype

thatdoesnot.

Inthe

either/orsystem

,there

isno

roomfor

"perhapssom

eO

fthemm

ightbe

goodfor

something

sometim

e:'This

means

thatthedigitalsystem

isrelatively

imm

uneto

artifactsand

caneven

eliminate,

orat

leastm

inimize,

allunw

antedsignal

elements

thatwill

interferew

ith,ratherthanpreserve,the

integrityofthe

originalsignal.For

example, w

henyou

printseveralcopiesofa

letterthatyoujusttyped

with

yourw

ord-processingprogram

,the

thirdor

fourthcopy,

and

eventhe

hundredthone

theprinterdelivers,looks

justasclean

andsharp

asthe

original.W

henyou

usedigital

videorecorders,

allsubsequent

generationsproduce

picturesand

soundthat

areidenticalto

theoriginalrecording,

anattribute

calledtransparency.T

hefirstcopy

oftheoriginalis

calledthe

firstgeneration,thesecond

copy,thesecond

generation, and

soforth.

This

isnotthe

casew

ithan

analogsignal.In

itstrue

natureofbeing

seamless,

allvaluesare

equal:the

analogsignal

doesnot

distinguishbetw

eenthe

desirablesignalelem

entsand

theartifacts

(signalnoise).This

problemis

compounded

insub­

sequentanalogdubs.W

ithanalog

equipmenteach

generationadds

anotherlayerofartifacts, m

uchlike

howprinted

textdeterioratesw

henyou

progressivelyduplicate

photocopies.Analog

videoequipm

entwillyield

marked

picturedeterioration

afterjusta

fewgenerations.

WH

YD

IGitA

L?

The

secondm

ethodis

toload

severaltrucks

with

boxesthat

containenough

piecesto

getstarted

with

theassem

blyof

some

ofthe

hoses.T

hefirst

hosesare

alreadyfunctional

(thebeginning

ofthefile

canb

eopened)w

hilethe

restofthemarrive

oneafter

theother

(streaming).

Com

pressionY

oucarm

otcompress

analogsignals.T

hism

eansthatyou

mustdealw

iththe

com­

pleteanalog

videoand

audioinform

ationduring

capture,transport,

andstorage.

As

youhave

justread,digitalcom

pressionnotonly

savesstorage

spaceb

utspeeds

upsignaltransport.

I..

Alowdigitalsam

plingrateresultsin

thelossofsignalinform

ation.

IMA

GE

FO

RM

AT

ION

AN

DD

IGIT

AL

VID

EO

II

!I

,,

I

Time

Fewersamplesperse<ond

B

Com

pressionan

dTransport

The

sampling

gavethe

personin

thepacking

divisionanother

idea:b~causeeach

pieceofhose

hasa

specificnum

ber,youm

aybe

ableto

repackthe

pIecesso

th~tall

the

straightpieces

canb

esqueezed

inone

small

boxand

thecurved

onesIn

another(losslesscom

pression).This

would

certainlysave

space(sm

aller~es)and

allowsm

allertruckstobe

usedfortransport(sm

allerbandwidth).O

r,he

mIghteven

getaway

with

throwing

away

some

ofthepieces

beforepacking

them,

suchas~e

partsthatm

akeup

longstretches

beforethe

firstcurve,which

s~rveno

re.al~cnon

when

simulating

waves

(lossycom

pression).Nobody

would

noncethe

mISSIng

parts

anyway

The

smartpackerw

asrighto

nb

oth

counts.W

hiletesting

howm

uch

he

couldthrow

away,how

ever,he

alsodiscovered

thatthe

more

hosepieces

(data)youthrow

away,the

lessaccurate

thereassem

blyofthe

waves

willbe

(lowpicture

andsound

quality)..

.T

hetransportation

manager

decidedth

atth

epieces

canbe

shipped.In~o

ways.O

new

ayis

toload

allthe

straightpiecesin

onetruck,allthe

curvedp~eces

In

dk

d-;......·"e

ofbothin

athird

With

thism

ethodthe

receIverhas

asec on

truc,

ana

llllAL

\ll•

tow

aituntilalltruckshave

arrivedan

dallboxes

areopened

(downloading)

before

theassem

blyofthe

variouscurved

hosescan

begin(opening

thefile).

Sam

plingA

nequally

smart

colleaguein

theassem

blydivision

~scoversthat,w

henyou

cuta

hoseinto

verysm

all,equally

spacedpieces

(sampling),

youdon

tn

eedall

thepieces

torepresentthe

hose'soriginalcurves

(~gh

sampling

rate).If,however,you

cutthehose

intojusta

fewlarge

piecesto

savetim

eand

money,they

canno

lo.n~eraccurately

simulate

theoriginalcurves

(lowsam

plingrate).A

sa

result,thedigItal

representationofthe

analogsignalw

ouldb

eofm

uchlow

erquality.SEE

3.6

CH

AP

TE

R3

Ahighdigitalsam

plingrateapproxim

atestheoriginalsignal.

Manysam

plespersecond

Time

IlllllllllllllllllllllltUltlllU

lllllllltItIIlUltIlllllllllJ

lll'''lltItlll!\IIII1l1

11

IIUl,.

A

3.6

SAMPLING

.I

Toco

nve

rtananalog

wave

into

ad

igita

lone,it

isd

ivide

dand

measured

at

eq

ua

llyspaced

mterva

s.

AA

hig

hsa

mp

ling

ratelooks

mo

relike

the

orig

ina

lwave;

mo

stofth

eo

rigin

alw

aveis

retained.

BA

low

sam

plin

grate

looksless

liketh

eo

rigin

alw

ave;n

ot

mu

cho

fthe

orig

ina

lwave

remains.

46

46

A

CHAPTER 3

A high digital sampling rate approXlmates the original signal.

Many samples per second

Time

B

IMAGE FORMATION AND DIGITAL VIDEO

A low digital sampling ratt rtsults In the loss of signal infonnatlon.

Ftwer samples per second -1. .. .1......l._L _L_~ .1.-,1........1 --'---'- ....L.-1..-J .---L.+

Time

3,fi SAMPLING · 1 To convert an analog wave into a digital one, It Is divided and measured at equally spaced interva s.

A A high sampllng rate looks more like the original wave; most of the original wave is retained.

8 A low sampllng rate looks less llke the origlnal wave; not much of the original wave remains.

Sampling An equally smart colleague in the assembly division ~scovers that

1when you cut

a hose into very small, equally spaced pieces (sampling), you don t need all the pieces to represent the hose's original curves (high sampling rate). If, however, you cut the hose into just a few large pieces to save time and money, they can no lo~~er accurately simulate the origlnal curves (low sampling rate). As a result, the digital

representation of the analog signal would be of much lower quality. HUi.6

Compmssjon and Tra.n1.sport The sampling gave the person in the packing division another idea: b~cause each piece of hose has a specific number, you may be able to repack the pieces so th~t all the straight pieces can be squeezed in one small box and the curved ones m another (lossless compression). This would certainly save space (smaller ~es) and allow smaller trucks to be used for transport (smaller bandwidth), Or, he ffilght even get away wtth throwing away some of the pieces before packing them, such as ~e parts that make up long stretches before the first curve, which serve no re.al funcnon when simulating waves (lossy compression). Nobody would notice the rmssingparts

anyway. Toe smart packer was right on both counts. . While testing how much he could throw away, however, he also discovered that

the more hose pieces {data) you throw away, the less accurate the reassembly of the

waves will be (low picture and sound quality). . . The transportation manager decided that the pieces can be sh1pped in two

ways. One way is to lo ad all the straight pieces In one truck, all the curved p!eces in a second truck, and a mixtUre of both in a third. With this method the receiver has to wrut until all trucks have arrived and all boxes are opened ( downloading) before

the assembly of the various curved hoses can begin ( opening the file).

WHY DIGIT_AL?

The second method is to load several trucks with boxes that contain enough pleces to get started with the assembly of some of the hoses. The first hoses are already functional ( the beginning of the file can be opened) while the rest of them arrive one after the other (streaming).

WHY DIGITAL? Returning from transporting garden hoses to the digital process, you may wonder why we bother with such a complicated system when we already have an analog signal that perfectly represents the original stlm ulus. Your computer and how you use It can give you all the answers.

This seemingly crude either/or, on/off principle lets you dub video and audio with virtually no deterioration, compress the signals to save storage space and facili­tate signal transport, and manipulate pictures and sound with relative ease.

Picture aml Sound Quality in Dubs Because each sample point is numbered through quantizing, the robust either/or digital system identifies only two types of data: the good type that contributes to the creation of pictures and sound and the bad type that does not. In the either/or system, there is no room for uperhaps some of them might be good for something sometlme.n This means that the digital system is relatively immune to artifacts and can even eliminate, or at least minimize, all unwanted signal elements that will interfere with, rather than preserve, the integrity of the original signal.

For ex.ample, when you print several copies of a letter that you just typed with your word-processing program, the third or fourth copy, and even the hundredth one the printer delivers, looks just as clean and sharp as the original.

When you use digital video recorders, all subsequent generations produce pictures and sound that are Identical to the original recording, an attribute called transparency. The first copy of the original is called the first generation, the second copy, the second generation, and so forth.

This is not the case with an analog signal. In Its true nature of being seamless, aD values are equal: the analog signal does not distinguish between the desirable signal elements and the artifacts (signal noise). This problem is compounded in sub­sequent analog dubs. With analog equipment each generation adds another layer of artifacts, much like how printed text deteriorates when you progressively duplicate photocopies. Analog video equipment will yield marked picture deterioration after Just a few generations.

Compnssian You cannot compress analog signals. This means that you must deal with the com­plete analog video and audio information during capture, transport, and storage. As you have just read, digital compression not only saves storage space but speeds up signal transport.

48

CH

AP

TE

R3

IMA

GE

FO

RM

AT

ION

AN

DD

IGIT

AL

VID

EO

WH

YD

IGIT

Al?

Man

ipu

lation

Because

digital videoan

daudio

signalsconsist o

fO's

and

l's, theycan

be

changed

bysim

plyrearranging

thenum

bers. The

fontsin

yourword-processing

program, the

openingtitles

ofyour favoritetelevision

show,

and

ananim

atedw

eather map

-all

arethe

result of calculatedm

anipulationof digital data. In

fact, youcan

createsyn­

theticim

agesby

applyingcertain

programs

that let you"paint by

numbers:' A

gain,

suchextensive

and

predictablem

anipulationis

no

tpossible

with

analogsignals.

(See

chapter 9for

more

about digital videow

izardry.)

Editing

isprobably

them

ost evident formo

f digital flexibility.O

ncethe

source

clipsare

on

your computer's

harddrive, you

canorder the

variousclips into

aspecific

sequence, and

then, ifyouo

r your client doesn't likeit, you

canchange

it aroundat

will.

Such

manipulation

would

takeconsiderably

more

effort and

time

if youw

ere

working

with

analogvideo.

As

youcan

see,the

digitalprocess

has

revolutionizedvideo

production;an

d

becausem

ost of thetricky

operationsdiscussed

aredone

bythe

computer, you

can

nowconcentrate

more

on

theaesthetic

elements

ofpicturean

dso

un

dcontrol.

~In

terlacedan

dP

rog

ressiveS

cann

ing

An

interlacedtelevision

frame

ism

adeup

oftw

oscanning

fields,w

hichare

necessary

for onecom

pletefram

e. Interlacedscanning

scansevery

other line, thengoes

backand

scansthe

linesthat w

ereskipped. P

rogressivescanning

scansevery

line. Inprogressive

scanning, eachscanning

cycleproduces

not fieldsbut a

complete

videofram

e. Thefram

e

rate, or refreshrate, can

vary.

~D

igita

lV

ideo

Scan

nin

gS

ystems

Them

ostprevalent digital television(D

TV) scanningsystem

sare

4BOp, 720p, and

10BOi.

AllD

TVsystem

sproduce

videaw

ithhigher resolution

thanstandard

analogtelevision,

improved

color, andm

oresubtle

shadingsbetw

eenthe

brightest anddarkest picture

areas. High-definition

television(H

DTV)

usesthe

720pand

10BOi scanning

systems;

high-definitionvideo

(HD

V) uses

the720p

and1OBOi scanning

systems

but hasless

color

information

andhighercom

pressionthan

HD

TV.

~V

ariable

Scan

nin

gS

ystems

Som

evideo

cameras, especially

high-enddigital cinem

acam

eras, havea

variable

scanningsystem

that canproduce

thestandard

movie

frame

rateof24

frames

per

second(fps) or even

lower, as

well as

theH

DTV

60fps

andeven

avery

highfram

erate

for slow-m

otioneffects.

~F

lat-pan

elD

isplays

Flat-panel displays

havem

yriadtiny

transistors(pixels)

that aresandw

ichedbetw

een

two

transparent panels. Thereare

liqUid

crystal display(LCD) and

plasma

panels. When

avideo

signal isapplied, the

pixelslight up

or orient liquidcrystals

orgasto

let light

shinethrough.

Theadvantage

offlat-panel displays

isthat they

canbe

quitelarge

while

remaining

relativelythin.

~A

nalo

gan

dD

igita

lS

ign

als

An~nalog

signal fluctuatesexactly

likethe

original stimulus. A

digital signal isbased

on

a.bma~

codeth

at uses

on/off, either/or valuesrepresented

byO~

and7~; it is

purposely

dlscontmuous.

~S

amp

ling

and

Qu

antizin

g

Digital signals

sample

theanalog

signal at equally

spacedintervals

andassign

each

samp~ea~pecific ~inary

nu

mb

er-th

eprocess

ofquantizing. Each

number consists

ofa

c~m.bm~tlono

fOs

and7~.

Thehigher the

sampling

rate, thehigher the

picturequality.

Digital S

ignalsare

veryrobust and

don

otdeteriorate

overmU

ltiplegenerations.

~C

om

pressio

nan

dC

odecs

Digitalsignals

canbe

compressed; analog

signalscannot. C

ompression

eliminates

red~ndantor unnecessary

pictureinform

ationto

increasestorage

capacityand

speed

upS

Ignal transport andvideo

andaudio

processing. Losslesscom

pressionrearranges

the

datainto

lessspace. Lossycom

pressionthrow

saw

ayredundant or unim

portant data.

Thereare

several codec(com

pression-decompression) system

sthat offer various

ways

anddegrees

ofcom

pression.

~D

ow

nlo

adin

gan

dS

treamin

g

Downloadi~gm~ansthat the

dataare

sent inpackets

that areoften

out oforder.

You

needto

walt until a

llpacketshave

arrivedbefore

youcan

openthe

file. Stream

ingm

eans

thatyoucan

openthe

fileand

listento

andw

atchthe

first pa

rt while

thedata

deliveryo

f

thebalance

ofthe

fileis

still inprogress.

~M

an

ipu

latio

n

!"hedigit~1 process

permits

agreat num

ber ofqU

ickm

anipulationso

faudioand

video

l~formatlOn.~onlinear

editingis

made

possiblethrough

sortingand

orderingdigital

Videoand

audiOfiles.

CHAPTER 3 IMAGE FORMATION AND DIGITAL VIDEO

Manlputat lon

Because digital video and audio signals consist of O's and l's, they can be changed by simply rearranging the numbers . The fonts in your word-processing program, the opening titles of your favorite televiSion show, and an animated weather map-all are the result of calculated manipulation of digital data. In fact, you can create syn­thetic images by applying certain programs that let you "paint by numbers." Again, such extensive and predictable manipulation is not possible with analog signals. (See chapter 9 for more about digital video wizardry.)

Editing is probably the most evident form of digital flexibility. Once the soUice clips are on your computer's hard drive, you can order the various clips into a specific sequence, and then, if you or yow client doesn't like it, you can change it around at will Such manipulation would take considerably more effort and time if you were working with analog video.

As you can see, the digital process has revolutionized video production; and because most of the tricky operations discussed are done by the computer, you can now concentrate more on the aesthetic elements of picture and sound control.

liJI. lnterlac•d and Progressive Scanning An Interlaced television frame Is made up of two scanning ~elds, which are necessary for one complete frame. Interlaced scanning scans every other line, then goes back and scans the lines that were skipped. Progres.stve scanning scans every line. In {}r09resslve scanning, each scanning cyde produces nor fields but a complete video frame. The frame rare, or refresh rate, can vary.

,... Digital Video Scanning Systems The most prevalent digital television (DTV} scanning systems are 480p, 720p, and 10801. All DTV systems produce video with higher resolution than standard analog television, improved color, and more subtle shadings between the brightest and darkest picture areas. High-definition television (HDTV) uses the 720p and 1 OBOi scanning systems; high-definition video (HOV) uses the 720p and 10801 scanning systems but hos less color Information and higher compression than HDTV.

fll,- Variable Scanning Systems Some video cameras, especially high-end digital cinema cameras, have a variable Kannlng system rhat can produce the standard movie frame rate of 24 frames per second (fps) or even lower, as weJI as the HDTV 60 fps and even a very high frame rate for slow-motion effects.

• Flat-panel Displays Flot-panel displays hove myriad tiny transistors (pixels) that are S<Jndwiched between two transparent panels. There are liquid crystal display (LCD} and plasma panels. When a video signal is applied, the pixels fight up or orient llquid crystals or gas to let fight shine through. The advantage of flat-panel displays is that they can be quite forge while remaining relatively thin.

WHY DIG/TAC!

Analog and Digital Signals

An ~nalog signal fluctuates exactly like the original stimulus. A digital signal Is based on a btna~ code that uses on/off, either/or values represented by o~ and 1 '.t; It is purpose/ discontinuous. Y

Sampling and Qu•ntlzlng

Dig/ta/ signals sample the analog signal at equally spaced intervals and assign each samp~e a ~pedfic binary number-the process of quantizing. Each number consists of a c~mb1n~t1on of O's and 1 ~-The higher the sampling rote, the higher the picture quality. Digital signals are very robust and do not deteriorate over multiple generations.

Compression and Codecs

Digital signals can be compressed; analog slgnafs cannot. Comp,ejjion elimlnates redundant or unnecessary pieture Information to increase storage capacity and speed up signal transport and video and audio processing. Lossless compression rearranges the data Jnto Jess space. Lossy compression throws away redundant or unimportant data. There are several codec (compress/on-decompression) systems that offer varioos ways and degrees of compression.

.. Downloading and Streamrng

Downloadl~g means that the data are sent in packets that are often out of order. You need to wait until all packets have arrived before yau can open the file. Streaming means that yau can open the file and listen to and watch the first port while the data delivery of the balance of the fife Is stilt Jn progress.

• Manipulation

:he digital process permits a great number of quick manipulations of audio and video information. Nonlinear editing is made possible through sorting and ordering digitaf video and audio files.