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.
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