analog_television.ppt

7/25/2019 analog_television.ppt

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ANALOG TELEVISION


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Persistenceof vision:

the eye (or the brain rather) can retain the sensation of an

image for a short time even after the actual image is

removed.

1 Frame merging

This allows the display of a video as successive frames as

long as the frame interval is shorter than the persistence

period, The eye will see a continuously varying image in

time.


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When the frame interval is too long, the eye observes frame

flicker. The minimal frame rate (frames/ second or fps or

H) re!uired to prevent frame flic"er depends on display

brightness, viewing distance.

Higher frame rate is re!uired with closer viewing and

brighterdisplay.

#$or T% viewing& ' * fps

$or +ovie viewing& - fps

$or computer monitor& fps


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2 Line merging

0s with frame merging, the eye can fuseseparate lines into

one complete frame, as long as the spacing between lines is

sufficiently small.

The ma1imum vertical spacing between lines depends on the

viewing distance, the screen sie, and the display brightness.

$or common viewing distance and T% screen sie, ' *lines per frame is acceptable


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!erging "i#els

2imilarly, the eye can fuse separate "i#elsin a line into one

continuously varying line, as long as the spacing between

pi1els is sufficiently small.


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$ Interlacing

$or some reason, the brighter the still image presented to the

viewer ... the shorter the persistence of vision.

3f the space between pictures is longer than the period of

persistence of vision then the image flic"ers. Therefore, to

arrange for two 4flashes4 per frame,

interlacing creates the flashes. The basic idea here is that a

single frame is scanned twice. The first scan includes only

the odd lines, the ne1t scan includes only the even lines.


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%asic &lack an' ()ite television

3n a basic blac" and white T%, a single electron beam is

used to scan aphosphor screen. The scan is interlaced, that

is it scans twice per photographed frame.

The information is always displayed from left to right. 0fter

each line is written, when the beam returns bac" to the left,

the signal isblan"ed.When the signal reached the bottom it

is blan"ed until it returns to the top to write the ne1t line


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Trace an' *etrace


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5T26 has '' vertical lines. However lines number -7 to *8

and '99 to '' are typically blan"ed to provide time for the

beam to return to the upper left hand corner for the ne1t scan.5otice that the beam does not return directly to the top, but ig

ags a bit.


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Vertical Scanning signal

The vertical scanning signal for conventional blac" and

white 5T26 is !uite straightforward. 3t is simply a positive

ramp until it is time for the beam to return to the upper left

hand corner. Then it is a negative ramp during the blan"ed

scan lines.


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+ori,ontal Scan signal

The )ori,ontal scan signal is very much the same. The

horiontal scan rate is '':;.; or 9',8- H. Therefore,

*8.* u2 are allocated per line. Typically about 9 u2 of this

is devoted to the blan"ing line on the horiontal scan. There

are - pi1els per horiontal scan line and so each pi1el is

scanned for appro1imately 9' ns.


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The electron beam is analog modulated across the horiontal

line. The modulation then translates into intensity changes

in electron beam and thus gray scale levels on the picture

screen


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Horiontal blan"ing signal and synchroniation pulse is

!uite well defined. $or blac" and white T%, the 4front

porch4 is . times the distance between pulses, and the

4bac" porch4 is .* times the distance between pulses.


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The vertical blan"ing signal also has a number of

synchroniation pulses included in it. These are

illustrated below.


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The television bandwidth is * +H.

The subcarrier for the color is 8.'7 +H off the carrier for the

monochrome information.

The sound carrier is -.' +H off the carrier for the monochrome

information.

There is a gap of 9.' +H on the low end and .' +H on the high end

to avoid cross tal" with other channels.


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TV Transmitter -%./0


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TV *eceiver -%./0


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OLO* TELEVISION


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Trirece"tor t)eor of vision

why we use >?@ monitors

3f you as" someone why re'3 green an' &l4e are used in

computer monitors the immediate answer is 4@ecause

these are the primary colors4.

3f you then as", 4@ut why are these the primary colorsA4

the answer you get is that 43f you mi1 light of these colors

together you can ma"e any color4.


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l i f ti t i i i TV


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olor information transmission in TV

3n the most basic form, color television could simply be

implemented by having cameras with three filters (red,

greenandblue) and then transmitting the three color signals

over wires to a receiver with three electron guns and threedrive circuits.

Bnfortunately, this idealied view is not com"ati&le with

the previously allocated 6 MHz bandwidth of a T% channel.

3t is also not compatible with previously e1isting

monoc)rome receivers5

Th f d l T% i f ll t t d t


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Therefore, modern color T% is carefully structured to

preserve all the original monochrome information and

=ust add on the color information on top.

To do this, one signal, called l4minance -60 has been

chosen to occupy the ma=or portion (- +H) of thechannel. C contains the brightness information and the

detail. C is the monochrome T% signal.

6onsider the model of a scene being filmed with three

cameras.


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0ssume that the cameras all ad=usted so that when pointed

at 4white4 they each give e!ual voltages. To create the C

signal, the red, green and blue inputs to the C signal must be

balanced to compensate for the color perception misbalance

of the eye. The governing e!uation is&

For example, in order to produce "White" light to the

human observer there needs to be 11 % blue, 30 % red and

59% green (=100%)

Thi i th 4 h 4 t f th T% i l 3t ffi i ll


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This is the 4monochrome4 part of the T% signal. 3t officially

ta"es up the first - +H of the * +H bandwidth of the T%

signal. However, in practice, the signal is usually band

limited to 8. +H.

Two signals are then created to carry the c)rominance -0information. , ? and @

signals by&


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Th iti l it f D i l th ti i


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The positivepolarity of D is purple, the negative is green.

The positive polarity of 3 is orange, the negative is cyan.

Thus, D is often called the 4greenpurple4 or 4purplegreen4

a1is information and 3 is often called the 4orangecyan4 or

4cyanorange4 a1is information.

3t turns out that the human eye is more sensitive to spatial

variations in the 4orangecyan4 than it is for the 4green

purple4. Thus, the 4orangecyan4 or 3 signal has a ma1imum

bandwidth of 159 !+, and the 4green purple4 only has a

ma1imum bandwidth of 59 !+,5

5o the D and 3 signals are both mod lated b a 8 '7 +H


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5ow, the D and 3 signals are both modulated by a 8.'7 +H

carrier wave. However, they are modulated out of ;

degrees out of phase5-8A!0 These two signals are then

summed together to ma"e the 6 or chrominance signal.

The nomenclature of the two signals aids in rememberingwhat is going on. The 3 signal is In-phase with the 8.'7

+H carrier wave. The D signal is in Quadrature(i.e. 9/-

of the way around the circle or ; degrees out of phase, or

orthogonal) with the 8.'7 +H carrier wave.

5ew chrominance signal (formed by D and 3) has the interesting property that


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5ew chrominance signal (formed by D and 3) has the interesting property that

the magnitude of the signal represents the color saturation, and the phase of the

signal represents the hue.

P)ase; Arctan -8< I0 ;)4e!agnit4'e ; s=rt -I2> 820 ;sat4ration

5ow, since the 3 and D signals are clearly phase sensitive some sort of phase

reference must be supplied. This reference is supplied after each horiontalscan and is included on the 4bac" porch4 of the horiontal sync pulse.

The phase reference consists of 79 cycles of the 8.'7 +H signal. 3t is called

the 4color burst4 and loo"s something li"e this


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onversion between !"# and $IQ

$ % &.'(( ! ) &.*+, " ) &. #

I % &.*(6 ! -&.',* " -&./' #

Q % &.'' ! -&.*'/ " ) &./ #

! %.& $ ) &.(*6 I ) &.6'& Q

" % .& $ - &.',' I -&.6, Q

# %.& $ -.&+ I ) .,&&8

%an'(i't) of )rominance Signals


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%an'(i't) of )rominance Signals

With real video signals, the chrominance component

typically changes much slower than luminance

$urthermore, the human eye is less sensitive to changes in

chrominance than to changes in luminance

The eye is more sensitive to the orange cyan range (3) (the

color of faceE) than to green purple range (D)

The above factors lead to

3& bandlimitted to 9.' +H and

D& bandlimitted to .' +H

!4lti"le#ing of L4minance an' )rominance


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!4lti"le#ing of L4minance an' )rominance

Fosition the bandlimited chrominance at the high end of the

luminance spectrum, where the luminance is wea", but still

sufficiently lower than the audio (at -.' +H).

The two chrominance components (3 and D) are multiple1ed

onto the same sub carrier using 8A!5

The resulting video signal including the baseband

luminance signal plus the chrominance components

modulated to! c is called com"osite vi'eo signal.


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3n 5T26 Guminance is 0+ %2@ the 6hroma is D0+


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3n 5T26 Guminance is 0+ %2@, the 6hroma is D0+

3D, and the 0ural $+.


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Transmitter %lock ?iagram


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olor ?eco'er


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%lock 'iagrams of TV receivers

PAL 3 SEA! an' NTS


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PAL 3 SEA! an' NTS

There are three ma=or T% standards used in the world today.

These are the

9. 0merican 5T26 (5ational Television 2ystems

6ommittee) color television system,

. Iuropean F0G(Fhase 0lternation Gine rate)

8. $rench$ormer 2oviet Bnion 2I60+ (2e!uential

6ouleur avec +emoire)


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The largest difference between the three systems is the


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g y

vertical lines. 5T26 uses '' lines (interlaced) while both

F0G and 2I60+ use *' lines.

5T26 frame rates are slightly less than 9/ the * H power

line fre!uency, while F0G and 2I60+ frame rates are

e1actly 9/ the ' H power line fre!uency.

Lines a. lines v. resolution aspect h.resolution frame rate

NTSC 525 484 242 4/3 427 29.94

!L "25 575 29# 4/3 425 25

S$C!% "25 575 29# 4/3 4"5 25


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olor Enco'ing Princi"les for t)e PAL


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g "

0ll three systems use the same definition for luminance&

The color encoding principles for the F0G system are the

same as those of the 5T26 system with one minor

difference.

3n the F0G system, the phase of the >C signal is reversed

by 97 degrees from line to line. This is to reduce colorerrors that occur from amplitude and phase distortion of the

color modulation sidebands during transmission.

2aying this more mathematically, the chrominance signal


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y g y, g

for 5T26 transmission can be represented in terms of the >

C and @C components as

The F0G signal terms its @C component B and its >C

component % and phaseflips the % component (line by line)

as&


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i i i f S A


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olor Enco'ing Princi"les for t)e SEA!

2I60+ system differs very strongly from F0G and 5T26

3n 2I60+ the >C and @C signals are transmittedalternately every line. (The C signal remains on for each

line). 2ince there is an odd number of lines on any given

scan, any line will have >C information on the first frame

and @C on the second.


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$urthermore, the >C and @C information is transmitted on

different subcarriers. The @C subcarrier runs at -.' +H

and the >C subcarrier runs at -.- +H.

3n order to synchronie the line switching, alternate >C and

@C sync signals are provided for nine lines during he

vertical blan"ing interval following the e!ualiing pulses

after the vertical sync.

S4mmar


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Television is the radio transmission of soundandpicturesin

the %H$ and BH$ ranges. The voice signal from a

microphone is fre!uencymodulated. 0 camera converts a

picture or scene into an electrical signal called the video orluminance 6signal, which amplitudemodulated

%estigial sideband 0+ is used to conserve spectrum space.

The picture and sound transmitter fre!uencies are spaced

-.' +H apart, with the sound fre!uency being the higher.


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T% cameras use either a vacuum tube imaging device such

as a vidicon or a solidstate imaging device such as the

chargedcoupled device (66J) to convert a scene into a

video signal.

0 scene is scanned by the imaging device to brea" it up into


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segments that can be transmitted serially. The 5ational

Television 2tandards 6ommittee (5T26) standards call forscanning the scene in two *K line fields, which are

interlaced to form a single ''line picture called a frame.

3nterlaced scanning reduces flic"er.

The field rate is ';.;- H, and the frame or picture rate is

;.; H. The horiontal line scan rate is 9',8- H or *8.*

s per line.

The color in a scene is captured by three imaging devices, which brea" a


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picture down into its three basic colors of red, green, and blue using

color light filters. Threecolor signals are developed (, #, $). These

are combined in a resistive matri1 to form the $signal and are combined

in other ways to form theIand Qsignals.

The and & signals amplitudemodulate 8.'7+H subcarriers shifted

;from one another in balanced modulators producing !uadrature J2@

suppressed signals that are added to form a carrier composite color

signal. This color signal is then used to modulate the 0+ picture

transmitter along with the 'signal.

.

0 T% receiver is a standard superheterodyne receiver with


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separate sections for processing and recovering the sound

and picture. The tuner section consists of >$ amplifiers,mi1ers, and a fre!uencysynthesied local oscillator for

channel selection. Jigital infrared remote control is used to

change channels in the synthesier via a control

microprocessor.


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The tuner converts the T% signals to intermediate fre!uencies of -9.'

+H for the sound and -'.' +H for the picture. These signals are

amplified in 3$ amplifiers. The sound and picture 3$ signals are placed

in a sound detector to form a -.'+H sound 3$ signal. This is

demodulated by a !uadrature detector or other $+ demodulator to

recover the sound. $re!uencymultiple1ing techni!ues similar to those

used in $+ radio are used for stereo T% sound. The picture 3$ is

demodulated by a diode detector or other 0+ demodulator to recover

the 'signal.


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.The color signals are demodulated by two balanced

modulators fed with 8.'7+H subcarriers in

!uadrature. The subcarrier is fre!uency and phase

loc"ed to the subcarrier in the transmitter by phase

loc"ing to the color subcarrier burst transmitted on

the horiontal blan"ing pulse.


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.To "eep the receiver in step with the scanning process at

the transmitter, sync pulses are transmitted along with the

scanned lines of video. These sync pulses are stripped off

the video detector and used to synchronie horiontal and

vertical oscillators in the receiver. These oscillators

generate deflection currents that sweep the electron beam in

the picture tube to reproduce the picture.


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.The color picture tube contains three electron guns that generate narrow electron

beams aimed at the phosphor coating on the inside of the face of the picture tube.

The phosphor is arranged in millions of tiny red, green, and blue color dot triads or

stripes in proportion to their intensity and generate light of any color depending

upon the amplitude of the red, green, and blue signals. The electron beam is

scanned or deflected horiontally and vertically in step with the transmitted video

signals. Jeflection signals from the internal sweep circuits drive coils in a

deflection yo"e around the nec" of the picture creating magnetic fields that sweep

the three electron beams.


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The horiontal output stage, which provides horiontal sweep,

is also used to operate a flybac" transformer that steps up the

horiontal sync pulses to a very high voltage. These are

rectified and filtered into a 8 to 8'"% voltage to operate the

picture tube. The flybac" also steps down the horiontal

pulses and rectifies and filters them into lowvoltage dcsupplies that are used to operate most of the circuits in the

analog_television.ppt

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