ANALOG TELEVISION. Persistence of vision: the eye (or the brain rather) can retain the sensation of an image for a short time even after the actual image.

ANALOG TELEVISION

Persistence of 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.

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

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

Hz) required to prevent frame flicker depends on display

brightness, viewing distance.

• Higher frame rate is required with closer viewing and

brighter display.

– For TV viewing: 50- 60 fps

For Movie viewing: 24 fps

For computer monitor: > 70 fps

2 Line merging

• As with frame merging, the eye can fuse separate lines into

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

sufficiently small.

• The maximum vertical spacing between lines depends on the

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

• For common viewing distance and TV screen size, 500- 600

lines per frame is acceptable

3 Merging pixels

• Similarly, the eye can fuse separate pixels in a line into one

continuously varying line, as long as the spacing between

pixels is sufficiently small.

4 Interlacing

• For some reason, the brighter the still image presented to the

viewer ... the shorter the persistence of vision.

• If the space between pictures is longer than the period of

persistence of vision then the image flickers. Therefore, to

arrange for two "flashes" 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 next scan includes only the even lines.

Basic black and white television

• In a basic black and white TV, a single electron beam is

used to scan a phosphor screen. The scan is interlaced, that

is -- it scans twice per photographed frame.

• The information is always displayed from left to right. After

each line is written, when the beam returns back to the left,

the signal is blanked. When the signal reached the bottom it

is blanked until it returns to the top to write the next line

•

Trace and Retrace

• NTSC has 525 vertical lines. However lines number 248 to

263 and 511 to 525 are typically blanked to provide time for

the beam to return to the upper left hand corner for the next

scan. Notice that the beam does not return directly to the top,

but zig-zags a bit.

Vertical Scanning signal

• The vertical scanning signal for conventional black and

white NTSC is quite straightforward. It 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 blanked

scan lines.

Horizontal Scan signal

• The horizontal scan signal is very much the same. The

horizontal scan rate is 525*29.97 or 15,734 Hz. Therefore,

63.6 uS are allocated per line. Typically about 10 uS of this

is devoted to the blanking line on the horizontal scan. There

are 427 pixels per horizontal scan line and so each pixel is

scanned for approximately 125 ns.

• The electron beam is analog modulated across the

horizontal line. The modulation then translates into intensity

changes in electron beam and thus gray scale levels on the

picture screen

• Horizontal blanking signal and synchronization pulse is

quite well defined. For black and white TV, the "front

porch" is 0.02 times the distance between pulses, and the

"back porch" is 0.06 times the distance between pulses.

• The vertical blanking signal also has a number of synchronization pulses included in it. These are illustrated below.

• The television bandwidth is 6 MHz.

• The sub-carrier for the color is 3.58 MHz off the carrier for the

monochrome information.

• The sound carrier is 4.5 MHz off the carrier for the monochrome

information.

• There is a gap of 1.25 MHz on the low end and 0.25 MHz on the high

end to avoid cross talk with other channels.

•

TV Transmitter (B&W)

TV Receiver (B&W)

COLOR TELEVISION

• One of the great electrical engineering triumphs was the

development of color television in such a way that it

remained compatible with black and white television.

• A major driving force behind the majority of current color

TV standards was to allow black-and-white TVs to continue

to be able to receive a valid TV signal after color service

was in place.

Trireceptor theory of vision

why we use RGB monitors

• If you ask someone why red, green and blue are used in

computer monitors -- the immediate answer is "Because

these are the primary colors".

• If you then ask, "But why are these the primary colors?" --

the answer you get is that "If you mix light of these colors

together you can make any color".

Color information transmission in TV

• In the most basic form, color television could simply be

implemented by having cameras with three filters (red,

green and blue) and then transmitting the three color signals

over wires to a receiver with three electron guns and three

drive circuits.

• Unfortunately, this idealized view is not compatible with

the previously allocated 6 MHz bandwidth of a TV channel.

It is also not compatible with previously existing

monochrome receivers.

• Therefore, modern color TV is carefully structured to

preserve all the original monochrome information -- and

just add on the color information on top.

• To do this, one signal, called luminance (Y) has been

chosen to occupy the major portion (0-4 MHz) of the

channel. Y contains the brightness information and the

detail. Y is the monochrome TV signal.

• Consider the model of a scene being filmed with three

cameras. One camera has a red filter, one camera a green

filter and one camera a blue filter.

• Assume that the cameras all adjusted so that when pointed

at "white" they each give equal voltages. To create the Y

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

balanced to compensate for the color perception misbalance

of the eye. The governing equation 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%).

• This is the "monochrome" part of the TV signal. It officially

takes up the first 4 MHz of the 6 MHz bandwidth of the TV

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

limited to 3.2 MHz.

• Two signals are then created to carry the chrominance (C)

information. One of these signals is called "Q" and the

other is called "I". They are related to the R, G and B

signals by:

• The positive polarity of Q is purple, the negative is green.

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

Thus, Q is often called the "green-purple" or "purple-green"

axis information and I is often called the "orange-cyan" or

"cyan-orange" axis information.

• It turns out that the human eye is more sensitive to spatial

variations in the "orange-cyan" than it is for the "green

purple". Thus, the "orange-cyan" or I signal has a maximum

bandwidth of 1.5 MHz and the "green purple" only has a

maximum bandwidth of 0.5 MHz.

• Now, the Q and I signals are both modulated by a 3.58 MHz

carrier wave. However, they are modulated out of 90

degrees out of phase.(QAM) These two signals are then

summed together to make the C or chrominance signal.

• The nomenclature of the two signals aids in remembering

what is going on. The I signal is In-phase with the 3.58

MHz carrier wave. The Q signal is in Quadrature (i.e. 1/4

of the way around the circle or 90 degrees out of phase, or

orthogonal) with the 3.58 MHz carrier wave.

• New chrominance signal (formed by Q and I) has the interesting property

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

of the signal represents the hue.

Phase = Arctan (Q/ I) = hue

Magnitude = sqrt (I 2+ Q 2) = saturation

• Now, since the I and Q signals are clearly phase sensitive -- some sort of

phase reference must be supplied. This reference is supplied after each

horizontal scan and is included on the "back porch" of the horizontal sync

pulse.

• The phase reference consists of 8-10 cycles of the 3.58 MHz signal. It is

called the "color burst" and looks something like this

Conversion between RGB and YIQ

Y = 0.299 R + 0.587 G + 0.114 B

I = 0.596 R -0.275 G -0.321 B

Q = 0.212 R -0.523 G + 0.311 B

R =1.0 Y + 0.956 I + 0.620 Q

G = 1.0 Y - 0.272 I -0.647 Q

B =1.0 Y -1.108 I + 1.700 Q

Bandwidth of Chrominance Signals

• With real video signals, the chrominance component

typically changes much slower than luminance

Furthermore, 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 (I) (the

color of face!) than to green- purple range (Q)

• The above factors lead to

I: bandlimitted to 1.5 MHz and

Q: bandlimitted to 0.5 MHz

Multiplexing of Luminance and Chrominance

• Position the bandlimited chrominance at the high end of the

luminance spectrum, where the luminance is weak, but still

sufficiently lower than the audio (at 4.5 MHz).

• The two chrominance components (I and Q) are multiplexed

onto the same sub- carrier using QAM.

• The resulting video signal including the baseband

luminance signal plus the chrominance components

modulated to f c is called composite video signal.

• In NTSC Luminance is AM VSB, the Chroma is QAM

I&Q, and the Aural FM.

Transmitter Block Diagram

Color Decoder

Block diagrams of TV receivers

PAL , SECAM and NTSC

• There are three major TV standards used in the world today.

These are the

1. American NTSC (National Television Systems Committee) color television system,

2. European PAL (Phase Alternation Line rate)

3. French-Former Soviet Union SECAM (Sequential Couleur avec Memoire)

• The largest difference between the three systems is the

vertical lines. NTSC uses 525 lines (interlaced) while both

PAL and SECAM use 625 lines.

• NTSC frame rates are slightly less than 1/2 the 60 Hz power

line frequency, while PAL and SECAM frame rates are

exactly 1/2 the 50 Hz power line frequency. Lines a. lines v. resolution aspect h.resolution frame

rate• NTSC 525 484 242 4/3 427 29.94

• PAL 625 575 290 4/3 425 25

•SECAM 625 575 290 4/3 465 25

Color Encoding Principles for the PAL

• All three systems use the same definition for luminance:

• The color encoding principles for the PAL system are the

same as those of the NTSC system -- with one minor

difference.

• In the PAL system, the phase of the R-Y signal is reversed

by 180 degrees from line to line. This is to reduce color

errors that occur from amplitude and phase distortion of the

color modulation sidebands during transmission.

• Saying this more mathematically, the chrominance signal

for NTSC transmission can be represented in terms of the

R-Y and B-Y components as

• The PAL signal terms its B-Y component U and its R-Y component V and phase-flips the V component (line by line) as:

Color Encoding Principles for the SECAM

• SECAM system differs very strongly from PAL and NTSC

• In SECAM the R-Y and B-Y signals are transmitted

alternately every line. (The Y signal remains on for each

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

scan, any line will have R-Y information on the first frame

and B-Y on the second.

• Furthermore, the R-Y and B-Y information is transmitted on

different subcarriers. The B-Y sub-carrier runs at 4.25 MHz

and the R-Y subcarrier runs at 4.4 MHz.

• In order to synchronize the line switching, alternate R-Y

and B-Y sync signals are provided for nine lines during he

vertical blanking interval following the equalizing pulses

after the vertical sync.

Summary

• Television is the radio transmission of sound and pictures in

the VHF and UHF ranges. The voice signal from a

microphone is frequency-modulated. A camera converts a

picture or scene into an electrical signal called the video or

luminance Y signal, which amplitude-modulated

• Vestigial sideband AM is used to conserve spectrum space.

The picture and sound transmitter frequencies are spaced

4.5 MHz apart, with the sound frequency being the higher.

• TV cameras use either a vacuum tube imaging device such

as a vidicon or a solid-state imaging device such as the

charged-coupled device (CCD) to convert a scene into a

video signal.

• A scene is scanned by the imaging device to break it up into

segments that can be transmitted serially. The National

Television Standards Committee (NTSC) standards call for

scanning the scene in two 262½ line fields, which are

interlaced to form a single 525-line picture called a frame.

Interlaced scanning reduces flicker.

• The field rate is 59.94 Hz, and the frame or picture rate is

29.97 Hz. The horizontal line scan rate is 15,734 Hz or 63.6

s per line.

• The color in a scene is captured by three imaging devices, which break a

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

color light filters. Three-color signals are developed (R, G, B). These

are combined in a resistive matrix to form the Y signal and are

combined in other ways to form the I and Q signals.

• The I and Q signals amplitude-modulate 3.58-MHz subcarriers shifted

90 from one another in balanced modulators producing quadrature DSB

suppressed signals that are added to form a carrier composite color

signal. This color signal is then used to modulate the AM picture

transmitter along with the Y signal.

• .

• A TV receiver is a standard superheterodyne receiver with

separate sections for processing and recovering the sound

and picture. The tuner section consists of RF amplifiers,

mixers, and a frequency-synthesized local oscillator for

channel selection. Digital infrared remote control is used to

change channels in the synthesizer via a control

microprocessor.

• The tuner converts the TV signals to intermediate frequencies of 41.25

MHz for the sound and 45.75 MHz for the picture. These signals are

amplified in IF amplifiers. The sound and picture IF signals are placed

in a sound detector to form a 4.5-MHz sound IF signal. This is

demodulated by a quadrature detector or other FM demodulator to

recover the sound. Frequency-multiplexing techniques similar to those

used in FM radio are used for stereo TV sound. The picture IF is

demodulated by a diode detector or other AM demodulator to recover

the Y signal.

• .The color signals are demodulated by two balanced

modulators fed with 3.58-MHz subcarriers in

quadrature. The subcarrier is frequency- and phase-

locked to the subcarrier in the transmitter by phase-

locking to the color subcarrier burst transmitted on

the horizontal blanking pulse.

• .To keep 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 synchronize horizontal

and vertical oscillators in the receiver. These oscillators

generate deflection currents that sweep the electron beam

in the picture tube to reproduce the picture.

• .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 horizontally and vertically in step

with the transmitted video signals. Deflection signals from the internal sweep

circuits drive coils in a deflection yoke around the neck of the picture creating

magnetic fields that sweep the three electron beams.

• The horizontal output stage, which provides horizontal sweep,

is also used to operate a flyback transformer that steps up the

horizontal sync pulses to a very high voltage. These are

rectified and filtered into a 30- to 35-kV voltage to operate the

picture tube. The flyback also steps down the horizontal

pulses and rectifies and filters them into low-voltage dc

supplies that are used to operate most of the circuits in the