COURSE: Consumer Electronics

COURSE CODE: 22425

CHAPTER : 3. TV Fundamentals &

Transmitter
Marks: 16 Marks.
Course Outcomes:
A. Troubleshoot different types of microphones and speakers.
B. Maintain audio systems.
C. Analyse the composite video signal used in TV signal transmission.
D. Troubleshoot colour TV receivers.
E. Maintain various consumer electronic appliances.

Contents:
3.1. Concept: Aspect ratio, image continuity, interlaced scanning. scanning
periods — horizontal and vertical, vertical and horizontal resolution
3.2. Vestigial sideband transmission, bandwidth for Colour signal, characteristics
of colour signal, compatibility.
3.3. Colour theory, Grassman's law, additive and subtractive colour mixing
Composite Video Signal — Pedestal height, Blanking pulse, colour burst,
Horizontal sync pulse details, Vertical sync pulse details, equalizing pulses,
3.4. CCIR-B standards for colour signal transmission and reception, Positive and
Negative modulation, merits and demerits of negative modulation
3.5. Block diagram of Colour TV Transmitter.
 TV Fundamentals

3.1 Basic fundamentals of colour & monochrome television.

Aspect Ratio:

The aspect ratio of an image describes the proportional relationship between its
width and its height. The frame adopted in all television systems is rectangular with
width/height ratio, i.e., aspect ratio = 4/3.

Figure: Different Aspect Ratio

𝑊𝑖𝑑𝑡ℎ 𝑜𝑓 𝑡ℎ𝑒 𝑆𝑐𝑟𝑒𝑒𝑛 4

𝐴𝑠𝑝𝑒𝑐𝑡 𝑅𝑎𝑡𝑖𝑜 = 𝐻𝑒𝑖𝑔ℎ𝑡 𝑂𝑓 𝑡ℎ𝑒 𝑆𝑐𝑟𝑒𝑒𝑛
= 3

------- (1)

Why Width is more than height???

✓ In human affairs most of the motion occurs in the horizontal plane and so a
larger width is desirable. The eyes can view with more ease and comfort when
the width of a picture is more than its height.
✓ The usage of rectangular frame in motion pictures with a width/height ratio of
4/3 is another important reason for adopting this shape and aspect ratio. This
enables direct television transmission of film programs without wastage of
any film area.

Notes By: Tejas Shah

 TV Fundamentals

Image Continuity:
✓ While televising picture elements of the frame by means of the scanning
process, it is necessary to present the picture to the eye in such a way that an
illusion of continuity is created and any motion in the scene appears on the
picture tube screen as a smooth and continuous change.
✓ To achieve this, advantage is taken of ‘persistence of vision’ or storage
characteristics of the human eye. This is came from the fact that the sensation
produced when the light is incident on eye’s retina, it does not disappear
immediately after the light is removed but persists(stays) for about 1/16th of a
second.
✓ Thus if the scanning rate per second is made greater than sixteen, or the
number of pictures shown per second is more than sixteen, the eye is able to
integrate(mix) the changing levels of brightness in the scene.
✓ So when the picture elements are scanned rapidly enough, they appear to the
eye as a complete picture unit, with none of the individual elements visible
separately.
✓ In present day motion pictures twenty-four still pictures of the scene are
taken per second and later projected on the screen at the same rate.
✓ Each picture or frame is projected individually as a still picture, but they are
shown one after the other in rapid succession to produce the illusion of
continuous motion of the scene being shown.

Scanning:
The scene is scanned rapidly both in the horizontal and vertical directions
simultaneously to provide sufficient number of complete pictures or frames per

Notes By: Tejas Shah

 TV Fundamentals

second to give the illusion of continuous motion. Instead of the 24 as in commercial

motion picture practice, the frame repetition rate is 25 per second in most television
systems.
● Horizontal scanning:
✓ Fig. shows the trace and retrace of several horizontal lines. The linear rise
of current in the horizontal deflection coils deflects the beam across the
screen with a continuous, uniform motion for the trace from left to right.

Figure: Horizontal Scanning.

✓ At the peak of the rise, the sawtooth wave reverses direction and decreases
rapidly to its initial value. This fast reversal produces the retrace or flyback.
The start of the horizontal trace is at the left edge of raster. The finish is at
the right edge, where the flyback produces retrace back to the left edge.

● Vertical scanning.

✓ The sawtooth current in the vertical deflection coils moves the electron
beam from top to bottom of the raster at a uniform speed while the
electron beam is being deflected horizontally. Thus the beam produces
complete horizontal lines one below the other while moving from top to
bottom.

Notes By: Tejas Shah

 TV Fundamentals

✓ The trace part of the sawtooth wave for vertical scanning deflects the beam
to the bottom of the raster. Then the rapid vertical retrace returns the beam
to the top. Note that the maximum amplitude of the vertical sweep current
brings the beam to the bottom of the raster.

Number of scanning lines:

✓ Most scenes have brightness variations in the vertical direction. The ability of
the scanning beam to allow reproduction of electrical signals according to
these variations and the capability of the human eye to resolve these distinctly,
while viewing the reproduced picture, depends on the total number of lines
employed for scanning.
✓ It is possible to arrive at some estimates of the number of lines necessary by
considering the bar pattern shown in Fig., where alternate lines are black and
white. If the thickness of the scanning beam is equal to the width of each white
and black bar, and the number of scanning lines is chosen equal to the number
of bars, the electrical information corresponding to the brightness of each bar
will be correctly reproduced during the scanning process.

Figure: Scanning of alternate Black & White lines for calculation of TOTAL NO. OF LINES

✓ The maximum number of alternate light and dark elements (lines) which can
be resolved by the eye is given by
1
𝑁𝑣 = ----------(2)
αρ

Notes By: Tejas Shah

 TV Fundamentals

where 𝑁𝑣= total number of lines (elements) to be resolved in the vertical

direction,
α = minimum resolving angle of the eye expressed in radians, and
ρ = D/H = viewing-distance/picture height.
✓ For the eye this resolution is determined by the structure of the retina, and the
brightness level of the picture. it has been determined experimentally that
with reasonable brightness variations and a minimum viewing distance of
four times the picture height (D/H = 4), the angle that any two adjacent
elements must subtend at the eye for distinct resolution is approximately one
minute (1/60 degree).
Substituting these values of α and ρ we get
1
𝑁𝑣 = π 1 ≃ 860
( 180
× 60 ×4)

----------(3)
✓ In practice however, the picture elements are not arranged as equally spaced
segments but have random distribution of black, grey and white depending on
the nature of the picture details or the scene under consideration.
✓ Statistical analysis and subjective tests carried out to determine the average
number of effective lines suggest that about 70 per cent of the total lines or
segments get separately scanned in the vertical direction and the remaining
30 per cent get merged with other elements due to the beam spot falling
equally on two consecutive lines.
✓ Thus the effective number of lines distinctly resolved, i.e.,

𝑁𝑟 = 𝑁𝑣 × 𝑘

---------(4)
Where k is the resolution factor whose value lies between 0.65 to 0.75.
Assuming the value of k = 0.7.

Notes By: Tejas Shah

 TV Fundamentals

We get, 𝑁𝑟 = 𝑁𝑣 × 𝑘 = 860 × 0. 7 = 602

----------(5)

Interlace scanning:

✓ In television pictures an effective rate of 50 vertical scans per second is

utilized to reduce the flicker. This is accomplished by increasing the downward
rate of travel of the scanning electron beam, so that every alternate line gets
scanned instead of successive line.
✓ Then when the beam reaches the bottom of the picture frame it quickly returns
to the top to scan those lines that were missed in the previous scanning.
✓ Thus, the total numbers of lines are divided into two groups called ‘fields’.
Each field is scanned alternately. This method of scanning is called ‘interlaced
scanning’.
✓ In the 625 line TV system, for successful interlaced scanning, the 625 lines of
each frame or picture are divided into sets of 312.5 lines and each set is
scanned alternately to cover the entire picture area.

Figure: Principle of Interlaced Scanning

Notes By: Tejas Shah

 TV Fundamentals

✓ To achieve this, the horizontal sweep oscillator is made to work at a frequency

of 15625 Hz (i.e. 312.5 x 50 = 15625) to scan the number of lines per frame, but
the vertical sweep circuit is run at a frequency of 50 Hz (i.e. 25 x 2 = 50Hz)
✓ Note that since the beam is now deflected from top to bottom in half the time
and horizontal oscillator still operating at 15625 Hz, only half the total lines
(i.e. 312.5) get scanned during each vertical sweep.
✓ Since the first field ends in a half line and the second field starts middle of the
line on top of the screen, as shown in fig., the beam is able to scan the
remaining 312.5 alternate lines during its downward journey.
✓ The beam scans 652 lines per frame at the same rate of 15625 lines per second.
Therefore, with interlaced scanning the flicker effect is eliminated without
increasing the speed of scanning, which in turn does not need any increase in
channel bandwidth.

Scanning periods:

✓ The wave shapes of both horizontal and vertical sweep currents are shown in
Fig.
✓ As shown there the retrace times involved (both horizontal and vertical) are
due to physical limitations of practical scanning systems and are not utilized
for transmitting or receiving any video signal.

Notes By: Tejas Shah

 TV Fundamentals

Figure: Scanning Durations

✓ The nominal duration of the horizontal line as shown in Fig. is 64 μs (1/15625 =
64 μs), out of which the active line period is 52 μs and the remaining 12 μs is
the line blanking period. The beam returns during this short interval to the
extreme left side of the frame to start tracing the next line.
✓ Similarly with the field frequency set at 50 Hz, the nominal duration of the
vertical time period (see Fig.) is 20 ms (1/50 = 20 ms). Out of this period of 20
ms, 18.720 ms are spent in bringing the beam from top to bottom and the
remaining 1.280 ms is taken by the beam to return back to the top to
commence the next cycle.
✓ Since the horizontal and vertical sweep oscillators operate continuously to
achieve the fast sequence of interlaced scanning, 20 horizontal lines get traced
during each vertical retrace interval. Thus 40 scanning lines are lost per
frame, as blanked lines during the retrace interval of two fields.
✓ This leaves the active number of lines, Na, for scanning the picture details
equal to 625 – 40 = 585, instead of the 625 lines actually scanned per frame.

Scanning sequence:

Notes By: Tejas Shah

 TV Fundamentals

Figure: Scanning sequence/ line details for scanning

Vertical resolution:

Definition:

The ability of the scanning system to resolve picture details in vertical direction is known as
vertical resolution.

1)Vertical resolution is a function of scanning lines into which the picture is divided in
the vertical plane.

2)The maximum number of dark and white elements which can be resolved by the
human eye in the vertical direction in a screen of height H decided by the number of
horizontal lines into which picture is split while scanning.

Thus, vertical resolution can be expressed as,

Notes By: Tejas Shah

 TV Fundamentals

𝑉𝑟 = 𝑁𝑎 × 𝑘 ----------(6)

Vr = Vertical resolution

Na = Active number of lines.

K-kell factor or resolution factor

Vr= ___________________________________________

Horizontal Resolution:

Definition:

The ability of the scanning system to resolve the picture details in the horizontal direction is
known as horizontal resolution.

1) While aiming at equal vertical and horizontal resolutions and assuming the same
Kell factors the effective number of alternate black and white segments (N) that get
scanned in one horizontal line are-

𝑁 = 𝑁𝑎 × 𝐴𝑠𝑝𝑒𝑐𝑡 𝑅𝑎𝑡𝑖𝑜 × 𝑘

-------------(7)

N= ____________________

Vestigial sideband transmission:

✓ In the video signal very low frequency modulating components exist along
with the rest of the signal. These components give rise to sidebands very close
to the carrier frequency which are difficult to remove by physically realizable
filters.

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Notes By: Tejas Shah

 TV Fundamentals

✓ Thus it is not possible to go to the extreme and fully suppress one complete
sideband in the case of television signals. The low video frequencies contain
the most important information of the picture and any effort to completely
suppress the lower sideband would result in objectionable phase distortion at
these frequencies.

Figure: VSB
✓ This distortion will be seen by the eye as ‘smear’(spreaded) in the reproduced
picture. Therefore, as a compromise, only a part of the lower sideband is
suppressed, and the radiated signal then consists of a full upper sideband
together with the carrier, and the vestige (remaining part) of the partially
suppressed lower sideband. This pattern of transmission of the modulated
signal is known as vestigial sideband or A5C transmission. In the 625 line
system, frequencies up to 0.75 MHz in the lower sideband are fully radiated.

Definitions:

Contrast:

This is the difference in intensity between black and white parts of the picture over
and above the brightness level.

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Hue:

This is the predominant spectral colour of received light which means it is the actual
colour seen by the eye. Red, Green, Blue, Yellow, Magenta, represent different in the
visible spectrum.

Saturation:

It represents the spectral purity of a colour light. It is the amount of white light that is
mixed with a colour. A fully saturated colour will have no white light mixed with it.
For example, a Pure Red without White is a saturated colour.

Luminance or Brightness: This is the amount of light intensity as perceived by the

eye regardless of the colour. In black and white pictures, better lighted parts have
more luminance than the dark areas.

Viewing distance:

✓ The viewing distance from the screen of the TV receiver should not be so large
that the eye cannot resolve details of the picture. The distance should also not
be so small that picture elements become separately visible. The above
conditions are met when the vertical picture size subtends an angle of
approximately 15° at the eye.
✓ The distance also depends on habit, varies from person to person, and lies
between 3 to 8 times the picture heights.
✓ Most people prefer a distance close to five times the picture height.

Compatibility:

Compatibility implies that

● The colour television signal must produce a normal black and white picture on
a monochrome receiver without any modification of the receiver circuitry and

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 TV Fundamentals

● A colour receiver must be able to produce a black and white picture from a
normal monochrome signal. This is referred to as reverse compatibility.

To achieve this, that is to make the system fully compatible the composite colour
signal must meet the following requirements:

✓ It should occupy the same bandwidth as the corresponding monochrome

signal.
✓ The location and spacing of picture and sound carrier frequencies should
remain the same.
✓ The colour signal should have the same luminance (brightness) information as
would a monochrome signal, transmitting the same scene.
✓ The composite colour signal should contain colour information together with
the ancillary signals needed to allow this to be decoded.
✓ The colour information should be carried in such a way that it does not affect
the picture reproduced on the screen of a monochrome receiver.
✓ The system must employ the same deflection frequencies and sync signals as
used for monochrome transmission and reception.

3.2 Colour Theory:

✓ All light sensations to the eye are divided (provided there is an adequate
brightness stimulus on the operative cones) into three main groups. The optic
nerve system then integrates the different colour impressions in accordance
with the curve shown in Fig. to perceive the actual colour of the object being
seen.

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 TV Fundamentals

Figure: Wavelength of different colour

✓ This is known as additive mixing and forms the basis of any colour television
system. A yellow colour, for example, can be distinctly seen by the eye when the
red and green groups of the cones are excited at the same time with
corresponding intensity ratio. Similarly and colour other than red, green and
blue will excite different sets of cones to generate the cumulative sensation of
that colour.
✓ A white colour is then perceived by the additive mixing of the sensations from
all the three sets of cones.
✓ Mixing of colours can take place in two ways—subtractive mixing and
additive mixing.
✓ In subtractive mixing, reflecting properties of pigments are used, which
absorb all wavelengths but for their characteristic colour wavelengths. When
pigments of two or more colours are mixed, they reflect wavelengths which are
common to both. Since the pigments are not quite saturated (pure in colour)
they reflect a fairly wide band of wavelengths. This type of mixing takes place
in painting and colour printing.

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 TV Fundamentals

Figure: subtractive mixing

✓ In additive mixing which forms the basis of colour television, light from two
or more colours obtained either from independent sources or through filters
can create a combined sensation of a different colour. Thus different colours
are created by mixing pure colours and not by subtracting parts from white.

Figure: additive mixing

Additive Colour Mixing Subtractive Colour Mixing

Additive mixing of three primary In subtracting mixing reflecting

colours red, green and blue with properties of pigments are used which
1. 1.
proper proportions can create any absorb all wavelengths but for their
colour. characteristics colour wavelengths.

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 TV Fundamentals

Different colours are created by Different colours are created by

2. mixing pure colours hence used in 2. subtracting parts from white so not
TV. suitable for TV.

For example, For example,

Red + Blue = Magenta White – Green = Magenta

3. 3.
Red + Green = Yellow White – Blue = Yellow

Green + Blue = Cyan White – Red = Cyan

Additives primaries are Red, Subtractive primaries are Magenta,

4. 4.
Green, and Blue. Yellow, and Cyan.

Grassmann's Law:

✓ The eye is not able to distinguish each of the colours that mix to form a new
colour but instead perceives only the resultant colour. The subjective
impression which is gained when green, blue and red lights reach the eye
simultaneously may be matched by a single light source having the same
colour.
✓ In addition to this, the brightness (luminance) impression created by the
combined light source is numerically equal to the sum of the brightness
(luminance) of the three primaries that constitute the single light.
✓ This property of the eye of producing a response which depends on the algebraic sum
of the red, green and blue inputs is known as Grassman’s Law.
✓ White has been seen to be reproduced by adding red, green and blue lights.
The intensity of each colour may be varied. This enables simple rules of
addition and subtraction.

3.3 Composite Video Signal:

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 TV Fundamentals

✓ In monochrome TV, the composite video signal consists of –

1. Camera signal corresponding to light intensity in the picture.
2. Blanking pulses to make retrace invisible.
3. Synchronizing pulses to keep scanning at receiver in synchronous with
transmitting end.
✓ A horizontal synchronizing pulse is sent at the end of line period, vertical sync
pulse is needed after each field of scanning. In colour TV, the video signal has
additional information about colours and colour sync to Synchronize colour
reception. Fig. shows composite video signal for three lines having different
brightness level of black and white picture.
✓ Video signal varies between certain amplitude limits. The level of video signal
when picture information being transmitted corresponds to maximum
whiteness is referred to as peak white level.
✓ Peak white level is fixed at 12.5 percent of maximum value of signal and black
level is fixed at 72-75 percent. Sync pulses are added at 75 percent.
✓ Picture information may vary between 10 percent to about 75 percent of
composite video signal depending on relative brightness of picture. Lowest 10
percent is not used to avoid noise effect.
✓ The electrical signal formed by scanning the picture image is called video
signal.

Definition: The video signal containing the horizontal and vertical sync and blanking pulses is
called as Composite Video Signal.

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 TV Fundamentals

Figure: Composite Video Signal (CVS) for 3 horizontal lines

D.C. component of the video signal:

✓ In addition to continuous amplitude variations for individual picture

elements, the video signal has an average value or dc component
corresponding to the average brightness of the scene. In the absence of dc
component the receiver cannot follow changes in brightness.

Pedestal height:

✓ Pedestal height is the distance between the pedestal level and average value (dc
level) of the video signal. This indicates average brightness since it measures
how much the average value differs from black level.
✓ The output signal from TV camera is of very small amplitude. Hence, it is
amplified by multistage high gain amplifiers. Sync and blanking pulses are
added to it and then signal is clipped at proper value to form pedestal.
✓ Pedestal height determines brightness of scene. Large pedestal height makes
picture brighter and vice versa. Operator who observes the picture in studio
adjusts level for desired brightness by adding dc component to ac signal.

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 TV Fundamentals

Blanking pulses:

✓ The composite video signal contains blanking pulses to make retrace line
invisible.
✓ This is done by increasing the signal amplitude slightly more than the black
level during retrace period
✓ Composite video signal contains horizontal and vertical blanking pulses.
✓ Repetition of rate of horizontal blanking pulses per frame is 15625 Hz (line
frequency)
✓ Vertical blanking pulse frequency is 50Hz (field frequency)
✓ Sync pulses are having amplitude in upper 25 percent of video signal.

Figure: Horizontal & Vertical Blanking pulses

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 TV Fundamentals

Horizontal sync details:

✓ The horizontal blanking period and sync pulse details are illustrated in Fig. The
interval between horizontal scanning lines is indicated by H.

Figure: Horizontal line & sync Details

✓ out of a total line period of 64 μs, the line blanking period is 12 μs. During this
interval a line synchronizing pulse is inserted. The pulses corresponding to the
differentiated leading edges of the sync pulses are actually used to synchronize
the horizontal scanning oscillator.
✓ The line blanking period is divided into three sections. These are the ‘front
porch’ (1.5 μs), the ‘line sync’ pulse (4.7 μs) and the ‘back porch’ (5.8 μs).

Front porch:

✓ This is a brief cushioning period of 1.5 μs inserted between the end of the
picture detail for that line and the leading edge of the line sync pulse.

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 TV Fundamentals

✓ This interval allows the receiver video circuit to settle down from whatever
picture voltage level exists at the end of the picture line to the blanking level
before the sync pulse occurs.

“Despite the existence of the front porch when the line ends in an extreme white detail, and the
signal amplitude touches almost zero level, the video voltage level fails to decay to the blanking
level before the leading-edge of the line sync pulse occurs. This results in late triggering of the
time base circuit thus upsetting the ‘horz’ line sync circuit. As a result the spot (beam) is late in
arriving at the left of the screen and picture information on the next line is displaced to the left.
This effect is known as ‘pulling-on-whites’.”(given as viva point of view)

Line sync pulse:

✓ After the front porch of blanking, horizontal retrace is produced when the sync
pulse starts. The flyback is definitely blanked out because the sync level is
blacker than black.
✓ Line sync pulses are separated at the receiver and utilized to keep the receiver
line time base in precise synchronism with the transmitter. The nominal time
duration for the line sync pulses is 4.7 μs.
✓ During this period the beam on the raster almost completes its back stroke
(retrace) and arrives at the extreme left end of the raster for scanning next
line.

Back porch:

✓ This period of 5.8 μs at the blanking level allows plenty of time for line flyback
to be completed. It also permits time for the horizontal time-base circuit to
reverse direction of current for the initiation of the scanning of next line.
✓ The back porch also provides the necessary amplitude equal to the blanking
level (reference level) and enables to preserve the dc content of the picture
information at the transmitter.

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 TV Fundamentals

✓ At the receiver this level which is independent of the picture details is utilized
in the AGC (automatic gain control) circuits to develop true AGC voltage
proportional to the signal strength picked up at the antenna.
✓ It also contains colour burst signal for colour picture reproduction.

Vertical sync details:

✓ The basic vertical sync added at the end of both even and odd fields is shown in
Fig. Its width has to be kept much larger than the horizontal sync pulse, in
order to drive a suitable field sync pulse at the receiver to trigger the field
sweep oscillator.
✓ The standards specify that the vertical sync period should be 2.5 to 3 times the
horizontal line period. If the width is less than this, it becomes difficult to
distinguish between horizontal and vertical pulses at the receiver.
✓ In the 625 line system 2.5 line period (2.5 × 64 = 160 μs) has been allotted for the
vertical sync pulses.

Figure: Vertical sync details

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 TV Fundamentals

✓ a vertical sync pulse commences at the end of 1st half of 313th line (end of first
field) and terminates at the end of 315th line. Similarly after an exact interval
of 20 ms (one field period) the next sync pulse occupies line numbers— 1st,
2nd and 1st half of third, just after the second field is over.
✓ This alignment of vertical sync pulses, one at the end of a half-line period and
the other after a full line period (see Fig.), results in a relative misalignment of
the horizontal sync pulses and they do not appear one above the other but
occur at half-line intervals with respect to each other.
✓ looking further along the waveform, we see that the leading edge of the
vertical sync pulse comes at the wrong time to provide synchronization for the
horizontal oscillator.
✓ Therefore, it becomes necessary to cut slots in the vertical sync pulse at
half-line-intervals to provide horizontal sync pulses at the correct instances
both after even and odd fields.
✓ The technique is to take the video signal amplitude back to the blanking level
4.7 μs before the line pulses are needed. The waveform is then returned back to
the maximum level at the moment the line sweep circuit needs
synchronization.
✓ Thus five narrow slots of 4.7 μs width get formed in each vertical sync pulse at
intervals of 32 μs. The trailing but rising edges of these pulses are actually
used to trigger the horizontal oscillator.
✓ The resulting waveforms together with line numbers and the differentiated
output of both the field trains are illustrated in Fig. below. This insertion of
short pulses is known as notching or serration of the broad field pulses.

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Figure: Serration

✓ The vertical oscillator trigger potential level marked as trigger level in the
diagram intersects the two filter output profiles at different points which
indicates that in the case of second field the oscillator will get triggered a
fraction of a second too soon as compared to the first field.
✓ Note that this inequality in potential levels for the two fields continues during
the period of discharge of the capacitor once the vertical sync pulses are over
and the horizontal sync pulses take-over.
✓ Though the actual time difference is quite short it does prove sufficient to
upset the desired interlacing sequence.

Figure: Half line Discrepancy

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 TV Fundamentals

Equalizing pulses:

✓ To take care of this drawback which occurs on account of the half line
discrepancy five narrow pulses are added on either side of the vertical sync
pulses. These are known as pre-equalizing and post-equalizing pulses.
✓ Each set consists of five narrow pulses occupying 2.5 lines period on either
side of the vertical sync pulses. Pre-equalizing and post equalizing pulse
details with line numbers occupied by them in each field are given in Fig.

Figure: Pre & Post equalizing pulses.

✓ The effect of these pulses is to shift the half-line discrepancy away both from
the beginning and end of vertical sync pulses.
✓ Pre-equalizing pulses being of 2.3 μs duration result in the discharge of the
capacitor to essentially zero voltage in both the fields, despite the half-line
discrepancy before the voltage buildup starts with the arrival of vertical sync
pulses.
✓ Post-equalizing pulses are necessary for a fast discharge of the capacitor to
ensure triggering of the vertical oscillator at proper time. If the decay of
voltage across the capacitor is slow as would happen in the absence of
post-equalizing pulses, the oscillator may trigger at the trailing edge which

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 TV Fundamentals

may be far-away from the leading edge and this could lead to an error in
triggering.

Colour burst signal:

✓ The transmitted signal does not contain the subcarrier frequency but it is
necessary to generate it in the receiver with correct frequency and phase
relationship for proper detection of the colour sidebands. To ensure this, a
short sample of the subcarrier oscillator, (8 to 11 cycles) called the “colour
burst” is sent to the receiver along with sync signals. Subcarrier frequency is
4.43MHz.

Figure: Colour burst signal

✓ The colour burst is gated out at the receiver and is used in conjunction with a
phase comparator circuit to lock the local subcarrier oscillator frequency and
phase with that at the transmitter.
✓ As the burst signal must maintain a constant phase relationship with the
scanning signals to ensure proper frequency interleaving, the horizontal and
vertical sync pulses are also derived from the subcarrier through frequency
divider circuits.

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Figure: complete Line details

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3.4 CCIR B standards for Colour signal transmission & reception:

Reception

Camera output R, G, and B video signals

Luminance signals Y=0.30R+0.59G +0.11B

Colour difference signals chosen for

(B-Y) and(R-Y)
transmission

Suppressed carrier amplitude modulation

Type of colour signal modulation Of two subcarriers in quadrature having
same numerical value.

Colour difference signals U=0.493(B-Y) V=0.877(R-Y)

Composite colour signal Y+U sin ωm t+-Vcos ωmt

Amplitude of modulated Chroma

u2+v2
signal

Colour subcarrier frequency 4.433185 MHz

Duration of burst 10+1

Chroma encoding Phase and amplitude modulation

Bandwidth for colour signals (u and v) Fsc-1.3 MHz to fsc+0.6 MHz

Transmission

No. of lines per picture (frame) 625

Field frequency (Fields/second) 50

Interlace ratio, i.e., No. of

2/1
fields/picture

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Picture (frame) frequency, i.e.,

25
Pictures/second

Line frequency and tolerance in

lines/second,(when operated
15625 ± 0.1%
non-synchronously)

Aspect Ratio (width/height) 4/3

(i) Line: Left to right

Scanning sequence
(ii) Field: Top to bottom

System capable of operating

independently of power supply YES
frequency

Approximate gamma of picture signal 0.5

Nominal video bandwidth, i.e.,

highest video modulating frequency 5
(MHz)

Nominal Radio frequency bandwidth,

7
i.e., channel bandwidth (MHz)

Sound carrier relative to vision carrier

+5.5
(MHz)

Sound carrier relative to nearest edge

– 0.25
of channel (MHz)

Nearest edge of channel relative to

–1.25
picture carrier (MHz)

Fully radiated sideband Upper

Nominal width of main sideband

5
(upper) (MHz)

Width of end-slope of full (Main)

0.5
sideband (MHz)

Nominal width of vestigial sideband 0.75 MHz

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Vestigial (attenuated) sideband Lower

Peak white level as a percentage of

10 to 12.5
peak carrier

FM, ± 50 KHz
Type of sound modulation

Pre-emphasis 50 μs

Resolution 400 max

Why AM is preferred for video (picture) transmission & FM is

preferred for Audio signal transmission?

AM is preferred for picture because the following reasons,

✓ The distortion which arises due to interference between multiple signals is

more objectionable in FM than AM because the frequency of the FM signal
continuously changes.
✓ Hence, hardly any steady picture is produced.
✓ Alternatively if AM were used, the multiple signal paths can at most produce a
ghost image which is steady.
✓ In addition to this, circuit complexity and bandwidth requirements are much
less in AM than FM.

FM is preferred for sound because the following reasons,

✓ The bandwidth assigned to the FM sound signal is about 200 kHz of which not
more than 100 kHz is occupied by sidebands of significant amplitude.
✓ The latter figure is only 1.4 per cent of the total channel bandwidth of 7 MHz.
Thus, without encroaching much, in a relative sense, on the available band
space for television transmission all the advantages of FM can be availed.

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Positive and Negative modulation:

Positive Modulation Negative Modulation

When increase in brightness of that When increase in brightness

picture results in an increase of the reduces amplitude of the
1. 1.
amplitude of modulated envelope.it modulated envelope, it is called
is called positive modulation. negative modulation.
White level of video signal White level of video signal
2. corresponds to 100% total 2. correspondence to 12.5% of the
magnitude. total amplitude.

Noise pulses do not affect

Noise pulses are seen as less
4. synchronization but cause white 4.
annoying black spot.
spot in the picture
If peak power available from
More power is required with less transmitter is considered them less
5. 5.
efficiency power is required for more
efficiency.
Black level of video signal
6. correspondence to 25% of total 6. Blanking level starts at 75%
magnitude.

7. 7.

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Effect of Noise Interference on Picture Signal:

✓ In negative system of modulation, noise pulse extends in black direction of

the signal when they occur during the active scanning intervals. They extend in
the direction of sync pulses when they occur during blanking intervals.
✓ In the positive system, the noise extends in the direction of the white during
active scanning, i.e., in the opposite direction from the sync pulse during
blanking.

Figure: Effect of noise in modulation techniques

Effect of Noise Interference on Synchronization:

✓ Sync pulses with positive modulation being at a lesser level of the modulated
carrier envelope are not much affected by noise pulses.
✓ However, in the case of negatively modulated signal, it is sync pulses which
exist at maximum carrier amplitude, and the effect of interference is both to
mutilate(serious damage) some of these, and to produce lot of spurious
random pulses.

Peak Power Available from the Transmitter:

✓ With positive modulation, signal corresponding to white has maximum

carrier amplitude. The RF modulator cannot be driven harder to extract more

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 TV Fundamentals

power because the non-linear distortion thus introduced, that would affect the
amplitude scale of the picture signal and introduce brightness distortion in
very bright areas of the picture.
✓ In negative modulation, the transmitter may be over-modulated during the
sync pulses without adverse effects, since the non-linear distortion thereby
introduced, does not very much affect the shape of sync pulses. Consequently,
the negative polarity of modulation permits a large increase in peak power
output and for a given setup in the final transmitter stage the output increases
by about 40%.

Use of AGC (Automatic Gain Control) Circuits in the Receiver:

✓ In negative system of modulation, stable reference level is the peak of sync

pulses which remains fixed at 100 per cent of signal amplitude and is not
affected by picture details. This level may be selected simply by passing the
composite video signal through a peak detector.
✓ In the positive system of modulation the corresponding stable level is zero
amplitude at the carrier and obviously zero is no reference, and it has no
relation to the signal strength.

Merits of Negative Modulation:

✓ Lesser noise interference on picture signal.

✓ Possible to obtain larger peak power output.

✓ Less picture signal distortion.

✓ Easy to develop true AGC voltage.

✓ More efficient operation.

✓ More power available from the transmitter

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 TV Fundamentals

✓ Saving in transmission power

Demerits of Negative Modulation:

✓ The synchronization of the receiver is affected by spurious random pulses

produced due to the effect of noise.
✓ The loss of horizontal and vertical synchronization may cause diagonal or
vertical rolling of picture.

3.5 Block diagram of Colour TV transmitter:

Figure: Block Diagram of Colour TV transmitter

A PAL colour TV transmitter consists of following three main sections.

1. Production of Luminance (Y) and Chrominance (U and V) signals

2. PAL encoder
3. Video and Audio modulators and transmitting antenna

Production of Luminance (Y) and Chrominance (U and V) signals:

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 TV Fundamentals

✓ Colour camera tube produces R, G and B voltages pertaining to the intensity of

red, green and blue colours respectively in pixels. The luminance signal Y is
obtained by a resistive matrix, using grassman's law. Y=0.3R+0.59G+0.11B.
✓ For colour section Y is inverted colours R&B obtained from the colour camera
tubes are added to it to get (R-Y) and (B-Y) colour difference signal. These
signals are weighted by two resistive matrix network which gives U & V signals
as U=0.493 (B-Y) & V=0.877(R-Y)

PAL encoder:

✓ PAL switch which operates electronically at 7812.5Hz with the help of bistable
multivibrator and feeds the sub-carrier to balanced modulator with phase
difference of +90 degree on one line and -90 degree on the next line.
✓ The PAL encoder consists of a subcarrier generator and two balanced
modulator with filters to produce modulated subcarrier signal. These signals
are added vertically to give Chroma signal (C). Then Chroma signal is mixed
with Y signal along with sync. And blanking pulses to produce Colour
Composite Video Signal (CCVS).

Video and Audio modulators and transmitting antenna:

✓ CCVS amplitude modulates the main video carrier. It is followed by a sharp

VSB filter to attenuate the LSB to give AMVSB signal for transmitter. Audio
signal modulates separate carrier. This modulation is FM type.
✓ AMVSB video signal along with audio signal passes to the transmitting
antenna through Diplexer Bridge which is a wheatstone's bridge.

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TV channel allocation for band I & band III:

Picture carrier
Ch Sound carrier
Band Frequency range Frequency
No. Frequency (MHz)
(MHz)
1 41–47 (not used)

BAND I 2 47–54 48.25 53.75

(41-68
MHz) 3 54–61 55.25 60.75

4 61–68 62.25 67.75

5 174–181 175.25 180.75

6 181–188 182.25 187.75

7 188–195 189.25 194.75

BAND
8 195–202 196.25 201.75
III
(174-230
9 202–209 203.25 208.75
MHz)
10 209–216 210.25 215.75

11 216–223 217.25 222.75

12 223–230 224.25 229.75

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Notes By: Tejas Shah