Colout TV Fundamentals

 Electrical output of camera tube is not linear function of
light input.
 Light output of picture tube is also not a linear function of
electrical input.
 Exponent of transfer function called Gamma.
Gamma Correction

 γ = 1
 System is linear.
 Overall γ = 1.2 gives slightly more
pleasing picture.
 B/W PT - γ = 2
 Colour PT - γ = 2.2
 Hence gamma of transmission
chain should be 0.5.
Gamma correction

 Delta Gun Colour Picture Tube
 Gun-in-Line or Precision-in-line Colour Picture Tube
 Single gun or Trintron Colour Picture Tube
Colour Picture Tube

 Radio Corporation of America RCA
 3 separate guns for 3 colours.
 Guns equally spaced at 120 ,tilted towards axis.
 Screen coated with 3 different phosphors for R, G, B.
 Triad contains 1 dot of each colour.
 3,33,000 triads.
 Eye integrates 3 colour information to give a sensation of
combined hue.
 Shadow mask.
 One hole per triad.
 All stray electrons (80%) get collected by mask.
 Hence need higher EHT.
Delta Gun Colour Picture Tube

Gun viewed from base
120º120º
120º
Electron
Gun
Tube
Neck

Electron Guns with mask and screen

 Convergence is difficult.
 Very elaborate arrangements to overcome it.
 Focus not sharp over entire screen.
 As focus and convergence planes for three guns
separated by 120º are not same.
 Mask passes only 20% of electrons.
Disadvantages

Precision-in-line Colour Picture Tube

 3 Guns in line simplifies convergence adjustment.
 Colour phosphors in form of vertical stripes.
 Distance same as in Delta gun.
 More efficient.
 Larger % of electrons pass through mask.
 Fewer convergence adjustments.
 Most widely used.
Precision-in-line Colour Picture Tube

 By Sony Corporation, Japan
 Single gun having three cathodes.
 Simplified construction as only one electron assembly.
 Three phosphor triads arranged in vertical strips.
 Each strip is few thousandth of a cm.
 A metal aperture grill has one slot each triad.
 Grill has greater electron transparency.
 Three beams appear to emerge from same point.
TRINTRON Colour Picture Tube

 Each beam must fall at center of corresponding dot.
 Irrespective of position of triad on screen.
 Circular magnets called purity magnets on neck of yoke
does the alignment.
 If all tabs moved together, they change direction of field.
 If tabs are separated, magnetic field reduces.
Purity and Convergence

 2, 4, 6 pole magnets to achieve collective or individual beam
deflection.
 Error may also occur if yoke is not positioned properly.
Purity

 Technique that all electrons hit the same part of the screen
to produce 3 coincidental rasters.
 Errors–
 Non-coincidental convergence plane.
 Non-uniformity of deflection field.
 Flat surface of picture tube screen.
Convergence

 Correction:
 Static and dynamic convergence.
 Static convergence by permanent magnet.
 Once correctly set, brings the beam into convergence in the
central area of the screen
 Dynamic convergence converges beam over rest of the
screen.
 Dynamic convergence achieved by additional winding in
series with yoke coil.
 Achieved by continuously varying magnetic field.
 Instantaneous strength depends upon position of the spot
on the screen.
Convergence

• Static pincushion may cause purity problem.
• Dynamic pincushion correction.
• This stretches the horizontal width and vertical size of
raster at edges.
Pincushion correction

• Due to Earth magnetic field and other nearby magnetic
fields, mask and mounting frame may get magnetized.
• Will cause purity error causing in colour patches.
• Can be repaired by influencing the screen by an alternating
magnetic field which gradually reduces to zero.
Degaussing

• At switch ON, thermistor is cold and has high resistance.
• More AC passes through coil and varistor, causing alternating
magnetic field.
• Thermistor gradually heats up, reducing its resistance.
• More current flows through thermistor.
 Lesser current through coil and varistor.
 Heats thermistor further
• This continues till
 current through thermistor becomes maximum.
 Current through varistor and coil gradually reduces to zero.
• Components chosen to give initial surge of 4ampere to D.Coil.
• Final current less than 25mA in less than a second.
Automatic degaussing

• R G B must combine to give to give white.
• 3 phosphors have different efficiencies.
• 3 guns may not have identical emission and cutoff point
(Ip/Vgk).
• Tint appears instead of pure grey shades for monochrome
transmission.
• Suitable adjustment required to produce correct monochrome
information with no colour tint for all settings of contrast
control.
Grey Scale Tracking

• Appearance of tint instead of pure grey shades in areas of
low brightness.
• Necessary to bring cutoff points in coincidence.
• Achieved by making screen grid voltage (1st anode) different
for each cathode.
• Potentiometers are normally provided in DC voltage supply
to 3 screen grids.
Grey Scale Tracking
Adjustment on low light

• All levels of light must be correctly reproduced.
• Need to Compensate for slightly different slopes and also for
substantially different phosphor efficiencies.
• Achieved by the video signal Y drive to the 3 guns.
• Red has lower efficiency, maximum video signal Y fed to red
cathode.
• By potentiometer B and G inputs are varied to produce
optimum reproduction of high lights.
Grey Scale Tracking
Adjustment on high light

 3 Systems---
 American NTSC(National Television System Committee)
 German PAL( Phase Alteration by Line)
 French SECAM(Sequential couleures a memoire)
 All systems are good.
 India adopted PAL for 625 line CCIR-B standard for B/W.
 NTSC and PAL being similar are dealt together.
Colour signal
transmission and reception

 Should accommodate hue and saturation in same band of
7MHz.
 Colour signal should not disturb B/W information and vice versa.
 Done by Frequency interleaving.
Colour signal transmission

 Video signal band bears a definite relation with scanning
frequencies.
 Energy content of video signal is contained in individual energy
“bundles”.
 Bundles occur at harmonics of line frequency.15625Hz
 Components of each bundle separated by multiple of field
frequency. 50, 100, 150…
 Each bundle has peak at exact line harmonic gradually reducing
in amplitude on either side.
 Amplitudes of bundles reduce towards higher harmonics of line
frequency.
Frequency interleaving

Energy bundle
Amplitude
fH = 15625Hz

 Vertical sidebands contain lesser energy than Horizontal
because of lower rate of scanning frequencies.
 Overall Energy contents decreases to very small value beyond
3.5MHz from picture carrier.
 Part of B/W BW unused at spacing between the bundles.
 Used for colour energy bundles.
Energy bundle

 Colour subcarrier so chosen that side band energy
frequencies exactly fall between harmonics of line
frequencies.
 Hence colour subcarrier chosen to be odd multiple of half
of line frequency.
 To avoid any cross talk between B/W and colour info,
higher portion of B/W BW is chosen to place coloue BW.
 Hence Colour subcarrier is 567 times half line frequency.
 PAL = 567x 15625/2 = 4.43MHz
 NTSC = 455x 15750/2 = 3.58MHz
Colour Energy bundle

Interleaving of Energy bundles

 Study show that eye can perceive colour in object areas of 1/25th
of screen width or more.
 For smaller areas, Eye can only perceive its brightness.
 Between 0 – 0.5MHz, all colours can be seen.
 0.5 – 1.5MHz, only two primary colours seen. G, B.
 Eye can not distinguish purple and green-yellow hue.
 For smaller objects, colour not visible, only shades of grey.
 Hence colour B/W = ±1.5MHz = 3MHz (DSB-SC)
Band width of Colour Signal

 (B-Y), (R-Y).
 Carrier = 4.43MHz.
 Second carrier by giving 90º phase shift to carrier.
 Called Quadrature modulation.
 Resultant subcarrier phaser called chrominance signal C.
 Instantaneous value of C represents colour saturation at that
time.
 Phaser of C varies from 0º to 360º , represents hue.
Modulation of Colour Signal

 (R-Y) = R - 0.59G – 0.3R – 0.11B
= 0.7R – 0.59G – 0.11B
 (B-Y) = B - 0.59G – 0.3R – 0.11B = 0.89B – 0.59G – 0.3R
Modulation of COLOUR Signal

 Pure red –
 R = 1v
 B = G = 0v
 (R-Y) = ?
 (B-Y) = ?
(B-Y)t
-(R-Y)t
(R-Y)t
-(B-Y)t

 Pure red –
 R = 1v
 B = G = 0v
 (R-Y) = 0.7R
 (B-Y) = -0.3R
0.7
-0.3
0.76
(B-Y)t
-(R-Y)t
(R-Y)t
-(B-Y)t

 Pure red –
 R = 1v
 B = G = 0v
 (R-Y) = 0.7R
 (B-Y) = -0.3R
 C = √[(R-Y)2 + (B-Y)2]
 θ = tan-1 [(R-Y)/(B-Y)]
C(t)
θ(t)

 Similarly can be found for pure Blue, Green, Magenta, Cyan,
Yellow

 (R-Y) and (B-Y) modulate colour subcarrier using DSB-SC.
 Avoids interference due to colour subcarrier.
 Avoids power wastage.
 In B/W transmission, (R-Y), (B-Y) are zero. No colour carrier.
Hence no chrominance signal.
 But we need to generate Colour subcarrier at receiver for
detection of DSB-SC.
 It should have same frequency and phase.
 Hence 8-11 cycles of subcarrier sent as pilot for synchronization.
 Rides the back porch of Blanking pulse.
Colour burst signal

 Does not interfere with sync
as small in amplitude and
occurs after the sync.
 Gated out and used to sync
colour subcarrier.
 H and V sync also derived
from it as they bears a
constant relation with it.
Colour burst signal

 (R-Y) and (B-Y) modulate 4.43MHz colour subcarrier and
4.43MHZ with 90º phase shift.
 Gives C, θ(t)
 C when added to Y, rides over Y.
Chrominance Signal Weighing Factor

 Over-modulation must be avoided.
 Over modulation reduced by reducing (R-Y) and B-Y) before
modulating colour carrier.
 (R-Y) and B-Y) scaled down.
 (R-Y)’ = 0.877(R-Y)
 (B-Y)’ = 0.493(B-Y)
 And then modulate the carrier.
Weighing Factor

 New values still over modulates.
 In real life, such pure colours never occurs.
 Hence amplitude levels never reach shown peak values.
 No over modulation in real signals.
 At receiver, (R-Y) and (B-Y) need to be scaled up to bring
them to original value.
 Needs controlled amplification.
 Due to weighing, there is change in amplitude and phase of
various colours.
Weighing Factor

 Colour B/W reduced to 2MHz.
 Eye’s resolution of colours along reddish blue –
yellowish green axis on colour circle is much less than
colours which lie around yellowish red – greenish blue
axis.
 Two new signals Q and I chosen to represent these areas.
NTSC SYSTEM

 33º counterclockwise to (R-Y).
 Maximum colour resolution.
 (R-Y) = 0.74.
 (B-Y) = -0.27
 I = 0.74(R-Y) – 0.27(B-Y)
 I = 0.60R – 0.28G – 0.32B
 Covers orange hue and blue-green (Cyan) hue.
 Eye is more sensitive to colours around it.
 More bandwidth is allowed for I.
I Signal

 33º counterclockwise to (B-Y).
 (R-Y) = 0.48.
 (B-Y) = 0.41
 Q = ?

Q Signal

 33º counterclockwise to (B-Y).
 (R-Y) = 0.48.
 (B-Y) = 0.41
 Q = 0.21R - 0.52G + 0.31B
 Covers reddish blue (Magenta) and yellow-green hue.
 Eye is less sensitive to colours around it.
 Less bandwidth is allowed for Q.
Q Signal

 0 to 0.5MHz – Both I and Q are active.
 0.5 to 1.5MHz – Q drops out. Only I remains.
 DSC-SC allowed for Q.
 BW = 1MHZ
 VSC allowed for I.
 Upper side band = 0.5MHz
 Lower side band = 1.5MHz
 BW = 2MHZ
 Total BW = 2MHz for colour, 6MHz for NTSC system.
Bandwidth of I and Q Signal

 500ns delay to y signal.
 Chrominance signal during active trace time.
 Burst signal during retrace time.
 Colour killer ckt kills video amplifier during retrace to
prevent burst affecting the picture.
 Done by application of blanking pulses to turn the bias of
amplifier off.
 Final colour amplifiers to compensate for weighing factor.
 R-Y boosted by 1.4
 G-Y boosted by 2.03
 B-Y attenuated by 0.7
NTSC Receiver

 Sensitive to transmission path difference which
introduces phase errors that result in colour changes in
picture.
 Phase changes can also take place when changeover
between local and TV network system take place.
 Chroma phase angle is also affected by the level of the
signal while passing through various circuits.
 Cross talk between demodulator outputs at the
receiver can cause distortions.
 All above require automatic tint control with provision
of manual control.
Limitations of NTSC Receiver

 At Telefunken laboratory in the Federal Republic of
Germany.
 Phase error susceptibility of NTSC system has been largely
eliminated.
PAL Colour system

 Weighted (R-Y) and (B-Y) are used.
 (R-Y) and (B-Y) both are given same B/W of 1.3 MHz.
 Gives better colour reproduction.
 Vestigial Sideband modulation used.
 USB – attenuation slope starts at 0.57MHz.
 (5-4.43 = 0.57MHz)
 LSB – Extends to 1.3MHz before attenuation begins.
 Colour subcarrier = 4.43361875MHz.
 Gives better cancellation of dot pattern interference.
Features of PAL Colour system

 (R-Y) and (B-Y) modulated with subcarrier using QAM as in
NTSC but with a difference.
 Phase of one carrier is 0º.
 Phase of other carrier shifts between +90 º to -90 º at alternate
lines .
 Hence the name Phase Alteration by Line(PAL).
 This cancels hue errors resulting from unequal phase shifts in
the transmitted signal.
 Shifting of phase occurs during line blanking interval to avoid
visible disturbance.

 Weighed (B-Y) and (R-Y) are-
 U = 0.493(B-Y)
 V= 0.877(R-Y)
 CPAL = USinωst ± Vcosωst
 = √(U2 + V2) Sin(ωst ± θ)
 Where θ = V/U
+

 PAL receiver requires three types of carrier.
 Subcarrier at 0º phase to detect U signal
 Subcarrier at +90º phase at every second line for V.
 Subcarrier at -90º phase at every second line for V.
 V demodulator must be switched at half horizontal line
frequency rate.
 Hence PAL Colour burst has two components.
1. -(B-Y) (same as NTSC), but of amplitude 1/√2 of NTSC burst.
2. (R-Y) component which is reversed in phase from line to line,
amplitude same as –(B-Y)
PAL Burst

 Resultant burst swings ± 45º about –(B-Y) axis from line to line.
 Sign of (R-Y) burst always same as that of (R-Y).
 Hence called swinging burst.
PAL Burst

 Multiple path transmission results in phase errors.
 Called differential phase error.
 Changes hue in reproduced picture.
 PAL has built-in protection against phase errors.
 Provided picture content almost remains same from line to line.
Cancellation of phase errors

 Line N and N+1 response shown.
 U and V are detected on two separate detectors.
 Resultant R has same amplitude on both line.
No Error
Line N
NTSC Line
Line N+1
PAL Line

 Suppose R suffers phase error of angle δ .
 Resultant phasor will swing between R1 and R2.
 Phase error cancels if two lines are displayed simultaneously.
With phase Error

 Practically, two lines are displayed in sequence.
 Colours reproduced by two successive lines will be slightly on
either side of actual hue.
 Since lines are scanned at very fast rate, eye perceives a colour
that lie between two colours reproduced by R1 and R2.
 Hence colour viewed will be more or less the desired hue.
 PAL superior to NTSC in this front.
Cancellation of phase errors

 Small errors not visible to eye.
 Beyond a limit, eye starts to see the error in alternate lines.
 Remedy – Delay line employed to do the averaging of alternate
line colour information first then present the colour to viewer.
 Hence called Delay line PAL or PAL-D
PAL - D

 PAL-D removes phase errors and Differential phase error
problem .
 Manual hue control not needed..
 BUT..
 Delay line technique of reception results in reduction in
vertical resolution of chrominance signal.
 More complicated and expensive due to delay lines, adding,
subtracting…
 Receiver cost higher.
Merits and Demerits of PAL

 Earlier – 819 lines system with B/W of 14MHz.
 NOW –
 625 lines per frame.
 25 frames per second.
 50 fields per second
 Line frequency – 15625HZ
 Field frequency – 50Hz
 Video B/W – 6MHz
 Channel B/W – 8MHz
 Picture – FM
SECAM System

 Sound carrier 5.5MHz away from Picture carrier.
 Colour Subcarrier ---4.4375MHz away from Picture carrier.
 Y signal same as NTSC/PAL.
 Weighing factor –
 IDRI = -1.9(R-Y)
 IDBI = 1.5(B-Y)
SECAM System

 French “Sequential a Memoire”.
 Only one of two colour difference signal transmitted at a
time.
 (R-Y) transmitted in one line and (B-Y) in next line.
 Sequence repeated for full raster.
 Due to odd number of lines, (R-Y) and (B-Y) alternate each
picture.
 Subcarrier is frequency modulated by colour difference
signal before transmission.
 Magnitude of frequency deviation represents colour
saturation and rate of deviation its hue.
SECAM System

 Ultrasonic delay line of 64µs used as one line memory device.
 Produces decoded output of both colour difference signal
simultaneously.
 Modulated signals routed to their correct demodulators by an
electronic switch operating at line frequency.
 Switch driven by Bistable Multivibrator.
 Determination of proper sequence of colour lines in each field is
accomplished by Identification pulse .
 Ident generated and transmitted during vertical blanking
interval.
Principle of SECAM SYSTEM

 FM for colour.
 Phase distortions in transmission path will not change hue of
picture area as two carriers are at same phase..
 Limiter to remove amplitude variation.
 Subcarrier for (R-Y) = 282fH = 4.40625MHz
 Subcarrier for (B-Y) = 272fH = 4.250MHZ
 Suppresses dot pattern interference on B/W TV.
Modulation of Subcarrier

 Colour difference signals band limited to 1.5MHZ and pre-
emphasised.
 Linear deviation for (R-Y) = 280DR KHz
 MAX DEVIATION: 500KHZ and 350KHZ as below.
 +(R-Y) = -500KHz
 -(R-Y) = +350KHz
 Linear deviation for (B-Y) = 230DB KHz
 MAX DEVIATION: 500KHZ and 350KHZ as below.
 +(B-Y) = +500KHz
 -(B-Y) = -350KHz
Modulation of Subcarrier

 It is necessary for receiver to determine which line is being
transmitted.
 Ident pulses are generated during Vertical blanking pulses.
 Ident signal is carrier duly modulated by a saw-tooth signal.
 Sawtooth signal is positive going for DR and negative going
for DB.
 At receiver, generate positive and negative control signal
after detection.
 Regulates instant and sequence of switching.
Ident Signal Generator

 Y created as in PAL.
 DR and DB fed to switch for switching per line.
 Sync pulse generator generates sync.
 Added to Y.
 Controls signal and subcarrier switching through switching
control.
 Also controls Ident generator to be added to DR and DB during
blanking.
 DR and DB fed to FM modulator after 1.5MHZ LPF and pre-
emphasis.
 HF pre-emphasis increases amplitude of subcarrier as its
deviation increases, to further improve S/N.
SECAM CODER

SECAM DE-CODER
(R-Y)
(B-Y)
VR, VG, VB
(R-Y)
(B-Y)
VR, VG, VB
VR, VG, VB
(R-Y)
(B-Y)
VR, VG, VB
VR, VG, VB

 By Japan Broadcast Company NHK.
 Offers wide screen format.
 Does not show scan lines even from close.
 Has twice vertical and horizontal resolution.
 Improves colour rendition.
 Stereophonic sound.
 1125/60 HDTV comparable to 35mm theatre film.
 22GHz band for HDTV direct broadcast to TV.
HDTV

 Three parts:
 Generating equipment Camera, VCR
 Transmitters and links – microware, satellite.
 Large screen TV receiver, projection TV
 Highly sensitive equipments developed for each of above.
HDTV

 Aspect ratio 19:9.
 Gives channel B/W = 26.7MHz.
 60Hz field preferred to reduce flicker.
HDTV

Colout TV Fundamentals

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Viewers also liked

Viewers also liked (15)

Similar to Colout TV Fundamentals

Similar to Colout TV Fundamentals (20)

More from Madhumita Tamhane

More from Madhumita Tamhane (20)

Recently uploaded

Recently uploaded (20)

Colout TV Fundamentals