CHAPTER 26 COLOUR SIGNAL TRANSMISSION AND RECEPTIONCOLOUR TELEVISION SYSTEMS1. The American NTSC (National Television Systems Committee) system.2. The German PAL (Phase Alteration by line )Systems3. The French SECAM (Sequential Couleures memoire) system.
ENERGY CONTENT DECREASES 3.5 Mhz from the picture carrier. ENERGY BUNDLES HARMONICS OF LINE50 HZ FREQUENCY (H) =64 MICROSEC)
COLOUR SIGNAL TRANSMISSION• Colour video signal contains two independent informations:-- HUE SATURATION
• FREQUENCY INTERLEAVING1. It is possible in TV transmission because of the relationship of the video signal to the scanning frequencies which are used to develop it.2. Energy content of the video signal is contained in individual energy “ bundles ” which occur at harmonics of the line frequency ( 15.625 , 31.250 …. KHz).3. The components of each bundle being separated by a multiplier of the field frequency (50,100,……Hz).4. The shape of each energy bundle shows a peak at the exact harmonics of the horizontal scanning frequency .5. The lower amplitude excursions that occur on either side of the peaks are spaced at 50 Hz intervals and represent harmonics of the vertical scanning rate6. The vertical sidebands contain less energy than the horizontal because of the lower rate of vertical scanning .7. Energy content progressively decreases with increase in the order of the harmonics and is very small beyond 3.5 Mhz from the picture carrier.
1. A part of the bandwidth in the monochrome television signal goes unused because of spacing between the bundles2. This available space could be occupied by another signal.3. Here the colour information is located by modulating the colour difference signals with a carrier frequency called “ Colour Subcarrier”.4. The Carrier frequency is so chosen that its sidebands frequencies fall exactly mid – way between the harmonics of the line frequency.5. The frequency of the sub carrier must be an odd multiple of half the line frequency.6. It is 567 times one half the line frequency in the PAL system.7. (2* 283+1) 15625/2=4.43MHz.
• BANDWIDTH OF COLOUR SIGNAL TRANSMISSION1) The Y signal is transmitted with full frequency bandwidth of 5 MHZ for maximum horizontal details in monochrome.2) For very small details the eye can perceive only the brightness but not the colour.3) Perception of the colours by the human eye which are produced by combinations of the 3 primary colours is limited to objects which have relatively large coloured areas ( = 1/25 th of the screen width or more )4) On scanning they generate video frequencies which do not exceed 0.5 MHz.5) For very fine colour details produced by frequencies from 1.5 MHz to 5 MHZ, all persons with normal vision can see only changes in brightness even for the coloured areas.6) Therefore maximum bandwidth necessary for colour signal transmission is around 3 MHz.(+ 1.5 MHZ.)
Y = 0.59 G + 0.3 R + 0.11 B(R-Y) = R – (0.59G + 0.3 R + 0.11 B) = 0.7 R – 0.59 G -0.11B(B-Y) = B- ( 0.59 G + 0.3 R + 0.11 B) = 0.89 B -0.59 G -0.3 R* The amplitude of C , the chrominance signal corresponds to the magnitudes of colour difference signals , its instantaneous value represents colour saturation at that instant.• Maximum amplitude corresponds to greatest saturation• Zero amplitude to no saturation i.e. white.• Instantaneous value of the C phasor angle Ɵ which may vary from 0 degree to 360 degree represents hue of the colour at that moment.
• COLOUR BURST SIGNAL• Suppressed carrier double sideband technique is used in modulating colour difference signals with the colour subcarrier frequency.• This is done by Balanced Modulators.• The carrier is suppressed to minimize interference produced by the chrominance signals on both the monochrome receivers when they are receiving colour transmissions and in the luminance channel of colour receivers themselves.
• No reduction is made in the amplitude of Y signal.• As the transmitter radiates weighted chrominance signals values, these must be increased to the uncompensated values at the colour TV receiver for proper reproduction of different hues.• This is done by adjusting gains of the colour difference signal amplifiers.
• The compensation of the chroma signal values results in a change of chroma phase angles.• In the NTSC system phase angles are measured relative to the – (B-Y) phasor.• This location has been designated 0 degree or the reference phase position on the phasor diagram .• Because this is also the phase of the colour burst that is transmitted on the back porch of each horizontal sync pulse.• Primary colours are 120 degree apart and complementary colours differ in phase by 180 degree from their corresponding primary colours.
• NTSC COLOUR TV SYSTEM1. It is compatible with the American 525 line monochrome system.2. In order to save bandwidth :-3. Advantage is taken of the fact that4. Eye‟s Resolution of colours along the REDDISH BLUE –YELLOWISH GREEN AXIS is much less than those colours which lie around the YELLOWISH RED – GREENISH BLUE AXIS.5. Therefore two new colour video signals corresponding to these colour regions:- I and Q signals are generated
I = 0.74 (R-Y) – 0.27 (B-Y) Q= 0.48 (R-Y) + 0.41(B-Y) I SIGNAL1. I Signal lies in a region 33 counter clockwise to + (R-Y) where the eye has the maximum colour Resolution.2. I = 0.60R-0.28G-0.32B3. It is located at an angle of 57 with respect to the colour burst in the Balanced Modulator circuits. Q SIGNAL1. Q signal is derived from colour difference signals by suitable matrix .2. Q = 0.21R-0.52G+0.31B3. It is located 33 counter clockwise to the + (B-Y) signal and is thus in QUADRATURE with the I signal.
1. Q Signal covers the regions around Magenta (Reddish – Blue ) and Yellow –Green shades.2. Orange hues correspond to phase angles centered around + I .3. Complementary BLUE - GREEN (CYAN) hues are located around the diametrically opposite –I signal .
1. As the eye is capable of resolving fine details in these regions , I signal is allowed to possess freq. upto 1.5 MHZ.2. Eye is least sensitive to colours that lie around the + Q signals, and therefore it is allowed a bandwidth of only + 0.5MHz with respect to the colour subcarrier.3. Both I and Q signals are active upto 0.5 MHz and being at right angles to each other , combine to produce all the colours contained in the chrominance signal.4. The Q signal drops out after 0.5 MHz and only I signal remains between 0.5 and 1.5 MHz to produce colours , the finer details of which the eyes can easily perceive.5. Only one colour difference signal is needed for producing colours which are a mixture of only two colours.6. Thus the Q signal is not necessary for producing colour lying in the region of Orange ( red + green) and Cyan ( Green + blue ) hues.7. Hence at any instant when Q= 0 , and only I signal is active the colours produced on the screen will vary from the reddish orange to bluish green.
BANDWIDTH REDUCTION1. Q SIGNAL Double sideband transmission is allowed . Occupies a channel bandwidth of 1 MHz ( + 0.5 MHz)2. I SIGNAL Upper sideband is restricted to maximum of 0.5 MHz Lower sideband is restricted upto 1.5 Mhz.3. For colour signal transmission B.W. of 2 MHz is necessary.4. This is saving of 1 MHz as compared to a bandpass requirement of 3 M/HZ if (B-Y) and (R-Y) are directly transmitted.
EXACT COLOUR SUBCARRIER FREQUENCY IN THE NTSC SYSTEM1. 3.579545 MHz.2. To maintain compatibility between Monochrome and colour systems.3. Interference between chrominance signal and higher video freq. is minimized by using suppressed colour subcarrier transmission and by using a notch filter in the path of luminance signal.4. When a colour transmission is received on a monochrome receiver a dot pattern structure appears along each raster line on the receiver screen .5. This is caused by the colour signal freq. that lie in the pass band of the video section of the receiver.6. Such an interference can be eliminated if the subcarrier freq. is maintained at the exact value .
1. If the colour subcarrier freq. is a multiple of line freq. (n * Fh) the phase position of the disturbing colour freq. will be same on successive even or odd fields.2. Thus black and white spots will be produced at the same spots on the screen and will be seen as a persistent dot pattern interference .3. However if a half line offset is provided by fixing the subcarrier freq. to be an odd multiple of the half line freq. , the disturbing colour signal freq. will have opposite polarity on successive odd and even fields.4. Thus as the same spot on the display screen a bright dot image will foloow a dark one alternately.5. The cummulative effect of this on the eye would get averaged out and the dot pattern will be suppressed.
THE COLOUR SUBCARRIER FREQUENCYFsc = 3.583125= ( 2 * 227+1) * 15750/2MHz.1. The sound carrier and the colour subcarrier beat with each other in the detector and an objectionable beat note of 0.92 MHz is produced. ( 4.5 – 3.58)= 0.922. This interferes with reproduced picture.3. Hence sound carrier freq. must be an exact multiple of an even harmonic of the line freq. .4. Location of sound carrier cannot be disturbed so 4.5 MHz is made to be the 286 th harmonic of the horz. Deflection freq.5. Therefore Fh = 4.5 MHz /286 = 15734.26 Hz.This is close to 15750 Hz for horz. Scanning for monochrome transmission .6. Field freq. Fv is changed to be 15734.26/262.5=59.94Hz in place of 60 Hz.7. The slight difference of 15.74 Hz in line freq. and 0.06 Hz in field freq. has no effect on the deflection oscillators.
The colour subcarrier freq. must be an odd multiple of half the line frequency to suppress dot pattern interference.ThereforeFsc = (2n+1) Fh/2= 455 * 15734.26/2=3.579545 MHz.Cystal controlled oscillator is used to generate this freq.
LIMITATIONS OF NTSC SYSTEM1. It is sensitive to transmission path difference which introduce phase errors that results in colour changes in the picture.2. At the transmitter , phase changes in the chroma signal takes place when changeover between programmes of local and television network systems takes place and when video tape recorders are switched on.3. The chroma phase angle is also effected by the level of the signal while passing through various circuits .4. Crosstalk between demodulator outputs at the receiver causes colour distortion.5. Hence automatic tint control with provision of a manually operated tint control is used.
PAL COLOUR TV SYSTEM1. Developed at Telefunken Laboratories in the Federal Republic of Germany.2. Phase errors susceptibility of the NTSC system has been eliminated.
MAIN FEATURES OF PAL SYSTEM:---1. The weighted (B-Y) and (R-Y) signals are modulated without being given a phase shift of 33 degree as in NTSC system.2. On modulation both the colour difference quadrature signals are allowed the same bandwidth of about 1.3 MHz. This results in better colour reproduction.3. The chroma signal is of VSB type . The upper sideband attenuation slope starts at 0.57 MHz ie. 5- 4.43 =0.57 MHz. but the lower sideband extends to 1.3 MHz before attenuation begins.4. The colour sub carrier freq. chosen to be 4.43361875 MHz. It is an odd multiple of one quarter of the line freq. instead of half line offset as used in NTSC system. This results in better cancellation of dot pattern interference.
5. The weighted (B-Y) and (R-Y) signals are modulated with the sub carrier in the same way in NTSC system (QAM) but with the difference that phase of the sub carrier to one of the modulators (V) is reversed from + 90 degree to -90 degree at the line freq. Hence it is named as phase alteration by line from this mode of modulation.6. This method of modulation cancels hue errors which results from unequal phase shifts in the transmitted signals.
• IN PAL SYSTEM:- The (B-Y) and (R-Y) subcarrier components in the chrominance signal are scaled down by multiplying them with the “ Weighting ” Factors.1. The weighted signals are known as U = 0.493(B-Y) V = 0.877(R-Y) C = U Sin ωt + V Cos ωt PAL s s ------------------ = √ U² + V ² sin (ω t + Ɵ ) sWhere tan Ɵ = V/U The Switching action occurs during the line blanking interval to avoid any visible disturbance.
• THE PAL BURST:-1. If PAL signal is applied to an NTSC Type decoder , The (B-Y) output would be U as required (R-Y) output would alternate as +V and –V from line to line.2. The V demodulator must be switched at half the horz. Line freq. to give „+V‟ only on all successive lines.COLOUR BURST:-1. 10 cycles at 4.43 MHz. is sent out at start of each line.2. To synchronize receiver colour oscillator for reinsertion of the correct carrier into the U and V demodulators.
• PAL COLOUR BURST:---1. IN NTSC SYSTEM:- Burst has the phase of – (B-Y) and a Peak to Peak amplitude equal to that of the sync .2. IN PAL SYSTEM:-- Burst has two components 1. - (B-Y) component as in NTSC but with only 1/√2 of NTSC amplitude. 2. (R-Y) component which like all the (R-Y) information is reversed in phase from line to line.3. This + (R-Y) burst signal has an amplitude equal to that of the –(B-Y) burst signal , so that the resultant burst amplitude is the same as in NTSC.4. The burst phase actually swings + 45 degree about the – (B-Y) axis from line to line.5. The sign of (R-Y) burst component indicates the same sign as that of the (R-Y) picture signal.
6. Thus the necessary switching mode information is always possible.7. Since the colour burst shifts on alternate lines by + 45 degree about the zero reference phase it is often called the swinging burst.
• CANCELLATION OF PHASE ERRORS:-1. Chroma signal is susceptible to phase shift errors both at the transmitter and in the transmission path.2. This effect is called as DIFFERENTIAL PHASE ERROR and it results in changes of hue in the reproduced picture.3. This results from a phase shift of the colour sideband frequencies with respect to colour burst phase.4. The PAL system has built in protection against such errors provided the picture content remains almost the same from line to line .
1. Fig shows phasors representing particular U and V chroma amplitudes for two consecutive lines of a field.2. Since there is no phase errors the resultant phasor ® has the same amplitude on both the lines.3. Detection along the U axis in one synchronous detector and along the V axis in another , accompanied by sign switching in the latter case yields the required U and V colour signals.4. Thus the correct hues are produced in the picture.
Fig 26.13 b1. If during the transmission the phasor R suffers a phase shift by an angle ∂.2. The corresponding changes in the magnitude of U and V would mean a permanent hue error I the NTSC system.3. In PAL system , the resultant phasor at the demodulator will swing between R1 and R2 .4. Phase error would cancel out if the two lines are displayed at the same time .5. The lines are scanned in sequence and not simultaneously .6. The colours produced by two suuccessive lines , therefore will be slightly on either side of the actual hue.7. Since the lines are scanned at a very fast rate the eye due to persistance of vision will perceive a colour that lies between the two produced by R1 and R2 resp.8. Thus the colour seen would b e more or less be the actual colour .
• PAL D COLOUR SYSTEM:-1. Basic principle of PAL system:- Use of Eye as the averaging mechanism for the correct hue.2. Beyond a certain limit the eye does see the effect of colour changes on alternate lines , and system needs modification.3. Hence a delay line is introduced to do the averaging first and then present colour to the eye.4. This is known as PAL –D or Delay Line PAL method and is most commonly used in PAL colour Receivers.5. Fig shows a basic circuit for separating individual U and V products from the chrominance signal .6. For convenience both U and V have been assumed to be positive and they correspond to some shade of magenta (purple).
1. For the FIRST line When the V modulator product is at + 90 degree to the + U axis phasor can be expressed as ( U + jV) .2. This is called the NTSC Line.3. On NEXT line when V Phase is switched to – 90 degree , the phasor becomes (U – jV) and the corresponding line is called the PAL LINE.4. DELAY LINE To delay the chrominance signal by almost exactly one line period of 64 µs.5.
For the chosen HUE : -1. If the present incoming line is an NTSC line, the signal entering the delay line and also reaching the adder and subs tractor is (U+jV) .2. But then the previous line must have been the PAL Line i.e. (U-jV) and this signal is simultaneously available from the delay line.3. The result is that the signal information of two picture lines, though transmitted in sequence are presented to the adder and subs tractor circuits simultaneously.4. The adder yields a signal consisting of U information only but with twice the amplitude (2U).5. The Subtraction circuit produces a signal consisting only of V information , with an amplitude twice that of the „V‟ modulation product.
1. For precise addition and substraction of direct and delayed line signals , the delay line must introduce a delay which is equivalent to the duration of an exact no. of half cycles of the chrominance signal.2. This requirement would not be met if the line introduced a delay of exactly 64 µs.3. µs At a freq. of 4.43361875 MHz the no. of cycles which take place in 64 µs. are 4.43361875 * 10 ^6 * 64 * 10 ^-6 = 283.7485998 Hz.= 283.75 Hz.4. A Delay line which introduces a delay equal to the duration of 283.5 subcarrier cycles is therefore suitable .5. This is equal to a time delay of 63.943 µs ( 1 / f * 283.5) sc
CHOICE OF COLOUR SUBCARRIER FREQUENCY1. If the sub carrier frequency is chosen on the half line offset basis as is done in the NTSC system , an annoying vertical line up of dots occurs on certain hues.2. This is due to phase reversal of subcarrier at line frequency .3. This is overcomed by giving a quarter line offset instead and Fsc is made an odd multiple of one quarter of the line freq.4. For optimum results this is slightly modified by adding 25 Hz to it, to provide a phase reversal on each successive field.5. Thus the actual relationship between Fsc , Fh AND Fv is6. Fsc =Fh ( 2* 567 +1) + Fv = Fh (1135) + Fv = Fh (284-1/4) +Fv 4 2 4 2 2
Putting the values of Fh and Fv gives:-Fsc = 4.43361875 MHz.
• THE PAL CODER1. One switching cycle takes two lines, The square wave switching signal from the multivibrator to the electronic phase switch is of half line freq. 15625/2 = 7.8 KHz.2. Each burst gate is controlled by delayed pulses at f rate obtained H from the frequency dividing circuit.3. The gating pulses appear during BACK PORCH PERIOD.4. During this intervals the (B-Y) U modulator yields a sub carrier burst along –U (R-Y) (V) mudulator gives a burst of same amplitude but having a phase of + 90 degree on alternate lines relative to the –U phasor.5. At the output of 2 modulators the two burst components combine in the adder to give an output which is vector sum of 2 burst inputs.6. This is a subcarrier sinewave ( = 10 cycles ) at =45 degree on one line and – 45 degree on next line with ref. to – U phasor.
AFT (Automatic frequency tuning circuit)1. Controls local oscillator freq. to obtain a picture IF of exactly 38.9 MHz. at the converter output.2. The discriminator measure IF and develops a dc control voltage proportional to the freq. deviations if any.3. This error voltage is applied to the reactance section of local oscillator to maintain its freq. at the correct value.LUMINANCE CHANNEL:-NOTCH FILTER1. Attenuates the subcarrier by about 10 db.2. To suppress appearance of any dot structure on the screen along with the picture tube.
CHROMA BANDPASS AMPLIFIER EMITTER FOLLOWER STAGE Q1 L1 & C3 TUNED CKT BW 4.43 MHZ BW is adjusted using R5 andR6.NEGATIVE GOINGPULSES DRIVE Q1 INTO CUT OFF DURING COLOUR BURST INTERVALS ANDPREVENT IT FROM REACHING THE DEMODULATORS
COLOUR SIGNAL PROCESSING :-BURST BLANKING 1. The pulses drive Q1 into cutoff during colour burst intervals and thus prevent it from reaching demodulators.BANDPASS STAGE 1. Tuned Circuit :- L1 and C3 2. R5 and R6 adjusts necessary bandwidth centered around 4.43 MHz.AUTOMATIC COLOUR CONTROL(ACC) 1. The biasing of amplifier Q2 is determined by dc control voltage fed to it by ACC circuit. 2. The ACC circuit develops a dc control voltage proportional to amplitude of colour burst. 3. This voltage when fed at input of Q 2 shifts its operating point to change the stage gain.
AUTOMATIC COLOUR CONTROL(ACC)1. The biasing of amplifier Q2 is determined by dc control voltage fed to it by ACC circuit.2. The ACC circuit develops a dc control voltage proportional to amplitude of colour burst.3. This voltage when fed at input of Q 2 shifts its operating point to change the stage gain.4. Thus net overall chroma signal output from the bandpass amplifier tends to remain constant.
MANUAL COLOUR CONTROL (SATURATION)1. D2 :- colour control diode.2. R3 , R4 and R5 :-Colour control potentiometer.3. When diode is excessively F.B. (R5 at +30 V) it behaves like a short circuit. The chroma signal gets shorted to via C1 to gnd. And there is no input to the demodulators. B/W picture is produced on the screen.4. If R5 is so adjusted that F.B. on D 1 is almost Zero D1 offers very high impedance to the signal. All signal feeds into the amplifier and a large signal voltage appears at the output of chroma bandpass amplifier.This produces picture with maximum saturation.
If forward bias on d2 is zero diode would present a very high impedance , so all signal feeds into the amplifier , produces picture with maximum saturationCHROMASIGNAL COLOURD2 IS CONTROLFORWARD DIODEBAISED BY When d2 is forwardA VOLTAGE biased (R5 at +30V) itDIVIDER behaves like shortR3,R4, AND ckt.via C1 to gnd andR5. there is no input to COLOUR CONTROL demodulators . So b/w POTENTIOMETER picture is produced.At other settings of R5, conductance of D1 would cause the signal current to divide between theemitter of Q3 and C1 resulting in intermediate levels of picture colour saturation.
No hue of tint control is provided in PAL receivers because of theinbuilt provision for phase shift cancellation.In some receiver designs the saturation control is combined with thecontrast control. 1.
Positive feedback improves quality factor of tuned ckt.AC coupling Emitter follower Colour killer diode d4 Tuned Potential amp divider 3.3k7.8 khz andswitching rate 680ohmsof the R-YsignalD3 conducts onnegative halfcycles Q5 last stage of bandpass amplifier
Input 2 Input 1 Class B or C tuned amplifierLarge amplitude delayedhorz retrace pulsesdrives amplifier out of Centre freq. 4.43 Mhz.cutoff and allows the Bandpass of amplifer isamplifier to function 0.6 MHz.normally. Common emitter Separates colour burst from the config. chrominance amplifierFor proper operation the timing of the horizontal pulse and the colour burst must be suchthat the amplifier is turned on and kept operative during the time when the colour burst isat the input of the burst gate.Under this condition the output of the burst gate contains only the amplified colour burst.
Total external capacitance required across the crystal to make the oscillator work at4.43361875 MHz is around 20 pF. Removes harmonic content in the oscillator output. Tuned CKT.Variable capapcitance diode Emitter follower Q9 Crystal controlled subcarrier oscillator of inverted colpitts type
Two inputs:Locally generatedref. subcarrier &Transmitted burst. Automatic phase control CKT.
SECAM COLOUR TV SYSTEM• It was developed in France.• NTSC and PAL systems transmit and receiver two chrominance signals simultaneously,• While SECAM system is “sequential a memoire system” i.e. only one of the two colour difference signals is transmitted at a time .
SECAM system• The subcarrier is frequency modulated by the colour difference signals before transmission.• The magnitude of frequency deviation represents saturation of the colour and rate of deviation its fineness.
• If RED difference signal is transmitted on one line then the blue difference signal is transmitted on the following line.• This sequence is repeated for the remaining lines of the raster.• Because of the odd no. of lines per picture, if nth line carries (R-Y) signal during one picture, it will carry (B-Y) signal during scanning of the following picture.• AT the receiver an ultrasonic delay line of 64 microsec is used as a one line memory device to produce decoded output of both the colour difference signals simultaneously.• The modulated signals are routed to their correct demodulators by an electronic switch operating at the rate of line frequency.• The switch is driven by a bistable multivibrator triggered from the receiver‟s horizontal deflection circuitry.
• The determination of proper sequence of colour lines in each field is accomplished by identification (Ident ) pulses which are generated and transmitted during vertical blanking intervals.
SECAM III• It is 625 line 50 field system with a channel bandwidth of 8 Mhz.• The sound carrier is +5.5 MHz relative to the picture carrier.• The nominal colour subcarrier frequency is 4.4375 MHz. Actually two subcarrier frequencies are used.
• The Y signal is obtained from the camera outputs in the same was as in the NTSC and PAL systems.• However different weighting factors are used and the weighted colour difference signals are termed DR and DB where
SECAM III• I DR I = 1.9 (R-Y)• I DB I = 1.5 (B-Y)
Modulation of the subcarrier• The use of FM for the subcarrier means that phase distortion in the transmission path will not change the hue of picture areas.• Limiters are used in the receiver to remove amplitude variations in the subcarrier.• The limitation of subcarrier , 4.4375 MHZ away from the picture carrier reduces interference and improves resolution.
• In order to keep the most common large deviations away from the upper end of the video band, a positive frequency deviation of the subcarrier is allowed for a negative value of (R-Y) .• Similarly for the blue difference signals a positive (B-Y) value.• Therefore , the weighted colour signals are :• DR= - 1.9 (R-Y) ( THE minus sign for indicates that negative values of (R-Y) are required to give rise to positive frequency deviations when the subcarrier is modulated.• DB= 1.5 (B-Y)• In order to suppress visibility of dot pattern on monochrome reception two different subcarriers are used.
• For red difference signal it is• 282 fH = 4.40625MHz• And for the blue difference signal it is• 272 fH = 4.250 MHz.
• PRE-EMPHASIS• The colour difference signals are bandwidth limited to 1.5 MHz.• On modulation the subcarrier is allowed a linear deviation = 280 D R KHz for the red difference signals and 230 DB KHZ for the blue difference signals .• The maximum deviation allowed is 500 KHz in one direction and 350 KHz in the other direction for each signal although the limits are in opposite directions for the two chrome signals.
• After modulating the carrier with the pre-emphasized and weighted colour difference signals (D R and DB ) , another form of pre-emphasis is carried out on the signals.• This takes the form of increasing the amplitude of the subcarrier as its deviation increases.• Such a pre-emphasis is called high frequency pre- emphasis .• It further improves signal to noise ratio and interference is very much reduced.
Line identification signal• The switching of D R and DB signals line by line take place during the line sync pulses period.• The sequence of switching continues without interruption from one field to the next and is maintained through the field blanking interval.
• However it is necessary for the receiver to be able to deduce as to which line is being transmitted .• Such an identification of the proper sequence of colour lines in each field is accomplished by identification pulses that are generated during vertical blanking periods.
• The signal consists of a saw tooth modulated subcarrier which is positive going for a red colour difference signal and negative going for the blue colour difference signal.• At the receiver the ident pulses generate positive and negative control signals for regulating the instant and sequence of switching.
SECAM III CODER• The colour camera signals are fed into a matrix where they are combined to form the luminance• Y= 0.3 R +0.59 G +0.11 B• And colour difference signals.• The SECAM weighting and sign factors are applied to the colour difference signals so that the same subcarrier modulator can be used for both the chrominance (DR and DB) signals. The ident signal is also added in the same matrix.• An electronic switch which changes its mode during every line blanking interval directs DR and DB signals to the frequency modulator in a sequential manner i.e. when DR is being transmitted on the line, then DB is not used and vice versa.
Sync pulse generation and control• The line frequency pulses from the sync pulse generator are passed through selective filters which pick out the 272 nd and 282 nd harmonics of FH.• These harmonics are amplified and used as the two subcarrier references.• The sync pulse generator also synchronizes the switching control unit which in turn supplies operating pulses to the electronic switch fro choosing between DR and DB signals.