COLOUR TV CH-5

PAL-D Receiver

Tuner - picture IF of exactly 38.9 MHz & sound IF of exactly 33.4 MHz. AFT (automatic frequency tuning )= circuit actually controls the local oscillator frequency to obtain a picture IF of exactly 38.9 MHz at the converter output. Saw filter= surface acoustic waves is a non- electromagnetic waves travel along the surface of a piezoelectric substrate. Sound Strip = The volume and tone controls are associated with the audio amplifier, the output of which feeds into the loudspeaker.

AGC, Sync-separator and Deflection Circuits = provide necessary wave-forms for dynamic convergence and pincushion correction. Sync Separator : It separates the horizontal sync signal & vertical sync from composite video signal. Deflection circuit = develop horizontal, vertical scanning, form waveform for dynamic converence and pincushion correction. Luminance Channel= foundation of full colour picture gets build up with luminance image.

Delay Line (64us) : reproduces the brightness & colour information. Matrix : The primary colour signals R, G, B, obtained the matrix section are given to the output video amplifiers circuit. Vertical deflection = Vertical oscillator to get sawtooth voltage to drive at 50Hz to vertical deflection coil. The current in the coil rises linearly. Horizontal deflection = Horizontal oscillator gives the output rectangular pulses at 15626Hz for sawtooth current in the horizontal deflection coil. So, the rising current deflection beam from left to right from the screen.

EHT & Other D.C voltages= input to this section is deflection pulses from line output stage & EHT rectified after necessary boosting. EHT transformer supplies +22KV (EHT) accelerating potential to final anode of picture tube. EHT transformer also produces AFC fly back pulses. Focus and screen voltage are produced by EHT Transformer fed to picture tube at G2 and G4 (greed). Heater filament gets supply by EHT transformer pin no. 9.

Colour Burst Signal A short wave of 8 to 11 cycles called the ‘colour burst’ is sent to the receiver along with sync signals.

Colour Burst Signal Suppressed carrier in modulated signal(transmitting), it is necessary to generate by demodulating of colour signal( receiver), This signal generated must be exactly same phase & frequency as the transmitter. This is located in the back porch of the horizontal blanking pedestal. The colour burst does not interfere with the horizontal sync because it is lower in amplitude and follows the sync pulses

PAL-D DECODER

PAL-D DECODER Chroma Signal selection:- Select the chrominance signal and reject all other unwanted components of composite signal. Colour killer circuit:-This circuit becomes ‘ON’ and disables the chroma amplifier during monochrome reception. Thus it prevents coloured interference on the screen. Sync demodulator: - The o/p from adder and subtractor consist of two independent RF signals such as (U and V). They yield an accurate and constant no-colour difference signal voltage varies. Colour difference amplifier and matrixing:- Three colour difference signal amplified and fed to appropriate grids of the picture tube matrix is designed to produce (R-Y)(G- Y)(B-Y) signal from I and signal .

Burst gate amplifier:-To separate the colour burst from the chrominance signal. Reference oscillator:-Generates exactly right frequency with same phase reference as the original subcarrier. ACC amplifier:- A second o/p from the gate burst amplifier is connected to a dc voltage by rectifier circuit and then fed to the ACC amplifier. PAL delay lines:-Averaging and separates U and V modulating signal. V signal demodulator:- Produces V colour difference signal. U signal demodulator :-Produces U colour difference signal.

Principle of PAL-D DECODER

The object of the delay line is to delay the chrominance signal by almost exactly one line period of 64 µs. The chrominance amplifier feeds the chrominance signal to the adder, the subtractor and the delay line. The delay line in turn feeds its output to both the adder and subtractor circuits. The result is that the signal information of two picture lines, are presented to the adder and subtractor circuits simultaneously. U information with twice the amplitude (2U) & V information, with an amplitude twice that of the ‘V ’ modulation product.

Yagi antenna

Yagi- uda antenna is directional TV receiving antenna. Its an array consisting of driven element (Dipole)& one or more parasitic element ( reflector & director). Director – Director face towards transmitting antenna & collects the maximum signal strengths.so no of directors are more than one.

Reflector – Reject unwanted signals from opposite side. Dipole- collects all signal strength from director and fed to TV receiver through parallel wire. Application- Used as VHF & UHF TV receiving antenna.

Colour Signal Processing

First stage of chroma band-pass amplifier The signal available at the output of video detector is given some amplification (video preamplifier). Chrominance band-pass amplifier- the chroma band-pass amplifier selects the chrominance signal and rejects other unwanted components of the composite signal. The burst blanking, colour level control and colour killer switch also form part of this multistage amplifier.

Burst Blanking

The output from the video preamplifier is fed to the first stage of chroma band-pass amplifier through an emitter follower stage (Q1). Negative going horizontal blanking pulses are coupled to the base of Q1 through diode D1. The pulses drive Q1 into cut-off during colour burst intervals and thus prevent it from reaching the demodulators.

Band-pass Stage- The emitter follower output is fed to the band-pass stage through a tuned circuit consisting of L1 and C3. The bandwidth centred around 4.43 MHz is adjusted by R5 and R6. The tuning of L1 also incorporates necessary correction on account of vestigial sideband transmission of the chrominance signal.

Automatic Colour Control (ACC) The biasing of amplifier (Q2) in Fig. is determined by the dc control voltage fed to it by the ACC circuit

The ACC circuit is similar to the AGC circuit used for automatic gain control of RF and IF stages of the receiver. It develops a dc control voltage that is proportional to the amplitude of colour burst.

This voltage when fed at the input of Q2 shifts its operating point to change the stage gain. Thus net overall chroma signal output from the band-pass amplifier tends to remain constant.

Colour (Saturation) Control The chroma signal from the first stage is applied through R1 and R2 to the emitter of Q3 and cathode of D2, the colour control diode.

The diode is forward biased by a voltage divider formed by R3, R4 and R5 the colour control potentiometer. When the diode is excessively forward biased (R5 at + 30 V position). Under this condition the chroma signal gets shorted via C1 to ground and there is no input to the demodulators. As a result, a black and white picture is produced on the screen

If R5 is so adjusted that forward bias on D2 is almost zero, the diode would present a very high impedance to the signal. Under this condition all the available signal feeds into the amplifier and a large signal voltage appears at the output of the chroma band-pass amplifier. This is turn produces a picture with maximum saturation.

Colour Killer Circuit The last stage of band-pass amplifier depends on the state of the colour killer circuit.

the 7.8 KHz input from the APC (automatic phase control) circuit applied via C1 to the base of tuned amplifier Q6. The D.C operating amplifier Q6.

Autotransformer forming by L & C & enable a waveform of about 25v peak to peak developed at collector of Q6. This waveform is fed through C2 to D2 (Half wave rectifier). R & C form a low pass filter which provide steady D.C level output.

This is colour killer voltage which is used to control conduction of the second stage of chroma amplifier. This prevent any stray colour signal reaching R,G,B amplifier and hence no colour noise appears on black and white picture during transmission. Colour killer circuit provides +13.5v to the second stage of chroma amplifier.

Separation of U and V Signals Chroma signal is applied to Q1. Chroma signal is applied to delay line through transformer T1.

This signal after delay line appears across A winding of T2 transformer. Voltage induced into winding A and B is equal in magnitude but opposite in phase due to signal from delay line. Whereas voltage induced into winding A and winding B is equal in magnitude and same phase. This means that direct and delayed signals have same phase in one winding but are of opposite phase in second winding. Thus results in separation of U and V signal.

Colour signal matrixing

Compatibility Considerations

1] The luminance signal Y = 0.3 R + 0.59 G + 0.11 B. R = G + B i.e. 0.59 + 0.11 = 0.7 So R=0.7, G= 0.59 to make 0.7 (add 0.2) ,B =0.11(+0.6) Substituting the values of R, G, and B we get Y = 0.3 (0.7) + 0.59 (0.2) + 0.11(0.6) = Y= 0.394 (volts). 2]The colour difference signals are: (R – Y) = 0.7 – 0.394 = + 0.306 (volts) (B – Y) = 0.6 – 0.394 = + 0.206 (volts)

Reception at the colour receiver— At the receiver after demodulation, the signals, Y, (B – Y) and (R – Y), become available. Then by a process of matrixing the voltages B and R are obtained as: R = (R – Y) + Y = 0.306 + 0.394 = 0.7 V B = (B – Y) + Y = 0.206 + 0.394 = 0.6 V

Rearranging the above expression we get: (G – Y) matrix—The missing signal (G – Y) that is not transmitted can be recovered by using a suitable matrix based on the explanation given below: Y = 0.3 R + 0.59G + 0.11B Rearranging the above expression we get: 0.59(G – Y) = – 0.3 (R – Y) – 0.11 (B – Y) Substituting the values of (R – Y) and (B – Y) (G – Y) = – (0.51 × 0.306) – 0.186(0.206) = – 0.194 G = (G – Y) + Y = – 0.194 + 0.394 = 0.2 = – 0.51(R – Y) – 0.186 (B – Y)

Unsuitability of (G – Y) Signal for Transmission (G – Y) = – 0.51(R – Y) – 0.186 (B – Y). Since the required amplitudes of both (R – Y) and (B – Y) are less than unity, (R – Y) = – 1.97(G – Y) – 0.37 (B – Y). ( B – Y) = – 5.4(G – Y) – 2.8 (R – Y). extra amplifiers requires, need more gain, problem in s/n ratio.

RGB Drive amplifier

RGB Drive amplifier It consists of three video amplifiers for driving three cathode of picture tube. Chroma IC gives input (decode R, G, B)to the video amplifiers. It consists of three transistor i.e Q1, Q2, Q3 are high frequency transistors

Q1 is RED signal amplifier (CE configuration) 150v dc supply is filtered by L2 & C9. C7 & C8 is bypass capacitors. R12 & R15 provides negative feedback to improve d.c stabilty. L3 is used to extend bandwidth & C1 is used to improve response.

EHT Generation In monochrome TV receiver EHT potential generates by L.O.T ( Line output transformer) up to 16kv. In colour TV receiver potential generates by diode split addition techniques up to 25kv.

Line output transformer

EHT is a voltage generator, which generates around 17KV for B/W TV & 25KV for colour TV. The principle of auto transformer action V=L di/dt Need of EHT :- It is used to generate high voltage which acts as a anode voltage for picture tube . Picture tube has inner coating called aquading coating & outer coating as graphic coating which acts as a capacitor, that charges during the horizontal retrace. In order to strike the electron on the face plate of the picture tube, we need to accelerate the electron which will be attracted by high voltage.

First Anode of picture tube (G2) (screen grid) is obtained for separately at collector of Q2. This is rectified by D1and then filtered by C10. Output DC voltage is 550 to 800 V. Any failure of G2 means no beam current and hence no spot is produced on screen.

Focus anode (G3) potential needed is 6. 5kV to 7. 5kV Focus anode (G3) potential needed is 6.5kV to 7.5kV.It is obtained from diode split winding (D2,D3 and D4). Each stage produces potential of 8kV.

Diode split addition techniques

In colour TV to generate EHT up to 25 Kv the diode split addition technique is used. 3 layers of secondary windings are wound on ferroxide core of line o/p transformer. Each winding is identical to another and has same no.of turns therefore same voltage induced in each section.

Every time flyback derived i/p pulse gets applied to primary winding. The layers are close to each other, thus interlayer capacitor C3-2& C2-1 exits between each of them. If diode is connected between ends of 1 layer of winding & ac voltage induced in each layer charge the capacitors.

Since these capacitance are in series, total voltage appearing at o/p terminal is sum of all voltage. 3 windings are designed that voltage induced in each layer is 8.33 KV. This makes total potential equal to 25 KV & forms EHT supply source.