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Colour Television Systems- Vikas Kaduskar
NTSC coder
NTSC Colour Receiver
Limitations of NTSC
PAL System Features, Swinging Burst, Phase
errors cancellation
PAL Coder
PAL-D colour Receiver.
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NTSC colour Receiver:
A signal from the selected channel is processed in usual
way by the tuner, IF & Video detector stages.
Sound signal is separately detected, demodulated &lified before feeding it to loudspeaker.
At the output of video detector, composite video &
chrominance signals reappear in their pre-modulated form,Y signal is processed as in a monochrome receiver except
that video amplifier needs a delay line- introduces a delay
of 500 ns which is necessary to ensure time coincidence of
luminance & chroma signals.Decoding of chroma ( C) signal: Chroma signal is
avilable at video pre amplifier, chrominance signal has
color information during active trace time of picture & burst
occurs during blanking time when there is no picture.
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NTSC Colour Receiver continued
Chrominance Bandpass Amplifier: purpose is to separate
chrominance signal from composite video signal, amplify it
& then pass it on to synchronous demodulators. Colour
burst is prevented from appearing at its output by horizontal
blanking pulses which disables the bandpass amplifier
during horizontal blanking intervals (blanking pulses aplliedto colour killer circuit which biases off chromi. Amplr.
Colour Demodulators: - synchronous demodulators used-
combination amplitude & phase detectors, each has 2 inputs
ci.e chroma signals & constant amplitude output of local
subcarrier oscillator.
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NTSC colour receiver continued .
Colour Matrix: designed to produce (R-Y), (G-Y0 & (B-
Y) signals fro I & Q signals. There are also color
difference amplifiers,While amplifying signals chroma
signal compression required( weigting facor)( i.e for R-Y,
1.14=1/0.877; B-Y 2.03), grids of picture tube are fed
+ve color differ. Signals & cathode receive
VeY
signal.
Burst separator: extract 8 to 11 cycles of reference burst
transmitted at back porch og HORZ sync.output also fed
to phase descriminator Ckt(AFPC).
Colour killer circuit: when ckt On & disables chroma
bandpass amplr for monochrome reception. Noise-
confetti-looks like large spot in colour
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NTSC limitations: NTSC is sensitive to transmission path
differences introduces phase errors-result incolour changes in picture.
Chroma phase angles effected by level of signals
passing through various circuits.
Crosstalk between demodultors at receiver-color
disrortion.
All these require Automatic Tint control (ATC) .
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PAL colour System Main features:
1) Weighted (B-Y) & (R-Y) modulated without beinggiven phase shift 33.
2) On modulation, both colour-difference signals areallowed same BW-1.3 MHz results in better colour
reproduction.3) CSC-chosen to be 4.43361875 MHz. it is odd multiples
of quarter line freq.-results in cancellation of dot patterninterference.
4) Weighted signals modulated-QAM as same as in NTSCbut phase of subcarrier to one of modulators (Vmodulator) is reversed from +90 to -90 at line freq.(assystem derives name Phase alteration by line).
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PAL system
B-Y=U(along X-axis) & (R-Y)=V from line to line.
+/-V or +/-(R-Y) burst signal has an amplitude equal tothat of(B-Y).
Burst phase actually swings 45 about (b-Y) axis from
line to line.
Since colour burst shifts on alternate line by 45 about
zero reference phase it is often called swinging burst.
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PAL Coder:
The gama corrected R, G ,B signals matrixed to
form Y & weighted colour difference signals.
BW of(B-Y & R-Y is restricted to 1.3 MHz) by low
pass filters. in this process suffer delay , a delay
line is inserted in path of Y signal. Weighted colour diff. signals are fed in to
balanced modulators, sinusoidal subcarrier is fed
directly to U modulator & passes 90 phaseswitching circuit before entering V modulator.
Squrewave swiching signal from Multivibrator to
electronic phase switch is half line frequency.
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PAL-D Decoder
The general pattern of signal flow in PAL & NTSc
receivers is nearly same. Tuner, sound section &deflection ckt in a similar manner. Main differ lies
in chroma section of receivers.
Chroma signal path: o/p from IF section detectedat video detector , then amplified which contains Y
signal mixed with chrominance. Chrominance fed
to gated burst chroma bandpass amplr. Chroma bandpass amplr:it is tuned to accept only
C signal out of composite video signal.it is designed
to have center freq 4.43 MHz with BW 2MHz.
Colour burst not allowed to ass thr it.
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i/p of chroma bP amplr is shut down by a burst blanking ckt at end
of each active line
Colour killer ckt.: it is actuated by 7.8 KHz tuned amplr. ,killer ckt.
inoperate chroma BP amplr. During monochrome reception. Separation of U & V modulator products:
Delay line- adder-U demodulator; delay line-substractor-V-
demodulator.
Synchronous demodul: U& V demodulat-
Subcarrier generation & control: it should have same freq & phase
as that fed to modulators at transmitter, for this a crystal
controlled osclt. Is built in chroma section. It is forced to work at
correct freq by action of automatic freq. & phase control-calledphase descriminator. Which compares burst signal obtained frm
gated burst amplr & locally gene subcarrier ti develop control vtg.
7.8 KHz amplr:
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