video Engineering-Colour Systems

7/31/2019 video Engineering-Colour Systems

1/15

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.


2/15


3/15


4/15

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 &amplified 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.


5/15

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.


6/15

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


7/15

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) .


8/15

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).


9/15


10/15

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.


11/15


12/15

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.


13/15


14/15

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.


15/15

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:

Documents

video Engineering-Colour Systems