color signal processing

7/23/2019 Color Signal Processing

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• Monochrome receivers are still made, of course, butmainly for use as second receivers , for kitchens, as

portables, and so on. However,, we will see that what wehave learned in our study of monochrome receivers isdirectly applicable to color television.

Introduction



Definition of Terms

• Chroma (Chrominance) – the color signal

• Luma (Luminance) – the signal that provides brightnessinformation in a video system

•

H- length of horizontal line

• Comb filter - a filter that can pass (or reject) a fundamentalfrequency and its harmonics

• Yoke - the assembly that contains the deflection coil and is

mounted on the neck of a CRT.

• I – in-phase component of the color signal

• Q - quadrature component of the color signal



Antenna

RF Amp Mixer

Local Oscillator

PictureIF Amp

Second IFDetector

Audio IFAmp

AudioDetector

AudioAmp

ChromaBandpass

Amp

ColorDemodulator

Matrix

VideoDetector

DelayLine

LumaAmp

Adder

VerticalAmp

SyncSeparator

VerticalOscillator

HorizontalOscillator

HorizontalAmp

Tuner

AC line

Main Power Supply

Keyed AGC

DeflectionYoke

CRT

Mixer

HV Supply

LV Supply

Power toOther Stages

Color television Receiver

Figure 19.21



Figure 19.21 is the block diagram of a typical color receiver.

Similarities to monochrome receivers

• Tuner

• IF video sections

• Same audio section

• Raster and synchronizating circuitry

Differences to monochrome receivers

• Varactor tuners

• Frequency synthesis

• Remote control

• Incorporate stereo sound

•

Higher accelerating voltage for CRT

Essential differences between monochrome and color receivers:The differences are mainly related to the way in which the chroma signal is demodulated and theresult is combined with luma. The composite color signal must be converted back into an RGBsignal before it is applied to the color CRT.



Antenna

RF Amp Mixer

Local Oscillator

PictureIF Amp

Second IFDetector

Audio IFAmp

AudioDetector

AudioAmp

ChromaBandpass

Amp

ColorDemodulator

Matrix

VideoDetector

DelayLine

LumaAmp

Adder

VerticalAmp

SyncSeparator

VerticalOscillator

HorizontalOscillator

HorizontalAmp

Tuner

AC line

Main Power Supply

Keyed AGC

DeflectionYoke

CRT

Mixer

HV Supply

LV Supply

Power toOther Stages

Color television Receiver

Figure 19.21



Differences of Color Receiver from Monochrome Receiver

• Separate sound IF Detector- separates out the 4.5 Mhz sound IF signal

• Following the video detector, the chroma signal must be separated from the luma andsent to the color demodulators. There are two common ways to do this.

• From figure 19.21, the simplest way is to apply the signal to a bandpass amplifierwith its center frequency at the color subcarrier frequency of approximately3.58 MHz and a bandwidth of about 1 MHz. This will remove most of the luma,though of course the highest frequency luma components will pass through thefilter.

•

There is a better way to separate luma from chroma by using a comb filter.Figure 19.22 shows how this works. The composite signal is applied to the inputof 1H delay line, that is, one of a delay of exactly one horizontal line period ,or 63.5 µs. The original and delayed signals are added to give the luminanceoutput and subtracted to produce the chroma.



Comb Filter

The operation of a comb filter can be understood by considering asignal with a period of 1H (63.5µs) the delay results in a phase shift of360°, and the delayed signal is indistinguishable from the original.

Assuming that the amplitudes are made equal, the output is thesummer is enhanced, while subtractor has zero output. The samelogic hold for any harmonic of the horizontal scanning frequency.



Delay Line63.5 µs

Delay Line63.5 µs

Σ

Σ

Luma

Chroma

CompositeInput

Comb Filter

Figure 19.22



Color Demodulation using I and Q axes

Once the chroma signal has been separated from the luma, it must still bedemodulated. The obvious solution would be to demodulate the QUAM color signalinto the original I and Q signals using two balanced modulators, each supplied with acarrier of the correct phase. Of course, those carriers have to be synchronized with theoriginal subcarrier, a relatively easy job since the color burst provides a sample once

per line.

How it works?

The extra filters take into account the different bandwidths of the I and Q signals, andthe extra delay line compensates for the different delays in filters of differentbandwidth. The AFPC(automatic frequency phase control), a system that phase-locksa crystal oscillator to the exact frequency and phase of the color burst. This in turnrequires that the burst be separated from the rest of the signal. Its position right after horizontal sync makes this fairly easy. The block marked color killer does what it says: itturns off the color circuitry when there is no color burst.



Matrix3.58 MHz

Color Signal

Bandpass

Amp

Q

Demodulator

LPF Cutoff

0.5 MHz

I DemodulatorDelay Line

LPF Cutoff

1.3 MHz

90° Phase

Shifter

Color Killer

3.58 Mhz

Oscillator

AFPC

Burst DetectorHorizontal

Sync Keying

Pulse

R - Y

G - Y

B - Y

Color Demodulation using I and Q axes

Figure 19.23



Color Demodulation using R-Yand B-Y axes

It is possible to simplify the color circuitry somewhat by using two different axes to demodulatethe color signal.

How it works?

Figure 19.24 shows a demodulator using the R-Y and B-Y axes. The number of filters is reducedand there is a need for a delay line- since both of the demodulated signals are combinations ofI and Q, there is point in using bandwidth larger than that of the Q signal, which is 500 kHz. Therelative simplicity of the R-Y/ B-Y demodulator is paid for in reduced horizontal resolution forcolor.

Whichever color demodulator is chosen, the output will consist of three signals, R-Y, B-Y, and G-Y. These can simply be added to the luminance signal to produce the primary signals (red,green and blue) that are required by the CRT.

The color and tint controls adjust the color circuitry. The tint control alters the phase of theinjected carriers to the demodulators, and the color control varies the gain of the colorbandpass amplifier.



Matrix3.58 MHz

Color Signal

Bandpass

Amp

R-Y

Demodulator

B-Y

Demodulator

90° Phase

Shifter

Color Killer

3.58 Mhz

Oscillator

AFPC

Burst DetectorHorizontal

Sync KeyingPulse

R - Y

G - Y

B - Y

Color Demodulation using R-Yand B-Y

axes

Figure 19.24



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color signal processing

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