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Chapter: Television and Video Engineering : Colour Television Systems

Important Short Questions and Answers: Colour Television Systems

Television and Video Engineering - Colour Television Systems - Important Short Questions and Answers: Colour Television Systems

COLOUR TELEVISION SYSTEMS

 

 

Distinguish between S-PAL and D-PAL.

The use of eye as the averaging mechanism for the correct hue is the basic concept of simple ‘PAL’ system. Beyond a certain limit, the human eye see the effect of color changes on alternate lines hence the system needs modification.

Considerable improvement found in the system of a delay line is used to do the averaging first and then present the color to the eye. This is called PAL-D or delay line PAL method and is most commonly employed in PAL receivers.

 

State the type of circuits used to separate horizontal and vertical sync pulses.

Integrator(LPF) and differentiator(HPF)

 

What are trap circuits?

In the IF amplifier circuitry, provision must be made for rejection of signals from adjacent channels. For this purpose special tuned circuits, called trap circuits, are connected in the signal path in such a way that the offending frequencies are removed.

These trap circuits are disposed at convenient places in the IF amplifiers. Their position will vary from receiver to receiver, but generally they are placed in the input circuit of the first IF amplifier.

 

Why different bandwidths are assigned to Q and I signals?

NTSC system is compatible with 525 line American system. In order to maintain compatibility two new color difference signals are generated and they are represented as I and Q.

Since eye is capable of resolving finer details in the regions around I, it is allowed to have a maximum bandwidth of 1.5MHz. The bandwidth of Q signal is restricted to 0.5MHz.

 

What is the significance of color killer circuit?

 The color killer is an electronic stage in color TV receiver sets which acts as a muting circuit to cut off the color amplifiers when the TV receives a monochrome signal.

 If black and white signal is received, no burst signal is present. So, there is no need for chromo section and the output of chromo amp is zero. For provide this color killer circuit is used.

 

What are the demerits of PAL systems?

The use of phase alteration by line technique and associated control circuitry together with the need of a delay line in the receiver makes the PAL system more complicated and expensive. The receiver cost is higher for the PAL color system.

 

Why is it necessary to employ an L-C instead of R-C filter to remove IF ripple from the detected output?

 To avoid over attenuation of the video signal while filtering out the carrier components. So L-C component is employed.

 

Why is the color signal bandwidth requirement much less than that of Y signals?

 By conducting many experiments with different viewers, it is found that , only the color region having an area more than 1/25 th of screen width will give useful color information.

So for color signal large bandwidth is not necessary.

 

Why is the color burst signal transmitted after each scanning line and why PAL color burst is often called the swinging burst?

 In PAL system the two carrier components are suppressed in the balanced quadrature modulator it is necessary to regenerate at the receiver for demodulation.

 For this, 8 to 10 cycles of the color subcarrier oscillator output at the encoder are transmitted along with other sync pulses. This sample of the color subcarrier called color burst and it is placed at the back porch of each horizontal blanking pulse pedestal.

 The PAL burst phase actually swings 45 about the –U axis from line to line and indicates the same sign as that of the V signal; thus the switching mode information is the swinging burst. This is known as swinging burst.

 

Justify the choice of 3.579545 MHz as the subcarrier frequency in the NTSC system.

The color subcarrier frequency in the NTSC system has been chosen to have an exact value equal to 3.579545 MHz.

The reason for fixing it with such a precision is to maintain compatibility between monochrome and color systems. Any interference between the chrominance signal and higher video frequencies is minimized by employing suppressed carrier (color subcarrier) transmission and by using a notch filter in the path of the luminance signal.

However, when a color transmission is received on a monochrome receiver a dot pattern structure appears along each raster line on the receiver screen.

This is caused by the color signal frequencies that lie within the pass-band of the video section of the receiver.

 

List the functions of the following stages of a PAL-D color receiver mentioning their input and output.         a) Burst phase discriminator b) Color killer

Burst phase discriminator

 The burst phase discriminator is used to compare the phase of the color burst and sub carrier frequency generated by the reference oscillator.

 If there is any phase difference which will produce DC control voltage (error voltage) which is fed to reference oscillator.

 In this way the frequency and phase are kept locked in synchronism with the color burst.

Color killer

 The color killer is an electronic stage in color TV receiver sets which acts as a muting circuit to cut off the color amplifiers when the TV receives a monochrome signal.

 If black and white signal is received, no burst signal is present. So, there is no need for chromo section and the output of chromo amp is zero. For provide this color killer circuit is used.

 

What are the merits of using an RF amplifier before the frequency converter?

 In areas where the signal strength is somewhat less, raising the antenna or using an antenna that is directional and has higher gain results in an acceptable picture.

 However, in deep fringe areas where the signal from the desired station is very weak and fails to produce any worthwhile picture, an additional RF amplifier external to the receiver becomes necessary. Such amplifiers are known as booster amplifiers and are normally mounted on the antenna mast close to the antenna terminals.

 A booster amplifiers is a broad-band transistor RF amplifier designed to have a reasonable gain but a very high internal signal-to-noise ratio. It may be emphasized that a booster capable of providing a high gain but incapable of providing a good signal-to-noise ratio will give a picture with lot of snow.

 Similarly, a booster amplifier having minimum internal noise but low gain will fail to provide a satisfactory picture. Thus a booster amplifier must have both the attributes, i.e. reasonable gain and high signal-to-noise ratio.

 

What is gamma correction?

A color camera is used develop three voltages proportional to red, green and blue color contents of the picture. These voltages are represented as R,G,B and correction is applied to these voltages to compensate for any nonlinearity of the system and that of the picture tube. This is called gamma correction. i.e. the camera tube output voltage amplitudes are normalized to I V p-p level.

 

Give the exact color sub carrier frequency of NTSC system.

Color sub carrier frequency of NTSC system is exactly 3.579545 MHz.

 

What is the use of color subcarrier oscillator?

The function of subcarrier oscillator is to generate a carrier wave output at 3.57MHz and feed it to the demodulators. The subcarrier frequency is maintained at its correct value and phase by the APC circuit.

 

Give the sound and picture IF values of PAL-D TV receiver.

•        Video IF--------     38.9 MHz

•        Sound IF--------     33.4 MHz

 

List the features of PAL color system.

The weighted (B-Y) and (R-Y) signals are modulated without being given a phase shift of 33 as is done in the NTSC system.

On modulation both the color difference signals are allowed the same bandwidth of about 1.3MHz.

The color subcarrier frequency is chosen to be 4.43MHz.

The weighted color difference signals are quadrature modulated with the subcarrier.

 

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