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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