111). Write notes on luminance channel.
The video amplifier in the luminance channel is Dc coupled and has the same
bandwidth as in the monochrome receiver. It is followed by a delay line to
compensate for the additional delay the color signal suffers because of limited
bandpass of the chrominance amplifier .This ensures time coincidence of the
luminance and chrominance signals. The channel also includes a notch filter
which attenuates the subcarrier by about 10db.This helps to suppress the
appearance of any dot structure on the screen along with the color picture.
112). What is the use of chrominance bandpass amplifier?
The chroma bandpass amplifier selects the chrominance signal and rejects
other unwanted components of the composite signal.
113). What do you mean by automatic color control?
The ACC circuit is similar to the AGC circuit used for automatic gain
control of RF and IF stages of the receiver.It develops a dc control voltage that is
proportional to the amplitude of the color burst.
114). Write short notes on color killer circuit.
When a monochrome transmission is received there is no input to the color
killer and no positive voltage is developed . Therefore no input is given to the
second chroma amplifier from the color killer circuit ,it blocks the second chroma
amplifier.Thus it prevents the color noise on black and white picture.
115). Merits of PAL system.
The problem of differential phase errors has been successfully overcome
in the PAL system.
116). Demerits of PAL system.
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 colour system.
117). What do you mean by automatic frequency tuning?
AFT is used to improve the stability of the oscillator circuit ,some drift does occur
on account of ambient temperature changes ,component aging ,power supply
voltage fluctuation and so on. The fine tuning control is adjusted to get a sharp
picture.
118). Write short notes on burst seperator.
The burst seperator circuit has the function of extracting 8 to 10 cycles of
reference color burst which are transmitted on the back porch of every horizontal
pulse .the circuit is tuned to the subcarrier frequency and is keyed on during the
flyback time by pulses derived from the horizontal output stage.
119). 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.
120). How the phase error is cancelled in the PAL system.
In PAL system phase shift error is cancelled by reversing the phase angle
of v signal on alternate lines.
121). Give the abbreviation for NTSC, SECAM ,and PAL.
NTSC -National Television systems committee
SECAM –Sequential –a-Memoire
PAL - Phase Alteration by Line
122). What do you understand by PAL –D Colour system.
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 colour 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 employe in PAL receivers.
123). Write short notes on colour subcarrier frequency of PAL D system.
The color sub carrier frequency of 4.43MHz is produced with a crystal controlled
oscillator .To accomplish minimum raster disturbance through the color
subcarrier it is important to maintain correct frequency relationship between the
scanning frequencies and subcarrier frequency .Therefore ,it is usual to count
down from the subcarrier frequency to twice the line frequency pulses .
124). write short notes on AGC circuit.
AGC circuit is used to control the gain of RF and IF amplifiers .The change in
gain is achieved by shifting the operating point of transistors used in the
amplifiers.The operating point is changed by a bias voltage that is developed in
the AGC circuit.
125). What do you mean by Peak AGC system?
The system based on sampling the sync tip levels is known as “peak”
AGC system.The Peak AGC system is also called as non-keyed AGC sytem.
126). What are the two types of AGC control?
Forward AGC control
Reverse AGC control
127). What is Forward AGC control?
In any transistor amplifier ,gain is varied by shifting the operating point either
towards collector current cutoff or saturation. This actually varies beta of the
transistor and hence the stage gain changes.When gain is changed by shifting the
operating point towards current cutoff ,then it is called “Reverse AGC”.
128). What is forward AGC?
In any transistor amplifier ,gain is varied by shifting the operating point either
towards collector current cutoff or saturation. This actually varies
beta of thetransistor and hence the stage gain changes.When gain is changed by shifting the
operating point towards collector current saturation ,then it is called “Forward
AGC”.
129). List the draw backs of nonkeyed AGC.
The AGC voltage developed across the peak rectifier load tends to increase during
vertical sync pulse periods because the video signal amplitude remains almost at the
peak value every time vertical sync pulses occur. This results in a 50Hz ripple over
the negative AGC voltage and reduces gain of the receiver during these intervals.The
reduced gain results in weak vertical sync pulse which in turn can put the vertical
deflection oscillator out of synchronism causing rolling of the picture.
130). Merits of Keyed AGC system.
AGC voltage developed is a true representation of the peak of fixedsync
level and thus corresponds to the actual incoming signal strength.
Noise effects are minimized because conduction is restricted to a small
fraction of the total line period. 
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