Important Short Questions and Answers: Monochrome Television Transmitter and Receiver

MONOCHROME TELEVISION TRANSMITTER AND RECEIVER

State the main function of UHF tuners.

The purpose UHF tuner unit is to amplify both picture and sound signals picked up the antenna and to convert the carrier frequencies and their associated side bands into intermediate frequencies.

What is dipole array?

Dipole antenna is used for band I&III transmitters. It consists of dipole panels mounted on the four sides at the top of the antenna tower.

Each panel has an array of full wave dipoles mounted in front of reflectors. To get a unidirectional pattern, the four panels mounted on the four sides of the tower are so fed that the current in each lags behind the previous by 90 degree.

This is done by changing the field cable length by λ/4 to the two alternate panels and by reversal of polarity of the current.

State the application of a diplexer.  Mention the need for diplexer?  What is a diplexer?

The outputs of both the video and the audio transmitter are combined by the diplexer circuit and given to a common broadcast transmitting antenna.

Why is de-emphasis done in TV receivers?

To eliminate the high frequency noises present in the received signal can be the LPF is used as the De-emphasis circuit in TV receiver.

List the functions of TV tuner.

The purpose tuner unit is to amplify both picture and sound signals picked up the antenna and to convert the carrier frequencies and their associated side bands into intermediate frequencies.

Mention the use of ACC amplifier.

 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.

What are the requirements of TV broadcast transmission?

 Height of the transmitting antenna

 Transmitter power

 Transmission bandwidth(7 MHz)

What are the advantages of AGC?

 AGC circuit is used to control the gain of RF and IF amplifiers .The changes 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.

Why EHT is needed in TV circuits?

 The EHT (Extra High Tension or HV to the CRT) is generated from a secondary winding on the flyback transformer having several thousand turns of very fine wire.

 If the EHT voltage drops, then the electrons will be accelerated less and will move through the deflection field at a lower velocity. As a result they will be easier to deflect by the magnetic field, and the picture size will grow. Without special measures, brighter pictures will be larger. The measure is to feed some EHT information or beam current information to the deflection circuits, reducing the deflection current amplitude a bit for bright pictures.

 The EHT information is also used to protect the flyback transformer from overload. As the load increases, the average primary current rises. Ultimately it may reach a level where the transformer core may go into saturation. This causes large peak currents in the HOT which might lead to destruction.

Mention the use of ACC amplifier.

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.

Define luminance and hue.

Luminance can be defined as the quantity of light intensity emitted per square centimeter of an illuminated area.

Saturation refers to the spectral purity of the color light. It indicates the degree by which the color is diluted by white.

What is the purpose of an AGC circuit in a TV receiver?

AGC circuit controls the gain of RF and IF stage to enable almost constant signal voltage at the output of video detector, despite changes in the signal picked up by the antenna. 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 bias circuit.

Define contrast ratio.

The ratio of maximum to minimum brightness relative to the original picture is called contrast ratio.

In broad daylight the variations in brightness are very wide with ratio as high as 10000 : 1

Why is AM preferred over FM broadcasting the picture signal?

If FM is adopted for picture transmission, the changing beat frequency  between the multiple paths delayed with respect to each other would develop a bar interference in the image with a shimmering effect as the bars continuously changes as the beat frequency changes therefore ,no study picture is produced.

Apart from that, circuit complexity and BW requirements are much less in AM than FM. Hence AM is preferred to FM for broadcasting the picture signal.

List any three requirements to be satisfied for compatibility in television systems.

It should has the same bandwidth as the corresponding monochrome signal.

The color signal should have the same brightness information as that of monochrome signal.

The location and spacing of the picture and sound carrier frequencies should remain the same.

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.

List the requirements of receiving antenna.

It must pick up the desired signal and develop maximum signal voltage from the available field strength.

It must discriminate against unwanted signals like EMI, ghost signals, co-channel interferences.

It must capable of wide band operation.