Saturday, February 11, 2006

BLOCKDIAGRAM AND ITS BRIEF DESCRIPTION - Part 3

TRANSMITTER

The output of the video camera is fed to transmitter as modulating waves and these waves are super imposed over the carrier and transmitted as modulated waves. The carrier is designed for transmitting the picture details. At the receiving end, a small television set of 4” screen is used.

The transmitter circuit generates a continuous frequency of 100MHz approximately, which is used to form a permanent link between the transmitter and receiver, and this is known as carrier frequency. The output of video camera is fed to this carrier input as a modulating wave. This is a frequency modulated radio transmitter. The radiating power of the transmitter is less than 20 mw, so that the range between transmitter and receiver can be less than 25 feet. The detailed description is provided in the next chapter. For the demonstration purpose either black & white television set or computer is used, the block diagram of this simple TV along with its brief description is provided in this chapter. The details are as followed.

The block diagram of simplified block & white TV receiver shown below



In above block diagram, the receiving antenna intercepts radiated RF signals and the turner selects desired channels frequency band and converts it to common IF band of frequencies. The receiver employs two or three stages of IF amplifiers. The output from the last IF stage is de-modulated to recover the video signal. This signal that carries picture information is amplified and coupled to the picture tube, which converts the electrical signal back into picture elements of the same degree of black and white. The picture tube is very similar to the cathode-ray tube used in an oscilloscope. The glass envelope contains and electron-gun structure that produces a beam of electrons aimed at the fluorescent screen. When the electron beam strikes the screen. Light is emitted. A pair of deflecting coils mounted on the neck of picture tube in the same way as the beam of camera tube scans the target plate deflects the beam. The amplitudes of currents in the horizontal and vertical deflecting coils are so adjusted that the entire screen called raster, gets illuminated because of the fast rate of scanning.

The video signal is fed to the grid or cathode of picture tube. When the varying signal voltage makes the control grid less negative, the beam current is increased, making the spot of light on the screen brighter. More negative grid voltage reduces brightness. If the grid voltage is negative enough to cut-off the electron beam current at the picture tube, there will be no light. This state corresponds to black. Thus the video signal illuminates the fluorescent screen from white to black through various shades of grey depending on its amplitude at any instant. This corresponds to brightness changes encountered by the electron beam of the camera tube while scanning picture details element by element. The rate at which the spot of light moves is so fast that the eye is unable to follow it and so a complete picture is seen because of storage capability of the human eye.

The path of sound signals is common with the picture signal from antenna to video detector section of the receiver. Here the two signals are separated and fed to their respective channels. The frequency modulated audio signal is demodulated after at least one stage of amplification. The audio output from the FM detector is given due amplification before feeding it to the loudspeaker.

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