tailieunhanh - Thông tin thiết kế mạch P6

THE TELEVISION TRANSMITTER The transmission of video images depends on a scanning device that can break up the image into a grid and measure the brightness of each element of the grid. This information can be sent serially or in parallel to a distant point and used to reproduce the image. It is evident that the smaller the size of the grid element, the better the definition of the image. One of the simplest devices which can measure the brightness of light is the phototube | Telecommunication Circuit Design Second Edition. Patrick D. van der Puije Copyright 2002 John Wiley Sons Inc. ISBNs 0-471-41542-1 Hardback 0-471-22153-8 Electronic 6 THE TELEVISION TRANSMITTER INTRODUCTION The transmission of video images depends on a scanning device that can break up the image into a grid and measure the brightness of each element of the grid. This information can be sent serially or in parallel to a distant point and used to reproduce the image. It is evident that the smaller the size of the grid element the better the definition of the image. One of the simplest devices which can measure the brightness of light is the phototube. It consists of a cathode which is coated with a material which gives off electrons when light is shone on it and an anode which can collect the emitted electrons when a suitable voltage is applied to it. The cathode and anode are enclosed in an evacuated glass envelope. The number of electrons emitted by the cathode is proportional to the intensity of the light impinging on it. Assuming complete collection of the electrons the current in the resistor R shown in Figure will be proportional to the light intensity and so will the voltage across R. A primitive video signal can be generated by using a 3 x 3 matrix made up of phototubes as shown in Figure . For simplicity we assume that the tree is black and its background is white. A suitable lens focuses the image of the tree onto the matrix of phototubes. It is clear that the voltage output from phototubes 1 1 1 3 3 1 and 3 3 will be high all others will be low. The voltages so obtained can be transmitted and used to control the brightness of a corresponding 3 x 3 matrix of lights at a distant point giving a vague idea of what the tree looks like The picture detail can be improved by increasing the number of elements in the matrix so that each element corresponds to the smallest area possible. The assumption of a black tree on a white background is no longer .