1. PICTURE TUBE

2. ELECTROSTATIC FOCUSSING The electric field due to the positive potential at the accelerating grid extends through the opening the of the control grid right to the cathode surface. The orientation of this field is such that besides accelerating electrons down the tube. It also brings all the electrons in the stream into a tiny spot called the cross over. This is known as the first electrostatic lens action. The electrode voltages are so chosen or the electric field is so varied that the second point where all the electrons get focused is the screen of the picture tube. Electrostatic focusing is preferred over magnetic focusing because it is not affected very much by changes in the line voltage.

3. DEFLECTION YOKE It may be noted that a perpendicular displacement results because the magnetic field due to each coil reacts with the magnetic field of the electron beam to produce a force that deflects the electrons at right angles to both the beam axis and the deflection field.

4. Deflection Yoke Akshay Jilowa G.P.C.G. Jalandhar4

5. BEAM DEFLECTION As already stated the electron beam must attain a very high velocity to deliver enough energy to the atoms of the phosphor coating. Because of this the electrons of the beam remain under the influence of the deflecting field for a very short time. This necessitates application of high deflecting fields to achieve the desired deflection. It is very difficult to generate such high voltages at the deflection frequencies. On the other hand with magnetic deflection it is a large current that would be necessary to achieve the same deflection.

6. Picture Tube The beam is deflected by a pair of deflecting coils mounted on the neck of the picture tube in the same way and rate as the beam scans the target in the camera tube. The amplitudes of the 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. Akshay Jilowa GPCG Jalandhar 6Punjab EDUSAT Society

7. Scanning The scene is scanned rapidly both in the horizontal and vertical directions simultaneously to provide sufficient number of complete pictures or frames per second to give the illusion of continuous motion. Scanning of the element is done at a very fast rate and this process is repeated a large number of times per second to create an illusion of simultaneously pick up & transmission of picture detail Scanning may be identified as a particular process which permits the conversion of information existing in space and time coordinates into time variations only. The electrical information obtained from the TV camera tube is generally referred to as video signal Akshay Jilowa GPCG Jalandhar 7Punjab EDUSAT Society

8. Horizontal Scanning Akshay Jilowa GPCG Jalandhar 8Punjab EDUSAT Society

9. Vertical Scanning Akshay Jilowa GPCG Jalandhar 9Punjab EDUSAT Society

10. Horizontal and Vertical Scanning Akshay Jilowa GPCG Jalandhar 10Punjab EDUSAT Society

11. COLOUR TELEVISION DISPLAY TUBES The colour television picture tube screen is coated with three different phosphors, one for each of the chosen red, green and blue primaries. The three phosphors are physically separate from one another and each is energized by an electron beam of intensity that is proportional to the respective colour voltage reproduced in the television receiver. Akshay Jilowa G.P.C.G. Jalandhar11

12. COLOUR TELEVISION DISPLAY TUBES The object is to produce three coincident rasters with produce the red, green and blue contents of the transmitted picture. While seeing from a normal viewing distance the eye integrates the three colour information to convey the sensation of the hue at each part of the picture. Akshay Jilowa G.P.C.G. Jalandhar12

13. COLOUR TELEVISION DISPLAY TUBES Based on the gun configuration and the manner in which phosphors are arranged on the screen, three different types of colour picture tubes have been developed. These are:- 1. Delta-gun 2. Guns-in-line or Precision-in-line (P-I-L) 3. Trintron Colour Akshay Jilowa G.P.C.G. Jalandhar13

14. Delta-gun colour picture tube Akshay Jilowa G.P.C.G. Jalandhar 14 (a) guns viewed from the base (b) electron beams, shadow mask and dot-triad phosphor screen (c) showing application of ‘Y’ and colour difference signals between the cathodes and control grids

15. DELTA-GUN COLOR PICTURE TUBE This tube was developed by Radio Corporation on America (R.C.A). It employs three separate guns on for each phosphor. The guns are equally spaced at 120 degree interval with respect to each other and tilted inwards in relation to the axis of the tube. They form an equilateral triangular configuration.

16. Drawbacks of the Delta-gun Tube Convergence is difficult and involves considerable circuit complexity and service adjustments. In most delta-gun tubes, four static convergence magnets and a dynamic convergence assembly are employed. The focus cannot be sharp over the entire screen because the focus and convergence planes cannot remain coincident for the three beams which emanate from guns positioned at 120° with respect to each other around the tube axis. The electron transparency of the mask is very low since it intercepts over 80 percent of the beam currents. Akshay Jilowa G.P.C.G. Jalandhar16

17. PRECISION-IN-LINE (P.I.L.) COLOUR PICTURE TUBE Akshay Jilowa G.P.C.G. Jalandhar 17 (a) in-line guns (b) electron beams, aperture grille and striped three colour phosphor screen(c) mountings on neck and bowl of the tube.

18. PRECISION-IN-LINE COLOR PICTURE TUBE This tube as the name suggests has three guns which are aligned precisely in a horizontal line. The gun and mask structure of the P.I.L tube together with yoke mounting. The inline gun configuration helps in simplifying conversions adjustments. The color phosphors are deposited on the screen in the form of vertical strips in triads which are repeated along the breadth of the tube. To obtain the same color, finest as in a delta gun tube the horizontal spacing between the strips of the same color in adjacent traids is made equal to that between the dots of the same color in the delta gun tube.

19. Trintron (cathodes in-line) colour picture tube Akshay Jilowa G.P.C.G. Jalandhar 19 (a) gun structure(b) electron beams, vertical-striped three colour phosphor screen (c) constructional, focus and convergence details

20. THANKS Akshay Jilowa G.P.C.G. Jalandhar20