15_tv - Television How the Pictures are Painted How the...

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Television Television How the Pictures are Painted How the Pictures are Painted How the Signal is Communicated How the Signal is Communicated
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UCSD: Physics 8; 2007 2 The Main Job The Main Job The main job of a television is to paint a new picture The main job of a television is to paint a new picture 30 times a second 30 times a second sounds hard Caesar’s dictum: divide and conquer Can I make a dot on a screen? Can I make it any color I want? Can I put it in any position I want? Can I scan across and make many dots in a row? Can I do this all fast enough? Can I find anything decent to watch?
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UCSD: Physics 8; 2007 3 CRT makes light CRT makes light Cathode Ray Tube Cathode Ray Tube Cathode rays = beam of fast moving electrons TV “tube” shoots an electron beam at a phosphor screen electric or magnetic fields deflect the beam phosphor on inside of front screen lights up (glows) when hit by electrons E-field metal plates - - - - - - - + + + + + + + screen electron beam cathode emitter + +
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UCSD: Physics 8; 2007 4 Getting two dimensions Getting two dimensions Now orient two sets of plates Now orient two sets of plates orthogonal to each other and you can orthogonal to each other and you can deflect deflect left-right left-right AND AND up-down up-down If you “sweep” the electric field on If you “sweep” the electric field on one set of plates (let’s say the one set of plates (let’s say the left- left- right right ), you will draw a “trace” across ), you will draw a “trace” across the phosphor screen the phosphor screen This is how an oscilloscope works: This is how an oscilloscope works: – the beam is repeatedly swept left-to- right at a constant rate – meanwhile, an input signal “tells” the up-down plates what to do end result is a time-trace of what the input signal did beam is turned off for return trip to left A CRT uses coils of wire to make electro-magnets (instead of plates shown here), steering electrons with magnetic fields rather than electric fields
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UCSD: Physics 8; 2007 5 Drawing the Picture Drawing the Picture Scan, or Scan, or raster raster the beam from left to right to make a the beam from left to right to make a horizontal stripe of the picture horizontal stripe of the picture Turn off beam, and do a “carriage return/line feed” Turn off beam, and do a “carriage return/line feed” Upon arrival at lower right, scoot back to upper left Upon arrival at lower right, scoot back to upper left with beam off with beam off Now do it all over again… Now do it all over again…
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UCSD: Physics 8; 2007 6 TVs on TV TVs on TV Often when you see a TV or a monitor Often when you see a TV or a monitor on TV on TV , you see , you see a dark band moving slowly down the screen a dark band moving slowly down the screen Or a fast flicker of a computer monitor This is because the video camera is This is because the video camera is
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This note was uploaded on 04/10/2008 for the course PHYS 8 taught by Professor Tytler during the Spring '08 term at UCSD.

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15_tv - Television How the Pictures are Painted How the...

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