Unformatted text preview: Television
How the Pictures are Painted How the Signal is Communicated UCSD: Physics 8; 2 The Main Job The main job of a television is to paint a new picture 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? Making a dot where we want it electric or magnetic fields deflect the beam phosphor lights up (glows) when hit by electrons UCSD: Physics 8; 2 TV "tube" shoots an electron beam at a phosphor screen +
+ + + + + + + cathode emitter E-field electron beam
- - - - - - - metal plates + UCSD: Physics 8; 2 Getting two dimensions Now orient two sets of plates orthogonal to each other and you can deflect left-right AND up-down If you "sweep" the electric field on one set of plates (let's say the leftright), you will draw a "trace" across the phosphor screen This is how an oscilloscope works: the beam is repeatedly swept left-toright 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 TVs use coils of wire instead of plates, steering electrons with magnetic fields rather than electric fields Drawing the Picture UCSD: Physics 8; 2 Scan, or raster the beam from left to right to make a horizontal stripe of the picture Turn off beam, and do a "carriage return/line feed" Upon arrival at lower right, scoot back to upper left with beam off Now do it all over again... UCSD: Physics 8; 2 TVs on TV Often when you see a TV or a monitor on TV, you see a dark band moving slowly down the screen Or a fast flicker of a computer monitor This is because the video camera is also recording 30 frames per second strobes the screen mid-paint dark band is faded phosphor from earlier paint job the slow motion is due to camera not being exactly at 30 Hz if it were exactly so, the band would appear frozen UCSD: Physics 8; 2 Monkey with Brightness The electron beam is generated from a hot electrode called a cathode cathode is heated by a filament, like a light bulb thermionic effect: boiling off electrons electrons are attracted by positive plate, and collimated by magnetic field The intensity of the electron beam depends on the current through the cathode vary the current vary the brightness UCSD: Physics 8; 2 Making Color So far we can paint black and white pictures (actually grayscale) by varying brightness Generating color requires three tricks: use colored phosphors (red, green, blue) instead of just white use three separate electron guns, one for each color mask the phosphor screen so that the green gun can only hit green phosphor, etc.
C RGB Triads UCSD: Physics 8; 2 UCSD: Physics 8; 2 Numerical Logistics Need to paint a new picture every 33 ms but 30 Hz update might appear to flash solution: interlace draw every other line in 1/60th of a second, then go back and fill in the gaps appears completely smooth to our eyes Need to paint 525 lines total that's how many are in each frame and doing this 30 times per second means 15,750 lines per second means horizontal sweep has 15.75 kHz frequency it's the high-pitched whine you hear when a TV is on This is the way you have to modulate the horizontal steering mechanism to raster lines UCSD: Physics 8; 2 Telling the TV what to do The video signal (such as what comes from your DVD or VCR) tells the TV how and when to sweep Every 63.5 s, get a signal to start a new line (5 s dip) called horizontal sync At bottom of screen, get 400 s dip to signal arrival at bottom called vertical sync In between is brightness information: E.g., 2 V means white, 0.5 V means black Frequency Allocation 2, 3, 4 cover 5472 MHz 5, 6 cover 7688 MHz 713 cover 174216 MHz 1483 cover 470890 MHz (UHF) UCSD: Physics 8; 2 TV channels require 6 MHz of bandwidth each Amplitude modulation (AM) + all kinds of sneaky tricks had to add on color after-the-fact basically treat video signal as waveform, and AM accordingly audio is FM (Frequency Modulation), however UCSD: Physics 8; 2 Liquid Crystal Displays (LCDs) LCD displays use totally different technology LCD is backlit with fluorescent tubes (usually around side) each pixel "addressed" with horizontal conductor on back, transparent vertical on front, forming matrix of connectivity amount of current through pixel determines its brightness filters in front of R, G, B pixels create color each pixel usually has own transistor (TFT on glass!) and capacitor to switch and hold charge UCSD: Physics 8; 2 Plasma Displays Plasma is an ionized gas atoms of neon and xenon have an electron stripped off Electrons collide with phosphor, which gives off light Plasma displays use the same phosphors as CRTs, accounting for the extremely accurate color reproduction Burn-in is a problem: static images burn the phosphor, to leave a permanent "ghost" http://en.wikipedia.org/wiki/Plasma_displays Rear glass Horizontal electrodes Phosphor Vertical electrodes L i g h t
Phosphor Front glass UCSD: Physics 8; 2 High Definition Television (HDTV) All digital (except in Japan) Similar to DVD encoding Digital Versatile Disk, ne Digital Video Disk 111 111 111 100 100 100 001 001 001
Data Compression Advanced micro-computers Two-dimensional Fourier Transforms Variable length encoding Motion estimation Differential encoding Backward prediction 111111111100100100001001001 Data Compressio n >100:1 11111000100101 UCSD: Physics 8; 2 References and Assignments Extensive support for this lecture from: http://electronics.howstuffworks.com/tv.htm Read pp. 478488, 495496 (for Thursday, 5/25) HW 6 due 5/25: 13.E.19, 13.E.21, 13.E.22, 13.E.24, 13.E.25, 13.E.26, plus additional required problems accessed via assignments web page Q/O #4 due Friday (5/26) by 6PM ...
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- Fall '10
- Physics, Magnetic Field, Cathode Ray Tube