 |
|
|
|
|||||
|
| اعلانات |
|
|||||
|
|
| اعلانات اضافية ( قم بتسجيل الدخول لاخفائها ) | |
|
|
|
|||||
|
التلفزيون الملون
Color TV Screen A color TV screen differs from a black-and-white screen in three ways:
The following figure shows how the shadow mask works:   When a color TV needs to create a red dot, it fires the red beam at the red phosphor. Similarly for green and blue dots. To create a white dot, red, green and blue beams are fired simultaneously -- the three colors mix together to create white. To create a black dot, all three beams are turned off as they scan past the dot. All other colors on a TV screen are combinations of red, green and blue. Color TV Signal A color TV signal starts off looking just like a black-and-white signal. An extra chrominance signal is added by superimposing a 3.579545 MHz sine wave onto the standard black-and-white signal. Right after the horizontal sync pulse, eight cycles of a 3.579545 MHz sine wave are added as a color burst.  Following these eight cycles, a phase shift in the chrominance signal indicates the color to display. The amplitude of the signal determines the saturation. The following table shows you the relationship between color and phase: ColorPhaseBurst0 degreesYellow15 degreesRed75 degreesMagenta135 degreesBlue195 degreesCyan255 degreesGreen315 degrees A black-and-white TV filters out and ignores the chrominance signal. A color TV picks it out of the signal and decodes it, along with the normal intensity signal, to determine how to modulate the three color beams.
التعديل الأخير تم بواسطة : ahmedfarghaly بتاريخ 17-04-2008 الساعة 04:27 PM |
|
|||||
|
TV Broadcasts Now you are familiar with a standard composite video signal. Note that we have not mentioned sound. If your VCR has a yellow composite-video jack, you've probably noticed that there are separate sound jacks right next to it. Sound and video are completely separate in an analog TV. You are probably familiar with five different ways to get a signal into your TV set:
The first four signals use standard NTSC analog waveforms as described in the previous sections. As a starting point, let's look at how normal broadcast signals arrive at your house. A typical TV signal as described above requires 4 MHz of bandwidth. By the time you add in sound, something called a vestigial sideband and a little buffer space, a TV signal requires 6 MHz of bandwidth. Therefore, the FCC allocated three bands of frequencies in the radio spectrum, chopped into 6-MHz slices, to accommodate TV channels:
The composite TV signal described in the previous sections can be broadcast to your house on any available channel. The composite video signal is amplitude-modulated into the appropriate frequency, and then the sound is frequency-modulated (+/- 25 KHz) as a separate signal, like this:   To the left of the video carrier is the vestigial lower sideband (0.75 MHz), and to the right is the full upper sideband (4 MHz). The sound signal is centered on 5.75 MHz. As an example, a program transmitted on channel 2 has its video carrier at 55.25 MHz and its sound carrier at 59.75 MHz. The tuner in your TV, when tuned to channel 2, extracts the composite video signal and the sound signal from the radio waves that transmitted them to the antenna. |
|
|||||
|
Inside a CRT As you can see in the below drawing, there's not a whole lot to a basic cathode ray tube.   There is a cathode and a pair (or more) of anodes. There is the phosphor-coated screen. There is a conductive coating inside the tube to soak up the electrons that pile up at the screen-end of the tube. However, in this diagram you can see no way to "steer" the beam -- the beam will always land in a tiny dot right in the center of the screen. That's why, if you look inside any TV set, you will find that the tube is wrapped in coils of wires. On the next page, you'll get a good view of steering coils. |
