A color TV signal starts off looking just like a black-and-white signal. An extra chrominance signal is added by superimposing a 3. Right after the horizontal sync pulse, eight cycles of a 3. Following these eight cycles, a phase shift in the chrominance signal indicates the color to display.
The amplitude of the signal determines the saturation. Here is the relationship between color and phase:.
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. 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.
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. To the left of the video carrier is the vestigial lower sideband 0.
The sound signal is centered on 5. As an example, a program transmitted on channel 2 has its video carrier at 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. VCRs are essentially their own little TV stations. Almost all VCRs have a switch on the back that allows you to select channel 3 or 4.
The video tape contains a composite video signal and a separate sound signal. The VCR has a circuit inside that takes the video and sound signals off the tape and turns them into a signal that, to the TV, looks just like the broadcast signal for channel 3 or 4. The cable in cable TV contains a large number of channels that are transmitted on the cable.
Your cable provider could simply modulate the different cable-TV programs onto all of the normal frequencies and transmit that to your house via the cable; then, the tuner in your TV would accept the signal and you would not need a cable box.
Unfortunately, that approach would make theft of cable services very easy, so the signals are encoded in funny ways. The set-top box is a decoder. You select the channel on it, it decodes the right signal and then does the same thing a VCR does to transmit the signal to the TV on channel 3 or 4. Large-dish satellite antennas pick off unencoded or encoded signals being beamed to Earth by satellites. First, you point the dish to a particular satellite, and then you select a particular channel it is transmitting.
The set-top box receives the signal, decodes it if necessary and then sends it to channel 3 or 4. Small-dish satellite systems are digital. A digital TV decodes the MPEG-2 signal and displays it just like a computer monitor does, giving it incredible resolution and stability. There is also a wide range of set-top boxes that can decode the digital signal and convert it to analog to display it on a normal TV.
For more information, check out How Digital Television Works. Your computer probably has a " VGA monitor " that looks a lot like a TV but is smaller, has a lot more pixels and has a much crisper display. The CRT and electronics in a monitor are much more precise than is required in a TV; a computer monitor needs higher resolutions.
In addition, the plug on a VGA monitor is not accepting a composite signal -- a VGA plug separates out all of the signals so they can be interpreted by the monitor more precisely. Here's a typical VGA pinout:. This table makes the point that the signals for the three beams as well as both horizontal and vertical sync signals are all transmitted separately.
See How Computer Monitors Work for details. For more information on television, display types and related topics, check out the links on the next page. Sign up for our Newsletter!
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Do you understand the technology behind TVs? The only way we can see that this is actually happening is to blow the dots up so big that our brains can no longer assemble them, like this: Advertisement. TV Motion and Your Brain " ". The Cathode Ray Tube " ". Inside a CRT " ". TV Steering Coils " ".
Note the large black electrode hooked to the tube near the screen -- it is connected internally to the conductive coating. TV Phosphors " ". Electrical fields may cause a large number of ghost sightings. Learn about electrical fields and find out how electrical fields relate to ghosts. Intensity information for the beam as it paints each line Horizontal-retrace signals to tell the TV when to move the beam back at the end of each line Vertical-retrace signals 60 times per second to move the beam from bottom-right to top-left.
Composite Video Signal " ". TV Buying Guide. What Is 4K? Best TVs by Brand. Best TVs by Size. Best TV Accessories. In This Article. What Is a Smart TV? How Smart TVs Work. App Platforms by Brand. Benefits of Smart TVs. Additional Features. Extra Costs and Limitations. Extra: Smart TV Alternatives. Shopping Tips.
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Video Projector vs. TV: Which is Best for You? Home Theater Receiver Connections Explained. Main transmitters broadcast a larger number of channels compared to relay transmitters. All transmitters carry the most popular channels. To find out where your nearest transmitter is and see what channels you should be getting, just put your postcode into our Freeview Channel Checker.
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