How Television Works
The Cathode Ray Tube
Almost all TVs in use today rely on a device known as the Cathode Ray Tube, or CRT, to display their images. LCDs and Plasma displays are sometimes seen, but they are still rare when compared to CRTs. It is even possible to make a television screen out of thousands of ordinary 60-watt light bulbs! You may have seen something like this at an outdoor event like a football game. Let's start with the CRT, however, because CRTs are the most common way of displaying images today.
The terms anode and cathode are used in electronics as synonyms for positive and negative terminals. For example, you could refer to the positive terminal of a battery as the anode and the negative terminal as the cathode. In a cathode ray tube, the "cathode" is a heated filament (not unlike the filament in a normal light bulb). The heated filament exists in a vacuum created inside a glass "tube". The "ray" is a stream of electrons that naturally pour off a heated cathode into the vacuum. Electrons are negative. The anode is positive, so it attracts the electrons pouring off of the cathode. In a TV's cathode ray tube, the stream of electrons is focused by a focusing anode into a tight beam and then accelerated by an accelerating anode. This tight, high-speed beam of electrons flies through the vacuum in the tube and hits the flat screen at the other end of the tube. This screen is coated with phosphor, which glows when struck by the beam.
A phosphor is any material that, when exposed to radiation, emits visible light. The radiation might be ultraviolet light or a beam of electrons. Any fluorescent color is really a phosphor - fluorescent colors absorb invisible (to us) ultraviolet light and emit visible light at a characteristic color. In a CRT, phosphor coats the inside of the screen. When the electron beam strikes the phosphor it glows. In a black-and-white screen there is one phosphor that glows white when struck. In a color screen there are three phosphors arranged as dots or stripes that emit red, green and blue light. There are also three electron beams to illuminate the three different colors together. There are thousands of different phosphors that have been formulated. They are characterized by their emission color and the length of time emission lasts after they are excited
As you can see in this 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, you can see in this diagram 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. The following pictures give you three different views of a typical set of steering coils:
(Note in the picture above the large black electrode hooked
to the tube near the screen - it is connected internally to the conductive coating)
The steering coils are simply copper windings . These coils are able to create magnetic fields inside the tube, and the electron beam responds to the fields. One set of coils creates a magnetic field that moves the electron beam vertically, while another set moves the beam horizontally. By controlling the voltages in the coils you can position the electron beam at any point on the screen.
In this figure, the blue lines represent lines that the electron beam is "painting" on the screen from left to right, while the red dashed lines represent the beam flying back to the left. When the beam reaches the right side of the bottom line it has to move back to the upper left corner of the screen, as represented by the green line in the figure. When the beam is "painting" it is on, and when it is flying back it is off so that it does not leave a trail on the screen. The term Horizontal retrace is used to refer to the beam moving back to the left at the end of each line, while the term vertical retrace refers to its movement from bottom to top.
As the beam paints each line from left to right, the intensity of the beam is changed to create different shades of black, gray and white across the screen. Because the lines are spaced very closely together, your brain integrates them into a single image. A TV screen normally has 525 lines visible from top to bottom.
All TVs use an interlacing technique when painting the screen. In this technique, the screen is painted 60 times per second but only half the lines are painted per frame. The beam paints every other line as it moves down the screen, for example every odd-numbered line. Then the next time it moves down the screen it paints the even-numbered lines, alternating back and forth between even-numbered and odd-numbered lines on each pass. The entire screen, in two passes, is painted 30 times every second. The alternative to interlacing is called progressive scanning, which paints every line on the screen 60 times a second. Most computer monitors use progressive scanning because it significantly reduces flicker.
Because the electron beam is painting all 525 lines 30 times per second, it paints a total of 15,750 lines per second. (Some people can actually hear this frequency as a very high-pitched sound emitted when the television is on.)
When a television station wants to broadcast a signal to your TV, or when your VCR wants to display the movie on a video tape on your TV, the signal needs to mesh with the electronics controlling the beam so that the TV can accurately paint the picture that the TV station or VCR sends. The TV station or VCR therefore sends a well-known signal to the TV that contains three different parts:
A signal that contains all three of these components is called a composite video signal. If your VCR has a composite video input capability, it is normally a yellow RCA jack. One line of a typical composite video signal looks something like this:
The horizontal retrace signals are 5 microsecond (abbreviated as "us" in the figure) pulses at zero volts. Electronics inside the TV can detect these pulses and trigger the beam's horizontal retrace with them. The actual signal for the line is a varying wave between 0.5 volts and 2.0 volts, with 0.5 volts representing black and 2 volts representing white. This signal drives the intensity circuit for the electron beam. In a black and white TV this signal can consume about 3.5 MHz of bandwidth, while in a color set the limit is about 3.0 MHz.
A vertical retrace pulse is similar to a horizontal retrace pulse but is 400 to 500 microseconds long. The vertical retrace pulse is serrated with horizontal retrace pulses in order to keep the horizontal retrace circuit in the TV synchronized.
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.
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, 8 cycles of a 3.579545 MHz sine wave are added as a color burst.
Following these 8 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:
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.
Getting the Signal to You
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 have 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 5 different ways to get a signal into your TV set:
The first four signals all use standard NTSC analog waveforms as described in the previous sections. Let's look at how normal broadcast signals arrive at your house as a starting point.
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.
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 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. The TV programs are encoded in MPEG-2 format and transmitted to earth. The set-top box does a lot of work to decode MPEG-2, then converts it to a standard analog TV signal
and sends it to your TV on channel 3 or 4.
Digital Television (DTV) is an exciting new digital broadcast standard that will provide vastly improved picture and sound quality compared to current NTSC (National Television Standards Committee) analog technology. DTV includes both single channel High Definition Television (HDTV) and multiple channel Standard Definition Television (SDTV). DTV will eventually replace analog television and will enable broadcasters to send a choice of more varied information over the airwaves, cable and satellite, including:
Both HDTV and SDTV will offer the exciting experiences of clearer, more detailed digital video and audio than todays NTSC.
Broadcasters are free to meet your viewing needs. They may choose the type and number of signals they transmit within their allotted bandwidth (6 MHz) and transmission rate (19.3 Mpbs). Depending on the features of your DTV receiver, a few of the benefits you will experience are:
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.