How a standard Television works and what understanding of Physics was needed to develop it.
00 years ago it was merely a scientists dream. 70 years ago people such as Zworkyn and John Logie Baird proved the basics possible. 50 years ago owned only by the wealthy, they began to change the world. Today almost every household in Europe has at least one, they are used for entertainment, information and education.
This report aims to describe how a standard Television works and what understanding of Physics was needed to develop it.
(1) Background
Unlike modern Television sets the earliest were almost completely mechanical. The dream that was Television was a machine that could reproduce captured images using light, unlike photographs and film Television would store pictures electronically. The original mechanical Televisions could achieve this but were not alike Televisions based on the Cathode Ray tube (see next page). It is not surprising that the inventor of Television is greatly debated, there are claims of it being a Scotsman, a Russian and the Japanese. It is also a matter of opinion, a Russian called Zworkyn is accredited as inventing the first electrical Television, whilst John Logie Baird is accredited as the inventor of the first commercially possible mechanical Television. It also seems to depend on National opinion. One American Physicist and Historian explained how American companies invented the Television with "People from foreign countries contributed a little here and a little there over the years". When this American was talking about Television it seems he was talking about the commercial Television developed for broadcasting in America from 1939 but does not explain this clearly, however he does mention how the British Broadcasting corporation introduced Television broadcasting in 1936, 3 years before America.
Sources claiming it was a Scotsman include www.Kinema.com, Dorling Kindersley Science Encyclopedia
The definition of Television from the Oxford Dictionary is "A system for reproducing on screen visual images transmitted by radio signals"
The scanning method of mechanical Televisions used a disk patented by a Russian called Nipkow as early as 1884. The Disk he proposed was cut with 18 holes in a spiral pattern and was capable of scanning a scene in horizontal lines. The idea was that a photoelectric cell placed behind the disk would record the light intensity that was received through each hole by translating the light intensity into a voltage. A similar disk facing a screen, with a light bulb instead of a photoelectric cell would reproduce the picture by varying in intensity at a synchronised time to the original disk. This same idea was used in John Logie Bairds Television except this time the technology was more advanced, the signal could be transmitted though radio waves. (Example: see (1)).
Nipkows machine was only capable of 4000 dots every second.
Although John Logie Baird became famous his Television was not used widely. Instead electronic recording and displaying methods were used.
Zworkyn, was a Russian who travelled to America to develop his Television system. Eventually his work produced a camera and display that seem very alike today's Television. His camera called the Iconoscope, stored light intensity electronically before displaying on the Cathode Ray Tube, which he developed for Television. The Iconoscope used either a metal conductor covered with a sheet of silver caesium or aluminium film coated with potassium Hydride, unknown because I found conflicting sources, it is likely one was an earlier model. Both Silver Caesium and Potassium Hydride are photosensitive, when they are exposed to light electrons are lost in a number proportional to the intensity of light falling on them. The photosensitive dots would then have become positive, the charge was detected by an electron gun, which scanned the material. The picture was then shown using a Cathode Ray tube, which Zworkin pioneered. The equipment Zworkyn introduced was preparation for commercial Television.
All Televisions rely upon the detection of light intensity, only possible through the discovery and understanding of the photoelectric effect. This is the effect that light has on substances, causing them to lose electrons to their surroundings. When it was discovered the intensity of light was proven to compare directly with the number of electrons being released, the greater the intensity: the greater the number of electrons released. Also discovered was the effect of different light on the emission of electrons. Physicists at the time (late 19th Century) could not explain this, this was before the Quantum theory had been proposed. The only light theory was an early wave theory, which would not have explained why different light frequencies had different effects upon the release of electrons. The classic wave theory models light as having all the characteristics of a wave, believing the only differences between blue light and red light to be frequency and wavelength. This theory would have explained the photoelectric effect by energy given to electrons by oscillating. The findings would suggest that on certain substances a low intensity of high frequency light would release electrons that could not be released by a large intensity of low frequency light.
Einstein developed Plancks theory that when energy was absorbed it turned into Quanta into the modern quantum theory, 18 years after the discovery of the photoelectric effect by Hertz.
Einstein said that light is made from very small particles of energy, which he named quanta, that travel through the air arriving randomly, meaning that their arrival was not constant. The quantum theory and Plancks relationship between energy and frequency explained the photoelectric. Einstein said that one photon, no more, releases an electron from material. The photons will not be able to release an electron unless they have ...
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Einstein developed Plancks theory that when energy was absorbed it turned into Quanta into the modern quantum theory, 18 years after the discovery of the photoelectric effect by Hertz.
Einstein said that light is made from very small particles of energy, which he named quanta, that travel through the air arriving randomly, meaning that their arrival was not constant. The quantum theory and Plancks relationship between energy and frequency explained the photoelectric. Einstein said that one photon, no more, releases an electron from material. The photons will not be able to release an electron unless they have enough energy to release an electron from a material, called the activation energy. As it was found that different materials released electrons at different light intensities the relationship:-
E =hf
(Where E is the energy of a photon, h is the Planck constant and f is the frequency of light)
can be used to explain the relationship of the energy of a photon.
Kinetic energy = hfW = EW (2)
(Where W is the energy required to free an electron from the particular material)
If a photon with more energy than the activation energy releases an electron the extra energy must be lost as all electrons have the same charge. The extra energy must be lost as heat or as kinetic energy in the released electron so:
mv2 = hv - W
2
(2) Basics Of Television
The picture above shows a basic Cathode Ray Tube. A, the cathode is the source of the electrons, which are responsible for the overall picture. C, represents an anode, responsible for directing and accelerating the electrons. There can be up to 6 of these in a television. The rear of the screen is coated in small phosphor dots which the electron beams are aimed towards creating colour on the front of the screen.
The cathode is the negative terminal in a circuit. Its use in the Cathode Ray tube is similar to its use in electrolysis where the cathode releases electrons, which go through a molten substance, acting as a conductor, to the positive anode. This completes a circuit. In the Cathode Ray tube rather than a molten substance there is a vacuum. This allows electrons to pass through which would not happen air as the electrons would collide with molecules.
Colour Television is only possible as the three primary colours of light can be mixed in varying proportions to create almost any other colour. These primary colours are red, blue and green.
An electron gun is the part of the Cathode ray tube containing the cathode and the anodes. Colour Television needs three electron beams while a black and white Television only needs one, but this does not mean three electron guns are needed (see developments of colour TV).
Black and white Televisions use one beam as they only send a set intensity of electrons to each pixel, per pass. intensity set intensity of electrons to one pixel at a certain time. In a colour Television there are three beams as three different, controlled, intensities of electrons are needed at any one time, one beam for each pixel. The variables that I have found for an electron beam are:-
* The colour of the phosphor the beam will hit, determining the Intensity of electrons that are needed to reach the screen
* The positioning of the beam
Before the beam reaches the screen of a television, it must pass through the grid that stops dispersed electrons from hitting the screen, this is called a shadowmask and it has a current.
The electrons are accelerated towards the grid in two ways, either the voltage across the cathode, which releases the electrons, or the voltage across the grille is varied. An increase in the voltage of the cathode or the grid (anode) increases the potential difference across the tube therefore causing more electrons to be released. (Source: Television magazine)
A coil filament, often Tungsten, is covered with aluminium oxide heats the cathode in a standard Television. The cathode surrounds the filament (see fig 14) to absorb as much heat as is possible. Electrons are emitted when the cathode reaches a certain temperature by Thermionic emmission. A characteristic of all metals, electrons are emitted when a temperature of between 1000K and 3000K is reached caused by the free electrons in the metals heating up and becoming energised enough to leave the metal. As the metal in this case is a cathode the heater assists the flow of electric current through the vacuum.
A noticeable development in Televisions is that they have become much lighter over the years. An example of how this is achieved is by the change in design of filaments, now much smaller and lighter, emitting a smaller amount of energy but concentrating this more on the cathode which is also smaller. These advances also mean that the cathode takes less time to heat up. Older Televisions take a long time to warm up.
The energy required to remove an electron from a material is called the work function, measured in electron Volts it is a measure of kinetic energy. An electron Volt is the energy acquired by one electron passing through a potential difference of 1 volt in a vacuum. ( As Current = Energy x Voltage this is 1.6 x 10-19J, the same as an electron's charge)
eV = mv2
2
Properties of electron beams
* Travel in straight lines
* Can be deflected by an electric field
* Can be deflected by a magnetic field
A cathode releases a "cloud" of electrons, they then have to travel through a small hole in a grid. After this they are under the influence of an anode/ electromagnet which is under high voltage. (see magnets). The negative force of the anode and the negative force on the electron cause the two to repel (see magnets). The combination of the two repelling forces will cause a force to be exerted on the electron by the anode and on the anode by the electron. As every force has an equal and opposite reaction force the force on the electron will be great in comparison with the electrons size.
Force = Mass x acceleration
(Mass of electron = 1.6 x 10-19)
Acceleration = Force
Mass
Even if the force on the electron is very small the acceleration will be great. With no particles to slow the electrons greatly they retain much of their velocity, more anodes are needed however, to keep the velocity high and to focus the beam very finely. (A high velocity is required so that when the electrons hit the screen they have lots of kinetic energy to be transformed into light energy to produce a bright glow).
After the electrons have passed the first anode the may pass several more, depending on the Television. The anodes are arranged so that they force the electron towards the screen.
The magnetic force on an electron is given by the equation:
FM = Bev
Where Fm is the magnetic force acting on the electron, B is the magnetic field in flux, e is the charge on an electron and v is the velocity of the electron.
The electrons are focused into a beam, and aimed towards the particular pixel by other anodes.
The electrons are then put under the influence of two Anodes, one to accelerate the electrons and one to focus them into a beam. As two masses with the same charge, positive or negative, repel each other, the negative electrons and the negative anode repel. The high power Anode causes the electrons to be accelerated greatly. The other anode uses the same property to concentrate the electrons into a fine beam. I suspect that a fine beam is needed to make the picture sharp. I' have noticed that in old televisions the picture is not very sharp and the screen appears to 'glow' a lot more than newer televisions. The high speed of electrons is vital as to give them enough kinetic energy so that when they hit the Phosphor enough light energy is released to make the television bright and easily visible.
Magnets
As magnets effect electrons, electromagnets in a Television can be used to direct electrons to the desired position on the screen.
Force on electron
Where both current and velocity are in the same direction:-
Force (F) = -(constant) x charge (q) x Current (I) x velocity (v)
Distance (r)
Where r is the distance between the two particles
I do not want to look at this particularly into too much depth but the relationships seem clear.
Electromagnets
When magnetic metal is unaffected by an external field movement of electrons in a magnet is at random but when there is an external field the electrons align. (A solenoid is a coiled wire that causes whatever it is wrapped around to become very magnetic, I have seen this used to make non magnetic objects magnetic.)
"The combined magnetic fields of all these aligned electrons add to the original magnetic field and enhance it drastically"
In a Television the magnets that direct electrons are made from solenoids within the CRT with the power to direct the electrons to any point on the screen. The magnets effect the electrons perpendicular to the direction of their current
Interesting Facts
www.sciencejoywagon.com claims that 900,000 electrons are needed for one image on a colour Television. Using this fact I can predict the current that is likely to be travelling through the vacuum in the cathode ray tube. Each electron has a current of -1.6*10-19C. Every second there are 24 images on a television screen:-
24*900,000 = 21.6million electrons per second
Charge transferred/ second (Current) = 21600000*1.6*10-19 = 3.456*10-12 Amps
Phosphors
Looking at a television set in a dark room I noticed something else. If the television set is on and then turned off, the screen glows faintly and a vague outline of the last image remains. (I noticed this occurred more noticeably on a black and white television). The continuing glow is very much like that given out by 'glow in the dark' paint. When the TV set has been turned on and the phosphors have been 'charged' with electrons the phosphor will continue to glow, strongly at first before eventually becoming dull. Glow in the dark paint shares the same characteristics, when the paint is exposed to light it seems to take in energy from the light and then glow in the dark.
Howstuffworks.com had an article stating that there are three characteristics that make phosphors differ.
. The type of energy they absorb to become 'energised'. (In television this must be an electric charge given by the electrons.)
2. The frequency of light they emit. (The manipulation of the substances by doping gives a range of light. This being the important factor that makes the phosphors suitable for television.)
3. The length of time they glow after they have been energised. I expect that the phosphors on a television set have a short 'glowing time'. Without this I expect the picture would be very unclear as light would be emitted when it is not wanted.
Phosphors are man made materials. I found several examples of these including Zinc sulphide doped with small amounts of silver. The silver in this case is the 'activator' which causes the substance to glow in particular colour.
Television signal
The Television signal is sent as a voltage so that in Black and white Television all that is needed, for the picture, is one varying voltage. On a black and white television the screen is black when it is off so when the desired pixel shade is black no electrons need be sent to the screen. When the screen is white the maximum voltage is used to send the maximum number of electrons to the pixel, resulting in the brightest possible phosphor glow. The scanning pattern of a Television screen is shown by the image below. The electron beams are all aimed along these lines, starting from the upper left corner and finishing at the bottom right. The blue lines represent the lines of pixels where the beam must strike. The red dotted line represents the movement of the gun (not emitting electrons) as it travels from line to line, called flyback. The blue line represents the electron guns movement after it has scanned a full image and prepares to start again.
Analogue televisions receive a continuous signal that represents the whole screen. In Europe the image on the screen changes 25 times a second, therefore a continuous signal must represent a whole image every 0.04seconds. This meaning that receiving Televisions do not need to share the same number of pixels per line, as long as the electron beam scans the screen at the same rate, a similar picture will be shown.
Figure GSIT demonstrates this.
In the period in which the beam changes line the electron gun has to be inactive or there would be interference on the screen. At this time the signal has to be 'blanked'.
With everything taken into account the actual picture signal of a black and white Television should consist of a varying voltage representing the intensity of light needed for a line of pixels, followed by a period of no signal as the electron gun is repositioned to a different line. 25 times a second there should be an even greater blanked period while the electron gun repositions itself from bottom right of the screen, to top left. Synchronisation is achieved by telling the electron gun when to do what a with negative signal. The signal is negative so that there will be no confusion with the signal when it is 'painting', and is used twice. First as the gun finishes a line and secondly just before the blanking period finishes to tell the gun to start the next line. When the scan reaches the bottom right the signal uses several negative signals to tell it to go back and start again.
Considering all the time that the electron gun is not 'painting' the screen, the phosphors must hold some of the charge or the whole image would not appear at the same time.
Televisions also use interlace which means that they only scan half the screen every second before returning to scan the rest. As the electricity we get through our mains is 50Hz Televisions use this as timing, they go from top left to bottom right 50 times a second coinciding with this frequency, but the only scan every other line every time. Computer monitors scan the whole screen.
The image above demonstrates a US Television signal. The 42?s line represents the signal as the screen is painted. The dip represents the signal to tell the electron gun to get start a new line.
This image contradicts what I wrote about the signal being negative (reversed) yet even with a small voltage such as 0.5 volts when the electron gun is repositioned would cause disturbance on the screen, therefore I expect that the other source, 21 is correct.
Signal facts
* Television signals use electromagnetic radiation at frequencies at Ultra High Frequency, this contains any electromagnetic wave between 300MHz and 3000MHz. Television signals only use between 470MHz and 850MHz. This high frequency wave is needed to contain all the information for colour Television signal.
* The monochrome signal, this being the part of the signal that contains the intensity information for the light (this was the original signal used in Black and White Televisions, keeping this part of the signal allowed black and white Televisions to work on the same signal as colour Televisions)
* The colour signal, containing the information for the intensity of each colour needed, corresponding to the electron gun electron intensity
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: 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.
The sound signal The television bandwidth is 6 MHz. The sub-carrier for the colour is 3.58 MHz off the carrier for the monochrome information. The sound carrier is 4.5 MHz off the carrier for the monochrome information. There is a gap of 1.25 MHz on the low end, and 0.25 MHz on the high end to avoid cross-talk with other channels[12].
http://www.ee.washington.edu/conselec/CE/kuhn/ntsc/95x412.gifhttp://www.ee.washington.edu/conselec/CE/kuhn/ntsc/95x412.gif
Television has a maximum frequency bandwidth of 6 MHz. This says that the highest resolution signal is something like 1/6MHz or 166.7 nS. This is consistent with a 330 element scan line with a 8.7 uS blanking time.
The selection of the display primary colors occurred first -- due to the relatively small range of available phosphors. The camera filter characteristics were then established next in order to provide a matched system for accurate color representation. However, over the years, the camera and receiver filters have been deliberately mismatched in order to obtain higher brightness values. Typically, critical colors (such as flesh tones) are set to be accurately reproduced and non-critical colors are allowed to drift.
Therefore, modern color TV is carefully structured to preserve all the original monochrome information -- and just add on the color information on top. To do this, one signal, called luminance (Y) has been chosen to occupy the major portion (0-4 MHz) of the channel. Y contains the brightness information and the detail. Y is the monochrome TV signal.
*
Colour
In late 1949 the first colour television was introduced, the need for 3 separate electron guns meant that television technology had to improve, subsequently this TV only had a screen of 11cm. All of the early televisions used circular dots of phosphors arranged in a triangle (see figure 2, page 5), they were called Delta tubes. In 1972 Sony introduced a screen that used stripes of phosphors (see figure, page 5) which all Television screens now use. Delta gun tubes continue to use the circular phosphor dots rather than the stripes but they are not used for Television any more but they are used in computer monitors as they give a far higher definition when used for a very high resolution, with the use of many dots.
The electron guns are positioned in a triangle, perfectly similar to the triangles that appear on the screen. The electron guns are aimed so that the electron beams converge as they pass through the grille. The grille (see fig 2) is positively charged acting as an accelerator to the electrons and a stencil to make sure that the electrons only hit the desire phosphor dots. After the beams pass through the grille they hit the phosphor dots on the screen. With the delta tube (fig1) each electron beam has a separate set of electromagnets acting on it yet they are needed to travel to the same point.
It seems that Televisions stopped using Delta tubes because all three electron beams had different scanning patterns, rasters, as they had left the electron guns at different angles so would behave differently at different positions on screen. By looking at the diagram below it should be apparent that as the guns are quite far apart the beams will also be far apart so when they hit the screen they will not be the ideal distance apart.. The result of this is shown by fig 3
The Trinitron tube uses three electron beams that are in line, but only one gun. This means that the electron beams travel much closer together and the picture they produce is not made from round dots but the rectangular ones. By only having one electron gun with three cathodes, each beam can be put under the same magnetic field which means there is no need for calibration
In the Trinitron tube the magnetic field directs all three beams to a prism which directs the beams, instead of the solenoid electromagnets.
Analysis
Television was only made possible through comprehension of the phtoelectric effect which allowed scientists to detect light, as an intensity
