There are at present four types of camera tubes: Iconoscope, Im age Orthicon, Image Dissector, and Monoscope. A photograph of each type is given.
A brief discussion of the internal construction and operation of the camera tubes will assist the reader in connecting the scanning technique and transmitted picture to that of the picture tube for reproduction of the transmitted image.
ICONOSCOPE
The Iconoscope is simply a cathode-ray tube, specially designed for translating the scene to be transmitted into electrical impulses. It consists of an electron gun similar to that of the receiving tube, but instead of a fluorescent screen, a large rectangular plate of thin mica is used for a scanning area.
On the front side of this mica sheet are deposited many microscopic particles of cesium silver compound--a photo sensitive material. Each particle or globule is insulated from the other, which gives the mica sheet a mosaic appearance. The back of the mica sheet is covered with a conductive film which is connected to an output lead. The whole arrangement appears as myriads of small condensers or cells, with a common lead through which to discharge their stored energy.
In order to understand the action of the scanning beam, let us assume that no scene is being projected on the mica sheet or mosaic.
As the beam strikes the small particles of cesium silver, secondary emission takes place. The number of secondary electrons emitted is several times greater than the primary electrons in the beam which strike the particle. Since more electrons, which are of negative potential, are emitted than the number striking the particle, the potential of the particle will change in a positive direction. It will rise to an "equilibrium potential" of approximately positive three volts. The secondary electrons which have been emitted either go to the collector or to other parts of the mosaic.
Since each of the particles is insulated from all others, this "charge" cannot leak off. However, after the beam continues on its sweep, the particle which has been positively charged will attract secondary electrons which have been emitted from other particles in the mosaic. It will then change potential in a negative direction. Due to the abundance of free electrons on the face of the mosaic the particle will actually charge to approximately minus 1-1/2 volts. This action parallels very closely the method of obtaining bias in audio amplifiers known as "contact bias", with which we are familiar. The proximity of the grid to the cathode places it in a cloud of electrons which causes current flow in the grid circuit. In the case of the particle in the mosaic, however, there can be no current flow so the particle takes on a negative charge and maintains it until the beam again strikes it. It can now be seen that each particle changes from negative 1-1/2 volts to a positive three volts each time the beam strikes it.
The output from the Iconoscope is obtained from a resistive load which is connected between the conductive film on the back of the mosaic and ground. A certain capacitance exists between each of the particles and the conductive film. At the instant the beam strikes the particle, the charge on this capacitance cannot change, so the entire voltage will appear across the resistive load. A number of electrons equal to the amount lost by the particle will flow from ground to the conductive film to maintain the charge on the existing capacity. This current flow results in a four and one-half volt potential across the load.
Since in the above case no scene has been projected on the mosaic, the potential on each of the particles will change an equal amount. This results in no change in the amount of current flow in the load as the beam scans the mosaic.
Since there is no a-c component there is no output from the Iconoscope.
We have discussed the action of the tube with no image projected on the mosaic. In order to understand the action of the tube when illuminated areas are present on the mosaic, let us assume that half of it is illuminated. The cesium silver particles, as stated above, are photo sensitive ·and will emit electrons when struck with light. When the beam of electrons has passed over a particle which is being struck with light, the particle will attract free electrons. Since some electrons are being emitted due to the photo sensitive properties of the compound, the particle will not take on a charge of negative one and one-half volts.
Instead it will assume some charge in a positive direction from the negative one and one-half volts. The amount, of course, depends upon the amount of light present. For illustration purposes let us assume that the intensity of light present on the illuminated half of the mosaic is such that allows the illuminated particles to charge to a negative one volt. The particles in the non-illuminated area will charge to a negative one and one-half volts, as was the case of the non-illuminated mosaic. As the beam of electrons from the electron gun strike the non illuminated particles a change of four and one half volts takes place resulting in a four and one -half volt potential across the load. When the beam strikes the illuminated particles, however, the particle in charging to the "equilibrium potential" of plus three volts, will change only four volts since the original charge was only negative one volt. This results in a potential of four volts across the load giving an a-c component in the output.
When an image is projected on the mosaic each particle will charge to a certain potential depending on the amount of light present. As the scanning beam scans the mosaic horizon tally from left to right, and vertically from top to bottom, each particle will be returned to the "equilibrium potential" which causes current pulses to flow in the load resistor. This train of pulses, varying with the charge on the particles, constitutes the video signal.
The output from the Iconoscope is of negative polarity since there is less current flow when an illuminated particle is scanned than when a non-illuminated particle is scanned. Figure 24 shows the major elements of the Iconoscope.
COLLECTOR RINGS
Fig. 24. RCA Type 1850A Iconoscope (Drawing from Sample Courtesy RCA)
IMAGE ORTHICON
This tube is a more recent development than the Iconoscope and will function over a wide range of light values. It is ideal for out door pickups and other installations where the light available is not under the control of the pickup crew. For a better understanding of the operation of the Image Orthicon, refer to Figure 25 while studying the following paragraphs.
Light from the scene being televised is focused on the photo-cathode which is semi transparent. This photo-cathode emits electrons proportional to the amount of light striking the area. These electrons are accelerated toward the target by grid No. 6 and are focused by the magnetic field produced by an external coil. The target consists of a special thin glass disc with a fine mesh screen on the photo-cathode side. Focusing is also accomplished by varying the potential of the photo-cathode.
Fig. 25. RCA Type 2P23 Im age Orthicon (Drawings from Sample Courtesy RCA)
When the electrons strike the target, secondary emission from the glass takes place.
These secondary electrons are collected by the wire mesh , which is maintained at a constant potential of approximately one volt.
This limits the potential of the glass disc and accounts for its stability in varying intensities of light. As electrons are emitted from the photo-cathode side of the glass disc, positive charges are built up on the other side of the disc which vary with the amount of electrons which were emitted. Thus it can be seen that a pattern of positive charges are set up which correspond to the intensities of light of the scene which is being televised. This constitutes the image section of the Image Orthicon and the action described is completely independent of the electron beam and the scanning circuits of the tube.
The back side of the target is scanned with a low velocity beam from the electron gun. The beam is focused by t he magnetic field generated by an external coil and by the electrostatic field of Grid No. 4. The potential applied to Grid No. 5 adjusts the decelerating field between Grid No. 4 and the target. As the low velocity beam strikes the target it is turned back and focused on dynode No. 1, which is the first element of an electron multiplier. As the beam is turned back from the target, however, some electrons are taken from the beam to neutralize the charge on the glass. The greater the charge on the glass, the more electrons are taken from the beam. Thus, when the beam scans a more positively charged area, which corresponds to a brighter area in light intensity, fewer electrons are returned to dynode No. 1. This action leaves the scanned side of the target negatively charged while the opposite side is positively charged. Due to the extreme thinness of the glass disc target , however, these charges will neutralize themselves by conduction through the glass. This neutralization takes place in less than the time of one frame.
As the amplitude modulated stream of electrons strike dynode No. 1, secondary electrons are emitted. The number emitted is proportional to the number striking it. Several secondary electrons are emitted for each primary electron striking the element. These free electrons are then accelerated toward dynode No. 2 where, upon striking the element, more secondary emission takes place. This same process continues on through dynode No. 3, dynode No. 4, dynode No. 5 and the electrons are finally collected by the anode or plate.
Thus it can be seen that the electrons returned to dynode No.1 are amplified or multiplied many times before the signal reaches the anode.
The amount of multiplication per element is equal to the difference of secondary electrons emitted and the electrons striking the element.
The approximate gain of the multiplier section of this tube is 500. The load resistor of the Image Orthicon is connected from the anode to the power supply. More current flow in the multiplier, which corresponds to a dark area in the televised scene, causes more current flow in the load giving a negative output. A brighter area causes less current flow giving a positive output. Thus it can be seen that the output of the Image Orthicon is of positive polarity.
Figure 25 is an exploded view of the internal construction of the Image Orthicon.
IMAGE DISSECTOR
Fig. 26. Image Dissector Operation (Courtesy Farnsworth Television
and Radio Corp.)
Both camera tubes previously discussed are known as the "storage" type since their operation depends upon the neutralization of positive charges by the scanning beam. The Image Dissector, on the other hand, employs instantaneous scanning.
The tube consists of an evacuated glass cylinder which is closed at both ends. The elements within the tube are the photo-sensitive cathode, an anode, a shielded target having a small aperture and an electron multiplier. The cathode, upon which a cesium-silver oxide film has been formed, is placed at one end of the cylinder. The anode, whose purpose is to accelerate the electrons emitted from the photo-cathode, is a conductive coating on the inner surface of the cylinder. The target is placed near the other end of the cylinder, which is a plane glass end. The target is at the end of an electron multiplier which is used for amplification. In front of the target is a small aperture which will allow only a small portion of the electron image to fall on the target.
The entire cylinder is placed within a focusing coil which produces an axial magnetic field throughout the entire length of the cylinder. The horizontal and vertical deflection coils are also placed around the cylinder, and act also as a supporting framework.
In operation the scene to be televised is focused on the photo-cathode. Electrons are emitted from this cathode according to the amount of light striking that particular area. It can be said that an "electron image" is emitted from the cathode which corresponds to the optical image projected on the cathode.
This "electron image" is then accelerated toward the target by the anode which has a positive potential of several hundred volts. · The image is maintained in focus by the axial magnetic field of the focusing coil.
The "electron image" is deflected horizontally and vertically by the magnetic field set up by the sawtooth current flow in the deflection coils. As the "electron image" is deflected past the aperture, only a small portion of the image can strike the target. The image, however, is swept past the aperture in a series of 525 interlaced lines thirty times per second. Instead of a beam scanning the image, the entire image is scanned past the aperture which "dissects" the image. Thus the name, Image Dissector.
As the image is moved in front of the ,aperture a varying amount of electrons strike the target . The amount is dependent on the amount of light present in that particular area of the televised scene. As these electrons strike the target, secondary electrons are emitted which are drawn to the next element of the electron multiplier. Each element in the multiplier is maintained at a potential approximately 100 volts positive with respect to the preceding element. The electron multiplying action is similar to that which took place in the Image Orthicon . However , in the Image Dissector eleven stages are used to multiply the photo-cathode emitted electrons.
The amount of resolution obtainable from the Image Dissector is dependent on the size of the aperture in front of the target and to the ratio of the "electron image" to the optical image. As the aperture is made smaller the resolution increases, as is the case when the "electron image " is made larger. An aperture .012"x.012'' gives a good signal-to-noise ratio for 525-line resolution.
This size aperture is used in present day construction of Image Dissectors.
Figure 26 illustrates the operation of the Image Dissector.
Fig. 27. RCA Type 2F21 Monoscope (Drawing from Sample Courtesy RCA)
MONOSCOPE
Another cathode--ray tube used in transmission of the television signals is the Monoscope, shown in Figure 27.
It is used for testing and adjusting studio equipment and when its signal is transmitted by the station, it is useful for the proper adjustment of the receiving equipment. The primary difference of this tube from the other camera tubes discussed previously is the inclusion of a test pattern which is placed in the front of the tube envelope. This test pattern is then reproduced as the video signal.
The difference in amount of secondary emission of electrons between two materials is used. to produce the output. Usually a sheet of aluminum, which has high emission, is marked with high carbon content ink. Carbon has fairly low emission and as the electron beam scans the entire pattern , secondary electrons are emitted from both materials in proportion to their emission ratios. Any pattern with any line shape may be drawn on the aluminum sheet.
The Monoscope is a stable video signal source and, therefore, provides both the tele vision engineer and the service technician with a useful tool.
Fig. 28. Interlaced Scanning
SCANNING
All of the picture generating tubes discussed have associated external focus and deflection elements which cause the electron beam to scan the active picture surface at the front of the tube.
The video signal carries the picture information to be transmitted over the air.
Since the timing of the scanning process is very important, the video signal must contain other information in the form of electrical impulses. These impulses are termed blanking pulses, which blank out the return trace of the cathode-ray beam in the camera tube during fly-back time, and synchronizing pulses, which are utilized by the receiver to synchronize the horizontal and vertical sawtooth generators.
The path travelled by the beam across the screen of the picture tube should be identical to the path travelled by the beam in the camera tube so that the picture may be re constructed in the correct sequence at the receiver.
For picture resolution, the present standards for television broadcasts are 30 frames per second, each frame being constructed of 525 horizontal lines , using interlaced scanning. (If alternate lines be transmitted in such a way that two series of lines are necessary to produce a complete frame, the system is called interlacing.)
Therefore, to produce one frame of 525 lines interlaced, 262-1/2 horizontal lines are scanned on the first down sweep of vertical deflection, and the beam returns to the top and scans 262-1/2 alternate lines. The horizontal and vertical scanning traces are the result of passing current having a sawtooth form through the respective deflection coils. The rapid return of the electron beam, or retrace, for the start of the succeeding scanning function, is a result of the rapidly decreasing current in this sawtooth form. In or de r to produce interlaced scanning with 525 lines and 30 complete frames per second, the vertical sweep frequency must be 60 cycles per second, and the horizontal sweep frequency must be 15,750 cycles per second. Figure 28 further explains the complete scanning operation.