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A glossary of television terms most commonly used is presented on the following pages. These terms will assist the reader in acquiring knowledge in interpretation of the function of the numerous components involved as well as to review the numerous technical terms used in the text. Absorption Trap. A parallel-tuned circuit coupled either magnetically or capacitively to absorb and attenuate undesired frequencies. Accelerator. The second anode of a cathode-ray tube. This anode, operated at a high positive potential with respect to the cathode, increases the velocity of the electron stream and is therefore referred to as an accelerating anode. Accompanying Audio (Sound) Channel. The RF carrier frequency which supplies the sound that accompanies the picture. Also called co-channel sound frequency. Active Lines. The lines which produce the actual picture, as distinguished from those which occur during the blanking time. Adjacent Audio (Sound) Channel. The RF carrier frequency which carries the sound modulation associated with the next lower-frequency television channel. Amplitude Separation. Separation of signal components by virtue of their various amplitude excursions, usually accomplished by means of a clipper. Aperture Mask. A thin sheet of perforated material placed directly behind the viewing screen in a three-gun color picture tube to prevent the excitation of any one color phosphor by either of the two electron beams not associated with that color. Also called Shadow Mask. Aquadag Coating. A conductive coating formed by a colloidal solution of carbon particles on the surface of the glass envelope of picture tubes. It is also placed on the inside of the tube to collect secondary electrons emitted by the fluorescent screen. On the outside of the tube, it forms one plate (with the inner coating the second plate) of a capacitor for the high-voltage filter circuit. Array. A combination of antennas. Aspect Ratio. The ratio of the width to the height of a television picture. Under present television standards, the picture aspect ratio is 4 to 3. Background. Average illumination of a scene. Bands. This refers to a group of continuous frequencies occupying "room" in frequency space. Bandpass Filter. An electrical network designed to transmit a band of frequencies and to reject all other frequencies. Beam Current. The current in the stream of electrons in the cathode-ray tube. The beam current rarely exceeds 2502a and is normally less than 100µa. Black Level. In the television receiver, the video signal is applied to the cathode-ray tube. Portions of this signal drive the tube to cutoff and produce the black portions of the picture. Those portions of the video signal which drive the tube beyond cutoff are said to be below the black level. Blacker than Black. The region of amplitude excursion of the video signal which corresponds to levels lower than black in the picture. These are not seen on the picture tube when the background is correct. This region is occupied by sync signals. Blanking. The process of applying voltage to the cathode-ray tube to cut off the electron beam during the retrace or flyback periods. Blanking Pedestal. A voltage pulse used to drive the cathode-ray tube beyond cutoff during the time the spot is returning from right to left or from the bottom to the top of the picture. These blanking pedestals must be synchronized with the sweep circuits so that the beam is cut off at the right time. Brightness. The intensity of the light produced at the screen of a cathode-ray tube. Brightness Control. In the television receiver, the adjustment which varies the average illumination of the picture by varying the bias on the cathode-ray tube. Brilliance. Same as brightness. Cathode-Ray Tube. An electron tube which converts electrical energy into light by projecting a beam of electrons upon a fluorescent screen. The screen glows at the point where the electrons strike it, producing a spot of light. By deflecting the electron stream, the spot may be made to trace a pattern corresponding to the deflection voltage. Centering. The process of moving the center of the image to coincide with the center of the cabinet opening which frames the picture. Centering Control. An adjustment for moving the raster electrically in either a horizontal or vertical direction for framing the image. Clamper. A circuit which establishes the DC level of a waveform (the baseline of an AC wave with a DC component). Clampers are also known as DC restorers. Clipper. A circuit designed to remove all of a waveform above or below a given level. Clipping Level. The amplitude level at which a waveform is clipped. Contrast Control. An adjustment for increasing or decreasing the range of light intensities of an image by varying the amplitude of the picture signal. Contrast control in the television receiver corresponds to gain control in a sound receiver. Composite Signal. A television signal whose waveform is com posed of both video and synchronizing signals, each having different amplitude excursions. Composite Sync. A signal composed of horizontal sync signals, vertical sync signals, and equalizing pulses. Contrast. The total range of light intensities between the darkest and brightest portions of an image on the television screen. Cutoff Frequency. A frequency beyond which no signals are transmitted or utilized. It may refer to an upper limit, a lower limit, or both. Deflection. A process whereby an electron beam is deviated from its straight-line path by means of an electrostatic or electromagnetic deflection. Deflection Coils. Coils placed around the neck of a cathode-ray tube to deflect the electron stream. The magnetic field created by the flow of current through the deflection coils causes the electron stream to deviate from its normal path. This system is electromagnetic deflection. Demodulation. The derivation of a waveform having substantially the same form in time as the amplitude or frequency modulation of a carrier. Differentiating Circuit. A circuit arranged to derive an output potential which is proportional to the time rate of change of the input current. Dipole. A linear conductor whose length is approximately one half the optimum wavelength of resonance, generally used as a television antenna. It is usually divided in the middle into two arms, where the impedance is lowest (72 ohms theoretically) for connection to a transmission line lead-in. Director. A dipole placed in front of a dipole antenna, toward the transmitter, to narrow the angle of reception in order to obtain greater directivity. No connection is made to a director. Dissector. A type of pickup tube used in the television camera, more properly referred to as an image dissector. The scene to be televised is focused through a system of lenses upon a photosensitive surface. The electron emission from every point on this surface is directly proportional to the intensity of the light falling upon that point. Since emission takes place simultaneously from all points on the surface, an electron image corresponding to the optical image is formed. This electron image is deflected in such a manner that a small portion of it at a time passes through a window or aperture, on the other side of which is an electron-multiplier tube. The output contains signal currents corresponding to the optical image. Double-Tuned Circuits. These are circuits resonant to two frequencies, usually closely adjacent and coupled in such manner as to show two values of peak response, approximately equal, with a dip-response between. Dynode. An intermediate electrode between the cathode and plate of an electron-multiplier tube. The dynode emits many secondary electrons for each incident electron striking it. Echo. Usually a pulse signal of lower amplitude than the parent primary pulse from which it originates by reflection, and occurring at a later time than the primary pulse. An echo pulse usually exhibits some phase distortion. EIA Signal. This is a composite signal composed of video signals and EIA sync signals. It has been standardized by the Electronic Industries Association. Electron Gun. An arrangement of electrodes inside a vacuum tube which will direct electrons from many directions, falling upon one end of it into a beam emerging from the other end. The velocity of the emerging beam may differ from that of the entering electrons. Electron Multiplier. A device arranged to receive electrons at an input and to deliver a greater number of electrons to an output. The increase in number is due to multiplication by secondary emission in one or more stages. Electron Optics. This refers to the treatment of electric fields as lenses for electron beams, similar to treatment of ordinary lenses in ordinary optics in regard to ordinary light beams. Electrostatic Scanning. The deflection of electrons from a straight-line path by means of an electrostatic field of force, which depends upon the force at a distance between electric charges. Equalizing Pulses. Horizontal sync pulses occurring at twice line frequency and of half normal duration. Field. The picture information produced by scanning the image from top to bottom in the standard interlaced scanning system. The odd and even lines are scanned separately, thus two fields are necessary to produce the complete picture. Field Frequency. The number of fields scanned per second. Under present television standards, this frequency is 60 fields per second. Field Repetition Rate. The number of fields transmitted per second. Flyback. In cathode-ray tubes, the return of the spot between successive sweeps. Flyback is also known as retrace. In some oscilloscopes and in all television receivers, the cathode-ray tube is biased beyond cutoff during this period. Fluorescent Screen. The face of a cathode-ray tube when the inside of the glass is coated with phosphor. Focusing Control. The adjustment which varies the potential of the first anode in a cathode-ray tube. When it is properly adjusted, the stream of electrons converges to a sharp point at the exact instant it strikes the fluorescent screen. Folded Dipole. A dipole antenna in which the outer ends of the two arms are connected together by a linear conductor, located at a small distance, one inch or so, away. Surge impedance is 300 ohms. Frame. The total picture information contained in a scanned image. In the standard interlaced scanning system, one frame consists of two fields. The frame frequency is therefore equal to one-half of the field frequency, or 30 frames per second. Frequency Band. A region of frequencies, extending between limits, each frequency being adjacent to another, without gaps. Ghost. A duplicate image on the screen of a television receiver. The ghost image is caused by a reflected signal which arrives at the receiver a short time after the direct signal. Halation. The glowing of a phosphor on the fluorescent screen, in a region immediately surrounding the scanning spot. Height. The amplitude of a picture in the vertical direction. High Voltage. A potential, usually above 500 volts, utilized usually in television equipment for accelerating or speeding up an electron beam. High voltages can be dangerous to life. Hold Controls. The adjustments which control the free-running frequencies of the horizontal and vertical sweep oscillators in a television receiver. Horizontal. Pertaining to the line structure of a picture in a directional parallel to the ground; normally it refers to the dimensions of width. Horizontal Blanking. The application of cutoff bias to the cathode-ray tube during the horizontal retrace. Horizontal Centering Control. The adjustment which permits the television image to be shifted in the horizontal direction so that it may be centered on the screen. Horizontal Flyback. The return of the spot after each horizontal sweep. It is also known as horizontal retrace. Horizontal Frequency. The number of times per second the spot sweeps across the screen in the horizontal direction. It is also referred to as the horizontal repetition rate. In standard television practice, the horizontal frequency is 15,750 sweeps per second. Horizontal Resolution. The number of picture elements which can be distinguished in each line of the picture. Horizontal Retrace. The return of the beam across the width of the image after the scanning of one line. Iconoscope. A television pick-up tube consisting of a mosaic of photosensitive elements upon which an optical image may be projected through a window, and arranged to be scanned by an electron beam which releases the stored charges in the latent image on the mosaic and produces an electrical signal in time sequence with the scanning, at an output electrode. Image Dissector. A device for dissecting an electron image, picture element by picture element, to derive there from an electrical signal arranged in a time sequence. Image Orthicon. A television pickup tube which embodies the combination of dissector and orthicon principles to produce a very high level of light sensitivity. Integrating Circuit. A circuit arranged to derive an output potential which is proportional to the stored value of the input cur rent over each cycle. Interlaced Scanning. A system of scanning in which only a fraction of the image is scanned during each field. In the standard interlaced scanning system, the odd lines and the even lines are scanned as separate fields. Each field therefore contains 262.5 or the total 525 lines. Ion. An atom having more or less than its normal number of electrons. A balanced atom has an equal number of protons and electrons. If such an atom loses one of its electrons, it assumes a positive charge (positive ion). If the atom should gain additional electrons, it assumes a negative charge (negative ion). Ion Trap. An arrangement of magnetic fields which will allow an electron beam to pass through but will deflect ions. Keystone Distortion. A form of distortion which causes the television image to take the shape of a trapezoid even though the mosaic in the pickup tube is rectangular. Keystone distortion is due to the fact that the electron stream does not strike the mosaic at right angles. This distortion is normally corrected in the transmitting equipment. LIB Circuit. A time-determining circuit in which the time constant depends on the ratio of inductance to resistance. Line. One of the strips which make up a television image. The scanning path across the width of a television raster. Linearity. The uniform distribution of picture elements over the total area of the image. Such uniformity can be achieved only if the sweep waveforms are linear. Linearity Control. An adjustment in the vertical or horizontal sweep circuit which controls the linearity of the sawtooth and consequently the uniform distribution of the picture elements of the image. If the sawtooth is not linear, the spot sweeps across the screen at a varying rather than at a constant rate, with the ultimate result that the image is spread out near one edge of the picture and crowded toward the opposite edge. Line Doubling. The technique of inserting line sync pulses at double frequency during the preparatory interval that pre cedes the field sync signal. The pulse width of the doubled pulse is cut in half so that integrating circuits will not store up too much energy in this period. Line Sync. This refers to sync pulses at horizontal frequency. Monoscope. A pattern-signal generating tube which produces in the proper circuit a time sequence of pulses equivalent to a fixed television signal. The pattern usually contains a resolution chart. Mosaic. A photosensitive surface consisting of a large number of individual cesium-silver globules. (See Iconoscope.) Negative Transmission. Modulation of the picture carrier in such a manner that the dark portions of the image cause an increase in radiated power and the bright portions cause a decrease. Noise. The word noise has carried over from audio practice. It refers to random signals which produce a "salt-and-pepper" pattern over a picture which is called "noisy." Also called snow. Nonlinearity. The crowding of picture elements from side to side, or the crowding of lines at either top or bottom of the picture. Odd-Line Interlace. This refers to a double interlace system in which there is an odd number of lines in each frame, and in which also, therefore, each field contains a half line extra. Orthicon. A television pickup tube somewhat similar in structure to an iconoscope but with a translucent mosaic, a collector ring instead of a backing plate for deriving output signals, and operated on different principles whereby the scanning beam is at low velocity and always at right angles to the plane of the mosaic, which in practice avoids shading signals usually generated in the iconoscope. Overcoupled Circuits. Usually two, resonant circuits tuned to the same frequency but coupled so closely as to exhibit two response peaks with a slight valley between, in order to obtain broad-band response with substantially uniform impedance. Pairing. A partial failure of interlace in which the lines of alternate fields do not fall exactly between one another but tend to fall nearly on top of one another. The cause is usually improper timing of the field-deflection oscillator but is sometimes due to pickup of stray fields, and the result is a raster consisting of separated pairs of lines rather than with a continuous line structure. Pass Band. A band of frequencies which is transmitted freely without intentional attenuation or reduction in amplitude of signals. Peaking Coil. A small inductance placed in a circuit to resonate with the distributed capacitance at a frequency where it is required to develop peak response, as in a video amplifier near cutoff frequency. Pedestal. A pulse, such as the blanking pulse, used in television systems. (See Blanking Pedestal.) Phase Distortion. This refers to phase delays at different frequencies being of different magnitudes, which distorts peak values of the signal and spoils picture contrast and/or re solution. Phosphor. The chemical coating deposited on the face of a cathode-ray tube. This chemical produces light when born-barded by electrons. Various chemicals are employed in practice to produce different colors. Phosphorescence. Light given off by a phosphor after the exciting light or electron stream has ceased to act. The same as persistence and afterglow. Photocell. A device for converting variations of light intensity or color into equivalent electrical variations. Photoconductive. The name applied to a substance which changes its electrical conductivity under varying degrees of illumination. Photoemissive. The name applied to a substance which emits electrons when struck by light. Cesium and rubidium are examples. Pickup Tube. A tube used in the television camera for the purpose of converting the optical image into its electrical equivalent. Picture Frequency. The same as frame frequency. In standard practice, the picture frequency is 30 per second. Picture Element. An elementary area of an image field which represents one detail and is relatively uniform in illumination. The shape of a picture element in television is considered to be square, even though the aperture of scanning spot is round, because the lines are uniform and rectilinear. The ratio of the area of an image field to the area of a picture element is representative of the detail of a television image. For a 525-line television picture, the maximum detail which can be transmit ted (with equal horizontal and vertical resolution, or square picture elements) is about 330,000 picture elements. Polarity. This refer to the direction, plus or minus, of a potential peak of a voltage. Positive polarity of a video wave at the grid of a cathode-ray tube means that the potentials are in the right direction to give a positive or normal picture. In this case the pedestals, or blanking signals, have their peaks in the negative direction to cut off the beam current during the occurrence of black. Thus, in a positive picture, black is negative. Polarization. This refers to the direction of vibration of the electric field of force in a radiated wave. The magnetic field of force is perpendicular to the electric, and so it also is defined. Positive Transmission. Modulation of the picture carrier in such a manner that the bright portions of the televised scene cause an increase in radiated power, and the dark portions cause a decrease. Positive transmission is also called positive modulation. Raster. The rectangular area scanned by the electron beam in the picture tube. Reflections. This has two meanings in television; it refers to reflected waves from structures or other objects, and also to shadows in the picture produced by these reflected waves. Reflector. A dipole placed behind a dipole antenna, away from the transmitter, to intensify the received signal. No connection is made to a reflector. It is usually spaced away at one-quarter wave-length for the desired signal. Relaxation Oscillator. A relaxation oscillator is a generator of electric current waves whose amplitudes vary between negative cutoff and positive overload, as limits. In essence, a relaxation oscillator is a violently regenerative device for which many circuit arrangements exist in practice. Retrace. The return path of the electron beam as it is swept back across the raster on the cathode-ray tube face after the completion of each scanning line and field trace. RC Circuit. This refers to a time-determining network composed of resistors and capacitors in which the time constant is the product of resistance by capacitance. Resolution. That quality of a television image which enables an observer to distinguish fine detail. Resolution Chart. A test pattern containing a number of converging lines. The point on the screen where these lines seem to merge into one, determines the maximum resolution of the image. Resolution is normally indicated as the number of lines which can be distinguished as individual. Return Time. This is the time required for retrace or flyback of the electron beam at the end of the scanning of the raster. RF Response. This refers to the wide-band acceptance of signals in a television receiver and defines the selectivity for signals lying outside of the channel being received. Retrace Ghost. An image produced during the retrace period. It may be due to improper blanking of the camera at the transmitter. Return Period. The time required for the spot to return after each sweep. It is also referred to as return time. Sawtooth. A voltage or current waveform which rises linearly to its peak value and then drops rapidly back to its starting level. The sawtooth waveform is used extensively for sweep or scanning in oscilloscopes and television equipment. If the sawtooth is not linear, the spot will move across the fluorescent screen at a varying rate and the pattern will appear to be crowded toward one side. Scanning. The process of exploring an image, usually with an electron beam, in a predetermined pattern. In standard television practice, scanning of an image is accomplished in 525 horizontal lines. Scanning Spot. This refers to an electrical window which scans an image field. Usually it refers to the size of the cross section of an electron beam used in a television pickup tube. In the image dissector it refers to the size of the aperture across which the extended electron image is scanned. Separator. A clipping circuit used to remove a portion of a wave form by virtue of its amplitude. In the television receiver, a separator circuit is used to extract the synchronizing pulses from the composite signal. Second Anode. This usually refers in television practice to the highest potential connection of a cathode-ray tube. Connections to the second anode supply the power for giving the electron beam its final, high level of energy. Secondary Electron. An electron which has been knocked out of the surface of a metal during bombardment by other electrons, called primary electrons. Secondary Emission. This refers to the phenomenon of knocking secondary electrons out of a surface by means of bombarding that surface with primary electrons. Serrated Vertical Pulses. The wide vertical synchronizing pulse is divided into a number of narrower pulses in order to prevent loss of horizontal synchronization during vertical flyback. Series Peaking. The technique of introducing a peaking coil in series with a resistor as the plate load of a vacuum tube to produce peaking at some desired frequency in the pass band. Serrated Signal. This consists of serrated pulses for field synchronizing, plus a preparatory period in which line-doubling pulses are inserted in order to pass the integrating circuit ahead of time to give equal peaks on alternate pulses. Shunt Peaking. The use of a peaking coil in a parallel-circuit branch to feed signals from the output load of one vacuum tube to the input load of a following tube, for the same purpose as a series-peaking circuit, but with the added advantage of splitting up the distributed capacitances of the two tubes. Single Sideband. Transmission of a carrier and substantially only one sideband of modulation frequencies, usually the upper sideband in television practice. Spectrum. The frequency band over which radiations are spread. It is usually used in connection with light frequencies, but may refer both to visible and invisible radiations. Spot. This refers usually to the area on which an electron beam is focused. Spot Size. This refers to the size of an electron beam. Staggered Circuits. Circuits are said to be staggered when they are alternately tuned to two different frequencies, in order to obtain broad-band response. A complete stage of amplification in a staggered-circuit amplifier requires two vacuum tubes with output circuits tuned to different frequencies. The separation in frequency divided by the mean frequency of the two circuits is a coefficient of staggering and corresponds directly to coefficient of coupling in double-tuned circuits. Staggered Tuning. Alignment of successive tuned circuits to slightly different frequencies in order to widen the overall response. Sweep. Movement of the spot across the screen of a cathode-ray tube. Sweep is normally accomplished either by applying a sawtooth voltage to the deflection plates (electrostatic deflection) or by passing a sawtooth current through the deflection coils (electromagnetic deflection). Sync. This is an abbreviation for synchronization and applies to a timing signal for determining the point in time at which an electrical oscillation or process will start. Synchronization. Timing of an electrical action or waveform. In the television receiver, the horizontal and vertical sweep oscillators are synchronized or locked-in by the synchronizing pulses which accompany the transmitted signal. Synchronization Clipper. A circuit designed to remove the synchronizing pulses from the composite signal. Synchronization Pulses. Pulses transmitted along with the picture information and used to lock the frequency of the sweep generators in the receiver. Synchronization Separator. Same as synchronization clipper. Tearing. Splitting of the television picture due to improper synchronization. Test Pattern. A fixed television image used to determine the quality and correctness of adjustment of a television system. See Resolution Chart. Time Constant. The time required in an electrical circuit for potential or current to rise to approximately 63% of its steady, final value or to fall to approximately 37% of its initial value. Time Delay. The time lapse between an electrical occurrence at the start of a transmission and the reproduction of this occurrence at a remote point. Time-Determining Circuit. A circuit composed of energy storage components having a time constant designed to intro duce a predetermined amount of time delay. Timer (Generator). An equipment designed to generate standard sync signals for synchronizing all components of deflection apparatus in a television system. Trace. The path followed by the spot as it moves across the screen of a cathode-ray tube. Trap. A tuned circuit used to eliminate a given signal or to keep it out of a given circuit. For instance, in the television receiver, traps in the video circuits keep the sound signal out of the picture channel. One type of trap is simply a tuned circuit which absorbs the energy of the signal to be eliminated. Transmission Band. This refers to the band of frequencies utilized for transmitting information electrically. Transmission Line. A two-conductor circuit having uniform characteristics for transmitting electrical signals. UHF Waves. This refers to carrier frequencies in the ultra-high frequency spectrum between 300mc and 3000mc. Vertical. This refers to the dimension of height in the picture. Vertical Blanking. The application of cutoff bias to the cathode ray tube during the vertical retrace. Vertical Centering Control. The adjustment which shifts the image in the vertical direction so that it may be centered on the screen. Vertical Hold. The adjustment which varies the free-running frequency of the vertical sweep oscillator in the television receiver. When this adjustment is properly set, the incoming synchronizing pulses will "lock in" the frequency of the vertical oscillator. Vertical Resolution. This refers to the line structure of the image, that is, the number of lines or picture elements which can be resolved in the vertical direction. Vertical Retrace. The movement of the spot from the bottom of the image to the top after each vertical sweep. The cathode-ray tube is biased beyond cutoff during this time. Vertical Synchronization. Locking in of the vertical sweep oscillator by the incoming vertical synchronizing pulse. (See Vertical Hold). Vestigial-Sideband Transmission. A method of transmission in which one set of sidebands is largely, but not completely, eliminated. This system is employed in commercial television practice. Video Amplifier. The amplifier stages following the video detector in a television receiver. They are designed to have a flat response up to several megacycles. Video Detector. The demodulator circuit which extracts the picture information from the modulated carrier. Video Signal. This refers to a time sequence of electrical pulses generated at the signal plate of a television pickup tube. Video Waveform. This refers to the portion of the waveform which corresponds to the light and dark values in the picture as they are transformed into an electrical signal, but does not include the synchronizing waveform. Viewing Distance. This refers to the best distance to view a television picture from the standpoint of seeing all the detail which the picture is capable is resolving. Visible Spectrum. That portion of the spectrum of electromagnetic radiations which is visible to the human eye. Wave-Shaping Circuit. A circuit which alters the form of an electric wave to a different form. Wedge. A convergent pattern of black-and-white lines equally spaced, and used as a television test pattern. Width. The horizontal dimension of the television image. Width Control. The adjustment which varies the horizontal size of the television picture. This is accomplished by controlling the amplitude of the horizontal sawtooth. Yoke. An arrangement of deflection coils, usually including two sets of two coils each, for producing the magnetic deflection field for the electron beam in a cathode-ray tube. |