It will be instructive, at this time, to examine in somewhat greater detail the requirements of the sawtooth scanning waves which produce the raster. We have mentioned at several times in the text that the horizontal and vertical sawtooth motion of the electron beam of the picture tube must keep in step accurately with a similar set of sawtooth scanning movements occurring at practically the same instant in the camera tube at the transmitter.
To accomplish this synchronization, pulses of a distinctively different nature for the control of horizontal and vertical scanning respectively, are transmitted as an integral part of the television signal. These pulses occur between each horizontal frame. During the scanning of the frame itself the operation of the receiver is ''on it's own''. However, during the short interval which occurs between successive horizontal frames, the action of the deflection circuits of the receiver are the absolute "slave" of the transmitter, providing the set is well designed, operating properly, and being used in an area of adequate field strength.
While the method of separating or sort mg these pulses from the complex signal are properly a matter to be taken up later in this course, at this point we should examine the relationship in timing which these pulses bear to the control of the sawtooth scanning action of the receiver. Figure 65 shows the sequence of events which occur during the scanning of one horizontal line and the return of the electron beam to start the scanning of the next line.
It should be understood that the sawtooth motion is that of the electron beam of the cathode ray picture tube and not necessarily the for m of voltage applied to the deflecting plates of an electrostatic tube or the wave of current which must occur through the deflecting coils of a magnetically controlled tube. We will find somewhat later that it will be necessary to introduce deliberate distortion of these voltages and currents to accomplish the linear sweep and rapid flyback of the beam of electrons which is tracing the picture.
As a radio service technician, you have been familiar with alternating currents whose frequencies are in the order of millions of cycles or alternations per second. Up to this time it has not been necessary to consider these frequencies in terms of actual motion.
The sequence of events which happen in the picture tube of a television receiver involve the motion of an electron stream at speeds which have not as yet been accomplished mechanically even in rocket propulsion.
Fig. 65. Horizontal Scanning Wave.
Figure 65 shows the ideal sawtooth for the control of the horizontal scanning motion of a television set. At point 'A' the electron beam starts to cross the face of the picture tube, which we will assume is ten inches in diameter, and has an active picture length in the horizontal direction of eight inches. The beam has blanked out over the distance 'A' to 'B' and the picture itself starts at the point 'B'. Between the points 'B' and 'C' as the uniform motion progresses, the video modulation is active in producing the picture.
As we have previously stated, the picture frame consists of 525 horizontal lines which are reproduced each 1/30th of a second. (30 frames per second times 525 lines per frame equals 15,750 horizontal line s per second.) This means that the time which can be allowed for the trace of a line and its return to start another line is 1/15, 750th of a second.
At this point we should introduce the idea of talking about these extremely short time intervals in multiples of one millionth of a second. This unit of time is known as the "micro" second. Such a term should be familiar to you as a practical radio service technician as it is the length of time required for the completion of one cycle of carrier wave at the middle of the broadcast band or 1000 KC. The entire horizontal action including the tracing of the picture line and the return to start a new line occurs in 63.5 microseconds.
In order to comprehend what this means in terms of motion of the electron beam over the active part of the picture we can divide the eight inches of picture length by the time of scanning ( 53. 34 microseconds) and obtain a velocity of 2.37 miles per second. The retrace time which is shown in the figure between the point 'D' and 'E' is 7 microseconds and since this retrace is over the same eight inches of horizontal motion it is obvious that the speed of the spot (blanked out to produce no light) must be much more rapid. Actually this retrace in the ideal case amounts to a beam velocity of 21.64 miles per second.
As was previously stated, the sequence of events must occur in absolute synchronism with a similar sequence occurring -at the same instant in the camera tube at the transmitter.
In order to accomplish this, pulses are sent out from the transmitter between each horizontal trace. The shape of these pulses is shown above the sawtooth wave, in Figure 65. At the instant shown as 'F' enough voltage appears at the grid of the picture tube to "blank out" all light. The region from 'F' to 'G' is known in television slang as the "front porch". The region is slightly more than one millionth of a second in duration. At point 'G' the carrier wave of the transmitter instantly increases by approximately 25% of its average value. This sharp rise in carrier is utilized to "trigger" scanning generators in the receiver which are used to produce the required sawtooth motion of the electron beam of the picture tube. The exact means of utilizing the pulse to accomplish this triggering will be described later in this course. The horizontal beam does not trace a line parallel with the top of the picture but has a downward slope which is 1/60th of the vertical height of the picture. Refer to Figure 28, Page 24.) This motion in the vertical direction is under the control of a vertical scanning sawtooth which serves to move the picture to the bottom of the image and then rapidly return it to the top. The motion of the electron beam from the top to the bottom of the picture occupies 1/60th of a second. It is easy to see that this vertical scanning motion is very much slower than the horizontal line tracing action and requires 16,666 microseconds for its re petition. Pulses are sent out between successive fields to lock in, or control as a "slave", the vertical scanning oscillator of the receiver.
A cycle of the vertical deflection saw tooth wave is shown in Figure 66, together with an enlarged section of that part of the wave which occurs during blanking and retrace.
It will be seen that the portion of the television signal which controls vertical retrace and synchronization is much more complicated than the single horizontal pulses which occur between successive horizontal lines. In appearance the vertical synchronizing signal resembles a comb with uneven teeth. If its only function were to trigger the vertical oscillator and to blank out the picture tube screen during retrace, it could be made in the form of a single long rectangular pulse, whose time duration is that of from 20 to 22 horizontal lines (1250-- 1450 microseconds). However, it has to perform two other functions, namely: those of continuing to keep the horizontal scanning oscillator in step during the vertical retrace period and also to assure that alternate fields have proper inter lace of the horizontal lines.
Fig. 66. Vertical Scanning Wave
The first function of keeping horizontal synchronization correct is accomplished by the serrations (notches) 'B' and pulses 'A', 'C' and 'D' shown in Figure 66. The second function of controlling interlace is taken care of by the use of the equalizing pulses 'A' and 'C' shown in Figure 66, preceding and following the vertical sync pulse itself.
The exact composition of the complex wave which constitutes the television signal does not need to concern us at this time, be cause it involves an understanding of those portions of a television receiver which are included under "Cathode ray beam modulation and synchronization".
CONTROL OF SCANNING GENERATORS BY SYNC PULSES: We have seen that it is necessary for the scanning systems of the receiver to keep in accurate step with the scanning raster of the camera tube at the transmitter, and have described the type of synchronization pulses which are made part of the television signal to satisfy this requirement.
For the reproduced picture to be of satisfactory character, the picture elements of adjacent horizontal traces must line up with considerable accuracy, and the lines of alternate fields must interlace or space accurately between one another.
To avoid a displacement of more than one picture element in successive horizontal lines, the frequency stability of the horizontal oscillator must be 0.2 percent or better. Figure 67A serves to illustrate horizontal displacement.
To avoid "pairing" of the lines of successive fields (the lines lying on top of those of the preceding field instead of being properly interlaced), the stability of the vertical oscillator must be better than 0.05 percent. Figure 67B illustrates this displacement.
In each of the impulse generating circuits (cathode-coupled multivibrator and blocking oscillator), which have been described as suit able for television scanning, coupling means have been indicated in the grid circuits for the introduction of synchronizing pulse controls.
See Figures 60, 62 and 64.
Fig. 67. Picture Element Displacement which might Result from Scanning Oscillator
Instability
The horizontal and vertical pulses are "clipped" from the signal, amplified, and passed through circuits which ''classify; them so that each will control only its own scanning oscillator. The methods of accomplishing these operations will be described later. The end result is a short, sharp "pip" for the horizontal control and a long triangularly shaped pulse for the vertical control.
In considering the manner in which the pulse controls oscillator frequency, three factors are of importance:
1. The free running frequency of the sweep generator. --- This is the frequency which would be generated at any particular setting of the “hold" control, if the sync pulses are not present. It can be slower than the pulse repetition rate, faster, or in exact step.
It will be shown later that for proper stable operation of the receiver, the slow condition is required.
2. The firing point of the sweep generator.--- This is the grid bias voltage of the controlled tube which initiates conduction in the discharge tube, thus starting capacitor discharge and retrace of the scanning wave. It is this point in the cycle at which the oscillator is most sensitive to control by the sync pulse.
3. The synchronizing frequency.--- This is the rate at which the pulses are applied to the control input terminal of the oscillator, and is determined by the signal. See Figures 65 and 66.
Since the control action of the blocking oscillator can be more readily illustrated dia grammatically, we will consider its operation first.
PULSE CONTROL OF THE BLOCKING OSCILLATOR. As has been shown, the "firing" point of a blocking oscillator is the inst ant that the grid potential passes the cut-off point. When the oscillator is "free running" this point is determined by the time constant of the grid capacitance and resistance. If at any time during the discharge of the grid capacitor through the resistor, positive voltage were to be added to the grid potential from an external source, to the ex tent that the grid potential passed the cut-off point, the tube would start to conduct and the sawtooth forming capacitor would discharge.
This would start retrace and a new sawtooth scanning cycle would begin.
Figure 68 illustrates this action in detail.
At A is shown an enlarged portion of the blocking oscillator grid voltage wave, whose operation has been described on pages 47 and 48 and illustrated in Figure 63. The series of synchronizing pulses below "A" shows a series of pulses marked "0" whose leading edges are in exact "step" with the wave. In this case, the free running frequency of the oscillator is not affected by the pulses. They merely add to the grid voltage at the same instant that it was being driven positive by the plate current pulse. If, on the other hand, the sync pulses were occurring at the points indicated as "1", the addition of the pulse voltage, to the voltage due to the discharge of C1 through R1 (see Figure 62), is still short of the cut-off bias point, and will not "fire" the tube. However, if the pulses were occurring at times 2 or 3, the critical bias would be exceeded, the tube would immediately become conductive, and re trace would start at that point. The oscillator has been "forced" into step with the repetition rate of the sync pulses.
Fig. 68. Pulse Control of the Blocking Oscillator
At this time it is interesting to examine the differences in this forced drive action under the conditions; (1) oscillator running faster than sync pulse rate, (2) oscillator running slower than sync pulse rate.
Figure 68B shows the condition of an oscillator which is running faster than the sync pulse rate. A number of cycles occur before the contribution of the pulse to the grid voltage wave is able to "fire" the tube. This action occurs at point x. It would appear that after this "lock in" has taken place, and repetition of action ensues as at y, control would be satisfactory. Such is not the case, for two reasons, namely:
1. The synchronizing pulses are occurring during the active scanning interval, and the picture is divided in two during the "blanking" interval. This phenomenon is similar to the frequent occurrence in old time movies when the projector "got out of frame" and the picture was divided in the middle with the hero's feet at the top of the screen and his he ad at the bottom. This situation will be taken up in greater detail later when we discuss "controls" and the effect of their maladjustment.
2. In the fast running condition, the oscillator is very susceptible to "triggering" by static and automobile ignition interference.
The effect on the picture is to "tear out" horizontal lines.
Figure 68C shows the condition which occurs when the free running frequency of the oscillator is lower than the sync pulse repetition rate. It is evident that "lock in" occurs much faster and a more stable operation en sues. This is a desirable mode of operation because the sync pulses always occur at the end of scanning action and the possibility of "parting" the picture, as described, cannot occur.
For control of the blocking oscillator just described, the sync pulses are in the positive direction. When we reach the study of pulse clipping and amplification, we will find that a sync pulse can be made either positive or negative with respect to ground (or the chassis), depending upon the number of tubes through which it passes. Some economy in the number of tubes may occur if the pulse is negative when it reaches the pulse generator grid. Such a condition is ideal for the cathode-coupled multi vibrator.
PULSE CONTROL OF THE CATHODE COUPLED MULTIVIBRATOR. Figure 61 and the text of pages 45, 46 and 47 indicate that "tripping", or discharge action of the cathode coupled multivibrator, is initiated by a negative voltage pulse on the grid of the first tube.
Once this action starts, it immediately receives a contribution in the form of additional negative voltage from the cathode bias resistor, which is common to both tubes. While the control actions and principles Just described for the blocking oscillator hold true, it is not feasible to show them in diagram form. As a matter of fact, the small step in the grid volt age curve of Figure 61A (indicating the sync pulse contribution to the grid voltage), is really a matte r of "poetic license" so to speak, since the action is so rapid and cumulative that it is not possible to tell where pulse control stops and the circuit takes over.
SUMMATION OF PULSE CONTROL OF SCANNING GENERATORS
1. A positive synchronizing pulse is used to control the frequency of a blocking oscillator.
2. A negative pulse is used to control the frequency of a cathode-coupled multivibrator.
3. The free running frequency of the scanning oscillator should always be made just slightly less than the synchronizing pulse repetition rate. This is accomplished by adjustment of the "hold" or frequency control of the oscillator.
4. As the grid voltage of a pulse genera tor approaches the "trigger" point, the oscillator becomes increasingly sensitive to control by additional grid voltage. At this point the scanning can be "tripped" by interference. Special circuit combinations have been devised which are controlled by the "pattern" of the pulses, rather than by the individual pulses themselves. Such a system is relatively insensitive to interference, which seldom has a regular pattern, and its operation will be described when specific circuits are covered later in the course.
REQUIREMENTS FOR SCANNING WITH ELECTROSTATIC PICTURE TUBES
It was pointed out in discussing the saw tooth scanning raster, that we were considering the motion of the spot or "pencil" of electrons at the picture tube fluorescent screen surface.
In an electrostatic tube, we can produce the desired raster by applying sawtooth voltage waves of the shape and time requirements shown in Figures 65 and 66 respectively to the horizontal and vertical deflecting plates.
The use of electrostatically deflected picture tubes, until recently, has been confined to the 7 -inch diameter size or smaller, usually employed in table models selling in the lower price bracket. As this is being written, a 10 inch diameter, electrostatic ally deflected tube is being made available to the design engineer, and will undoubtedly be introduced in console type receivers.
The popular 7-inch type 7JP4 requires deflection voltages of the order of 200 volts per inch of picture tube screen to move the beam.
The picture size on this tube is 4x5-1/2 inches.
This means that horizontal deflecting plates will require a "peak to peak" voltage of the sawtooth wave of 5.5 times 200, or 1100 volts.
The sawtooth generators described usually are not designed to give voltages of this order directly. For this reason, amplifiers are used between the scanning generator circuit and the picture tube deflecting plates. As has been shown, the deflection and centering circuits of the tube are of a balanced or push-pull type . Therefore the deflection amplifier feeding them is made of the push-pull type , usually by means of a phase inverter.
Typical electrostatic deflection systems will be covered in detail later.
REQUIREMENTS FOR SCANNING WITH ELECTROMAGNETIC PICTURE TUBES
We have seen that electrostatically deflected tubes require only an amplified sawtooth wave of voltage to produce the desired pattern or raster. Magnetically deflected tubes, how ever, impose a new set of requirements due to the nature of the deflecting coils.
Earlier explained in detail, was the theory and mechanical arrangement of the horizontal and vertical deflecting coils. It was stated at that point, that a sawtooth wave of current through the coils could be made to produce the desired raster.
To re-state this in another manner:--The amount of deflection of an electron beam, in electromagnetically deflected cathode-ray tubes, is dependent upon the strength of the magnetic field produced by the external deflecting coils. The magnetic field produced is proportional to the amount of current passing through the coils and these fields cross the path of the electron beam within the neck of the tube.
We must supply a linear sawtooth of current through the coil so that the electron beam will trace the proper raster under the combined influence of the horizontal and vertical deflecting coils. Such a deflection will not be produced by a linear sawtooth of voltage across the terminals of either coil.
To understand why this is the case let us assume for a moment that the deflecting coil is a pure inductance (no resistive component). Reviewing our theory of ''Inductance'' and its circuit function we find:
1. Surrounding an inductance carrying current, there exists a magnetic field whose intensity at any point is proportional to strength of the current.
2. If the current is altered in value, the magnetic field is also altered, increasing or decreasing with corresponding changes of the current.
3. This magnetic field constitutes a storage of energy and requires for its production a definite expenditure of energy, dependent on the amount of magnetic flux and the ampere turns of the circuit.
4. The expenditure of new energy is required only when we are attempting to change the amount of flux associated with the inductor, by ch an gin g the current through it. This energy appears as a voltage of" self-induction" which opposes the voltage impressed on the circuit to start the current flow.
5. When current is increasing, due to an increase of the externally applied voltage, this opposing voltage of "self induction" tends to retard the flow of current, and make it "lag" behind the increase of applied voltage.
With this brief "refresher" in magnetic theory, we are ready to determine the shape of voltage wave which will be required across a pure inductance to produce a sawtooth wave of current through it. The effect of the resistance of the coil , and the consequent wave shape modification to account for it, will then be discussed.
In Figure 69, we see the resultant shape of current wave which would flow through a pure inductance if a symmetrical square wave of voltage were to be applied across its terminals. This type of wave, as we have seen, can be developed by a conventional or symmetrical multivibrator. At point A the voltage has suddenly been applied to the coil in much the same fashion as though a switch had been closed to connect the coil to a DC source of potential, such as a battery.
Fig. 69. Linear Rise and Fall of Current through a Pure Inductance under the
Application of a square Voltage Wave
Fig. 70. Voltage and Current Waveforms--- Inductance and Resistance Circuits
It will be noted that the current through the coil did not rise immediately to maximum.
The voltage of self induction of the coil opposed the sudden change, and the current therefore increased in a linear fashion over the portion of the cycle when the applied voltage was steady. At point B, the impressed voltage was suddenly removed (switch opened). At this point the current did not fall immediately to zero, since it was maintained by the energy "stored" in the magnetic field. The voltage of self induction of the coil served as the driving potential to produce the linear fall of current from point B to point C. We have now produced a "triangular" wave of current through the coil. If we can lengthen the "rise" portion of the curve with respect to the "decay" portion, we have attained our objective of producing the desired saw tooth scanning current wave. This can be accomplished by making the impressed voltage wave asymmetrical as shown in Figure 70B. resistance of the windings, as regards the required voltage waveform to produce a sawtooth of current.
Since it is not possible to build a practical deflection coil as a pure inductance, it is now necessary for us to consider the effect of the Figure 70 illustrates three types of circuits, and shows the voltage wave form which it is necessary to impress, in order to produce a sawtooth wave of current through each circuit.
Figure 70A shows the case of a pure resistance. Here the current is in phase with the voltage, and a sawtooth wave of voltage impressed across the resistor will cause a saw tooth wave of current through it. Energy loss occurs only in the form of heat, and the voltage required to produce a certain current is equal to the IR drop as determined by Ohm's law.
Figure 70C shows the circuit represented by a practical deflection coil. The voltage waveform will be seen as a combination of the sawtooth of A and the rectangular wave of B. In reality, this shape is the sum of an instantaneous pulse and a sawtooth. We might think of its function as follows:
1. The sawtooth or linear rise portion of the wave tends to produce a sawtooth wave of current through the resistive part of the circuit.
2. The instantaneous pulse portion of the wave forces a sawtooth wave of current through the inductive part of the circuit.
To produce this combination wave shape, additional circuit elements are added to the sawtooth capacitor charging circuit. When this is done, the circuit is then known as a "peaking" type of wave-shaping circuit. By choice of capacitor and resistor values, it is possible to make either the sawtooth portion of the wave or the impulse portion predominate. The de tails of its action will be described later.
It is interesting to note that the deflection requirement, of having one part of the wave predominate over the other, is di ct ate d by fundamental differences between the horizontal and the vertical deflection coils.
In the vertical deflecting coil of a typical television receiver, the resistive component predominates over the inductive component.
For this reason, the sawtooth portion of the wave predominates over the impulse portion.
As an example, this coil might have a resistance in a practical case of 68 ohms with an inductance of 50 milli-henries. When operating at the 60 cycle retrace rate, this presents a predominately resistive circuit.
In the horizontal deflecting coil of the same receiver, the conditions are reversed and the inductive component predominates.
The impulse portion is more important and the required wave shape approaches that of Figure 70B. To continue the practical example, we would find a resistance of only 14 ohms with an inductance of 8 millihenries. Since this coil operates at the much higher frequency of 15,750 cycles per second, the circuit is essentially inductive.
Fig. 71. Electrostatic and Electromagnetic Scanning Systems
Figure 71 represents a comparison, in block diagram form, between the basic elements of electrostatic and electromagnetic scanning circuits.
PEAKING CIRCUITS FOR ELECTROMAGNETIC DEFLECTION. A very simple change in the discharge tube circuit, which has been previously described and shown in Figure 64, makes it possible to generate the combination sawtooth and impulse wave required for electromagnetically deflected scanning.
The modified circuit is shown in Figure 72, and consists of the addition of resistor R2 in series with the discharge capacitor C2. The circuit action will be described in sequence:
1. The sawtooth forming capacitor C2, is charged from the B+ source through resistors R3 and R2. This charging action takes place during the portion of the cycle when the tube is not conducting.
2. The output voltage waveform of the circuit is taken across the series combination of R2 and C2. R2 is known as the "peaking" resistor.
3. During the charging portion of the cycle, the voltage across the capacitor is a sawtooth wave as has previously been explain ed.
4. When the tube suddenly becomes conductive due to a positive pulse on its grid, the voltage across C2 and R2 is suddenly shunted by the low plate resistance of the tube.
5. The voltage across the capacitor can not change instantly, since its discharge path through R2 and the plate resistance of the tube is not zero. The difference -in voltage must therefore appear suddenly across peaking resistor, R2. After this initial sudden change of voltage, the capacitor discharges exponentially, through R2 and the tube, until the tube again becomes nonconductive.
Fig. 72. Representative Peaking Circuit.
6. As the tube is cut off, the B+ potential is suddenly applied to the capacitor through R2 and R3 in series. Again, the capacitor voltage cannot rise instantaneously. The voltage across R2 must once more change abruptly after which the capacitor charges through R2 and R3 in its normal sawtooth fashion.
By changing the values of R2 and C2, the ratio of the amplitude of the peaking impulse to that of the sawtooth can be adjusted to match the inductance and resistance requirements of the particular deflecting coil.