SAWTOOTH GENERATOR

Abstract: In a sawtooth generator for a television deflection system, first (C₁) and second (C₂) capacitors are coupled in series to a first terminal (500a) of a switch (U 6a ). When the switch (U 6a ) is conductive, the two capacitors are discharged by the switch to form a sawtooth signal (V O ′) at the first terminal (500a) having a down-ramping portion that corresponds with a retrace interval. When the switch (U 6a ) is nonconductive, the two capacitors are charged by a source (R₂) of current to form an upramping portion of the sawtooth signal that corresponds with a trace interval. An S-shaping arrangement that is coupled to a junction terminal (703) of the two capacitors (C₁,C₂) includes a third capacitor (C₃), two resistances (R₈;R₄,R₅) and a clamping arrangement (D₁,D₂) that provide S-shaping of the sawtooth signal (V O ′) at both ends of the trace interval and dc restoring prior to the beginning of each trace interval.

Description: This invention relates to a sawtooth generator that produces S-corrected signal for a television deflection system. In particular, it relates to such generator that may be used in, for example, a deflection system, arranged for reducing the visibility of flicker. The threshold of perception of field flicker in a television display system is a function of the flicker frequency and the brightness of the display. Over the years, displays have increased in brightness to the point where flicker is noticeable even in relatively high field rate systems (e.g., the NTSC 60 Hz system) and clearly objectionable in lower field rate systems (e.g., the PAL 50 Hz system). A solution to this problem is to double the field rate of displayed images. In one prior art system, a video input signal is stored in a field memory. Each stored field is recovered or "read" twice from the memory and displayed at double the line rate and double the field rate of the incoming video signal, thereby doubling the flicker frequency of displayed images and thus reducing the visibility of flicker. In U.S. Patent US-A-4701793 entitled, TELEVISION DISPLAY SYSTEM WITH FLICKER REDUCTION PROCESSOR, issued to W. den Hollander, et al on 20 October 1987, a television display arrangement that provides flicker reduction is described. In the den Hollander arrangement, an interlaced baseband television input signal having a given field rate is provided. A timing unit, responsive to the video input signal, supplies a vertical synchronizing signal having a pulse waveform that repeats on a four-field basis. The pulses of the pulse waveform cause a corresponding phase modulation of the deflection current such that the durations of corresponding vertical scanning cycles vary on a field basis and repeat on a four field basis. A conventional vertical deflection circuit may include a sawtooth generator, synchronized to a synchronizing signal having a constant phase that generates a drive signal having a sawtooth waveform. In the sawtooth generator, a capacitor is discharged during retrace by a switch, and charged during trace by a current source. The drive signal is coupled to a switching circuit that produces a deflection current having a corresponding sawtooth waveform. The drive signal includes a ramping portion that corresponds with vertical trace such that the beginning time of the ramping portion corresponds with a deflection current that causes scanning at the top of the raster of the display. Because of the phase modulation of the pulse waveform of the vertical synchronizing signal that is required in, for example, the den Hollander, et al. arrangement, the peak amplitude of the drive signal at the end of vertical trace may also change on a field basis in accordance with the pulse waveform pattern. Different peak amplitudes of the drive signal may cause the beginning time of the ramping portion to be different relative to the corresponding pulse of the vertical synchronizing signal that causes such ramping portion. This is so because the discharge time of the capacitor of the sawtooth generator may vary in accordance with the peak amplitude of the drive signal that may be different in different fields. The result is that the phase of the trace portion of the deflection current may, disadvantageously, vary from that established by the phase modulated vertical synchronizing signal. Consequently, the requirement that even field should overlap even field, odd field should overlay odd field and even and odd field pairs should interlace, disadvantageously may not be fulfilled. Therefore, it may be desirable in each deflection cycle to prevent, for example, the beginning time of the ramping portion of the drive signal from being substantially affected by variations in an amplitude, such as the peak amplitude, of the drive signal. It may also be desirable to generate the drive signal such that a nonlinearity type distortion, that is discussed below, is corrected by S-shaping of the sawtooth waveform of the drive signal. Nonlinearity distortion may be attributable to the fact that the effective center of deflection of the electron beam is not coincident with the center of radius of curvature of the screen of the kinescope. A common method of compensating for such nonlinearity is to form a deflection current that, rather than being a substantially linear ramp during the recurrent scanning intervals, becomes a segment of a sinewave having a reduction in the rate of change of deflection current at the positive and negative extremes of the deflection current relative to that at the center. This S-shaping in turn creates a slight reduction in the rate of scanning of the electron beam near the top and bottom edges of the raster relative to the center and a reduction in such nonlinearity. In some prior art vertical deflection circuits, the S-shaping of the deflection current that corresponds with, for example, the one extreme of vertical trace is done by S-shaping the drive signal that is generated in the sawtooth generator. S-shaping of the deflection current that corresponds with the other extreme of vertical trace is controlled by, for example, a voltage that is developed in a capacitor that couples the drive signal of the sawtooth generator to a vertical amplifier. As explained in the patent to Den Hollander, et al., it may be desirable to DC-couple the drive signal to the deflection winding. Therefore, the above-mentioned way of S-shaping that utilizes capacitive coupling may not be suitable. FR-A-1467113 discloses a television deflection apparatus responsive to a synchronizing input signal at a frequency that is related to a deflection frequency, comprising: a controllable switch responsive to said input signal that operates at a frequency that is related to that of said input signal; first and second capacitors forming a series arrangement that is coupled to said switch; a current source coupled to said capacitors for charging said capacitors to generate a sawtooth signal having a ramping first portion that changes in a first direction when said switch is nonconductive such that when said switch becomes conductive said switch discharges said capacitors to form a ramping second portion of said sawtooth signal that changes in an opposite direction; and a source of a ramping voltage related to the generated sawtooth signal. The disclosure of this document corresponds to the introductory portions of the independent claims. In accordance with a first aspect of the present invention there is provided a television deflection apparatus responsive to a synchronizing input signal at a frequency that is related to a deflection frequency, comprising: a controllable switch responsive to said input signal that operates at a frequency that is related to that of said input signal; first and second capacitors forming a series arrangement that is coupled to said switch; a current source coupled to said capacitors for charging said capacitors to generate a sawtooth signal having a ramping first portion that changes in a first direction when said switch is nonconductive such that when said switch becomes conductive said switch discharges said capacitors to form a ramping second portion of said sawtooth signal that changes in an opposite direction; and a source of a ramping voltage related to the generated sawtooth signal; characterized by a first impedance coupled to said source of said ramping voltage and to a first junction terminal that is coupled between said first and second capacitors for generating a first current that flows in said first impedance that controls S-shaping of said sawtooth signal by reducing the rate of change of said sawtooth signal at one of the beginning and end of one of said first and second portions of said sawtooth signal in accordance with said first current that flows in said first impedance; a second impedance coupled to a terminal of said first impedance that is remote from said first junction terminal for bypassing a portion of a current that flows in a current path between said source of said ramping voltage and said first junction terminal for reducing said first current in said first impedance and that reduces the rate of change of said sawtooth signal at the other of the beginning, and end of said one of said first and second portions of said sawtooth signal to control S-shaping at said other of the beginning and end; a deflection winding; and means responsive to said sawtooth signal for generating a deflection current in said deflection winding that is S-corrected. In accordance with a second aspect of the present invention there is provided a television deflection apparatus responsive to a synchronizing input signal at a frequency that is related to a deflection frequency, comprising: a controllable switch responsive to said input signal for operating said switch at a frequency that is related to that of said input signal; first and second capacitors forming a series arrangement that is coupled to a terminal of said switch; a source of DC voltage coupled through a first resistor to said capacitors for generating a sawtooth signal at said terminal having a ramping trace portion that changes in a first direction when said switch is nonconductive and a ramping retrace portion that changes in an opposite direction, when said switch is conductive;characterized by an impedance having a first terminal that is coupled to a junction terminal of said first and second capacitors for generating a first current that is coupled to said junction terminal to control S-shaping of said sawtooth signal; means coupled to said first and second capacitors for clamping each corresponding voltage across each of said first and second capacitors, causing each to be at a corresponding predetermined level prior to a beginning time of said trace portion such that each of said corresponding predetermined levels across each of said capacitors is substantially unaffected by an amplitude of said sawtooth signal at an end of said trace portion; a deflection winding; and means responsive to said sawtooth signal that is S-corrected for generating a deflection current in said deflection winding that is S-corrected. In accordance with a third aspect of the present invention there is provided a television deflection apparatus responsive to a synchronizing input signal at a frequency that is related to a deflection frequency, comprising: a source of said synchronizing input signal; first and second capacitors forming a series arrangement; a source of a first current coupled to said capacitors for charging said capacitors to generate a sawtooth signal having a ramping trace portion that changes in a first direction during trace and a ramping retrace portion of said sawtooth signal that changes in an opposite direction during retrace; and a source of a first voltage at a frequency that is related to that of said input signal; characterized by a first impedance coupled between said source of said first voltage and a junction terminal of said first and second capacitors for generating a second current that is coupled to said junction terminal that provides S-correction to said sawtooth signal; switching means coupled to said first and second capacitors for establishing prior to a beginning of said trace portion of said sawtooth signal, respective voltages that are developed across the two capacitors at corresponding levels that are substantially unaffected by an amplitude of said sawtooth signal; a deflection winding; and means responsive to said sawtooth signal that is S-corrected for generating a deflection current in said deflection winding that is S-corrected. A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, of which: Figure 1 illustrates a vertical scan generator that includes a sawtooth generator embodying an aspect of the invention; Figures 2a and 2b illustrates waveforms useful for explaining the operation of the circuit of Figure 1; Figure 3 illustrates a block diagram of a television receiver that includes the generator of Figure 1; Figures 4A-4D are waveform diagrams illustrating the operation of the receiver that utilizes the generator of Figure 1; and Figure 5 illustrates waveforms useful for explaining the operation of the arrangement of Figure 1. Referring to Figure 1, a sync pulse 2V' is coupled to a vertical oscillator circuit 501 of a vertical scan generator 64, embodying an aspect of the invention, that may be used in conjunction with a 110° 45AX type picture tube. Pulse 2V', that is phase modulated, is generated in a manner that is described later on. Pulse 2V' has a nominal frequency of 2f v , where f v is the frequency of the vertical sync in a baseband television signal such as of the NTSC or the PAL standard. Pulses 2V' are separated by corresponding intervals of slightly different durations, with the nominal duration being equal to 1/2V. V represents the vertical scanning interval of 20 milliseconds, in, for example, the PAL standard. A ramp generator 500 that is controlled by oscillator circuit 501 includes a charging current generating resistor R2 that is coupled to a terminal 500a of series coupled capacitors C₁ and C₂. Resistor R2 charges capacitors C₁ and C₂ to form a trace portion of a voltage V₀′ at terminal 500a. Terminal 500a is driven by an open-collector switching transistor U6a of a comparator U6 for discharging capacitors C₁ and C₂ to form a vertical retrace portion of voltage V₀′. Voltage V₀′ is coupled to an inverting input terminal of a comparator U5. A noninverting input terminal of comparator U5 is DC biased by an arrangement that includes a variable hold control resistor R20. Pulse 2V′ is inverted by OR-gate 1000 and differentiated by a differentiating network that includes a capacitor C₄ and a resistor R18 to form pulse 2V″, which is coupled via a capacitor C₅ to the noninverting terminal of comparator U5. An output terminal of comparator U5 forms a voltage V U4 on a conductor U4a and is coupled to a noninverting input terminal of comparator U6. An inverting input terminal of comparator U6 is DC biased by a voltage across a resistor R24. During vertical trace, the voltage at the noninverting input terminal of comparator U5 is more positive than the voltage at the corresponding inverting input terminal. Consequently, voltage V U4 is more positive than the voltage at the inverting input terminal of comparator U6. Therefore, output transistor U6a of comparator U6 remains nonconductive. At the end of trace, when voltage V₀′ is at its peak, as illustrated at time t₀ in FIGURE 2b, a leading edge 900 of pulse 2V′ of FIGURE 1 occurs. As a result of the inversion and differentiation of its leading edge 900, the voltage at the non-inverting input terminal of comparator U5 becomes smaller than at the corresponding inverting input terminal. The result is that voltage V U4 changes to a level that is smaller than that at the inverting input terminal of comparator U6, causing switching transistor U6a of comparator U6 to become conductive. Conduction of transistor U6a discharges capacitors C₁ and C₂, causing voltage V₀′ to be at the saturation level of transistor U6a that is approximately zero volts. When, as a result of leading edge 900 of pulse 2V′, voltage U4a changes, a time-out capacitor C₆ causes a corresponding voltage change at a noninverting input terminal of a comparator U4, operating as a one-shot, to a level that is below that at the inverting input terminal of comparator U4. Consequently, comparator U4 sustains voltage V U4 at the low level for a predetermined interval that is determined by capacitor C₆ and resistors R₂₅ and R₂₆. As long as voltage V U4 is at the low level, voltage V₀′ remains approximately zero, because output switching transistor U6a of comparator U6 is conductive. Transistor U6a is conductive in intervals that are illustrated schematically in FIGURE 2a such as, for example, between times t₀ and t₂. Thus, the conduction time of transistor U6a is independent of the duration of input pulse 2V′. When voltage V U4 of FIGURE 1 changes to a high level, the trace portion of voltage V₀′ of FIGURE 2b, such as, for example, between times t₂ and t₃ of FIGURE 2b, begins. Thus, when leading edge 900 of pulse 2V′ occurs, capacitors C₁ and C₂ of FIGURE 1 begin discharging to the saturation voltage level of switching transistor U6a. In the event that pulses 2V′ of FIGURE 1 are missing, comparators U4, U5 and U6, that form vertical oscillator 501, produce the required pulses of voltage V U4 . Each pulse of voltage V U4 of FIGURE 1 causes output transistor U6a to clamp voltage V₀′ to its saturation voltage level for the predetermined duration of voltage pulse V U4 , such as until time t₂ of FIGURE 2b, thus preventing voltage V₀′ from upramping. Voltage V₀′ of FIGURE 1 is coupled via a transistor Q2, operating as an emitter follower, to produce a voltage V₀ of FIGURE 2b having a controllable average value that, like voltage V₀′, is also of a sawtooth waveform. An upramping, first, or trace portion of voltage V₀′ or V₀, occurs during an interval such as interval t₂-t₃. The first portion begins upramping from a predetermined constant level that is equal to the saturation voltage of output switching transistor U6a of comparator U6 of FIGURE 1 in a way that is not affected by phase modulation of pulse 2V′. Thus, the upramping first portion begins upramping at an instant, such as time t₂ of FIGURE 2b. A downramping second portion of voltage V₀ occurs, for example, during interval t₀-t₁. A flat third portion occurs during interval t₁-t₂. The phase modulation of the pulses of voltage V U4 , shown in FIGURE 2a, that are separated by intervals having correspondingly different lengths, causes correspondingly different peak values of ramp voltage V₀, to occur such as at times t₀, t₃, t₆ and t₉, respectively, of FIGURE 2b. The sum of the intervals that correspond with the second and third portions is a predetermined constant that is not affected by the peak value of voltage V₀ of FIGURE 1. The interval between the beginning of the downramping portion and the beginning time of the upramping portion of voltage V₀, such as interval t₀-t₂ of FIGURE 2b, may be maintained, illustratively, constant so as to be unaffected by the phase modulation of pulses 2V′ of FIGURE 1. Ramp generator 500 includes an S-correction arrangement 700, embodying aspects of the invention. A capacitor C₃ of arrangement 700 is coupled between the emitter electrode of emitter follower transistor Q2 and a terminal 701. Terminal 701 is coupled, via a series arrangement of a variable, linearity control resistor R4 and a resistor R5, to a junction terminal 703. Terminal 703 is coupled between capacitors C₁ and C₂. A diode D₁ that, during retrace, clamps voltage V₂, at the junction of capacitors C₁ and C₂, to a predetermined positive level is coupled between terminal 703 and the emitter of transistor Q2. A diode D₂ that discharges capacitor C₃, during retrace, is coupled in parallel with a resistor R8 and between ground and terminal 701. FIGURE 5 illustrates waveforms useful in explaining the operation of the circuit of FIGURE 1. At the beginning of vertical trace, voltage V₀′ of FIGURE 1 is approximately zero, as described before. A positive voltage V₂ at terminal 703, is equal to the sum of saturation voltage V₀′, the emitter-base forward voltage of transistor Q2, and the forward voltage of diode D₁. Also, voltage V₄ at terminal 701 is slightly negative as a result of the clamping operation of diode D₂. Because voltage V₂ is more positive than voltage V₄, capacitor C₂ begins discharging through resistors R5 and R4, and through diode D₁ and capacitor C₃, at a rate of change that initially is, substantially unaffected by the value of resistor R8, so as to reduce the value of voltage V₂. Later on, during trace, when an emitter voltage V₃, shown in FIGURE 5, of transistor Q2 of FIGURE 1 attains a sufficiently high voltage level, capacitor C₂ of FIGURE 1 is charged through capacitor C₃. The initial decrease in voltage V₂ causes the rate of change of voltage V₀′ to be higher at, for example, the center of trace than at its beginning. As described before, the difference between voltages V₂ and V₄ causes a partial discharge of capacitor C₂, at the beginning of trace. The discharge of capacitor C₂ causes a slower rate of increase of voltage V₃, as shown in FIGURE 5, that is developed at the emitter of transistor Q2 of FIGURE 1, than at the center of trace. Thus the slower rate of increase provides S-shaping at a first half of trace that occurs between the beginning and the center of trace, as shown in FIGURE 5. In accordance with an aspect of the invention, during a second half of vertical trace, S-shaping becomes increasingly affected by resistor R8 which increasingly loads capacitor C₂ as voltages V₂ and V₄ increase. Resistor R8 reduces, at the end of trace, the current supplied through capacitor C₃ that is available for charging capacitor C₂. Voltage V₄ is higher toward the end of trace than, for example, at the center of trace. Therefore, the current that is supplied through capacitor C₃ that is bypassed by resistor R8 and that is prevented by resistor R8 from flowing in capacitor C₂ becomes increasingly larger toward the end of trace. The result is that voltage V₃ is S-corrected at the second half of trace. It follows that an increase, for example, in the value of resistor R8 changes the rate of change of voltage V₀′ at the end of trace in an opposite manner to that caused by an increase of, for example, resistor R4. In accordance with another aspect of the invention, S-corrected ramp generator 500 produces voltage V₀′ in which the rate of change, in a given vertical trace, is independent of vertical frequency variations or of the trace time of a preceding vertical trace. Such result is obtained because of the clamping operation of transistor U6a, diode D₁ and diode D₂ that clamp voltages V₀, V₄ and V₂ during retrace. FIGURE 1, advantageously, also includes a switched vertical deflection circuit 100 of generator 64 that is controlled by a vertical control circuit 120 of generator 64. The operation of deflection circuit 100 and of control circuit 120 is described in detail in U.S. Patent No. 4,544,864 in the name of P.E. Haferl, entitled SWITCHED VERTICAL DEFLECTION CIRCUIT WITH BIDIRECTIONAL POWER SUPPLY. It should be understood that ramp generator circuit 500 may be employed as the sawtooth generator for other types of DC coupled vertical deflection output stages to obtain proper registration of the displayed image. Control circuit 120 provides, in accordance with voltage V₀, width modulated horizontal, or line rate, switching signals to a switching element 21, illustratively shown as comprising a field effect transistor 118 and an integrated antiparallel diode 19. Transistor 118 may comprise a MOS power field effect transistor. Using such power field effect transistor is particularly advantageous when the horizontal rate is substantially higher than the horizontal frequency in, for example, the PAL standard. High rate horizontal frequency may be used in computer monitors, video display terminals or in television receivers, operating with deflection currents at, for example, twice the horizontal and vertical deflection frequencies, respectively, for reducing the visibility of flicker. Switching element 21 is coupled to a terminal 126′ of a storage capacitor 126 via a winding 23 of a flyback transformer 125 that is series coupled with an energy storage coil 25. Terminal 126′ of capacitor 126 is coupled to a vertical deflection winding 27. The other terminal of vertical deflection winding 27 is coupled to a voltage supply designated +V1. A horizontal output stage 33 is switched at the horizontal deflection rate by signals applied from a horizontal oscillator and driver circuit 134. The switching operation of output stage 33 generates a horizontal deflection current i 2H at twice the horizontal frequency, f H , of the sync signal of a baseband video signal V BB that is mentioned later on. Output stage 33 produces a retrace voltage at the horizontal rate of 2f H across each winding of flyback transformer 125. Ramping voltage V O is fed to a noninverting input terminal of a comparator 66 of FIGURE 1 to control the switching operation of switching element 21. Horizontal retrace pulses from winding 124, applied via a resistor 74, charge a capacitor 75 to obtain a horizontal ramp which is compared with the vertical sawtooth of voltage V O . Comparator 66 serves as a pulse width modulator. An output terminal of comparator 66 that generated pulses at the horizontal rate 2f H , having variable duty cycle that vary in a vertical manner, provides gate drive to MOS power field effect transistor 118. The operation of deflection circuit 100 consists of the horizontal rate charge and discharge of storage capacitor 126 which supplies a vertical deflection current i₂₇ that flows in winding 27. Horizontal rate switching is performed by switching element 21. At the beginning of vertical trace, during each horizontal period, transistor 118 of switching element 21 is conductive for a negligible short interval that occurs immediately prior to each horizontal retrace. Consequently, the large retrace pulse in winding 23 is clamped by diode 19 and generates a current i₂₃ that is large in winding 23 flowing in a direction opposite to that of the arrow that causes capacitor 126 to be charged to a voltage that is more positive than voltage +V1. The resulting voltage at terminal 126′ that is more positive than voltage +V1 causes negative deflection current i₂₇ to flow in deflection winding 27, in a direction opposite to that of the arrow. As a result, deflection current i₂₇ charges capacitor C₁₂ positively. Control circuit 120 progressively increases, during the vertical scanning trace, the conduction interval of transistor 118 that occurs in each horizontal trace. This causes the negative portion of current i₂₃, flowing through diode 19, to decrease progressively and the positive portion of current i₂₃, flowing through transistor 118, to increase progressively. As a result, the average voltage across capacitor 126 at terminal 126′ decreases and causes, in turn, a decrease of deflection current i₂₇. At the center of vertical trace, equal portions of current i₂₃ that are at opposite polarities flow through diode 19 and transistor 118, respectively. Consequently, no charge is added to capacitor 126. Also, the voltage at terminal 126′ is equal to voltage +V1; therefore, deflection current i₂₇, at the center of vertical trace, is zero. During the second half of the vertical trace, control circuit 120 further increases progressively the conduction time of transistor 118. The sum of the portions of current i₂₃ at the opposite polarity, flowing through diode 19 and transistor 118, respectively, increases progressively in the positive direction from zero, causing capacitor 126 to discharge. The voltage at terminal 126′ decreases progressively and causes deflection current i₂₇ that flows in the direction of the arrow to increase. As a result, deflection current i₂₇ discharges capacitor C₁₂. At the end of trace, the conduction time of diode 19 is small compared with that of transistor 118. The conduction of diode 19 is controlled by transistor 118 and by energy storage coil 25 which determines the rate di/dt of current i₂₃ that is substantially constant during each horizontal retrace period. It follows that from the beginning to the end of vertical trace deflection current i₂₇ changes in an upramping manner and reverses polarity at approximately the center of vertical trace. During vertical retrace, transistor 118 is nonconductive and diode 19 conducts a large amount of negative current i₂₃; consequently, deflection winding 27 and capacitor 126 cause a half cycle of oscillation. A resulting vertical retrace voltage charges capacitor 126 to a voltage that is larger than voltage +V1 that causes deflection current i₂₇ to reverse its polarity. It can be seen that the current through resistor 122 is equal to the deflection current i₂₇. Therefore, the voltage developed across resistor 122 is proportional to current i₂₇, the vertical deflection current. The voltage developed across deflection current sampling resistor 122 is produced by deflection current i₂₇ and provides negative feedback to vertical control circuit 120. This feedback provides information to vertical control circuit 120 to enable the driving of transistor 118 into conduction for the appropriate duration each horizontal interval to generate vertical deflection current i₂₇. Current i₂₇, during vertical trace, is linearly proportional to sawtooth ramp voltage V O ′ or V₀. When the leading edge 900 of pulse 2V′ occurs, such as, for example, immediately prior to time t₀ of FIGURE 2b, the downramping, second portion of voltage V₀ begins. The beginning of the downramping portion of voltage V₀ causes deflection current i₂₇ in winding 27 of FIGURE 1 to begin its corresponding downramping retrace portion. When a trailing edge of the pulse of voltage V U4 occurs, the upramping trace portion of deflection current i₂₇ of FIGURE 1 begins. Voltage V₀ controls the instantaneous level of deflection current i₂₇ during the vertical trace portion of deflection current i₂₇. In each vertical scanning cycle, each of the voltages V₀, V₂, V₃ and V₄ is at the same corresponding predetermined voltage level when the trailing edge of the pulse of voltage V U4 occurs, because of the clamping operation as described before. Consequently, as described later on, proper image registration is obtained. The phase modulation of pulse 2V′ results in a modulation of the vertical trace time. Capacitors C₁, C₂ and C₃ provide ¨S" shaping, as described before. Clamping the voltage across each of capacitors C₁, C₂ and C₃ during pulse V U4 establishes in each such capacitor the corresponding predetermined charge or voltage level at the beginning of each trace interval in a way that is unaffected by the duration of the trace interval of any previous field. Without the clamping action of diodes D₁ and D₂, each of capacitors C₁, C₂, and C₃ would have carried slightly modulated charge from previous fields which may upset proper image registration. Because of the clamping action in capacitors C₁, C₂ and C₃, the ramping trace portion in each deflection cycle of both voltage V₀ and of deflection current i₂₇ of FIGURE 1 is in phase with, and follows phase variations of, the corresponding leading edge 900 of pulse 2V′. As described later on, the phase modulation of pulse 2V′ provides the precise timing that is required to provide a displayed image in which proper image registration occurs such that even fields overlay even fields, odd fields overlay odd fields and such that even and odd field pairs are interlaced. It should be understood that as a result of the way the waveform of voltage V₀ or V₀′ of FIGURE 2b is generated, the interval from the end of vertical trace of a given deflection cycle of deflection current i₂₇ of FIGURE 1 to the beginning of vertical trace of the following deflection cycle is also constant. The DC component of the vertical sawtooth of voltage V₀ or V₀′ of FIGURE 2b should be, preferably, preserved and transmitted to deflection winding 27 of FIGURE 1. DC coupling may, preferably, be used between sawtooth generator circuit 500 and vertical deflection circuit 100 as well as to deflection winding 27. DC coupling is preferred so that the phase modulation of pulse 2V′ does not change the level of deflection current i₂₇ that corresponds with a given level of voltage V₀ such as at the beginning of trace. Height control is provided by a variable resistor R6 that controls, by control of a source voltage V₅, the rate by which capacitors C₁ and C₂ are charged, during trace. Centering is provided by a variable resistor R10 that is responsive to voltage V₅ for controlling the average value of voltage V₀. Voltage V₅, at a terminal 999 of resistor R2, is proportional to both the sawtooth AC portion of voltage V₀ and also to its DC average level. Therefore, voltage V₅ forms a DC reference for deflection circuit 100 that maintains picture centering while picture height is being adjusted by varying resistor R6. The features of deflection current i₂₇, that were mentioned before, may be useful, for example, in a television receiver circuit of FIGURE 3, that is similar to that described in the aforementioned den Hollander patent, where the vertical deflection current is phase and amplitude modulated in accordance with a phase modulated vertical sync signal. The receiver of FIGURE 3, that generates pulse 2V′ of FIGURE 1, includes a tuner 10 having an input terminal 12 for connection to an antenna or other source of video input signal and having an output for supplying baseband video output signal V BB , that was mentioned before, to a video processing unit 14. For purposes of illustration, it will be assumed that the baseband video output signal is of the PAL standard. It will be appreciated, however, that the principles of the invention also apply to other standard interlaced video signal formats. Video processor 14 includes a PAL decoder that converts the input signal to Y, R-Y and B-Y component signals. The signal could be processed, if desired, in R, G, B component form. R, G, B components each have full video bandwidth while the color difference signals (R-Y, B-Y) have a lower bandwidth. Accordingly, a field store for color difference signals can be realized with fewer memory elements than would be the case if processing were done using R, G, B components. The Y, R-Y and B-Y component signals are low pass filtered by filters 16, 18 and 20 and converted to digital form by analog-to-digital (A/D) converters 22, 24 and 26 for storage in a memory 40. Filters 16-20 minimize aliasing and have cutoff frequencies of 7.5 MHz for Y and 2.8 MHz for the color difference signals R-Y and B-Y for the assumed PAL input signal. Lower cut off frequencies would be appropriate for NTSC signals. Converters 22-26 digitize the low pass filtered components to an 8-bit resolution using a sample clock CL that is phase locked to a multiple of horizontal sync in order to obtain a constant number of samples per horizontal line. After A/D conversion, the digitized components are applied to memory 40 via respective delay units 28, 30 and 32. The delay units may be variable and are included to equalize the delay times of the three input signal paths. The color difference components R-Y and B-Y are applied to memory 40 via a multiplex switch (MUX) 34 controlled by a horizontal line rate signal H. Switch 34 combines the two 8-bit wide color difference signals into a single 8-bit wide signal to minimizing storage requirements in memory 40. As one field of the multiplexed 8-bit signal and the 8-bit luma signal are stored in memory 40, a field previously stored is read out twice using a read clock signal 2 CL of double the write clock CL frequency. This doubles the field rate (100 Hz for PAL, 120 Hz for NTSC) and so reduces the perception of flicker where the signal is displayed on display unit 60. A multiplex switch 42 demultiplexes the color difference signals which, with the double field rate luma signal, are converted back to analog form by digital to analog converters 44-48. Low pass filters 50-54 suppress the repeat spectra after D/A conversion, suitable cut-off frequencies being 13.5 MHz for luma and 6.75 MHz for chroma. The double field rate analog signals are then converted to R G B form for application to display unit 60 which is synchronized by means of double speed horizontal deflection current i 2H and vertical deflection current i₂₇ provided by horizontal and vertical scan generators 62 and 64, respectively. Generator 62 generates deflection current i 2H at twice the frequency f H of the horizontal sync of baseband video output signal V BB . A field consists of 312.5 lines in the PAL standard. At double speed read-out, this field together with its repetition must consist of 625 lines. This can be realized when one of the two fields consists of 312 lines and the other consists of 313 lines. Memory 40 of FIGURE 3 is supplied with timing signals from timing unit 70 to provide the field sequence shown in FIGURE 4A in which 312 lines are produced in the first read cycle (fields A or B) and 313 lines are produced during the second memory read cycle (field A′ or B′) with the 313th line being blank. The double field rate vertical sync pulse 2V′ required for generator 64 has the pulse pattern illustrated in solid line in FIGURE 4B. For comparison purposes, pulses in dashed-line are provided, which represent a situation of equidistant double rate vertical sync pulses having a period of 312.5 line. The solid line pulses represent pulse signal 2V′ of FIGURES 1 and 3, which is periodic on a four-field basis. As shown, there are 312 lines in field A, 312.5 lines in the repeat field A′, 312 lines in field B and 313.5 lines in repeat field B′. Pulse 2V′ controls vertical deflection generator 100, as described before, to produce the vertical scan waveforms of vertical deflection current i₂₇. The vertical trace portions of current i₂₇ are shown schematically in FIGURE 4C. The scanning current waveform sequence of FIGURE 4c results in the interlace pattern shown in FIGURE 4D in which the first fields (A, A′) overlay first fields, the second fields overlay second fields (B, B′) and in which the first and second field pairs (AA′, BB′) are interlaced. For purpose of comparison, the dashed lines in FIG. 4D illustrate scan lines which would result if sync pulses 2V′ of FIGURE 4B were equidistant rather than being shifted or phase modulated. To ensure proper registration of displayed fields, the sawtooth voltage of FIG. 4C, provided by generator circuit 500, that was described before, always starts at the same value; all retrace times (T0-T0′, T1-T1′, T2-T2′, etc.) are equal. Timing signals for controlling the digital converters, memory, switches and scan generators are provided by timing unit 70 of FIGURE 3. Unit 70 generates, as described in the den Hollander patent, two-field and four-field pulse sequences for memory control and for scan generation of pulse 2V′ for ensuring that even fields overlay even fields, odd fields overlay odd fields, and that even and odd pairs of fields are interlaced when the double field rate signal is displayed. Ramp generator circuit 500 and vertical oscillator circuit 501 of FIGURE 1, embodying aspects of the invention, reduce or eliminate any tendency for variation in the phase of the trace portion of deflection current i₂₇ relative to the irregularly spaced double field rate vertical sync pulse 2V′ over the four field sequence.

Claims: A television deflection apparatus responsive to a synchronizing input signal (2V') at a frequency that is related to a deflection frequency, comprising: a controllable switch (U 6a ) responsive to said input signal that operates at a frequency that is related to that of said input signal (2V'); first (C₁) and second (C₂) capacitors forming a series arrangement that is coupled to said switch (U 6a ): a current source (R₂) coupled to said capacitors (C₁,C₂) for charging said capacitors to generate a sawtooth signal (V o ') having a ramping first portion that changes in a first direction when said switch (U 6a ) is nonconductive such that when said switch becomes conductive said switch (U 6a ) discharges said capacitors (C₁,C₂) to form a ramping second portion of said sawtooth signal (V o ') that changes in an opposite direction; and a source (500) of a ramping voltage (V₃) related to the generated sawtooth signal; characterized by; a first impedance (R₄,R₅) coupled to said source (500) of said ramping voltage (V₃) and to a first junction terminal (703) that is coupled between said first (C₁) and second (C₂) capacitors for generating a first current that flows in said first impedance (R₄,R₅) that controls S-shaping of said sawtooth signal (V o ') by reducing the rate of change of said sawtooth signal (V O ') at one of the beginning and end of one of said first and second portions of said sawtooth signal in accordance with said first current that flows in said first impedance (R₄,R₅); a second impedance (R₈) coupled to a terminal (701) of said first impedance (R₄,R₅) that is remote from said first junction terminal (703) for bypassing a portion of a current that flows in a current path between said source (500) of said ramping voltage (V₃) and said first junction terminal (703) for reducing said first current in said first impedance (R₄,R₅) and that reduces the rate of change of said sawtooth signal (V o ') at the other of the beginning and end of said one of said first and second portions of said sawtooth signal (V o ') to control S-shaping at said other of the beginning and end; a deflection winding (27); and means (100) responsive to said sawtooth signal (V o ') for generating a deflection current in said deflection winding (27) that is S-corrected. An apparatus according to Claim 1 characterized by a third capacitor (C₃) for capacitively coupling said ramping voltage (V₃) to said first junction terminal (703). An apparatus according to Claim 2 characterized in that said source of ramping voltage (500) comprises a voltage follower (Q₂) having an input terminal (500a) that is responsive to said sawtooth signal (V o ') and wherein said third capacitor (C₃) is coupled between an output terminal (emitter) of said voltage follower (Q₂) and a terminal (701) of said first impedance (R₄,R₅) that is remote from said first junction terminal (703). An apparatus according to Claim 2 characterized in that said first (R₄,R₅) and second (R₈) impedances comprise corresponding first and second resistances wherein said third capacitor (C₃) and said first resistance (R₄,R₅) form a current path for discharging said second capacitor (C₂) at said one of the beginning and end of said portion that corresponds with a trace interval of said deflection current and for charging said second capacitor (C₂) at said other of said beginning and end of said portion that corresponds with said trace interval such that, at said other of the beginning and end, said second impedance (R₈) reduces the rate by which said second capacitor (C₂) is charged. A television deflection apparatus responsive to a synchronizing input signal (2V') at a frequency that is related to a deflection frequency, comprising: a controllable switch (U 6a ) responsive to said input signal for operating said switch (U 6a ) at a frequency that is related to that of said input signal (2V'); first (C₁) and second (C₂) capacitors forming a series arrangement that is coupled to a terminal (500a) of said switch (U 6a ); a source (V₅) of DC voltage coupled through a first resistor (R₂) to said capacitors (C₁,C₂) for generating a sawtooth signal (V o ') at said terminal (500a) having a ramping trace portion that changes in a first direction when said switch (U 6a ) is nonconductive and a ramping retrace portion that changes in an opposite direction, when said switch (U 6a ) is conductive; characterized by an impedance (C₃,R₄,R₅) having a first terminal that is coupled to a junction terminal (703) of said first (C₁) and second (C₂) capacitors for generating a first current that is coupled to said junction terminal (703) to control S-shaping of said sawtooth signal; means (D₁,D₂) coupled to said first (C₁) and second (C₂) capacitors for clamping each corresponding voltage across each of said first (C₁) and second (C₂) capacitors, causing each to be at a corresponding predetermined level prior to a beginning time of said trace portion such that each of said corresponding predetermined levels across each of said capacitors is substantially unaffected by an amplitude of said sawtooth signal at an end of said trace portion; a deflection winding (27); and means (100) responsive to said sawtooth signal (V o ') that is S-corrected for generating a deflection current (i₂₇) in said deflection winding (27) that is S-corrected. An apparatus according to Claim 5 characterized in that said impedance includes a third capacitor (C₃) and wherein said clamping means (D₁,D₂) is coupled to said third capacitor (C₃) for clamping a voltage across said third capacitor (C₃), causing it to be at corresponding predetermined level prior to said beginning time of said trace portion. An apparatus according to Claim 5 characterized in that said clamping means comprises a diode (D₁,D₂). A television deflection apparatus responsive to a synchronizing input signal (2V') at a frequency that is related to a deflection frequency, comprising: a source of said synchronizing input signal (2V'); first (C₁) and second (C₂) capacitors forming a series arrangement; a source (R₂) of a first current coupled to said capacitors for charging said capacitors to generate a sawtooth signal (V o ') having a ramping trace portion that changes in a first direction during trace and a ramping retrace portion of said sawtooth signal (V o ') that changes in an opposite direction during retrace; and a source (500) of a first voltage (V₃) at a frequency that is related to that of said input signal (2V'); characterised by a first impedance (C₃,R₄,R₅) coupled between said source (500) of said voltage (V₃) and a junction terminal (703) of said first (C₁) and second (C₂) capacitors for generating a second current that is coupled to said junction terminal (703) that provides S-correction to said sawtooth signal (V o '); switching means (U 6a ) coupled to said first (C₁) and second (C₂) capacitors for establishing prior to a beginning of said trace portion of said sawtooth signal, respective voltages that are developed across the two capacitors at corresponding levels that are substantially unaffected by an amplitude of said sawtooth signal: a deflection winding (27); and means (100) responsive to said sawtooth signal (V' o ) that is S-corrected for generating a deflection current (i₂₇) in said deflection winding (27) that is S-corrected. An apparatus according to Claim 8 characterized in that said first impedance includes a third capacitor (C₃ and that a second impedance (R₈) is coupled to said third capacitor (C₃) for bypassing a portion of a current that flows in said third capacitor (C₃) for reducing said second current such that by reducing said second current, a rate of change of said sawtooth signal (V o ') at an end time of said trace portion becomes smaller.

Inventors: HAFERL, PETER EDUARD
Applicants: RCA THOMSON LICENSING CORPORATION
Application year: 1987
Main classification: H04N3
IPC codes: H04N3