Raster distortion avoidance arrangement

Abstract: Organ pipes distortions, caused by parasitic ringing in a horizontal deflection current (i₅) are suppressed by forming a current pulse (i₄) in a current injection arrangement (R1, C1,. D1). The current pulse is coupled to a junction terminal (100a) between a terminal of the horizontal deflection winding where a high retrace pulse voltage (V2) is developed and a low voltage terminal of a linearity correction inductance (L LIN ). A high voltage terminal of the linearity correction inductance is coupled to a horizontal output transistor (Q). The current pulse begins prior to the end of retrace and ends during the trace interval.

Description: The invention relates to a deflection circuit of a video display apparatus. So-called "organ pipes" distortion may appear in the left-hand side of a display screen of a cathode ray tube (CRT) as brightness modulated, vertical stripes. This type of distortion may result from horizontal deflection current ringing that produces a beam velocity modulation visible as vertical stripes. The deflection current ringing may be caused by a stray capacitance, associated with windings of a horizontal deflection winding, immediately after a retrace voltage pulse that is developed across the horizontal deflection winding ceases. The high rate of change of a retrace voltage pulse that is developed across the deflection winding, during the second half of horizontal retrace, produces a capacitive current pulse flowing in the horizontal deflection winding. This capacitive current pulse ends abruptly at the end of horizontal retrace, when the rate of change of the retrace voltage pulse becomes zero and may produce the deflection current ringing. Organ pipes distortions may last, for example, 5µs after a beginning time of the trace interval. Typically, the effect of organ pipes distortion is not visible to the user due to the usage of a high degree of overscan. However, it may be desirable to operate modern full square CRTs with only a low degree of overscan. Color television receivers or display monitors operating at higher scan rates than, for example, f H = 15,625 Hz, may be even more susceptible to organ pipes distortions. The ringing depends upon the horizontal winding stray capacitance and the retrace pulse voltage width and amplitude and not upon the deflection frequency. When a higher deflection frequency is used, the trace interval is shorter. Therefore, the ringing interval is proportionally greater relative to the trace interval. It may be desirable to suppress the deflection current ringing during the beginning of horizontal trace without using a high degree of overscan. The Article "IBM Technical Disclosure Bulletin vol. 29, no. 10, March 1987, Armonk (US), pages 4456 - 4458: 'Suppression of Horizontal Deflection Ringing"' discloses the clamping of ringing in a high-frequency flyback resonant circuit having features corresponding to the preamble of claim 1. Japanese Patent Kokai 62-254574 (Hitachi) shows a diode switching arrangement for reducing ringing at the beginning of horizontal scan by charging a capacitor from an external source and discharging the capacitor through a deflection coil in a direction opposite to the ringing so as to cancel the ringing. According to the invention there is provided a video display deflection apparatus as defined in claim 1: FIGURE 1 illustrates a horizontal deflection circuit with a ringing suppression arrangement, embodying an aspect of the invention; FIGURES 2a-2i illustrate waveforms useful for explaining the operation of the circuit of FIGURE 1 when the ringing suppression arrangement of FIGURE 1 is functional; and FIGURES 3a-3g illustrate waveforms that would be obtained in the circuit of FIGURE 1 should the ringing suppression arrangement of FIGURE 1 be removed. FIGURE 1 illustrates a horizontal deflection circuit 100, embodying an aspect of the invention, operating at a horizontal frequency f H = 15,625 Hz. Deflection circuit 100 includes a flyback transformer T, a deflection switching transistor Q, a damper diode D, a retrace capacitor C R , and S-shaping capacitor C s , a linearity coil L LIN with a parallel coupled damping resistor R D and a horizontal deflection winding L H . Linearity coil L Lin is coupled between winding L H and transistor Q. As shown adjacent to winding L H , a stray or interwinding capacitance C STRAY is associated with the windings of winding L H . Stray capacitance C STRAY is formed between horizontal deflection winding L H and, for example, a vertical deflection winding, not shown, a yoke ferrite core, not shown, and/or other components in the vicinity. In operation, a ringing suppression network 200, embodying an aspect of the invention, is coupled to a junction terminal 100a, between coil L Lin and winding L H . For explanation purposes only, assume that network 200 is disconnected from terminal 100a. In this case, deflection circuit 200 operates in a conventional manner. FIGURES 3a-3g illustrate waveforms useful for the explanation of the way Organ Pipes distortions that are produced when circuit 200 is disconnected. Similar symbols and numerals in FIGURES 1 and 3a-3g indicate similar items or functions. As a result of the switching operation of transistor Q of FIGURE 1, a retrace voltage V1 of FIGURE 3a is developed across deflection transistor Q of FIGURE 1. A retrace voltage V2 of FIGURE 3b is developed across winding L H of FIGURE 1. FIGURE 3c illustrates a deflection current i1 that is formed externally to winding L H and in a current path of winding L H of FIGURE 1. FIGURE 3d illustrates a waveform of a current i2 flowing through stray capacitance C STRAY associated with winding L H of FIGURE 1. Current i2 represents a difference current, measured between current i1 and a current i5. Current i5 flows to ground from an end terminal 100b of winding L H that is remote from terminal 100a. Current i2 may be measured using a current probe for obtaining the current difference between currents i1 and i5. During the second half of retrace, current i1 is smaller than current i5 because of the effect of current i2. Current i2 affects horizontal scanning because current i2 flows through the windings of horizontal deflection winding L H . Capacitive current i2 is dependent on the rate of change of retrace voltage V2. Current i2 has a positive peak amplitude in the vicinity of an end time, t1, of the retrace interval where the high frequency content of retrace voltages V1 and V2 is the largest. When the rate of change of voltage V2 becomes zero, current i2 becomes zero. The fast decrease of current i2 to zero, at the end of retrace, produces current ringing, as illustrated in FIGURE 3d. Organ Pipes distortions may be formed by current ringing in winding L H of FIGURE 1 following time t1 of FIGURE 3d, the end of retrace. The waveforms of FIGURES 3e-3g are shown in the vicinity of time t1 of FIGURES 3a-3d, but with expanded amplitude and with an expanded time base relative to those of FIGURES 3a-3d. Ringing of current i2 of FIGURE 3g subtracts from current i1 of FIGURE 3f and appears superimposed on deflection current i1 that is coupled to deflection winding L H of FIGURE 1. Current i1 of FIGURE 3f increases at time t1 by the same amount as current i2 of FIGURE 3g decreases. The ringing of current i2 during the 5µs interval of FIGURE 3g that follow time t1 is superimposed upon the downramping or trace portion of current i1 of FIGURE 3f that is coupled to winding L H of FIGURE 1. The ringing appears also superimposed on retrace voltage V2 of FIGURE 3e, at the beginning of trace. The ringing is caused by the fast fall time of current i2 of FIGURE 3g after time t1. In carrying out an inventive feature, ringing suppression network 200 of FIGURE 1, reduces such ringing. Network 200 includes a series arrangement of a resistor R1, a capacitor C1 and a parallel arrangement that includes a resistor R2 and a switching diode D1. Advantageously, linearity coil L Lin is interposed between deflection transistor Q and winding L H for separating winding L H from transistor switch Q. Linearity coil L Lin is non-saturated at the beginning of trace and operates as an inductor having an inductance of about 10%-20% of the inductance of winding L H . FIGURES 2a-2i illustrate waveforms useful for explaining the operation of the circuit of FIGURE 1 when network 200 is included in the circuit. Similar symbols and numerals in FIGURES 1, 2a-2i and 3a-3g indicate similar items or functions. In FIGURES 2d-2i, the amplitude and the time base are expanded in the vicinity of time t1 relative to those of FIGURES 2a-2c. Capacitor C1 of FIGURE 1 is charged, during retrace, via resistors R2 and R1 to a voltage V3 of minus 20 volts with respect to ground, as shown in FIGURE 2b. Retrace voltage V2 of FIGURE 2a drops to minus 100 volts at the vicinity of time t1, the end of retrace, and discharges capacitor C1 of FIGURE 1 via diode D1 and resistor R1. This injects a pulse of a current i4 of FIGURE 2c into a junction terminal 100a of FIGURE 1 between winding L H and linearity coil L Lin . Injected pulse current i4 is divided between coil L Lin and winding L H according to the ratio of the inductances of winding L H and linearity coil L Lin . A greater portion of pulse current i4 flows through linearity coil L Lin because of its relatively low inductance and subtracts from current i3 of FIGURE 2g. This causes a decrease of current i3 in the vicinity of time t1. The pulse of current i4 starts to increase at a fast rate just before time t1, it reaches its peak amplitude at time t1 and then has a slow fall time, as shown in FIGURE 2e. In accordance with an inventive feature, the suppression of the deflection current ringing is obtained by the decrease of current i3 of FIGURE 2g, during the aforementioned short interval in the vicinity of time t1. The decrease in current i3 is produced by injected current i4 of FIGURE 2e. As a result, deflection current i1 of FIGURE 2f is prevented from increasing after current i2 of FIGURE 2h abruptly decreases to zero. The smaller portion of current i4 of FIGURE 2e flows into winding L H and prevents deflection current i1 of FIGURE 2f from decreasing prior to time t1. Advantageously, the slow fall time of the pulse of current i4 of FIGURE 2e, following time t1, provides a smooth or gradual transient between currents i1 and i3. Following the short interval at the beginning of trace, when current i3 is smaller than current i1, currents i1 and i3 have the have same waveform and amplitude, during the remaining trace interval, as shown in FIGURES 2f and 2g, respectively. Advantageously, as a result of the current injection of current i4 of FIGURE 2e, the ringing that occur in current i2 of FIGURE 3g no longer occur in current i2 of FIGURE 2h. FIGURE 2i illustrates a waveform depicting a difference between currents i5 and i3. The waveform of FIGURE 2i shows that the greater portion of current i4 of FIGURE 2e in linearity coil L Lin of FIGURE 1 is subtracted from current i3 of FIGURE 2g. Current difference, i5-i3, of FIGURE 2i decreases slowly after time t1 of FIGURE 2h, providing a smooth transient of voltage V2 of FIGURE 2d at the end of retrace and the beginning of trace. The peak of current i4 of FIGURE 2e occurs at the end of retrace, when the rate of change of voltage V2 of FIGURE 2a is zero. Advantageously, as a result of current i4 of FIGURE 1, voltage V2 does not include ringing. A conventional East-West raster correction circuit may be coupled to end terminal 100b of winding L H that is remote from linearity coil L Lin , in a conventional manner not shown. In this way, both East-West raster distortion correction and suppression of Organ Pipes ringing may be maintained. An arrangement similar to that shown in FIGURE 1 may be implemented to provide horizontal deflection for a 34", 16:9 aspect ratio CRT, operating at a horizontal frequency 2f H . In this case, winding L H has an inductance of 350µH, resistor R1 has a value of 330 Ohm, resistor R2 has a value of 68 K-Ohm and capacitor C1 has a value of 470pF. The values of capacitor C1 and resistor R2 are selected in such a way that voltage V3 of FIGURE 2b is higher, or less negative, than voltage V2 of FIGURE 2a at time t1. The voltage difference, V3-V2, and the values of capacitor C1 of FIGURE 1 and of resistor R1 determine the peak amplitude of current i4 of FIGURE 2e. The fall time of the pulse of current i4 is determined by resistor R1 and capacitor C1 of FIGURE 1. Stray capacitance C STRAY is dependent on winding L H . The amplitude of current i2 strongly depends on the amplitude, pulse-width and the rate of change of voltage V2. Therefore, the values of resistors R1 and R2 and of capacitor C1 are selected based on such parameters. Advantageously, because current injection is provided in an AC coupling manner via capacitor C1, DC offset current in winding L H is prevented.

Claims: A video display deflection apparatus comprising: a deflection winding (LH); a retrace capacitance (CR) coupled to said deflection winding to form a retrace resonant circuit (100); first switching means (Q,D) coupled to said deflection winding for producing a deflection current (i 1 , i 5 ) and a retrace pulse voltage (V2) therein; an inductance (L LIN ) coupled in a current path of said deflection current between said deflection winding and said first switching means; a ringing suppression network (200) coupled to said inductance (L LIN ) and to said deflection winding (L H ) via a junction terminal (100a), that is in said current path between said inductance and said deflection winding; said ringing suppression network being adapted to generate a pulse of current having both a beginning time and an end time in a vicinity of an instant (t 1 ) when said retrace pulse voltage ceases, characterized in that said ringing suppression network (200) comprises a series arrangement of a first resistor (R1), a capacitor (C1) and a diode (D1), with a second resistor (R2) coupled in parallel with said diode, said capacitor (C1) being charged during retrace via said first and second resistors (R1,R2), said pulse of current being produced by the discharge of said capacitor (C1) via said diode (D1) and said first resistor (R1), said pulse of current being AC coupled via said capacitor (C1). An apparatus according to Claim 1 characterized in that said diode (D1) is coupled between said capacitor (C1) and said junction terminal (100a).

Inventors: HAFERL PETER EDUARD, HAFERL, PETER EDUARD
Applicants: RCA THOMSON LICENSING CORP, RCA THOMSON LICENSING CORPORATION
Application year: 1992
Main classification: H04N3
IPC codes: H04N3