Horizontal deflecting circuit

A power source to drive a deflecting output unit is fixed, thereby simplifying so that it is possible to cope with two or more different horizontal frequencies. A resonance capacitor 4a and an auxiliary resonance capacitor 4b are serially connected and a variable capacitive switching element 7 to short-circuit the capacitor 4b is connected. When a tracing period changes from Tt to Tt' (Tt<Tt') in correspondence to a change from a first horizontal frequency on the high side to a second horizontal frequency on the low side, by turning on the variable capacitive switching element 7, the capacitor 4b is short-circuited and a capacitance is increased from C1.cndot.C1'/(C1+C1') to C1, thereby enabling (Tr'=Tr.cndot.Tt'/Tt) to be satisfied. Even when a fixed power source is used, a voltage of a retrace pulse does not change and a fluctuation of another voltage such as an anode voltage or the like which is formed from the retrace pulse can be prevented.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The invention relates to a horizontal deflecting circuit suitable for use
 in a multiscan type television receiver which can receive video signals of
 different horizontal frequencies.
 2. Description of the Related Art
 Generally, in a television receiver corresponding to a plurality of
 different horizontal frequencies, in order to assure a necessary raster
 width, a power voltage which is different for every frequency is supplied
 to a horizontal deflecting circuit. FIG. 6 shows an example of such a
 horizontal deflecting circuit. In FIG. 6, reference numeral 21 denotes a
 variable voltage power source. A (-) terminal of the variable voltage
 power source 21 is connected to the ground. A (+) terminal of the power
 source 21 is connected to the ground via a coil 22, a horizontal
 deflecting coil 25, and an S-shaped correcting curve provided by capacitor
 26. A switching element 23 for horizontal output is connected between the
 ground and a node of the coil 22 and horizontal deflecting coil 25. This
 node is connected to the ground via a resonance capacitor 24.
 The variable voltage power source 21 supplies an electric power necessary
 for maintaining the resonance to the horizontal deflecting circuit through
 the coil 22. For a tracing period of time during which the horizontal
 outputting switching element 23 is turned on, a voltage of the capacitor
 26 is applied across the horizontal deflecting coil 25. When a capacitance
 of the capacitor 26 is enough large, a saw-tooth wave current having a
 predetermined inclination flows in the horizontal deflecting coil 25 and a
 saw-tooth wave current having a predetermined inclination also similarly
 flows in the coil 22.
 Subsequently, when the horizontal outputting switching element 23 is turned
 off for a retracing period of time, the current flowing in the horizontal
 deflecting coil 25 and coil 22 flows into the resonance capacitor 24, so
 that a voltage (called a retrace pulse) is generated across the resonance
 capacitor 24. A voltage of the retrace pulse generated across the
 resonance capacitor 24 reaches a peak value at time Tr/2 shown by the
 following equation (1).
EQU Tr/2=(.pi./2).times. (L.cndot.C) (1)
EQU L=L1.cndot.L2/(L1+L2)
 where,
 L1: inductance of the coil 22
 L2: inductance of the horizontal deflecting coil 25
 C: capacitance of the resonance capacitor 24
 In equation (1), ( ) denotes the square root of the value in the
 parentheses.
 A peak value Vcp of the retrace pulse is obtained by the following equation
 (2).
EQU Vcp=V.cndot.(1+(.pi./2).cndot.(Tt/Tr)) (2)
 where,
 V: voltage of the variable voltage power source 21
 Tt: tracing period
 After that, the resonance capacitor 24 discharges the accumulated charges.
 When the voltage across the capacitor 24 is equal to 0, the horizontal
 outputting switching element 23 is again turned on and the foregoing
 operation is repeated. Thus, the saw-tooth wave current flows in the
 horizontal deflecting coil 25. In an actual circuit, by a combination of a
 bipolar transistor and a diode is often used as a horizontal outputting
 switching element 23 and, when the voltage across the resonance capacitor
 24 exceeds a forward voltage of the diode, the circuit is automatically
 short-circuited.
 The applicant of the present invention has proposed a multiscan type
 television receiver disclosed in JP-A-61-96875 (Japanese Patent
 Application No. 218863/1984). According to a horizontal deflecting circuit
 in the television receiver, a variable voltage power source is used. The
 variable voltage power source is controlled so that the product of a power
 voltage from the variable voltage power source and a horizontal tracing
 time is made constant. A serial circuit of a plurality of capacitors and a
 connection switch is connected in parallel with a resonance capacitor. By
 controlling so as to increase an electrostatic capacitance of a horizontal
 retrace capacitor as a frequency of a horizontal sync signal decreases and
 to make a high output voltage constant, it is possible to cope with a
 change in horizontal frequency of about twice.
 However, in case of the horizontal deflecting circuit using the variable
 voltage power source, it is necessary that a dedicated variable voltage
 power source unit for the horizontal deflecting circuit is added
 separately from a main power supplying unit to output a predetermined
 voltage or the main power supplying unit itself is used as a variable
 voltage power source. There are consequently problems such that a
 construction of the power supplying unit becomes complicated, a circuit
 scale of the whole apparatus increases, and the costs rise.
 In case of setting a duration of the retracing period constant, as shown in
 the equation (2), since the voltage of the retrace pulse generated for the
 retracing period rises as the horizontal frequency decreases, there is a
 possibility that unless the change in power voltage can transiently trace
 the frequency change, a voltage higher than the ordinary one is applied to
 elements constructing the circuit. There are, thus, problems such that a
 circuit construction to prevent the reliability from being lost is
 necessary and, further, the circuit becomes complicated.
 Moreover, in a horizontal deflecting circuit of the conventional type such
 that the coil 22 provided between the horizontal deflecting coil 25 and
 variable voltage power source 21 is set to a transformer structure like a
 flyback transformer and a secondary output such as a high voltage or
 heater voltage which is applied to an anode of a cathode ray tube (CRT) is
 extracted from the secondary coil 22, by changing a power voltage that is
 supplied to the horizontal deflecting circuit, a peak value of the retrace
 pulse and a voltage for the tracing period change. Since the secondary
 output is formed by rectifying the retrace pulse and the voltage for the
 tracing period, the secondary output also fluctuates and there is a
 problem that a necessary constant voltage cannot be extracted.
 In FIG. 7, a solid line waveform and a broken line waveform indicate
 retrace pulses when horizontal frequencies are different. Although a
 voltage according to a turn ratio with a primary coil is generated in the
 secondary coil of the transformer, as shown in FIG. 7, a voltage from a
 mean value of 0, namely, AC=0 is rectified for the tracing or retracing
 period and is extracted. There is, consequently, a problem that the
 secondary output fluctuates.
 OBJECTS AND SUMMARY OF THE INVENTION
 It is, therefore, an object of the invention to provide a horizontal
 deflecting circuit which can simplify a whole circuit without needing to
 change a power voltage.
 Another object of the invention is to provide a horizontal deflecting
 circuit which can stably extract a secondary output.
 Still another object of the invention is to provide a horizontal deflecting
 circuit which can be easily constructed in common with a circuit in which
 a horizontal frequency is limited to one kind.
 To accomplish the above objects, according to the invention of claim 1,
 there is provided a horizontal deflecting circuit for displaying an image
 based on an input video signal of a first horizontal frequency and an
 image based on an input video signal of a second horizontal frequency
 different from the first horizontal frequency to a CRT, comprising: a
 serial connecting circuit of a horizontal deflecting coil and a correcting
 capacitor; switching means connected in parallel across the serial
 connecting circuit; a variable capacitive resonance capacitor connected in
 parallel across the serial connecting circuit; a primary coil whose one
 end is connected to the serial connecting circuit and whose the other end
 is connected to a fixed power source; and a secondary coil, connected to
 the primary coil, for forming another voltage from a retrace pulse that is
 generated in the primary coil, wherein a value of the variable capacitive
 resonance capacitor is changed so that a voltage of the retrace pulse in a
 case where the circuit is driven at the first horizontal frequency and
 that in a case where the circuit is driven at the second horizontal
 frequency are set to a substantially same value.
 According to the invention, a capacitance C1 of a resonance capacitor and a
 capacitance C1' of an auxiliary resonance capacitor are selected as a
 combination which satisfies a predetermined relation and, for example, a
 switch is provided in parallel with the auxiliary resonance capacitor.
 When a tracing period changes from Tt to Tt' (Tt&lt;Tt') in correspondence to
 a change from the first horizontal frequency on the high side to the
 second horizontal frequency on the low side, by turning on the switch, the
 other resonance capacitor is short-circuited and the capacitance is
 increased from C1.cndot.C1'/(C1+C1') to C1 so as to satisfy
 (Tr'=Tr.cndot.Tt'/Tt; Tr denotes a retracing period of the first
 horizontal frequency). Even if a fixed voltage power source is used, the
 voltage of the retrace pulse does not change and a fluctuation of the
 other voltage such as an anode voltage or the like which is formed from
 the retrace pulse can be prevented.
 The above, and other, objects, features and advantage of the present
 invention will become readily apparent from the following detailed
 description thereof which is to be read in connection with the
 accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 An embodiment of the invention will now be described hereinbelow with
 reference to the drawings. In FIG. 1 showing a whole construction,
 reference numeral 1 denotes a power source of a fixed output voltage.
 A (-) terminal of the power source 1 is connected to the ground and a (+)
 terminal is connected to the ground through a primary coil 2a of a flyback
 transformer 2, a horizontal deflecting coil 5, and an S-shaped correcting
 curved is provided by a capacitor 6. A switching element 3 for horizontal
 output is connected between the ground and a node of the primary coil 2a
 and the horizontal deflecting coil 5. This node is connected to the ground
 through a resonance capacitor 4a and an auxiliary resonance capacitor 4b
 which are serially connected. A variable capacitive switching element 7 is
 connected between the ground and a node of the capacitors 4a and 4b. When
 the variable capacitive switching element 7 is turned on, both ends of the
 capacitor 4b are short-circuited. It is now assumed that the resonance
 capacitor 4a and auxiliary resonance capacitor 4b have capacitance values
 which satisfy a predetermined relation as will be explained hereinlater. A
 pulse voltage according to a turn ratio between the primary coil 2a and a
 secondary coil 2b is generated in the secondary coil 2b of the flyback
 transformer 2. By supplying the pulse voltage to a high voltage rectifying
 circuit (not shown), a high voltage is formed and supplied to an anode of
 a CRT (not shown).
 The power source 1 supplies an electric power necessary to maintain the
 resonance to the horizontal deflecting circuit. First, when an input video
 signal of a first horizontal frequency on the high frequency side is
 displayed on the CRT, the variable capacitive switching element 7 is
 turned off. For a tracing period of time during which the horizontal
 outputting switching element 3 is turned on, the voltage of the capacitor
 6 is applied across the horizontal deflecting coil 5. When the capacitance
 of the capacitor 6 is sufficiently larger than those of the resonance
 capacitor 4a and auxiliary resonance capacitor 4b, a saw-tooth wave
 current having a predetermined inclination flows in the horizontal
 deflecting coil 5. Similarly, a saw-tooth wave current having a
 predetermined inclination also flows in the primary coil 2a.
 When the horizontal outputting switching element 3 is subsequently turned
 off for a retracing period, the current flowing in the horizontal
 deflecting coil 5 and primary coil 2a flows into the capacitors 4a and 4b
 and a voltage is generated across the capacitors 4a and 4b. The voltage of
 a retrace pulse generated across the capacitors 4a and 4b reaches a peak
 value at time Tr/2 shown by the following equation (3).
EQU Tr/2=(.pi./2).times. (L.cndot.C) (3)
EQU L=L1.cndot.L2/(L1+L2)
EQU C=C1.cndot.C1'/(C1+C1')
 where,
 L1: inductance of the primary coil 2a
 L2: inductance of the horizontal deflecting coil 5
 C1: capacitance of the resonance capacitor 4a
 C1': capacitance of the auxiliary resonance capacitor 4b
 In the equation (3), ( ) denotes the square root of the value in the
 parentheses.
 A peak value Vcp at that time is obtained by the following equation (4).
EQU Vcp=V.cndot.(1+(.pi./2).cndot.(Tt/Tr)) (4)
 where,
 V: output voltage of the fixed voltage power source 1
 Tt: tracing period
 After that, the capacitors 4a and 4b discharge the accumulated charges.
 When the voltage across the capacitors 4a and 4b is equal to 0, the
 horizontal outputting switching element 3 is again turned on and the
 foregoing operation is repeated. Thus, the saw-tooth wave current flows in
 the horizontal deflecting coil 5 and a retrace pulse shown by a solid line
 in FIG. 2 is generated across the primary coil 2a.
 At the second horizontal frequency on the low frequency side, the
 horizontal outputting switching element 3 is turned on/off for the tracing
 period and the retracing period in the ON state of the variable capacitive
 switching element 7 in a manner similar to the foregoing case. In this
 instance, now assuming that the tracing period changes from Tt to Tt' by a
 change in horizontal frequency, a peak value Vcp' of the retrace pulse has
 a value shown by the following equation (5) from the equation (4).
EQU Vcp'=V.cndot.(1+(.pi./2).cndot.(Tt'/Tr)) (5)
 By varying the retracing period Tr in correspondence to a change in tracing
 period so that Vcp=Vcp', the peak value of the retrace pulse can be made
 constant irrespective of a difference of the horizontal frequencies. That
 is, from
EQU V.cndot.(1+(.pi./2).cndot.(Tt/Tr))=V.cndot.(1+(.pi./2).cndot.(Tt'/Tr))
 it is sufficient to satisfy the following relation.
EQU Tr'=Tr.cndot.Tt'/Tt (6)
 Therefore, when the tracing period corresponding to the second horizontal
 frequency on the low side Is equal to Tt' (Tt&lt;Tt'), the variable
 capacitive switching element 7 is turned on and the auxiliary resonance
 capacitor 4b is short-circuited. A synthetic capacitance when the variable
 capacitive switching element 7 is OFF is equal to C1.cndot.C1'/(C1+C1'). A
 capacitance when the variable capacitive switching element 7 is ON is
 equal to C1. The capacitance value in case of only C1 is larger. Thus, the
 equation (6) is satisfied.
 The retrace pulse at this time is shown in FIG. 2. In FIG. 2, a waveform
 shown by a solid line (the first horizontal frequency) indicates a retrace
 pulse which is generated for the tracing period Tt and retracing period
 Tr. A waveform shown by a broken line (the second horizontal frequency)
 indicates a retrace pulse which is generated for the tracing period Tt'
 and retracing period Tr'. As shown in FIG. 2, it will be understood that
 the voltages which are generated for the tracing period and retracing
 period are constant even if the horizontal frequency changes. Therefore,
 the high voltage that is formed by the rectifying circuit connected to the
 secondary coil 2b of the flyback transformer 2 can be also made constant.
 FIG. 3 shows another embodiment in which the invention is applied to a
 horizontal deflecting circuit of what is called a diode modulator
 construction. In another embodiment, an npn-type transistor 3a and damper
 diodes 3b and 3c are provided in place of the horizontal outputting
 switching element 3 in the foregoing embodiment. A coil 10 for modulation,
 a second S-shaped correcting capacitor 11, a second resonance capacitor
 8a, a second auxiliary resonance capacitor 8b, MOS-type FETs 7a and 9 of
 an n channel as switching elements, and a variable load 12 are provided.
 According to the circuit construction of FIG. 3, even if the voltage across
 the horizontal deflecting coil 5 is varied to adjust a raster width, so
 long as the power voltage is constant, the voltage across the primary coil
 2a is also constant. The first resonance capacitor 4a and first auxiliary
 resonance capacitor 4b are selected as a combination which satisfies the
 equation (6) in a manner similar to the foregoing embodiment. With respect
 to the second resonance capacitor 8a and second auxiliary resonance
 capacitor 8b as well, they are selected as a combination which satisfies
 the equation (6) in a manner similar to the foregoing embodiment.
 The fixed voltage power source 1 is connected to the ground via the primary
 coil 2a of the flyback transformer 2, horizontal deflecting coil 5, first
 S-shaped correcting capacitor 6, modulating coil 10, and second S-shaped
 correcting capacitor 11. A collector of the transistor 3a is connected to
 a node of the primary coil 2a and horizontal deflecting coil 5. An emitter
 of the transistor 3a is connected to the ground. The serially connected
 two damper diodes 3b and 3c are connected between the ground and the node
 of the primary coil 2a and horizontal deflecting coil 5. Further, a serial
 circuit of the first resonance capacitor 4a, first auxiliary resonance
 capacitor 4b, second resonance capacitor 8a, and second auxiliary
 resonance capacitor 8b is connected between the ground and the node of the
 primary coil 2a and horizontal deflecting coil 5. A node of the damper
 diodes 3b and 3c, a node of the capacitors 4b and 8a, and a node of the
 S-shaped correcting capacitor 6 and modulating coil 10 are connected in
 common and the variable load 12 is connected to the common node.
 A drain and a source of the FET 7a are connected across the first auxiliary
 resonance capacitor 4b. A drain and a source of the FET 9 are connected
 across the second auxiliary resonance capacitor 8b. A terminal G1 is led
 out from a gate of the FET 7a. When the terminal G1 is set to the high
 level, the FET 7a is turned on. By the turn-on of the FET 7a, both ends of
 the capacitor 4b are short-circuited and only the resonance capacitor 4a
 functions. A terminal G2 is led out from a gate of the FET 9. When the
 terminal G2 is set to the high level, the capacitor 8b is short-circuited
 and only the resonance capacitor 8a functions.
 In the foregoing other embodiment, by turning on the FETs 7a and 9 when the
 horizontal frequency decreases and by short-circuiting the capacitors 4b
 and 8b, the resonance frequency can be reduced in a manner similar to the
 foregoing embodiment.
 FIG. 4 shows a modification of the other embodiment. As shown in FIG. 4,
 the other end of the modulating coil 10 is connected to the ground and the
 second S-shaped correcting capacitor 11 provided between the modulating
 coil 10 and the ground is inserted between the node of the first auxiliary
 resonance capacitor 4b and second resonance capacitor 8a and the node of
 the first S-shaped correcting capacitor 6 and modulating coil 10. Further,
 one end of the variable load 12 is connected to the node of the capacitors
 4b and 8a and the other end of the variable load 12 is connected to the
 ground. In the construction shown in FIG. 4 as well, an effect similar to
 that of the embodiment shown in FIG. 3 can be obtained.
 FIG. 5 shows still another embodiment. As shown in the diagram, the node of
 the primary coil 2a and horizontal deflecting coil 5 is connected to the
 ground via a resonance capacitor 4c. A serial circuit of a capacitor 4d
 and the variable capacitive switching element 7 is connected between such
 a node and the ground. When the variable capacitive switching element 7 is
 turned on, the capacitors 4c and 4d are connected in parallel and a
 synthetic capacitance increases.
 In the embodiment shown in FIG. 5, a capacitance of the sole capacitor 4c
 is set to be equal to the serial synthetic capacitance of the capacitors
 4a and 4b of the embodiment shown in FIG. 1, and a parallel synthetic
 capacitance of the capacitors 4c and 4d when the variable capacitive
 switching element 7 is turned on is set to be equal to the capacitance of
 the sole capacitor 4a of the embodiment shown in FIG. 1.
 Although the other embodiments have been described with respect to the
 construction using the FET as switching means, it is sufficient to use any
 means which functions as a switch. For example, another switching element
 can be also used.
 Although the above embodiments have been described with respect to the
 cases where the invention copes with the two different horizontal
 frequencies as examples, the invention can also cope with three or more
 different horizontal frequencies. In this case, it is sufficient to
 increase the number of auxiliary resonance capacitors and the number of
 switches which are connected in parallel or serially with them in
 correspondence to the number of different horizontal frequencies.
 According to the invention, the power voltage to supply an operating
 voltage to the horizontal deflecting circuit is fixed and it is possible
 to cope with two or more different horizontal frequencies. According to
 the invention, since there is no need to use the variable power source,
 the whole circuit can be simplified. The circuit can be used in common
 with a horizontal deflecting circuit of a television receiver
 corresponding to the single horizontal frequency. According to the
 invention, since a peak voltage of the retrace pulse is held constant, a
 fluctuation of another voltage such as an anode voltage or the like of the
 CRT can be prevented.
 The present invention is not limited to the foregoing embodiments but many
 modifications and variations are possible within the spirit and scope of
 the appended claims of the invention.