Switching apparatus for horizontal driving pulse

Abstract: In a horizontal deflection circuit, a waveform response of a horizontal output pulse must be adequately taken into account before and after a switching of the horizontal output pulse so that the waveform does not lack in shape and does not lose continuity at the switching. A switching apparatus for a horizontal driving apparatus includes (a) a horizontal driving pulse generator which generates a horizontal driving pulse from a horizontal synchronization signal, (b) an alternative signal generator which outputs an alternative signal different from the horizontal driving pulse, (c) a horizontal output control device which determines a timing to switch between the horizontal driving pulse and the alternative signal, and (d) a switching device which switches an output pulse with a control signal which is output from the horizontal output control device. The switching device switches only when both the output signal from the horizontal driving pulse generator and the output signal from the alternative signal generator change from a low level to a high level at the same time.

Description: The present invention relates to a switching apparatus for a horizontal driving pulse which is input into a horizontal deflection circuit in a television receiver. Broadcast satellite (BS) digital broadcasting has ushered in the use of a television (TV) receiver in recent years, and importance is placed on development of the capability that the TV is provided for various kinds of input signals. A conventional phase control device, which controls a horizontal driving pulse in a horizontal deflection circuit, is described as follows. FIG. 3 is a block diagram of a conventional phase control device which controls a horizontal driving pulse. Frequency discriminator 1 discriminates a type (e.g. an interlace signal, or a progressive signal) of input video signal (not shown) from a frequency of input horizontal synchronization (hereinafter referred to as horizontal sync, or H. Sync) signal A. Reference phase signal generator 2 outputs reference phase signal B which provides a phase of a horizontal deflection pulse optimized for the input video signal responsively to horizontal sync signal A and discrimination result J of frequency discriminator 1 (an output signal of discriminator 1). The phase of the horizontal deflection pulse determines the retrace-time of a scanning line. Timing signal generator 3 generates timing signal C, which is responsive to the input video signal, based on the horizontal sync signal A and the discrimination result J of the frequency discriminator 1. Sawtooth wave generator 4 outputs sawtooth wave signal D which has a phase responsive to timing signal C. Multiplier 6 multiplies reference phase signal B by an output signal of comparator 5, and outputs multiplier output G. Low-pass filter (LPF) 7 makes multiplier output G smoothed into average voltage Vav, which is a DC voltage, and outputs average voltage Vav. Comparator 8 compares sawtooth wave signal D with average voltage Vav, and outputs phase control signal H. Signal H is at a high level when sawtooth wave signal D is larger than average voltage Vav, and is at a low level in the other period. Horizontal driving pulse generator 20 outputs horizontal driving pulse I. Pulse I is synchronized with a rising edge of phase control signal H, and has a duty cycle which is determined by discrimination result J of frequency discriminator 1. Horizontal deflection circuit 13 receives horizontal driving pulse I, and outputs horizontal deflection pulse E. Comparator 5 compares horizontal deflection pulse E with a specified reference voltage, and generates signal F. Signal F is at a high level when a voltage of horizontal deflection pulse E is equal to or larger than the reference voltage. Timing signal generator 3 and sawtooth wave generator 4 constitute a sawtooth wave generator unit. The unit outputs a sawtooth wave signal having a phase responsive to a type of input video signal which is extracted from the horizontal sync signal. Comparator 5, multiplier 6, and LPF 7 constitute a phase difference voltage detector. The detector outputs a phase difference voltage which is the DC voltage (average voltage Vav) responsive to a phase difference between reference phase signal B and horizontal deflection pulse E. Furthermore, comparator 8 compares sawtooth wave signal D with phase difference voltage Vav, and outputs a phase control signal, which is used for the horizontal driving pulse to have a phase corresponding to the phase difference voltage. Alternative signal generator 14 generates and outputs an alternative signal with free running clock pulse generated therein. The alternative signal is asynchronous to an output signal of horizontal driving pulse generator 20. Horizontal output controller 16 detects whether an operation of a frequency change accompanying an input video signal change is in progress or not, and whether an operation of a power-on is in progress or not. Switching circuit 15 outputs the alternative signal to horizontal deflection circuit 13 while the operation of the frequency change is in progress or the operation of the power-on is in progress. The alternative signal is alternative to an output signal from horizontal driving pulse generator 20. The horizontal output controller switches an input signal of horizontal deflection circuit 13 between the output of the horizontal driving pulse generator and the output of the alternative signal generator. The apparatus having components mentioned above may malfunction for various reasons, for example, a signal waveform lacks in shape, a signal waveform loses continuity at the switching operation mentioned above, and so on. A horizontal driving transistor in the horizontal deflection circuit breaks if a waveform response before and after the switching operation is not taken into account. Further devices which control a horizontal driving pulse in a horizontal deflection circuit are given in prior art documents JP 10 123994 A and JP 08 148998 A. Document JP 10 123994 A describes a circuit for setting the horizontal oscillation frequency including a phase comparison circuit, a horizontal oscillation circuit, a horizontal output circuit and a control circuit. If the control circuit detects the presence of a frequency change of the H-sync signal, it decouples the horizontal oscillation circuit from the phase comparison circuit leaving the horizontal oscillation circuit in a state of free-running. This circuit is used to stabilize the horizontal oscillation frequency which may occur due to variations of circuits parameter with temperature and lifetime. Document JP 08 148998 A describes an PLL circuit comprising a phase comparator two VCO circuits. The desired VCO output signal is selected by a selector which is controlled by a CPU. It is the object of the present invention to provide for a switching apparatus which allows the waveform of the horizontal driving pulse not to lack in shape and not to lose continuity at the switching operation. This is achieved by the features as set forth in claim 1. As a result, the output signal waveform of the switching apparatus keeps continuity before and after the switching operation, and the horizontal deflection circuit does not have malfunctions which are a breakage of the horizontal driving transistor and the like. FIG. 1 is a block diagram of a switching apparatus for a horizontal driving pulse in accordance with an exemplary embodiment of the present invention. FIG. 2 shows waveforms of individual points in the switching apparatus for the horizontal driving pulse in accordance with the exemplary embodiment of the present invention. FIG. 3 is a block diagram of a conventional switching apparatus for a horizontal driving pulse. FIG. 1 is a block diagram of a switching apparatus for a horizontal driving pulse in accordance with an exemplary embodiment of the present invention. Horizontal driving pulse generator 1 generates driving pulse C, a phase of which is locked to horizontal synchronization signal A and output signal B from horizontal deflection circuit 4. Alternative signal generator 3 generates alternative signal D, which is asynchronous to driving pulse C, with free running clock pulse generated therein, and outputs the alternative signal. The output pulse signal from alternative signal generator 3 can be adjusted in frequency and duty cycle. Adjusting the frequency and the duty cycle allows a phase control device to have a horizontal driving pulse corresponding to various horizontal frequencies . Horizontal output control device 2 generates control pulse E when a rising edge of driving pulse C coincides with that of alternative signal D. Switch 5 selects driving pulse C or alternative signal D depending on control pulse E, and inputs selected signal F into horizontal deflection circuit 4. Operation of the switching apparatus mentioned above for the horizontal driving pulse is described with reference to FIG. 2. FIG. 2 shows signal waveforms output from individual points in the switching apparatus for the horizontal driving pulse in accordance with the exemplary embodiment of the present invention. In FIG. 2, driving pulse C is the driving pulse signal, a phase of which is locked to horizontal synchronization signal A and output signal B from horizontal deflection circuit 4. Alternative signal D or driving pulse C is selected by control pulse E. After that, the selected signal becomes waveform F. Control pulse E is generated only when a rising edge of driving pulse C coincides with that of alternative signal D. Hence it follows that waveform F does not lack in shape before and after the switching operation by the control pulse. That always keeps waveform F continuous. Consequently, the horizontal deflection circuit does not have malfunctions which are a breakage of the horizontal driving transistor and the like.

Claims: A switching apparatus for selecting a horizontal driving pulse (F) for a horizontal deflection circuit (4) for deflecting an input video signal during display of said input video signal, said switching apparatus comprising: a horizontal driving pulse generator (1) for generating a horizontal driving pulse (C) from a horizontal synchronization signal (A) of said input video signal; an alternative signal generator (3) for outputting an alternative signal (D) different from said horizontal driving pulse (C); a horizontal output control device (2) for detecting a simultaneous transition of said horizontal driving pulse (C) and said alternative signal (D) from a low level to a high level, for determining a frequency change accompanying an input video signal change or a power-on operation, and for generating a control signal (E) determining a time to switch between said horizontal driving pulse (C) and said alternative signal (D); and switching means (5) for switching one of said horizontal driving pulse (C) and said alternative signal (D), and for outputting a selected signal (F) upon detection of a simultaneous transition of said horizontal driving pulse (C) and said alternative signal (D) from a low level to a high level by said horizontal output control device (2). The switching apparatus as defined in claim 1, wherein the alternative signal is adjusted in a frequency and a duty cycle. The switching apparatus of claim 1 or 2, wherein the switching apparatus is used with a horizontal deflection circuit (4), said horizontal deflection circuit (4) carrying out a horizontal deflection with the horizontal driving pulse, said horizontal deflection circuit (4) receiving an output signal from the switching means (5).

Inventors: ANDO HIROSHI, MORIBE HIROSHI, MORITA HISAO, SHIBUTANI RYUICHI, TAKESHIMA MASAHIRO, TAKETANI NOBUO, ANDO, HIROSHI, MORIBE, HIROSHI, MORITA, HISAO, SHIBUTANI, RYUICHI, TAKESHIMA, MASAHIRO, TAKETANI, NOBUO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 2000
Main classification: H04N5
IPC codes: G09G1, H04N5, H04N3