JP5113503B2 - Video output circuit - Google Patents

Description

  The present invention relates to a video output circuit having a function of correcting a sag of an output video signal.

  The configuration of a conventional video output circuit is shown in FIG. 4 (see, for example, FIG. 3 of Patent Document 1). In FIG. 4, 1 is an input terminal, 2 is an output circuit, and 3 is an output terminal. C3 is a capacitor externally connected to the output side of the output circuit 2 in order to extract only the AC component of the video signal, and this capacitor C3 and the load resistor R9 (input impedance of the subsequent circuit) constitute a high-pass filter. The Therefore, since the DC component is removed from the output video signal by the capacitor C3, when the video signal shown in FIG. 5 (a) is input to the input terminal 1, the output terminal 3 is shown in FIG. 5 (b). The video signal appearing in the waveform becomes a differential waveform, and a so-called sag with an inclined top is generated. In FIG. 5A, 11 indicates a video signal period (256H period), 12 indicates a blanking period, and 13 indicates a horizontal synchronization signal portion.

  In order to prevent the occurrence of this sag, a sag correction circuit is usually incorporated in the video output circuit. FIG. 6 is a circuit diagram showing a configuration of a video output circuit incorporating the sag correction circuit (see, for example, Non-Patent Document 1). The sag correction circuit connects an external capacitor C4 between the inverting input terminal of the output circuit 2 and the load resistor R9 side of the external capacitor C3, and further, between the output terminal and the inverting input terminal of the output circuit 2. A resistor R10 having a resistance value sufficiently larger than the output impedance Zo of the output circuit 2 is connected.

  In this sag correction circuit, a signal that has passed through a high-pass filter composed of a capacitor C3 and a load resistor R9 passes through the capacitor C4 and is fed back to the inverting input terminal of the output circuit 2, thereby correcting the sag.

JP 2005-184056 A “Whether the video circuit should be AC-coupled”, MAXIM, [searched on December 4, 2007], Internet <URL: http://japan.maxim-ic.com/appnotes.cfm/am_pk/3768>

  However, in the sag correction circuit shown in FIG. 6, the impedance of the loop composed of the resistor R10 and the capacitors C3 and C4 must be made sufficiently small in the frequency band of the luminance signal and the chroma signal, but the resistor R10 ensures the gain. As described above, since the resistance value must be set sufficiently larger than the output impedance Zo of the output circuit 2, the values of the external capacitors C3 and C4 cannot be reduced, and the capacitors C3 and C4 are enlarged. There was a problem to do.

  Furthermore, the output waveform when the luminance level fluctuates becomes a waveform as shown in FIG. 7, and the signal may be lost if the dynamic range of the output circuit 2 is not sufficient. Since the values of the capacitors C3 and C4 and the dynamic range are in a trade-off relationship, there is a problem that the capacitors C3 and C4 cannot be reduced from this point.

  An object of the present invention is to provide a video output circuit in which sag correction is performed and an external capacity for AC component extraction and sag correction can be reduced.

A video output circuit according to a first aspect of the present invention includes an operational amplifier that inputs and amplifies an input video signal to a non-inverting input terminal, and a first low-pass filter that extracts a low-frequency component from the output video signal of the operational amplifier. , by adding the low-frequency component output from the first low-pass filter to the input video signal to be input to the non-inverting input terminal of the operational amplifier, an integrating circuit the output video signal of said operational amplifier and integrated video signal When an output circuit for the output video signal of the integrating circuit is input, a high-pass filter connected to the output side of the output circuit, the operational amplifier is taken out APL center voltage of the output video signal of said operational amplifier And a reference circuit including a second low-pass filter that is applied as a control voltage to the inverting input terminal .
According to a second aspect of the present invention, above the video output circuit according to claim 1, APL center voltage output from the reference circuit, the first predetermined value for the output voltage of the first low-pass filter When the output voltage of the first low-pass filter is clipped to the upper limit value, and when the output voltage of the first low-pass filter falls below a second predetermined value with respect to the output voltage of the first low-pass filter, the output voltage of the first low-pass filter And a clip circuit for clipping to the lower limit value.

According to the first aspect of the present invention, since the integration circuit previously gives the video signal a slope opposite to the slope of the sag generated at the top of the video signal by the high-pass filter on the output side of the output circuit, the sag is corrected. Can do. At this time, the capacity of the high-pass filter only needs to satisfy the function of AC extraction, and can be set to a small value.
Further, since the amplifier of the integrating circuit is controlled by the APL center voltage extracted by the reference circuit, the amplifier is controlled according to the luminance level, and the output circuit can operate with a smaller dynamic range.
According to the second aspect of the present invention, since the clip circuit can limit the level fluctuation of the output waveform in accordance with the luminance level, the output circuit can operate with a smaller dynamic range in this respect.

FIG. 1 is a block diagram showing a configuration of a video output circuit according to one embodiment of the present invention. It has the same reference numerals to the same components as those in FIG. Reference numeral 4 denotes a sag correction circuit connected between the input terminal 1 and the input side of the output circuit 2. The sag correction circuit 4 includes an integration circuit 5 for forming an integrated waveform opposite to the differential waveform shown in FIG. 5B, and a reference circuit 6 for generating a reference voltage for correcting the amplitude of the video signal. And a clip circuit 7 for limiting the fluctuation range of the integrated waveform to be created.

  FIG. 2 is a specific circuit diagram showing the configuration of the video output circuit. The integrating circuit 5 includes resistors R1 and R2 constituting an adder, an operational amplifier 51, resistors R3 and R4 for setting the gain of the operational amplifier 51, and a low-frequency component (integrated waveform) extracted from the output signal of the operational amplifier 51. 1 includes a resistor R5, a capacitor C1, and a buffer circuit 52 that constitute one low-pass filter.

  The reference circuit 6 includes a resistor R6 and a capacitor C2 that constitute a second low-pass filter that extracts an APL (average video level) center voltage from the output signal of the operational amplifier 51, and a buffer circuit 61.

  The clip circuit 7 includes resistors R7 and R8 connected in series to a current source 71, an NPN transistor Q1 having a base connected to one end of the resistor R7, and a PNP transistor having a base connected to a common connection point between the resistor R8 and the current source 71. Q2 and the emitters of the transistors Q1 and Q2 are commonly connected to the output side, and the transistor Q1 or the transistor Q2 is turned on by the output voltage of the first low-pass filter including the resistor R5 and the capacitor C1. V1 is a voltage that determines the upper limit value for clipping, and V2 is a voltage that determines the lower limit value for clipping.

  In this video output circuit, the video signal input to the input terminal 1 is amplified by the operational amplifier 51 via the resistor R1, input to the output circuit 2, and the first low-pass filter comprising the resistor R5 and the capacitor C1. To enter. The output signal of the first low-pass filter composed of the resistor R5 and the capacitor C1 is added to the input signal via the buffer circuit 52 in the adder circuit composed of the resistors R1 and R2.

  The video signal of the input terminal 1 (node a) is shown in FIG. 3 (a), the video signal of node b is shown in FIG. 3 (b), and the output of the node c on the output side of the first low-pass filter comprising the resistor R5 and the capacitor C1. The video signal is shown in FIG. Thus, the video signal of the node c has a waveform obtained by extracting a low frequency (DC) component from the output video signal of the operational amplifier 51.

  Therefore, by adding the signal of the node c to the input video signal of the node a by an adding circuit composed of resistors R1 and R2, the video signal of the node b on the input side of the operational amplifier 51 is shown in FIG. As shown in the figure, the signal is an integrated waveform with the top inclined in the opposite direction to the waveform shown in FIG.

  Therefore, if this signal is amplified by the operational amplifier 51 and outputted from the output circuit 2 in the direction of the output terminal 3, a high-pass filter (as shown in FIG. 5B) is generated by the capacitor C3 and the load resistor R9. ), The inclination of the top is corrected to the inclination in the opposite direction, and a video signal with the sag corrected is output to the output terminal 3 (node d) as shown in FIG.

  Here, the constants are set so that the cut-off frequency of the high-pass filter composed of the capacitor C3 and the load resistor R9 is the same as the cut-off frequency of the first low-pass filter composed of the capacitor C1 and the resistor R5. Then, the sag can be corrected with high accuracy.

  At this time, the capacitor C3 only needs to satisfy the function of AC extraction, and can be set to a small value. Further, since the cut-off frequency of the first low-pass filter composed of the capacitor C1 and the resistor R5 may be set to be the same as the cut-off frequency of the high-pass filter composed of the capacitor C3 and the load resistor R9, the resistor R5 is set to a large value. By doing so, the value of the capacitance C1 can be set small. Therefore, both of the capacitors C1 and C3 can be set to a small value, and the size can be reduced.

  On the other hand, the output signal of the operational amplifier 51 is also input to the second low-pass filter comprising the resistor R6 and the capacitor C2 of the reference circuit 6, where the APL center voltage of the video signal is taken out and the buffer circuit The reference voltage is input from the resistor R 4 to the inverting input terminal of the operational amplifier 51 via the resistor 61. Since the amplitude of the input video signal at the input terminal 1 is attenuated by the resistors R1 and R2, the amplitude is returned to the original value by the operational amplifier 51. At this time, the APL center voltage is fed back to the operational amplifier 51. Thus, an appropriate operating point is set according to the luminance level of the input video signal, and the output circuit 2 can operate with a smaller dynamic range.

  Here, when the resistance R5 of the first low-pass filter is set to a large value, if the resistance R6 of the second low-pass filter is set to the same value as this, the capacitance C2 can be set to a small value, thereby reducing the size. This is possible and suitable for external mounting.

Further, the APL center voltage output from the buffer circuit 61 of the reference circuit 6 is also input to the clip circuit 7. Thus, APL center voltage, if greater than the first predetermined value for the output voltage of the first low-pass filter, the voltage at the common junction of the resistors R7 and R8, the transistor Q1 is rendered conductive, The higher side of the output voltage (FIG. 3 (c)) of the first low-pass filter including the resistor R5 and the capacitor C1 is clipped to the upper limit value V1. If the APL center voltage is lower than the second predetermined value with respect to the output voltage of the first low-pass filter, the transistor Q2 becomes conductive by the voltage at the common connection point of the resistors R7 and R8, and the resistor The lower side of the output voltage (FIG. 3 (c)) of the first low-pass filter composed of R5 and the capacitor C1 is clipped to the lower limit value V2.

  As described above, the integrated waveform shown in FIG. 3C appearing at the node c is clipped when it is too high or too low with respect to the APL center voltage. Therefore, the output circuit 2 can operate with a smaller dynamic range.

It is a block diagram which shows the structure of the video output circuit of the Example of this invention. It is a circuit diagram which shows the structure of the video output circuit of the Example of this invention. (a)-(d) is a signal waveform figure of node a, b, c, d of FIG. It is a circuit diagram of a conventional video output circuit. (a), (b) is a signal waveform diagram of nodes a and b of the video output circuit of FIG. It is a circuit diagram of the conventional video output circuit which incorporated the sag correction circuit in the output circuit. (a), (b) is an output waveform diagram of the output circuit when the luminance level fluctuates.

Explanation of symbols

  1: input terminal, 2: output circuit, 3: output terminal, 4: sag correction circuit, 5: integration circuit, 6: reference circuit, 7: clip circuit.

Claims (2)

An operational amplifier that inputs and amplifies an input video signal to a non-inverting input terminal, and a first low-pass filter that extracts a low-frequency component from the output video signal of the operational amplifier, and outputs from the first low-pass filter and added to the input video signal for inputting the component to the non-inverting input terminal of the operational amplifier, an integrating circuit for the integrated video signal the output video signal of said operational amplifier,
An output circuit the output video signal of the integrating circuit is inputted,
A high-pass filter connected to the output side of the output circuit;
A reference circuit comprising a second low-pass filter to provide a control voltage to the inverting input terminal of the operational amplifier is taken out APL center voltage of the output video signal of said operational amplifier,
A video output circuit comprising: The video output circuit according to claim 1, APL center voltage output from the reference circuit, the first low-pass when it exceeds a first predetermined value for the output voltage of the first low-pass filter the output voltage of the filter is clipped to the upper limit value, the clip circuit second when below a predetermined value for the first output voltage of low-pass filter for clipping the lower limit value of the output voltage of the first low-pass filter A video output circuit comprising:

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