JPS58223968A - Black frame inserting circuit - Google Patents

Abstract

PURPOSE:To quicken the response time, by connecting each one end of the 1st and the 2nd resistors connected to each emitter of transistors (TRs) to each base of which a vidio signal and a black frame signal are inputted respectively and connecting an output terminal to the connecting point of the resistors. CONSTITUTION:A video signal of negative polarity is applied to a terminal 20, and a black frame inserting signal is applied from a terminal 29. A reference potential Vref is equal to a pedestal level of input signal of negative polarity. Trs 21, 25, resistors 23, 24 and a constant current source 22 constitute an attenuator circuit and Trs 27, 28, 30, 31 resistors 32, 33, 34 and a constant current source 26 constitute a switching circuit. The black frame inserting signal is set to a low level when being inserted and to high level when being not inserted.

Description

[Detailed Description of the Invention] The invention relates to a black frame insertion circuit that inserts a black frame into a video signal so as to insert a black frame at an arbitrary position in a projected image, and also includes an image of the inserted black frame itself. It is designed so that it can be seen.

An example of a conventional black frame insertion circuit is shown in FIG. This circuit is i
E A positive polarity video signal is applied from input terminal 1. When not inserting a black frame, the input video signal is connected to transistor 2.
The signal is output from the output terminal 11 through the Emino Tough's lower circuit consisting of the resistor 3 and the resistor 4. At this time, the switch circuit composed of the resistor 6 and the transistor 6 is OF
F state, and transistor 6 is in the cutoff region. On the other hand, when inserting a black frame, a black frame insertion signal is applied to the insertion signal terminal 10 and becomes a high potential. At this time, transistor 6
operates in the saturation region, and the potential at the output terminal 11 is approximately determined by the constant voltage source 7, and the black frame is inserted by matching this value to the pedestal level of the output signal. Therefore, in the conventional circuit, the problem is that the level of the black frame is always at the pedestal level, and that the response time is slow because the transistor 5, which performs switching operation, shifts from the cutoff region to the saturation region when the black frame is inserted. there were.

The present invention improves this problem, and one embodiment thereof is shown in FIG. 2, and its concrete operation will be explained below. A video signal of negative polarity is applied to the video signal input terminal 20, and a black frame insertion signal is applied from the insertion signal input terminal 29. Further, the value Vref of the reference potential at the reference potential terminal A36 is equal to the best signal level of the input signal of negative polarity. Transistors 21 and 25 + resistors 23 and 24. The constant current source 22 constitutes an attenuator circuit, and the transistor 27
．． 2B, 30, 31. The resistor 32, 33.34° constant current source 26 constitute a switching circuit. The black frame insertion signal is low level when inserted, and high when not inserted.
h level. When the black frame insertion signal is not input, the base potential of the transistor 27 is higher than the base potential of the transistor 28, so the transistor 28 is turned on and the current value of the constant current source 26 is changed from the collector terminal of the transistor 28. Current value 1 flows into the collector terminal of transistor 31 due to the action of the current mirror circuit composed of transistors 30 and 31. then,
The base terminal voltage of the transistor 26 is Vref-I+-R32, where the resistance value of the resistor 32 is Rs2.

If this value is always selected to be lower than the potential of the video signal input terminal 2o, the transistor 26 always operates in the cutoff region, and the transistor 26 operates as an emitter follower.
Resistance 24. Transistor 261 is no longer a load, and the input video signal passes through resistor 23 to transistor 36.
．． It is converted into a low output impedance by an emitter follower operation constituted by a constant current source 37, and is outputted from a mother video signal output terminal 38 having the same amplitude as the input video signal.

When the black frame insertion signal is input, the base terminal potential of the transistor 27 is lower than the base terminal potential of the transistor 28, so the transistor 28 is in the OFF state (
(cut-off region operation), and no current flows into the collector terminal of the transistor 30 at all, and no current flows into the collector terminal of the transistor 31 forming the current mirror circuit. Therefore, the base terminal potential of transistor 260 becomes Vref. Figure 3 shows the function of the attenuator circuit when the black frame insertion signal is input, and will be explained in detail. fll (I and R2, the current value of the constant current source 22 is Io, the transistor 26
The base terminal potential of transistors 21 and 26 is Vref,
The current value flowing out from the emitter terminal of each is l111
．． The potential difference between IR2 base and emitter is Vg.
A linear equation holds for x+ and VBI+2.

Io = IE+ + II+2 −
-(1)V+ -VBIII = Vref -VBI
+2-(R+ + R2)II+2...-(2) The solution is as follows: 0 However, VT is the built-in potential O O...(3) O... (4) Current 11 flowing out from the emitter terminal of transistor 26
12 is linearly approximated using equation (4), as shown in FIG. 4, and in the linear region, Vat+=V+sx2. This is the difference between the pedestal level Vref and the negative input signal level v1, that is, the output of the input signal. When inserting a black frame, the sum of the resistance values of resistors 23 and 24 and the constant current value of The product must be selected to be larger than the amplitude of the signal from the pedestal level to the white peak. In Figure 5, ramp wave a is input to video input terminal 2.
Input the black frame insertion signal from 0 to the insertion signal input terminal 29.
This shows the signal C that appears from the video output terminal 38 when the signal is input from the video output terminal 38. When a black frame is inserted, a signal of twice the size is sent to the output terminal. FIG. 6 shows an application example circuit for a positive polarity video signal. In this case, by using PNP transistors as the transistors 46 and 47, the black frame insertion circuit can be easily constructed.

As described above, according to the present invention, when a black frame signal is inserted, the response time is fast, the signal waveform is shortened by a certain ratio determined by the resistance values of the resistors 23 and 24 without the black frame falling to the black level, and the video signal behind the black frame is reduced. It has the effect of obtaining a black frame that is also visible.0

[Brief explanation of drawings]

Figure 1 is a circuit diagram showing an example of a conventional black frame insertion circuit;
The figure is a circuit diagram showing one embodiment of the black frame insertion circuit of the present invention, FIG. 3 is a circuit diagram of the same essential part, and FIGS. 4 and 5 are explanations for explaining the operation of one embodiment of the present invention. 6 are circuit diagrams showing essential parts of another embodiment of the present invention. 20...Video signal input terminal, 22, 26.37.
... Constant current source, 23, 24, 32, 33.34...
...Resistance, 29...Black frame insertion signal input terminal, 3
6...Reference potential terminal, 38...Video signal output terminal. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure 1/// Figure 2 5ref 6Wυ- Figure 3 77 &AID Figure 4

Claims (4)

[Claims] (1) The video signal is input to the base of the seventh transistor, and its emitter is connected to the constant current source and the first resistor, and the black frame signal is input to its base, and its emitter is connected to the second resistor. A black frame insertion circuit comprising: a second transistor connected to one end of the transistor; and an output terminal connected to the other end of each of the first and second resistors and connected to the connection point thereof. (2) When the black frame signal is input, the voltage at the base of the second transistor becomes approximately equal to the pedestal potential of the video signal input to the first transistor. Black frame insertion circuit described. (3) When the black frame signal is not input, the base potential of the second transistor is lower than the lowest potential of the video signal when the video signal input to the base of the first transistor has negative polarity. and when the video signal input to the base of the first transistor is of positive polarity, the potential is set to be higher than the maximum potential of the video signal. range 1
Black frame insertion circuit described in section. (4) The product of the sum of the resistance values of the first and second resistors and the current value of the constant current source is larger than the maximum amplitude of the video signal input to the base of the first transistor. Black frame insertion circuit 0 according to claim 1