JPH01192284A - Video mixing circuit - Google Patents

Abstract

PURPOSE:To improve the cross fade characteristic with no adjustment by including a gain control circuit of a parallel mixing system into a feedback loop of a negative feedback amplifier. CONSTITUTION:The feedback loop of a negative feedback amplifier 100 is connected in common to input terminals 14, 24, 34 of gain control circuits 10, 20, 30 via a feedback resistor 102 from an output terminal 101. Thus, the gain control circuits 10, 20, 30 are included in the feedback loop of the negative feedback amplifier 100. Then the excellent gain control circuit whose cross fade characteristic is 0% is obtained without any adjustment by the effect of the negative feedback.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a television studio transmission system, and more particularly to a video mixing circuit for television signals.

[Conventional technology] A conventional video mixing circuit of this type is shown in FIG.

To mix a plurality of video signals, there are a parallel method shown in FIG. 2(a) and a serial method shown in FIG. 2(b).

The parallel system in Fig. 2(a) uses multiple gain control circuits 10.2.
0.30 are placed in parallel, each gain control circuit 10, 20 .
The 30 output terminals 13.23°33 are connected to each other and mixed. The priority order of the signals to be mixed is set by the KEY circuit 40.

Normally, the control signal generated by the KEY circuit 40.

2. 3 is always added to 1 + 2 + 3 =
1, and at this time the cross-fade characteristics are in the best condition. In this parallel method, the number of stages is 1
The video characteristics are good because only one step is required, but it is necessary to adjust the cross-fade characteristics between each gain control circuit 10, 20.30, that is, between the gain control circuits 10.20, 20.30, and 10.30. However, as the number of signals to be mixed increases, it is difficult to obtain good loss fade characteristics between each channel.

In the series mixing method shown in FIG. 2(b), the output terminal 13 of the gain control circuit 10 is connected to the input of the gain control circuit 20, and the output terminal 23 of the gain control circuit 20 is connected to the gain control circuit 30.
is connected to the input of the signal generator to perform signal mixing. In this serial system, the priority of signals is given to the latter stage, and there is no need to match cross-fade characteristics between channels.

[Problem to be solved by the invention] In the conventional video mixing circuit parallel mixing method described above, the KEY
From determining the priority of the signals to be mixed in the circuit 40,
It was necessary to match the cross-fade characteristics between each channel, and it was difficult to obtain good loss-fade characteristics.

In addition, in the serial mixing method, priority is given to the latter stage, so there is no need to match the cross-fade characteristics between channels, but the increased number of stages is disadvantageous in terms of video characteristics.

[Means for Solving the Problems] The present invention solves the above problems, and by including a parallel mixing type gain control circuit in the feedback loop of a negative feedback amplifier, it is possible to produce images with good cross-fade without adjustment. The purpose is to provide a mixed circuit. In order to achieve this object, the present invention provides a parallel mixing type video mixing circuit in which a plurality of gain control circuits that input a video signal and whose gain changes according to an external control signal are connected in parallel. A negative feedback amplifier is provided for mixing the outputs of the circuits, and the output of the negative feedback amplifier is negatively fed back to each input of the plurality of gain control circuits.

[Example] Next, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram showing one embodiment of the present invention. Similarly to the parallel mixing method shown in FIG. 2(a), the gain control circuit 10,
20.30 are connected in parallel. The video signal is input to the input terminals 11, 21, 31 of the gain control circuit 10, 20°30.
is supplied to the control input terminals 12 and 2 from the KYE circuit 40.
Gain control is performed by a control signal supplied to 2° 32 and output to output terminals 13, 23, and 33.

The output terminals 13, 23, and 33 are connected to each other, and the gain-controlled video signals are mixed, and the output terminal 1
The signal is supplied to the next stage negative feedback amplifier 100 which is commonly connected to terminals 3, 23 and 33. Negative feedback amplifier 100 is a high gain video amplifier. The feedback loop of the negative feedback amplifier 100 is commonly connected from the output terminal 101 to the input terminals 14, 24.34 of each gain control circuit 10, 20.30 via a feedback resistor 102. Therefore, the gain control circuit 10°20
．． 30 will be included in the negative feedback loop of the negative feedback amplifier 100.

Next, the operation of the video mixing circuit will be explained.

Now, each gain control circuit 10, 20, 30 has a video signal V, .
V2. V3 is supplied and the gain control circuit 10.20
．． The control input terminals 12, 22 and 32 of 30 are connected to control signals, and 2. Suppose that 3 is supplied to . Further, when the gain of the negative feedback amplifier 100 is G and the feedback resistance is β, the output signal V0 is expressed by the following equation.

Vo=G (K+ (V+-Vo/lj) ”K2 (
V2-vo//3) 3 in 4m (V3-Vo/, 1))
vo(i+G(nil"K2"=43)/4)-GV
+ to +"GV2 to 2+ = GVaK3 If 100% feedback, then β = 1, and here the control signal is set to = 1 (full voltage), K2 = = O (OFF voltage).

In equation 1, + = 1. Substituting 3 = O into 2 =
Vo=V, and the V1 signal is output as is. Similarly, 2 = 1. + = Ks = O, then Vo = V
2, K3 = 1, K+ = K2 = 0(
In case 7), v0=v3, and the full voltage signal is output as is.

Next, consider the case of mixing.

ni, 2 stations, ni 2 = number, ni 3 = tail (Kl 0 to 2+ to 3=1) They are mixed one by one.

Here, if K1+, 2+, and 3>1, for example, ``+''
If K2 = 3 = mending, also + 2 + x, if <1, for example, = 2 =
When 3 = A, as above, equations 2, 3, and 54 give the same result. Also, if K, ≠ 2 ≠ 3, for example, 8=%, 2=
%, 3 = In this case, (5 formula) V + = v2 = Va = V p-p
and sult, type 2, type 3, type 4, and type 5 are all v. =IV,
-, next to,, 2. No matter what value 3 takes, it will never exceed IVp- due to the effect of negative feedback. Therefore, a good gain control circuit with no aberration and a cross-fade characteristic of 0% can be obtained.

[Effects of the Invention] As explained above, the present invention provides a parallel mixing type video mixing circuit in which a plurality of gain control circuits that input a video signal and whose gain changes according to an external control signal are connected in parallel.
A negative feedback amplifier includes a negative feedback amplifier that mixes the outputs of the plurality of gain control circuits, and the gain control circuits that are mixed in parallel are configured to negatively feed back the output of the negative feedback amplifier to each input of the plurality of gain control circuits. By including it in the feedback loop of the gain control circuit, the negative feedback effect ensures that the output always does not exceed the 100% level, no matter what the value of the control signal of the gain control circuit is. Good video mixing with cross-fade is achieved without adjustment. A circuit is obtained.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of the present invention, and FIG. 2 (
Figures a) and (b) are block diagrams of conventional parallel mixing type and series mixing type video mixing circuits, respectively. 10.20, 30: Gain control circuit 11.21, 31, 14, 24, 34: Input terminal 12.
22, 32: Control input terminals 13.23, 33. :Output terminal 40:KYE circuit

Claims (1)

[Claims] In a parallel mixing video mixing circuit in which a video signal is input and a plurality of gain control circuits whose gain changes according to an external control signal are connected in parallel, a negative feedback amplifier is used to mix the outputs of the plurality of gain control circuits. A video mixing circuit comprising: negative feedback of the output of the negative feedback amplifier to each input of the plurality of gain control circuits.