Apparatus and method for combining video signals to provide an output signal with full field coverage
BACKGROUND OF THE INVENTION 
     This invention relates to a method and apparatus for combining video signals. 
     A typical television broadcast is formed by a sequence of static video effects, each composed of a background and, optionally, at least one key. The key is an area of key video that appears to be in front of the background and is limited spatially by an associated key control signal. In the context of a news broadcast, the background of the current, or on-air, effect might be a studio backdrop and the foreground of the on-air effect might be a newscaster, while the background of the next effect in the sequence might be a scene of a sports events and the foreground of the next effect a sports announcer. Alternatively, the background of the on-air effect might be a scene of a sports event and the foreground of the on-air effect a sports announcer, while the next effect is the scene of the sports event without the sports announcer. Depending on the key control signal, the area of the key can change, for example when the sports announcer gestures or a weather forecaster points to a region of a weather map background, but nevertheless the effect is considered to be a static effect. 
     A dynamic effect, or transition, occurs when there is a change from one static effect to another static effect. A change in the area occupied by a key is not considered to be a transition. A mix/effects (M/E) device is used to provide a transition that is not visually disturbing to the viewer. 
     One method of providing a transition from the on-air effect to the next effect is to use a wipe border matte effect. The wipe border matte effect will be described with reference to FIG. 1, which illustrates a typical serial architecture for an M/E device. 
     The M/E device shown in FIG. 1 comprises six video mixers 9-14 connected in series. Each mixer receives a mix control signal at its X input and full-field video signals at its Y and Z inputs and provides an output signal represented by the equation: 
     
         Output=XY+(1-X)Z 
    
     The mix control signal can range in value between 0, or fully off, and 1, or fully on, and therefore the output signal can be all Y, all Z, or a linear mix of Y and Z. The mix control signal can vary as a function of position in the video field and/or as a function of time, from frame to frame. 
     Mixer 9 receives a border matte video signal at its Z input, and the outputs of mixers 9-13 are connected to the Z inputs of mixers 10-14 respectively. The desired output signal is provided at the output of mixer 14. 
     Mixers 9 and 10 receive at their Y inputs respective video signals of program A and program B, which are the backgrounds of the on-air effect and the next effect respectively. Mixers 11-14 receive at their Y inputs respective key video signals, designated key 1 video-key 4 video. Key control signals, designated key 1 control-key 4 control, are associated with key 1 video-key 4 video respectively and will be discussed further below. Each key control signal is similar to a video signal, and is used to define an area of the video field. 
     The mix control signals received by mixers 9 and 10 at their X inputs are referred to as transition control signals and are designated transition control signal T1 and transition control signal T2 respectively. The transition control signals T1 and T2 are generated by a transition control signal generator 16 which functions in response to signals provided by an operator interface 18. The mix control signals received by mixers 11-14 are provided by respective multipliers 21-24, each of which receives a key control signal at its X input. The Y inputs of multipliers 21-24 are connected to respective selector switches 31-34, each of which is able to select logical 0, logical 1, T1 or T2. The output signal of each multiplier is represented by the equation: 
     
         Output=XY 
    
     It will be apparent from FIG. 1 that the selector switches 31-34 can be set so that if the transition control signals T1 and T2 were set to 1 and 0 respectively for the entire field, an effect could be composed of program A background with key 1 video within the area defined by key 1 control. If, on the other hand, the transition control signals T1 and T2 were set to 0 and 1 respectively for the entire field, an effect composed of program B background with key 2 video within the area defined by key 2 control could be provided. If, as shown schematically in FIG. 1, the transition control signal T1 were 1 for a region at the right of the field and the transition control signal T2 were 1 for a region at the left of the field, an effect could be composed in which program A plus key 1 is shown at the right of the field, program B plus key 2 is shown at the left of the field, and the border matte appears as a vertical stripe down the center of the field. The transition control signals T1 and T2 can be varied dynamically in response to control signals provided by the operator interface 18, such that the size of the region for which the transition control signal T1 is 1 becomes smaller and the size of the region for which the signal T2 is 1 becomes larger. Thus, the vertical stripe sweeps across the field from left to right, providing a wipe from an on-air effect composed of program A plus key 1 to a next effect composed of program B plus key 2. 
     It will be appreciated that in the case of the example that has just been described, mixers 13 and 14 are not used and accordingly selector switches 33 and 34 are each set to the zero input. 
     The M/E device shown in FIG. 1 is subject to certain disadvantages. For example, during a mix transition or a soft wipe, when the mix control signal for a given key mixer is between 0 and 1, so that the associated key video should be present at the output of the mixer, the key video is partially transparent and a lower priority key video and/or background video might be visible through the higher priority key video. Also, in the ME device shown in FIG. 1, the background of the output effect is created by mixers 9 and 10 and the keys are added by mixers 11-14 after the background has been composed, and therefore the ME device is unable to provide a preview output of the next effect. 
     FIG. 2 shows a parallel M/E device in which each component of the desired output effect is multiplied by a control signal that limits the component to the desired area of the video field. The desired output effect is composed in the manner of a patchwork. A disadvantage of the M/E device shown in FIG. 2 is that the processing of the control signals can be quite complicated, and therefore the circuitry for generating the control signals is complex. As in the case of FIG. 1, a preview output of the next effect cannot be provided. 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the invention, apparatus for combining video signals to produce an output video signal representing a video effect comprises a first static effect channel for generating a first full-field video signal, a second static effect channel for generating a second full-field video signal, means for generating first and second transition control signals each having a full-field domain and a predetermined range, the sum of the first and second control signals nowhere exceeding the predetermined range, and arithmetic means for multiplying the first full-field video signal by the first transition control signal to generate a first addend signal, multiplying the second full-field video signal by the second transition control signal to generate a second addend signal, and summing the first and second addend signals to provide the output video signal. 
     In accordance with a second aspect of the invention, a method for combining video signals to provide an output video signal representing a video effect comprises generating a first full-field video signal representing a first static effect, generating a second full video field signal representing a second static effect, generating first and second transition control signals each having a full-field domain and a predetermined range, the sum of the first and second transition control signals nowhere exceeding the predetermined range, multiplying the first full-field video signal by the first transition control signal to provide a first addend signal, multiplying the second full-field video signal by the second transition control signal to provide a second addend signal, and summing the first and second addend signals to provide the output video signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention, and to show how the same may carried into effect, further reference will be made, by way of example, to the accompanying drawings in which: 
     FIG. 1 is a block diagram of a serial M/E device, 
     FIG. 2 is a block diagram of a parallel M/E device, 
     FIGS. 3A and 3B are block diagrams of an M/E device embodying the present invention, and 
     FIG. 4 illustrates various video fields and the manner in which they can be combined by the M/E device shown in FIG. 3. 
    
    
     DETAILED DESCRIPTION 
     The M/E device shown in FIG. 3 has three static effect channels 60, 80 and 100, composed of mixers 61-64, 81-84 and 101-104 respectively. Channels 60 and 80 are program channels and channel 100 is a transition channel. The input terminals of the three channels 60, 80 and 100 are connected to respective output terminals of a video/key input selector 120, whereby respective full-field video signals may be applied to the Z inputs of mixers 61, 81 and 101. Input selector 120 also has four key video output terminals designated key 1 video-key 4 video and four key control output terminals designated key 1 control-key 4 control respectively. The output key 1 video is connected to the Y inputs of mixers 61, 81 and 101, and the key 2 video, key 3 video and key 4 video outputs of input selector 120 are connected in similar manner to the other mixers of channels 60, 80 and 100. The X inputs of mixers 61-64, 81-84 and 101-104 are connected to respective switches 71-74, 91-94 and 111-114. The key 1 control output of input selector 120 is connected to one terminal each of switches 71, 91 and 111, and accordingly each of those switches can select either logical 0 or key 1 control. The key 2 control, key 3 control and key 4 control outputs of input selector 120 are connected in similar manner to the switches associated with the other mixers of channels 60, 80 and 100. In this fashion, each channel is able to generate a montage video signal representing a static effect composed of a background and one or more keys. 
     The outputs of mixers 64, 84 and 104 are connected to the Y inputs of respective multipliers 66, 86 and 106. The X inputs of mixers 66 and 86 receive transition control signals T1 and T2 from a transition control signal generator 124, while the X input of mixer 106 is connected to a switch 108 that is able to select either 0 or a signal NOT (T1+T2) provided by a positive non-additive mixer 126. As in the case of FIG. 1, the transition control signals T1 and T2 can each range between 0, or off, and 1, or on. The sum of the transition control signals T1 and T2 nowhere exceeds 1. The outputs of multipliers 66, 86 and 106 are connected to a summation circuit 128, the output of which is connected through a switch 130 to the program output terminal of the M/E device. The output of channel 80 is connected by a switch 132 to a preview output terminal of the ME device. 
     It can readily be seen that by appropriately setting the switches 71-74, 91-94, and 111-114, the three channels 60, 80 and 100 are able to provide montage video signals representing the three static effects illustrated adjacent the respective outputs of those channels. Accordingly, the settings of the switches 71-74, 91-94 and 111-114 enable the operator to determine directly the nature of the static effects that will be provided by the three channels. 
     If the transition control signal T1 is 1 for the entire field (which implies that signal T2 is 0 for the entire field), the effect provided at the output of summation circuit 128 is the same as that at the output of channel 60. If there is a transition such that signal T1 becomes 0 for the entire field and signal T2 becomes 1 for the entire field, the output effect is the same as that at the output of channel 80. If the transition control signal T1 is 1 for a region at the right of the field and is 0 for the rest of the field, and the transition control signal T2 is 1 for a region at the left of the field and is 0 for the rest of the field, then the output effect shown adjacent the output of summation circuit 128 is provided. If the size of the region for which the transition control signal T1 is 1 decreases and the size of the region for which signal T2 is 1 increases, an effect similar to that described with reference to FIG. 1 is provided. 
     With switches 130, 132 in the positions shown, the ME device shown in FIG. 3 provides a transition from the on-air effect to the next effect at the program output. A preview of the next effect is provided at the preview output. By toggling each of the switches 130 and 132 to the opposite state, the on-air effect is provided at the program output terminal and a signal representing the transition from the on-air effect to the next effect can be provided at the preview output. 
     Operation of the ME device shown in FIG. 3, as described so far, requires only one video mixer in each program channel and does not require a video mixer in the transition channel. However, it is desirable that each program channel should include at least two video mixers to provide a static effect composed of a background and two keys, and by connecting the mixers as shown it is possible to provide a transition in which the key changes but the background remains the same, or a transition in which the background changes but the key remains the same, or a transition in which both the background and the key change. It is desirable to include video mixers in the transition channel because it allows combination of the border matte with a key. If the key that is combined with the border matte is also included in the static effects at the outputs of channels 60 and 80, the key is preserved through the transition and appears in front of the border matte during the wipe, as shown in FIG. 4. FIG. 4 shows in the top row of fields the inputs to channels 60, 80 and 100, in the second row the outputs of channels 60, 80 and 100 and in the bottom row the transition control signals T1 and T2 and the result of combining the outputs of the three channels under control of the transition control signals, in the manner described with reference to FIG. 3. In the case of program channel 60, the static effect is composed of program A video plus keys 1 and 3, in the case of program channel 80, the effect is composed of program B video plus keys 2 and 3, and in the case of the transition channel 100, the effect is composed of the border matte plus key 3. Key 3 might be, for example, a title, in which case the transition takes place behind the title. 
     It is not essential that the background signals received by channels 60 and 80 be different. If the two signals are the same, the effect might be addition of a key to the on-air effect or removal of a key from the on-air effect. 
     The switch 108 allows the operator to prevent the output of channel 100 from contributing to the program output. Generally, this would arise where it is intended that the sum of T1 and T2 be 1 for the entire field. In such a case, there might be a possibility of NOT (T1+T2) being 1 at some point due to an overlooked circumstance, and it might then be desired that the field should be black rather than the color of the border matte at that point. 
     It will be appreciated that the invention is not restricted to the particular embodiment that has been described, and that variations may be made therein without departing from the scope of the invention as defined in the appended claims and equivalents thereof.