Special Video Effect System

The present invention relates to a video signal processing system and, more particularly, to a special video effect system for combining video signals while compression, enlargement or rotation processing is being applied thereto.

In a special video effect system for television broadcasting, a plurality of video signals are frequently combined on a TV picture screen by using a key signal. Further, by using a digital process, various special effects such as the compression, enlargement and rotation of a picture are frequently performed. Therefore, when the video signals are to be combined, such digital processes may be used. In this case, it is necessary to produce a special key signal for the combination of video signals in accordance with a desired special effect. A prior art special effect system includes a digital special effect unit for special video processing, a key signal generating unit and a a micro-computer (CPU) for controlling the two units. The CPU provides a control signal to a digital special effect unit and a key signal generating unit so that the desired special effect can be produced. In the digital special effect unit, special video processing corresponding to the desired special effect is performed in response to the control signal. In the key signal generating unit, a key signal suitable to the desired special effect is also generated in response to the control signal. In the key signal generating unit, a large-scale circuit configuration is not required when the desired special effect is relatively simple, for example, a wiping effect in the horizontal and/or vertical direction. However, when the desired special effect is complicated as, for example, when accompanied by a rotation process and a tilting process, the key signal generating unit according to the prior art is quite complicated and as complicated as the special effect unit.

It is, therefore, an object of the present invention to provide an uncomplicated special video effect system for a key signal generating unit which is operative even when a complicated video process is produced by a digital special effect unit to produce complicated special effects.

According to the present invention, there is provided a special video effect system which comprises: a special effect unit for processing a video signal; a key signal generating unit for generating a key signal; a key signal superimposing circuit for superimposing the key signal delivered from the key signal generating unit over a video signal to be transmitted to the special effect unit; a key signal separating circuit for separating the key signal from a processed video signal delivered from the special effect unit; and a mixing circuit responsive to the separated key signal delivered from the key signal separating circuit for mixing the video signal delivered from the special effect unit and another video signal.

FIG. 1 shows a block diagram of a video signal special effect system according to the prior art;

FIG. 2 shows a block diagram of one embodiment of the present invention;

FIGS. 3(a) and (b) are graphs which show input/output characteristics of a key signal superimposing circuit;

FIG. 4 shows one example of the key signal superimposing circuit; and

FIG. 5 is a diagram of a key signal separating circuit.

In order to facilitate understanding of the present invention, a video signal special effects system according to the prior art will be described first with reference to FIG. 1. In this Figure, a video signal from an input terminal 1 is converted into a digital signal by a A/D converter. From the A/D converter the signal is sent to a special effect unit 3, in which the video signal is enlarged, compressed or rotated through digital processing to obtain a desired special effect.

For this processing, a control signal is sent from a CPU 4 to the special effect unit 3. A digital processed video signal from the special effect unit 3 is converted into an analog signal by a D/A converter 5 and is sent to a mixing circuit 6. In the mixing circuit 6, another video signal from a terminal 7 and the video signal from the D/A converter 5 are composed or combined in response to the key signal.

On the other hand, in response to the control signal delivered from the CPU 4, a key signal generating circuit 8 produces the aforementioned key signal in digital form according to the desired special effect. The digital key signal may be binary in case the video composition effect is sharp or is a multi-value signal of four bits where, for example, a soft video composition effect for blurring the boundary of the composed video signals is desired.

The digital key signal from the key signal generating circuit 8 is converted into the analog form by a D/A converter 9 and is sent to the mixing circuit 6. According to the prior art, to produce a key signal which is usable for various special effects including complicated effects, the key signal generating circuit 8 is required to have a large memory capacity to hold at least one television screen (one frame or one field) and an attendant complicated circuit configuration.

An embodiment of the present invention, as illustrated in FIG. 2, differs from the prior art system of FIG. 1 in that there is disposed, ahead of the special effect unit 3, a key gating circuit 10 to which the key signal is supplied, and in that there is added, after the special effect unit 3, a key signal extracting circuit 11 in which the key signal is extracted from the video signal.

In combining a plurality of video signals to produce the special effect, most obtainable effects have a simple shape as compared to their original shape, although the final effect shape may be complex. To generate a key signal of a simple shape, a key signal generating circuit can be constructed of a combination of counter and comparator circuits without any use of a frame memory or a field memory.

In the present invention, a relatively simple key signal is gated with the video signal before special video processing on the video signal takes place. Consequently special video processing to for obtain a desired special effect is conducted on the video signal containing the key signal. The key signal in the video signal is therefore processed in accordance with the desired special effect. Later that key signal is extracted. This provides a remarkable benefit because it is unnecessary to independently provide a complicated process of producing a key signal which is suitable for various special effects.

The input/output characteristics of the key gating circuit 10 are plotted in FIGS. 3(a) and (b). FIG. 3(a) corresponds to the case in which the key signal has a logical value "1" and FIG. 3(b) corresponds to the case of a logical value of "0". In FIG. 3(b), when the logical value is "0", no signal is sent to the special effect unit 3. And as shown in FIG. 3(a), when the logical value is "1", the key gating circuit 10 operates such that, the a video signal having a level higher than the pedestal level P is passed unchanged, and the video signal lower than the pedestal level P is modified to have a level which is equal to the pedestal level P.

This key gating circuit can be easily constructed by using a read only memory (ROM) 12, as shown in FIG. 4. In this figure, a plurality of bits representative of the input digital video signal from the A/D converter 2 and one bit representative of the key signal form a reading address of the ROM 12. The ROM 12 is preloaded with contents corresponding to the characteristics of FIGS. 3(a) and 3(b). Therefore, the ROM 12 delivers a modified, key-gated digital video signal in which the level of the incoming signal is modified in accordance with the input/output relationship depicted in FIGS. 3(a) and 3(b) in response to the key signal. In the special effect unit 3, during a period corresponding to a key signal of "0", an optional color can be inserted as a background color.

In FIG. 5, showing the structure of the key signal extracting circuit 11 is illustrated. A comparator 13 compares the video signal delivered from the special effect unit 3 with a reference value R which is lower than the pedestal level P. In response to the result of the comparison, the comparator 13 delivers a key signal having the logical value "1", when the level of video signal is higher than the reference value R, and, otherwise, the key signal is set to a logical value "0". As is apparent from the input/output characteristics of FIG. 3(a), the output of the key gating circuit 10 is set to a minimum value which is not below the pedestal level P, when the key signal is "1". Therefore, the key signal can be easily extracted out from the overall signal because the reference value R is set at a lower level than that pedestal level P. The key signal thus extracted has a binary level and, it is converted into a multi-value form having a suitable gradient by an interpolating circuit 14 (FIG. 2), if necessary.

As described in detail above, according to the present invention, the key signal is associated with the video signal and fed to the special effect unit, and, then, it is extracted again after special effects processing has taken place. As a result, shaping of a complicated key signal is carried out by the special effect unit so that the key signal generating remains an extremely circuit simple circuit.