Digital soft edge video transition system

#The present invention relates generally to video special effects generators for combining multiple video input signals into a single video output signal, and more particularly, to a special effects generator utilizing digital circuitry for providing a gradual transition between two video input signal switched into the video output signal.

#Digital special effects generators are known. One such digital special effects generator is described in U.S. Pat. No. 3,821,468 issued June 28, 1974 to Albert E. Busch, assigned to the same assignee as the assignee of the present invention and incorporated herein by reference. While digital special effects generators such as the one described in the above-mentioned Busch reference provide a variety of special effects, transitions in the video output signal between two video input signals are abrupt, thereby resulting in a video display wherein the two video signals are displayed adjacent to each other with a sharp line separating the two displayed images. Such sharp transitions are desirable for many special effects; however, for other special effects such as vignette-type inserts, it is desirable to have a gradual transition between the two video signals to provide a soft edge effect in which the two video signals gradually blend into each other when displayed on the screen.

#Circuits for providing a gradual transition between video signals have been constructed. Such circuits utilize standard special effects generators that have been modified by adding an analog video switch that gradually increases the amplitude of one of the video signals while simultaneously decreasing the amplitude of the other video signal to provide the soft edge effect. A delay circuit is generally interposed between the analog switch and the circuitry driving it to provide a gradually changing drive signal to the analog video switch.

#The other objects and advantages of the present invention will be readily apparent from the following detailed description and drawings, wherein:

#FIG. 1 is a simplified schematic diagram in digital logic and block diagram form illustrating the operation of a preferred embodiment of the digital soft edge video transition system according to the invention;

#FIG. 2 is an illustration of the viewing screen of a television monitor showing one of the soft edge special effects provided by the circuit according to the invention; and

#Referring to FIG. 1, the digital soft edge transition circuit according to the invention comprises an end-of-transition locating circuit 10 and a switching waveform generator 12. Only a horizontal end-of-transition locating circuit 10 is shown in FIG. 1; however, a similar circuit may be utilized to provide a soft edge transition in the vertical direction. The end-of-transition locating circuit 10 includes a horizontal clock 14 that generates 512 pulses during each horizontal line scanning interval. The pulses from the horizontal clock 14 are applied to an 8-bit end-of-transition counter 16 by means of a 3-input NAND gate 18. A source of horizontal synchronizing pulses 20 is utilized to reset the counter 16 at the beginning of each horizontal scanning line. A divide-by-255 divider circuit 22 controls a latch 224 comprising a pair of NAND gates 26 and 28. The divide-by-255 circuit 22 counts the pulses from the horizontal clock 14 and provides an output signal indicative of the center of the horizontal scan after 255 pulses have been received.

#The soft edge transition circuit is controlled by a digital special effects generator 30. The digital special effects generator 30 is illustrated only in block diagram form for reasons of simplicity, but a much more detailed description of a typical digital special effects generator usable as the generator 30 is given in the referenced Busch U.S. Pat. No. 3,821,468 patent. The digital special effects generator 30 includes a lever counter 32 (similar to the lever counter 400 shown in FIG. 1 of the Busch patent), a horizontal counter 34 (similar to the horizontal counter 600), a vertical counter 36 (similar to the vertical counter 500), a series of comparators 36 (which may include a horizontal comparator such as the comparator 650, a circle comparator such as the comparator 760 and other comparators for providing different effects), an analog-to-digital converter 40 (such as the analog-to-digital converter 300) and a lever controlled potentiometer 42 (similar to the lever controlled potentiometer 101). The function of the special effects generator section 30 is to provide a transition signal for switching the output video signals in accordance with the position of the lever potentiometer 42.

#The switching waveform generator 12 includes a counter 44 that is driven by a voltage controlled oscillator 46. The frequency of the voltage controlled oscillator 46 is determined by the setting of a potentiometer 48. The output signal from the voltage controlled oscillator 46 is applied to an up counting input 50 of the counter 44 when a pair of NAND gates 54 and 56 are enabled, or to a down counting input 52 when the NAND gate 54 and a NAND gate 58 are enabled. A latch circuit 60 comprising a pair of NAND gates 62 and 64 is controlled by the horizontal synchronizing circuit 20 and the divider circuit 22 and determines which of the gates 56 and 58 is to be enabled.