| 摘要 |
Keying signals controlling the switching from a first to a second available signal source are available at the output of a comparator. The input of the comparator receives the output of a presettable up/down counter. The comparator has a first reference input which receives a first reference count (generally one half of the maximum count) at which the counter is switched from up to down operation. The second reference input receives a clip reference count which is less than the first reference count. If the oscillator oscillates at a frequency which is a predetermined multiple of the line frequency, and if the counter is reset at the line frequency, the keying signal will cause a vertical bar containing the picture information from the second signal source to appear within the remaining display which corresponds to the first signal source. The position of the bar can be varied by changing the preset input of the main counter, while its width can be varied by changing the clip reference. Squares can be generated by combining keying signals derived from horizontally synchronized and vertically synchronized apparatus of the above described type in an AND circuit. Many other shapes can be generated by changing the preset inputs and/or the clip reference input of either or both the horizontally and vertically synchronized apparatus. Basic shapes other than bars can be created by use of other inputs to the comparator (e.g. ramps) in conjunction with other comparator settings.
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