Video signal switching apparatus

Abstract: An apparatus for switching a video signal is comprised of a switch (7) for switching from one video signal to another, a memory (9) for storing an output of the switch, a detector (13) for detecting a switch signal, a switch control circuit (13) for controlling the switch in response to the switch signal, and a memory control circuit (15) for controlling writing and reading of the memory. When the switch signal is detected, the memory control circuit continues to write one video signal until a sync. (synchronizing) signal of that signal is detected and after a predetermined period commences writing another video signal when a sync. signal of the other video signal is detected.

Description: The present invention generally relates to a video signal switching apparatus and, more particularly, is directed to such an apparatus which is suitable for use in switching and displaying a plurality of video signals which are asynchronous with respect to one another. Figure 1 of the accompanying drawings shows an example of a prior art video signal switching apparatus. As will be seen from Figure 1, one of the video signals output from a plurality of video tape recorders (VTRs), that is, VTR-A 3 and VTR-B 5 is selectively switched through to an output terminal 20 by a selector 10. However, a home VTR, for example, is not provided with a gain locking system which enables the video signal to be synchronized with an asynchronous video signal so that, as shown in Figure 2, when the video signal is simply selected, the output from the selector 10 has the discontinuity indicated in Figure 2 and the horizontal synchronizing (sync.) signal is disturbed thus resulting in disruption of the image. Accordingly, it is an object of the present invention to provide an improved video signal switching apparatus in which the aforenoted shortcomings and disadvantages encountered with the prior art can be reduced or substantially eliminated. More specifically, it is an object of the present invention to provide a video signal switching apparatus' in which, even when a plurality of video signals are switched, the horizontal synchronizing signal can be prevented from being disturbed, thereby a satisfactory image being maintained. It is another object of the present invention to provide a video signal switching apparatus which can be applied to a camera switcher. According to the present invention there is provided an apparatus for switching a video signal comprising: a) switch means for switching from a first video signal to a second video signal; b) memory means for storing the output of said switch means; c) detector means for storing a switch signal; d) switch control means for controlling said switch means in response to said switch signal; and e) memory control means for controlling writing to and reading from said memory means, in a manner such that when said switch signal is detected, said memory control means continues to write said first video signal until a sync. (synchronizing) signal of that first video signal is detected and after a predetermined period writes said second video signal when a sync. signal of said second video signal is detected. The invention will be further described by way of non-limitative example with reference to the accompanying drawings, in which:- Figure 1 is a schematic block diagram showing an arrangement of a video signal switching apparatus according to an example of the prior art; Figure 2 is a timing chart used to explain the operation of the video signal switching apparatus of Figure 1; Figure 3 is a schematic block diagram showing an embodiment of a video signal switching apparatus according to the present invention; and Figure 4 is a timing chart used to explain operation or the like of a switch controller provided in the above video signal switching apparatus of the present invention. An embodiment of the video signal switching apparatus according to the present invention will hereinafter be described with reference to Figures 3 and 4. As shown in Figure 3, there is provided a video signal switching apparatus 1 to which the present invention is applied. In this video signal switching apparatus 1, video signals from a VTR-A 3 and a VTR-B 5 are input to a video memory ( a digital memory) 9 via a change-over switch (video signal switching and selecting means) 7, and an output terminal 20 and delivered to a display section (not shown) or the like. A change-over switch 7 is changed in position by a switching controller (switching detecting means) 13 which detects an output signal of a switching operation section 11 each time the VTR-A 3 or VTR-B 5 is selected by the switching operation section 11. The switching controller 13 is also supplied with output signals from the VTR-A 3 and the VTR-B 5 and supplies write (W) and read (R) timing signals through a memory controller 15 to the video memory 9 in response to the operation of the switching operation section 11 (first, second and third control means). It is possible that the write and read timing signals are asynchronous. The switching controller 13 is mainly formed of, for example, a microcomputer and detects vertical synchronizing signals (first and second vertical synchronizing signals, respectively) of the VTR-A 3 and the VTR-B 5 under the condition that the switching detecting signal is detected (first and second synchronizing signal detecting means). The processing done by the switching controller 13 will be described next. Referring to Figure 4, suppose that the VTR-A 3 outputs respective video signals a, b, c before and after two vertical synchronizing signals A 1 and A 2 , while the VTR-B 5 outputs respective video signals d, e, f before and after two vertical synchronizing signals B 1 and B 2 , and that the switching detecting signal S occurs at the timing shown. Then, despite this being during the signal interval b, the signal b will continue to be input to the video memory 9 for the time being. The controller 13 responds to the signal S by designating that a change-over between the signals is pending but does not immediately carry out the change-over. The controller 13 also monitors the signals from the VTRs for the presence of sync. pulses. Despite the occurrence of the vertical synchronizing signal B 1 of the VTR-B 5, the signal S continues to be held and the video signal b is continuously input to the video memory 9 until its vertical synchronizing signal A 2 is detected. Once the vertical synchronizing signal A 2 is detected, the switch controller 13 operates the change-over switch 7 in accordance with the thus held signal S, thus temporarily inhibiting both the outputs from the VTR-A 3 and the VTR-B 5 being input to the video memory 9 (the level at the W terminal goes high). After a switching operation, typically a clock in a synchronizing system is disturbed for a period of approximately 10H (H represents one horizontal scanning period) so that, if the vertical synchronizing signal B were to be input during this period, then the clock stabilization will be hindered. Therefore, it is preferable that the level at the W terminal is selected to be high at least for about 10H. When the vertical synchronizing signal B 2 of the VTR-B 5 is detected, the change-over switch 7 is operated so as to input the video signal f from the VTR-B 5 to the video memory 9. As described above, in this embodiment, if the switching operation section 11 is operated when the video signal of the VTR-A 3 is input to the video memory 9, the change-over switch 7 will not be changed in position until the input of at least the video signal b (one field time) to the video memory 9 is finished. The video signal f from the VTR-B 5 is input to the video memory 9 in synchronism with the vertical synchronizing signal B 2 . As described above, the signal written in the memory is read out therefrom on the basis of the reference clock, whereby the video signal in which neither the image signal nor the synchronizing signal are neither disturbed can be obtained at the output terminal 20. Accordingly, even if the synchronizing signals of the VTR-A 3 and VTR-B 5 are asynchronous, a synchronization disorder will not occur, thus making it possible to avoid disadvantages such as the disturbance of the image and so on. Thus, the image can be switched naturally. During a video edit, so-called cut-in and cut-out may be made without changing the operation of VTR-A 3 and VTR-B 5. Furthermore, in an apparatus such as the home VTR or the like using the asynchronous video signal, the video signal switching apparatus 1 can be utilized as a vertical blanking switch so that the video signal switching apparatus of the present invention can be applied, for example, to a camera switcher or the like. If the timing displacement of the vertical synchronizing signals of the VTR-A 3 and VTR-B 5 is less than 10H, then the video signal f of the VTR-B 5 is not written in the memory 9 and after one field period, the next video signal g is written in the memory 9. As will be understood from the above explanation, in accordance with the video signal switching apparatus of this invention, when the video signals are switched, only one video signal is input to the video memory until the vertical synchronizing signal provided immediately after one video signal is switched is detected, while only the other video signal is input to the video memory after the input of one video signal is finished and the vertical synchronizing signal of the succeeding other video signal is detected. That is, even if the switching operation is performed, one video signal of at least one field is constantly input to the video memory, while the other video signal is input to the video memory under the condition such that the input of one video signal to the video memory is completed. Therefore, since in the signal read out from the memory 9 neither the synchronizing signal of one video signal nor the synchronizing signal of the other video signal exists in the same field in a mixed state, the synchronization disorder can be avoided, so that the image can be switched satisfactorily without disturbance of image. Thus, even if a switching operation is performed at least one field portion of a first video signal is usually inputted to the video memory, and the other video signal is inputted to the video memory once the input of the one video signal has been completed. Having described the preferred embodiment of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to that precise embodiment and that various changes and modifications thereof could be effected by one skilled in the art without departing from the scope of the novel concepts of the invention as defined in the appended claims.

Claims: An apparatus for switching a video signal comprising: a) switch means (7) for switching from a first video signal to a second video signal; b) memory means (9) for storing the output of said switch means; c) detector means (13) for storing a switch signal; d) switch control means (13) for controlling said switch means in response to said switch signal; and e) memory control means (15) for controlling writing to and reading from said memory means, in a manner such that when said switch signal is detected, said memory control means continues to write said first video signal until a sync. (synchronizing) signal of said first video signal is detected and after a predetermined period writes said second video signal when a sync. signal of said second video signal is detected. An apparatus according to claim 1, wherein said sync. signal is a vertical synchronizing signal. An apparatus according to claim 2, wherein said switch control means (13) controls said switch means (7) to switch from said first video signal to said second video signal at the time when said vertical synchronizing signal of the second video signal is detected. An apparatus according to claim 1, 2 or 3 wherein said predetermined period is the duration of 10 horizontal scanning line periods. An apparatus according to any of the previous claims, wherein said first video signal and said second video signal are reproduced from respective video tape recorders.

Inventors: NAKAMURA MASASHI, OGINO AKIRA, NAKAMURA, MASASHI, OGINO, AKIRA
Applicants: SONY CORP, SONY CORPORATION
Application year: 1991
Main classification: H04N5
IPC codes: H04N5