Field of the Invention
The present invention relates in general to a signal converter device. In particular, the present invention relates to a signal converter device, which is used for dealing with the horizontal and vertical composite sync signal of a monitor.
In general, the color monitor of a personal computer is designed in RGB system. That is, the monitor generates the color pixels according to the R (the red video frequency signal), G (the green video frequency signal), B (the blue video frequency signal). Moreover, the monitor makes sure each frame's displaying mode of the pixels is based on the input sync signals.
The monitor displays the pixels by the combination of the continuous single frames. The frames are composed of a plurality of scanning lines. The operation of displaying the frames is used for scanning the scanning lines from the upper scanning line to the bottom scanning line. The vertical sync signal (written as V hereinafter) and the horizontal sync signal (written as H hereinafter) of the composite sync signal are used for vertical scanning the etch scanning line in turn and horizontal scanning the pixels of the scanning lines, respectively. Therefore, in order to display the pixels correctly, the monitor should be inputted the signals of R, G, B, V, H at least.
Normally, the generation of the video frequency signals R, G, B and sync signals V, H and the signals transmitting to the monitor are controlled by the video card or the display card in the computer system. Refer to FIG. 1, which depicts a part of structure of the computer system. The video card 10 is inset to the expansion of the computer system, and the video card 10 gets the displaying data from others parts of the computer system through the data bus, such as AGP or PCI. Then the video card generates the R,G, B,V,H signals according to the displaying data and sends the data to the monitors.
The video frequency signals R, G, B are outputted directly. However, the transmission of the sync signals may be done in different ways. One way is to use the separate sync signals. The vertical sync signal H and the horizontal sync signal V are inputted to the different terminals of the monitor, respectively. The other way uses the composite sync signals. The vertical sync signal H and the horizontal sync signal V are stacked to form the composite sync signal, then the composite sync signal is inputted to the specific terminals of the monitor, such as at the V terminal. At this time, the analog/digital converter circuit in the monitor will generate the clocks through the phase lock loop (written as PLL hereinafter) according to the composite sync signals.
The monitor must separate the composite sync signal from the vertical sync signal H and the horizontal sync signal V to display the pixels. The separating of the composite sync signal uses a specific circuit to integrate the composite sync signal and generates a shelter signal according to the polarization of the composite sync signal. Then, the shelter signal is inputted to the analog/digital converter circuit to turn off the PLL for interrupting the output of the clock.
However, there are many errors when dealing with the composite sync signal that may cause the display of a frame error.
Refer to FIG. 2, which depicts the timing of the composite sync signal HS and the shelter signal COAST-A of the prior art. To separate the composite sync signal correctly, the range of the shelter signal COAST-A must cover the range between the points B and K of the composite sync signal HS. Therefore, the clock outputted from the PLL will be blocked to prevent the abnormal pixels. However, because of the circuit characteristics and the delay of the signals, the shelter signal usually cannot cover the range between the B and K of the composite sync signal HS completely. When the range between the points B and C of the composite sync signal HS is not covered by the shelter signal COAST-A, the analog/digital converter circuit will keep supplying the clock in the period between the points B and C, and this situation will effect the performance of the display.