VIDEO SIGNAL GENERATION AND CAPTURE

Abstract: An apparatus for outputting a VGA video signal comprises means for providing a clock signal on at least one of the three VGA analog colour signals. Also provided is a method generating a VGA video signal, the method including the steps of: receiving a video signal conforming to a VGA standard; providing a clock signal on at least one of these three VGA analog colour signals; and outputting the video signal with the clock signal provided. An image capturing apparatus is also provided.

Description: VIDEO SIGNAL GENERATION AND CAPTURE This invention relates to video signal generation and capture and, in particular, to the generation of video image signals for capture by an image capturing device. There are many situations in which it is desirable to be able to capture a video image either from a display or from the signal representing the image for use in image processing, display, or image printing applications. For video signals which conform to the VGA standard the signal consists of three colour signals, red, green and blue, which contain the video image, and two synchronisation signals, HSync and VSync. The colour signals are analog and time continuous and represent the intensity of each colour at every instant of display time. The synchronisation signals synchronise the display on which the signal is to be reproduced so that the vertical and horizontal alignment of the image is correct for the data provided. With such signals, it is relatively simple to detect the VSync and HSync signals to determine when each frame or each row of a frame begins. It is not, however, a straightforward task to detect data relating to pixels in a particular column, as expensive clocking and reference circuitry is required to count from the nearest preceding HSync signal to the required column. This makes image capturing devices with which to capture images from selected pixels both expensive and not particularly accurate. The present invention is directed toward providing an apparatus for generation of a video image signal which can be readily captured by an image capturing device without the image capturing device requiring high speed clocking and processing circuitry. According to the present invention there is provided an apparatus for outputting a VGA video signal, the apparatus comprising: means for providing a clock signal on at least one of the three VGA analog colour signals. The clock signal may switch at a rate representing every pixel or every alternate pixel and the clock signal may have a delay which may be fixed or which may vary dependent upon the rate of change of the level of one or both of the colour signals. The clock signal may use the whole voltage range of the colour signal on which it is imposed or may, alternatively, use only a proportion of the voltage range, allowing the remainder to continue to be used to convey colour data. The proportion may be in the range of 25 to 75%. A clocking signal may be provided on two of the colour signals to improve clocking accuracy. The present invention also provides a video signal conforming to a VGA standard, in which at least one of the three VGA analog colour signals has a clocking signal provided on it. The present invention further provides a method of generating a VGA video signal, the method including the step of: receiving a video signal conforming to a VGA standard; providing a clock signal on at least one of these three VGA analog colour signals; and outputting the video signal with the clock signal provided. Because the present invention generates a video signal in which a clock signal is present in the portion of the signal representing image data, there is no need for image capturing circuitry to generate its own clock signal in order to calculate where data relating to a pixel to be captured is located. This reduces the cost of the image monitoring circuitry and improves the accuracy of image capture . The present invention further provides an image capturing apparatus for capturing an image from a video signal of the type described above. The image capturing apparatus comprises a comparator for comparing a clocking signal within a video signal with a threshold value; a counter for counting the number of clock transitions on a signal from a received HSync signal; and means for capturing a value for at least one pixel of at least one colour signal within the video signal. The image capturing device may compare the clocking signal within the video signal with plural threshold values. The image capturing device may include means for determining the one or more threshold values by employment of a reference colour signal. The image capturing device may have means for comparing the colour signal pixel values with a further threshold value. The image capturing device may be employed in combination with a device for printing captured images. One example of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a switching diagram for explaining a first example of the present invention; Figure 2 is a switching diagram showing a second example of the present invention; Figure 3 is a switching diagram showing a third example of the present invention; Figure 4 is a block diagram of an image capturing apparatus according to the invention; Figure 5 is a block diagram of a second image capturing apparatus according to the invention; Figure 6 is a switching diagram showing the operation of the example of figure 5; and Figures 7a and 7b are diagrams showing a threshold determination which can be employed in the examples of figures 5 and 6. Referring to Figure 1, the colour and HSync components of a first example video signal generated according to the present invention are shown. As can be seen from figure 1, the green (G) and blue (B) colour signals are of the standard VGA type, namely analog and time continuous, as is the HSync signal which represents the start of a row. On the red signal, however, a clocking signal has been provided. The clocking signal switches state between high and low at a rate such that each transition represents the location of a pixel in a row. An image capture circuit wishing to detect either one or both of the values of the blue and green colour signals can do so by using a comparator and counter to locate the exact location that it wishes to sample for the pixel of interest. A skilled reader will readily appreciate that this arrangement does not impose the requirement that an image capture device have an internal clock, all that is required is a comparator and counter, although it still requires a comparator of relatively high efficiency and high capture speed. To ease the requirements on any image capture device the clocking signal may switch at every other pixel (figure 2) . This may mean that only half as much image data for a given width can be provided within the video signal, but provides a longer period between clock transitions in which data can be captured and allows clock transitions to occur at the centre of an image pulse by placing the clock signal 90° out of phase to the image data signals. Another manner in which the requirements of any capture device can be eased is to delay the clocking signal by a predetermined amount. This will ensure that the capture circuit has sufficient time to capture image data once the appropriate clock signal transition has been detected. The fixed delay device only operates reliably, however, for a limited range of pixel rates. In order to overcome this the delay period may be varied by an amount relating to the rate of change of one or both of the remaining analog colour signals to ensure accurate data capture. As a skilled reader will appreciate, the use of one of the colour signal allocations as a clocking signal means that a monochromatic or reduced colour ranged image will be produced by a display reproducing a signal according to the invention. Figure 3 shows a signal according to the invention in which full colour is possible, but which still provides a clocking signal. Again, a clocking signal has been imposed on the red colour signal, but only half the available voltage range has been used for the signal, with the remaining half being allocated to a standard analog signal. An analog to digital convertor can be employed by an image capture circuit to separate the clock from the signal during capture. The display will display an image which looks essentially correct, except that there will be an increase in brightness on adjacent lines. Such an arrangement could be used in combination with the arrangement of figure 2 to reduce the requirements on any image capture circuit. It could also use one of the delay arrangements discussed above. Whilst half the allowable voltage range has been used in the example of figure 3, higher clock stability can be gained by using a different portion of the range (for example 25%) this will harm the signal to noise ratio, but provides two legal ranges, 0 to 25% and 75 to 100% thus increasing the abruptness of the clock signal transition and easing capture device requirements further. Figure 4 shows an image capture apparatus for capturing image data from a signal according to the invention. The clock signal is passed through a comparator 1 which compares it to a threshold value provided by a threshold setting device 2. In this example the threshold setting device 2 is a digital-to-analog converter which receives a threshold valve in digital form from a control means (not shown) and outputs an analog equivalent. The comparison removes noise and/or any image data on the signal. The output of the comparator 1 is then fed to the clock input of a data retrieved device 4, in this example an analog-to-digital convertor. The data retrieval device 4 receives one or more of the colour signals and, using the clock input as a reference, outputs data corresponding to each pixel. It will be appreciated that this image capture apparatus can be easily adapted to process all the signal formats discussed above. Figure 5 shows a second example second image capture apparatus for capturing image data from a signal according to the invention. This example again employs a latch 4 coupled to an image signal input G. This time, however, two comparators 1,5 are employed to control the latch output and a further comparator is provided on the image signal input line. Each comparator has in this example, a different comparator threshold. Such an arrangement is beneficial in that, for poor quality received signals, such as the type that are extremely noisy or affected by line impedances, can still be received. Figure 6 shows the manner in which the employment of two thresholds, P and M, generates two clock signals which, in turn, can be employed to provide a signal clock output signal from the latch 4. In this example a further threshold comparison is performed on the image signal G to compensate for noise and line impedance effects also. As can be seen from figure 6, this particular scheme uses one pixel per data point, but this is not essential. It will be appreciated that such an arrangement relies on the assumption that the image signal data to be captured undergoes similar effects to the clocking signal R, but this will usually be the case. Figure 7 shows how any of the thresholds employed in the above examples can be calculated prior to their employment. To do this a known test pattern is provided on the image signal G and an initial threshold set. Looking at either odd or even pixels only, the threshold is raised until a minimal capture error is recorded and the set point determined as the threshold point. If a second threshold is to be determined, such as the threshold M in the example of figure 5, such a threshold is then varied until, again, minimum capture errors are recorded. If a threshold for the image signal data is to be employed (as per the example of figure 5) and the minimum error is not within a range which is acceptable, then the image signal data threshold is adjusted and the process repeated for the other thresholds.

Claims: CLAIMS 1. An apparatus for outputting a VGA video signal, the apparatus comprising: means for providing a clock signal on at least one of the three VGA analog colour signals. 2. An apparatus according to claim 1, wherein the means for providing a clock signal provides a clock signal which switches at a rate representing every pixel or every alternate pixel. 3. An apparatus according to claim 1, wherein the means for providing a clock signal provides a clock signal having a delay. 4. An apparatus according to claim 3, wherein the delay is fixed. 5. An apparatus according to claim 3, wherein the delay is varied dependent upon the rate of change of the level of one or both of the colour signals. 6. An apparatus according to claims 1 to 5, wherein the means for providing a clock signal uses the whole voltage range of the colour signal. 7. An apparatus according to claims 1 to 5 , wherein the means for providing a clock signal uses only a proportion of the voltage range of the colour signal. 8. An apparatus according to claim 7, wherein the proportion is in the range of 25 to 75%. 9. An apparatus according to claims 1 to 8 , wherein the means for providing a clock signal provides a clock signal on two of the colour signals. 10. A video signal conforming to a VGA standard, and generated on an apparatus according to any of claims 1 to 9 at least one of the three VGA analog colour signals has a clocking signal provided on it. 11. A method of generating a VGA video signal, the method including the steps of: receiving a video signal conforming to a VGA standard; providing a clock signal on at least one of these three VGA analog colour signals; and outputting the video signal with the clock signal provided. 12. An image capturing apparatus for capturing an image from a video signal according to claim 10. 13. An image capturing apparatus according to claim 12, further comprising a comparator for comparing a clocking signal within a video signal with a threshold value and generating a clock signal therefrom; a counter for counting the number of clock transitions on the clock signal from a received HSync signal; and capturing means for capturing a value for at least one pixel of at least one colour signal within the video signal, the capturing means using the clock signal as a reference. 14. An image capturing device according to claim 12 or 13, further comprising means for comparing the clocking signal within the video signal with plural threshold values. 15. An image capturing device according to any of claims 12 to 14, further including means for comparing the colour signal pixel values with a further threshold value. 16. An image capturing device according to any of claims 12 to 15, further including means for determining the one or more threshold values by employment of a reference colour signal. 17. An image capturing device according to any of claims 12 to 16 employed in combination with a device for printing captured images.

Inventors: PESCHKE JAMES MATTHEW, PESCHKE, JAMES, MATTHEW
Applicants: DOMINO PRINTING SCIENCES PLC, PESCHKE JAMES MATTHEW, PESCHKE, JAMES, MATTHEW
Application year: 1997
Main classification: H04N7
IPC codes: G09G3, G09G5, H04N9