Video signal convertion

Abstract: A general purpose scan converter for converting video signals from one scan rate to another scan rate accepts NTSC, PAL, EGA, VGA, or other inputs in standard video formats and convert them to be displayed on a monitor. Several modes of operation including continuous frame grabbing, grab and hold image, and dynamic scaling are provided. In addition, window sizing and window location options are under user control. The input sources for video may be selected, and a robust voltage controlled oscillator is provided for timing. The converter device operates continuously in real time and provides a high resolution output signal to a monitor. Special effects such as clipping of overlapping windows, superimposing graphics or text, and animation are supported.

Description: This invention relates to a converter for, and a method of, converting a video signal. The invention can be embodied as a general purpose scan converter for accepting video input signals and converting them to be displayed on a video monitor having a different scan rate and providing special effects on the display. Various types of video converters are known in the art. In a typical video converter, a video input signal is provided at a certain scan rate and a processor in the video converter transfers the incoming video data to a converter. The converter provides the information converted into digital form to a monitor to be displayed on the monitor at a different scan rate. Typically the conversion is not in real time because the conversion is performed by first converting all of the red components of the video signal, then converting all of the green components, and then finally converting all the blue components. This type of scan conversion is relatively inexpensive to implement but is very slow. In another type of video converter called a Time Base Corrector (TBC), the video signal is stored in analog form using charge coupled device (CCD) circuitry. Thus, the entire incoming frame is stored at a real time rate. The output of the CCD is shifted at a different rate, thus providing scan conversion. In another type of video converter, the incoming video data is taken by components (usually only the black and the white components or colored components) at a reduced resolution or reduced color grade. The data is transferred in a mode conventionally referred to as burst mode, which allows the input circuitry to transfer the information through a system data bus to the video circuitry. This type of scan converter has some general purpose applications because it allows transfer to almost any sort of circuitry. It has the disadvantage that software in the scan converter device must know the hardware memory locations. Typically the resolution provided by this system is very poor and relatively small amounts of information can be transferred in real time. Another type of converter is a video card that can store a video frame at one rate, then, through the use of an expensive multiscan monitor, output the stored video frame at a non-interlaced rate. Another disadvantage is that the video card must continuously be programmed to go between the two scan rates. This converter also does not provide a real time display. Thus, in the prior art there is no true real time video conversion available that provides a digital output signal for use by a host computer and to be displayed on a video monitor of a different scan rate, i.e.., going from interlaced video to non-interlaced. There is no prior art high resolution and high speed video converters available for use in a computer video system which requires digital output data from the converter. There is disclosed in IBM Technical Disclosure Bulletin, Vol. 28, No. 6, Nov 1985, a video compatibility feature which allows a certain class of successor display adapters to operate in a cooperative manner with predecessor adapters. Therefore useful transformations can be applied to the video before display. In particular the region of the screen in which the picture appears can be chosen arbitrarily and that region might be clipped, so that the picture could be overlain with another picture, or the picture might appear on top of another picture. According to the present invention, there is provided a converter for converting a video signal from a first to a second scan rate comprising: a pixel buffer for receivinq a portion of the video signal at a first scan rate; an address generator for determining a display location for the video signal; a non-interlaced raster frame buffer operatively connected to the pixel buffer for receiving the video signal at a second scan rate differing from the first scan rate; a control sequencer operatively connected to the pixel buffer and to the address generator for providing the portion of the video signal stored in the pixel buffer and the display location to the frame buffer upon occurrence of a predetermined event; characterised by means for controlling video data written into the frame buffer on a per-pixel basis so as to provide clipping of multiple overlapping windows, wherein the windows are of any size and shape. The present invention also provides a method of converting a scan rate of a video signal comprising the steps of: storing a portion of the video signal which is received at a first scan rate; providing the stored portion of the video signal to a frame buffer at a second scan rate differing from the first scan rates upon occurrence of a predetermined event; determining a location on a display for the video signal; providing the portion of the video signal stored in the frame buffer to the display at the determined location; characterised by controlling video data written into the frame buffer on a per-pixel basis so as to provide clipping of multiple overlapping windows, wherein the windows are of any size and shape. Thus the invention may provide a general purpose video scan converter that accepts NTSC, PAL, EGA, VGA, or other standard video inputs typically in RGBS, S-video, or composite form and converts these video inputs so as to display them and superimpose them as a window on a non-interlaced video monitor. The upscan rate may be dependent on the frame buffer resolution. (An upscan converter accepts video of a low scan frequency and displays it on a high scan rate monitor without the loss of video information.) Several modes of operation may be provided including continuous frame grabbing, single frame grabbing (i.e., grab and hold image), and dynamic scaling. Each of these modes allows window sizing and location of the window in the video screen to be under user control. The device provided in accordance with the invention is advantageously a real time video converter providing high resolution imaqes. In accordance with the invention, an incoming video signal may be accepted by the device at a given rate and after conversion be displayed in real time on a monitor typically having a different resolution and a different scan rate than that of the incoming signal. In accordance with one embodiment of the invention, each picture element (pixel), is represented by 24 bits. Thus, eight bits are provided in the video signal for each of the three primary video colors. This provides 256 (2 8 ) intensity levels for each video color component. Thus, each pixel can be any one of up to 16,777,216 colors, i.e., 256 3 . As is known in the art, S-video is an emerging standard which is used on commercially available medium grade video tape recorders in order to improve the quality of the image shown on television. S-video is a two wire standard involving chrominance and luminance. Also as is known in the art, a typical television set is an interlaced monitor in which each picture is drawn so that at one pass of the screen all of the odd lines are drawn, and then in a second pass the even lines are drawn on the screen. Thus, it takes two fields, i.e., passes, to complete an entire frame. Non-interlaced monitors are conventionally high resolution monitors in which the entire picture is drawn in one complete pass, thus removing the artifact of screen refresh called flicker. Also, as is known in the art, continuous frame grabbing is the process of continuously converting incoming information and displaying it on a non-interlaced monitor. Continuous refers to the fact that this process occurs in real time, as in typical broadcast television. Single-frame grab and hold means taking one frame at a time or looking at a view of several frames and freezing that particular frame or frames. Typically, this is done in order to store a particular frame in a computer memory device such as a disk drive. This process is useful in tape editing or frame editing and for animation. Dynamic scaling refers to the process by which a full size screen image is displayed in a reduced size. In accordance with one embodiment of the invention, a video image is first converted to digital data form, and then the digital data is provided to a pixel buffer. The digital video data is written into the pixel buffer via control lines at a rate that is synchronous with the incoming video data. A control sequencer provides a write pulse to the pixel buffer which begins filling the pixel buffer. The control sequencer also controls the rate at which the data is read out of the pixel buffer. As data is extracted out of the pixel buffer, the control sequencer simultaneously toggles a frame buffer control line and writes the pixel buffer data into a frame buffer via a frame buffer data bus. A host processor can at any time change the screen position of the converted video by loading a different start address into an address generator. The host can also start and stop the conversion as well as dynamically scale the window size of the converted video image. The frame buffer provides the converted video signal to a monitor and/or to a digital memory device such as conventionally provided in a computer. The invention is further described below, by way of example, with reference to the accompanying drawings, in which Figure 1 depicts one embodiment of the invention in block diagram form. Figure 2 depicts a second embodiment of the invention in block diagram form. Figures 3a to 3i show a computer program in accordance with the invention. Figures 4-7 depict registers in accordance with the embodiment of Figure 2. Figures 8a-8i show schematically the embodiment of Figure 2. Figure 9 depicts a third embodiment of the invention in block diagram form. Figure 1 depicts in a block diagram a converter in one embodiment of the invention. As shown, digital video data, i.e., data which has already been converted to digital form from a host or from the conventional video analog signal format, is provided on line 2 to a pixel buffer 4. The pixel buffer 4 in turn provides the data on frame buffer data bus 6 to a conventional frame buffer 8. Frame buffer 8 includes the elements of what is conventionally termed a "dumb" frame buffer, i.e., having no built-in intelligence. Frame buffer 8 thus conventionally includes (not shown) conventional video RAM (VRAM), a conventional video RAM controller and digital-to-analog output converters. Also included in frame buffer 8 is timing circuitry to link these elements together, and to display the contents of the VRAM on a monitor 12. Thus the data is written into video RAM in frame buffer 8, the outputs of the video RAM are provided to conventional digital-to-analog converters (not shown) in frame buffer 8, and the output of the conventional digital analog converters is provided to video monitor 12 to display the image. Thus, the output of frame buffer 8 continuously refreshes the screen of monitor 12 with the data stored in the video RAM. Pixel buffer 4 preferably includes enough storage capacity to hold several pixels (in one embodiment 64 pixels) of video digital data information. The status of data (i.e., whether or not there is data in pixel buffer 4 that can be written to frame buffer 8) in pixel buffer 8 in turn is provided to a pixel and frame buffer control sequencer 14 on a status line 16. Control sequencer 14 is provided with timing signals in the form preferably of a clock signal 18 and a horizontal and vertical synchronization signal 20. Also provided to control sequencer 14 is a frame buffer busy status signal 22 from frame buffer 8 indicating whether frame buffer 8 is busy or not. Pixel buffer 14 preferably can contain (but is not restricted to) 24 bits per pixel of digitized video data. The output of pixel buffer 14 to frame buffer data bus 6 preferably includes eight bits each of red, green, and blue data. The red channel (not shown) is preferably the least significant byte and is on data bits 0 to 7. The green channel (not shown) is preferably on data bits 8 to 15 and the blue channel (not shown) is on data bits 16 to 23. Data bits 24 to 31 are typically not used. The host processor can also directly access VRAM in frame buffer 8 by providing signals on processor address/data/control bus 23 to frame buffer 8 while the scan conversion is in progress. This multi-tasking feature allows scan conversion to co-exist with direct host access to frame buffer 8, and has several modes of operation. These various modes allow the user to select how much time frame buffer 8 is to allocate to scan conversion versus host access (i.e., in one embodiment the modes are that scan conversion is in effect 95%, 50%, 30%, or 20% of the time, with direct host access the rest of the time). The multi-tasking feature is achieved by latching (holding) the processor address, data, and control signals on bus 23 during a host-to-frame buffer 8 access. The host is then held off for a period of time, typically 20 µsec. Frame buffer 8 then asserts the frame buffer busy signal on line 22 and the host transaction then takes place. During the actual transaction period, pixel buffer 4 begins to back up with data. This is not a problem because during video blanking pixel buffer 4 has ample time to empty. If 50% or less is selected for the proportion of time devoted to the scan conversion, the converter operates at a reduced rate (non-real time) i.e., every other frame is converted. This allows longer periods of time when the host processor has full control of frame buffer 8. The operation of the converter is halted by clearing (setting to zero) a start bit which is located in a control register of control sequencer 14, as further described below. Control sequencer 14 issues commands to frame buffer 8 on frame buffer control lines 28. These control lines 28 generate the VRAM timing that is required in order to write the pixel buffer 4 data into VRAM. The control sequencer 14 is normally in an IDLE state. When the start bit is set to one, control sequencer 14 controls all aspects of the pixel buffer 4 using buffer read/write line 15, status line 16, and output enable (OE) control line 17. During a frame buffer screen refresh/transfer cycle, the frame buffer busy bit (on line 22) will go to the value of one. This alerts control sequencer 14 and causes control sequencer 14 to temporarily shut down, i.e., go into a tri-state condition, and thus all output signals 6, 28, and 37 are in an inactive state. This allows the processor to regain control of frame buffer 8 at this time. Pixel buffer 4, however, continues to be filled with data and will begin to back up. However, this is not a problem because during conventional blanking at the end of each incoming video scan line, pixel buffer 4 will have ample time to unload its data to frame buffer 8. If pixel buffer 4 is backed up to the point where it is not empty at the start of a new video scan line, control sequencer 14 clears the contents of pixel buffer 4 by providing a command on a reset (not shown) line (one of the Buf R/W lines 15). Control sequencer 14 in turn is connected by stop/start line 30, address control line 32, and output enable (OE) control line 34 to address generator 36. An address register (see Figure 6) in address generator 36 is a read/write register containing a linear address value corresponding to the upper left corner of the window to be scanned, minus some number which is preferably 1,152. This is described in further detail below. The output of the address register is provided to adders located in the address generator block. The adders calculate the start address of every scan line based on the previous internal calculated address. With the exception of the first line of an odd field, a value which in one embodiment is 2,304 is added to the internally calculated address at the start of every new scan line. At the first line of an odd field (conventionally, odd and even field fields are provided alternatively to make up one screen image) a value (which in one embodiment is 1,152) is added to the internally calculated address. This value 1,152 is the number of visible horizontal pixels per line for one type of monitor. The result of this addition is loaded into counters in the address generator. The counters, under control of control sequencer 14, are incremented by one for every pixel that is to be written to frame buffer 8. The counters thus generate an incremental address which is provided to frame buffer 8 on frame buffer address bus 37. As described above, address generator 36 is under control of control signals 26 supplied by the processor. The input of address generator 36 is also connected to the processor address/data bus 24, so address generator 36 can be programmed for the start address. The above describes one embodiment of the invention. In accordance with a second embodiment of the invention, shown in block diagram form in Figure 2, additional circuitry and a user interface are provided. As shown in Figure 2, various types of video analog input signals 44, i.e., RGBS, S-video, and composite video are provided to a DC restore and decoder circuit 46. An RGBS signal refers to RGB (red, green, blue) and synchronization; this is the standard RGBS signal. S-video, as described above, is super video (Y/C). Composite video is a conventional one-wire video transmission (the accepted television format throughout the world) in which chrominance and luminance components are multiplexed together onto a single conductor. Thus, in accordance with the present invention, the embodiment shown in Figure 2 can accept all three of these types of video signals. In accordance with the invention, other types of analog video signals can also be converted. DC restore refers to the process of direct current restoration which conventionally reduces the voltage level of an incoming video signal from a particular DC voltage bias level to a reference voltage level. Thus, a voltage-to-voltage level adjustment is provided in dynamic fashion at every scan line, so that if the picture becomes brighter or dimmer this activity does not affect the DC signal level. Since in most cases the incoming signal is AC coupled, the complete DC component, typically a 1.0-volt offset, is removed and the signal then is floating at a bias level convenient for the decoder circuitry, preferably 3.5 volts. The video decoder in block 46 includes the analog components conventionally required to convert a composite video or S-video or RGBS signal into its red, green, and blue components. The decoder of block 46 also includes a conventional video switch which will switch in an RGBS video source as well as other types of video as described above. The decoded red, green, and blue components 50 are provided to conventional analog-to-digital converters (ADCs) in ADC and pixel buffer block 52. Each RGBS signal component is digitized in block 52 to an eight-bit value. Conventionally a value of zero is black and a value of FF is full on, i.e., maximum signal strength. The decoder 46 also provides a conditional horizontal synchronization pulse (CSYNC) signal 54 to the voltage controlled crystal oscillator (VCXO) and robust phase detector circuitry 56. The VCXO and robust phase detector 56 are conventional. The VCXO provides system clock signals and sample clock signal 58 to the ADCs and to control sequencer 62. The VCXO is conventionally phase-locked to the incoming conditioned horizontal synchronization pulse (CSYNC) 54. The VCXO produces a clock signal 58 which is, in accordance with one embodiment of the invention, 776 times faster (for NTSC) or 944 times faster (for PAL) than the horizontal synchronization pulse (CSYNC 54). These clock signal frequencies correspond to frequencies of respectively 12.2098 MHz and 14.750 MHz. The robust phase detector in block 56 is a circuit including an internal process which allows intelligent phase detection. Thus if the reference signal (i.e., CSYNC signal 54) ceases for a moment, the robust phase detector bounces over, i.e., flywheels over that missing portion of the signal. Thus, a slight interruption in incoming reference signal 54 is tolerated. In addition, robust phase detector 56 is capable of detecting large phase errors and readjusting itself dynamically to account for those errors. This is beneficial because in many inexpensive video tape recorders the synchronization signal 54 is injected externally and may be as much as half a line (180°) out of synchronization. The robust phase detector detects this 180° out-of-phase condition and recovers from this condition. Control line 59 connects the robust phase detector circuitry to control sequencer 62. Control line 59 tells the sequencer 62 when valid video data is available. In accordance with this embodiment of the invention, the analog-to-digital converters are included in the same block 52 as is the pixel buffer discussed in connection with the embodiment shown in Figure 1. The pixel buffer in block 52 may alternatively be in the form of a conventional FIFO memory or may be a series of latched flip-flop circuits. Thus, the information once converted to digital format by decoder 46 and the analog-to-digital converters in block 52 is provided to pixel buffer block 52. The pixel buffer as discussed above is in use during times when frame buffer 64 must regain or have control of the video RAM memory. There are short periods when that control is necessary in order to keep the update of monitor 80 screen from losing synchronization lock with monitor 80, because the contents of the video RAM must be displayed at a rate that is correct for the monitor and that is correct for the frame buffer. Pixel buffer 52 then buffers up the incoming data for a duration of several pixels before providing the data to frame buffer 64 on frame buffer data bus 57. Once memory control is relinquished, control sequencer 62 (which controls the pixel buffer 52) continues to update the video RAM in frame buffer 64. The size of pixel buffer 52 is determined by the difference between the absolutely lowest scan rate and the absolutely highest scan rate which must be dealt with. The larger this difference, the larger the size of pixel buffer 52 required. In accordance with one embodiment of the invention, pixel buffer 52 holds 64 pixels each of 24 bits going from a scan rate of 15.734 KHz to one of 64 KHz. The configuration of pixel buffer 52, as discussed above, may be conventional latched flip-flops or conventional FIFO-type memory. In accordance with one embodiment of the invention, the FIFO-type memory is used partly since it is less expensive. The use of the FIFO-type memory is also preferred because if the exact scan rate is not known, then the variable capacity of the FIFO-type memory is better able to cope with the variable scan rates. Thus, latched flip-flops would be used if the scanned difference between the highest scan rate and the lowest scan rate is known so that the exact number of latched flip-flops needed can be provided. Pixel and frame buffer control sequencer (the "control sequencer") 62 controls pixel buffer 52 and frame buffer 64 in similar fashion as described above in connection with the embodiment of Figure 1 Control sequencer 62 also provides DAC (digital-to-analog converter) control signals to decoder 56 on line 63. These signals allow the user to adjust the conventional brightness, contrast, hue, and saturation in decoder 46. Control sequencer 62 is connected to pixel buffer 52 by buffer read/write line 66, status line 68, and OE control line 70, and to frame buffer 64 by control lines 74. Control sequencer 62 is preferably a state machine for controlling the rate of incoming and outgoing signals. When control sequencer 62 is triggered to begin a frame grab, control sequencer 62 looks for a particular incoming signal such as the horizontal and vertical synchronization signal 20 or the signal indicating what line of the frame is being provided on line 59. At this active scan line of the frame, control sequencer 62 begins filling pixel buffer 52 upon receipt of a command on pixel buffer write (buf R/W) line 66 and also begins writing to video RAM in frame buffer 64 via frame buffer control lines 74. Also control sequencer 62 looks for the frame buffer busy signal on line 78. The frame buffer busy signal is a feedback signal indicating that frame buffer 64 is busy, thus warning control sequencer 62 that frame buffer 64 is about to perform a refresh cycle or host access and will require control of the video RAM in frame buffer 64. A host access to frame buffer 64 (as explained above) is performed by the host providing digital video data directly to host port 77 of frame buffer 64 on processor address/data/control bus 79. Host port 77 includes address/data/control buffers and latches (not shown). At this time control sequencer 62 acknowledges the frame buffer busy signal by making all signals to the frame buffer 64 inactive, i.e., goes to a tri-state, thus allowing frame buffer 64 to perform its task of sending data to monitor 80. At the same time, control sequencer 62 does not stop sampling incoming signals but instead directs them to pixel buffer 52. This buffering step is necessary due to the continuous nature of the incoming video signals 44. As soon as the frame buffer busy signal on line 78 indicates that frame buffer 64 is no longer busy, control sequencer 62 initiates a new cycle. The OE (output enable) control lines 70, 82 carry the control signals which tri-state the frame buffer bus lines for, respectively, pixel buffer 52 and address generator 84. As discussed above, control sequencer 62 also controls address generator 84 which in this embodiment includes mode control, address, and sizing registers. Address generator 84 in this embodiment is similar to the address generator in the above-described embodiment discussed in connection with Figure 1, and includes two stages. The processor data is provided on the processor address/data bus 86 to program address generator 84, mode control, address, and sizing registers. The processor (host) control lines 87 do the actual writing of the programmed information into the above-mentioned registers. This allows the user interface portion of the device to be active as discussed below. The address register of address generator 84 contains the start location of where the image is to be placed on the screen, which conventionally is the upper left hand corner of the image. This placement allows the possibility of windows, i.e., images imposed on the screen which are smaller size than the full monitor screen. A feedback loop (i.e., the stop/start line 88) is provided between control sequencer 62 and address generator 84. Thus the address is incremented by address generator 84, and the data is written to frame buffer 64 VRAM. Therefore the data to frame buffer 64 is provided from pixel buffer 52 on frame buffer data bus 57, and the address of the data is provided by address generator 84 on frame buffer address bus 92. When the frame buffer busy signal arrives on frame buffer busy line 78 to control sequencer 62, control sequencer 62 shuts down the data flow and also shuts down address 92 and control lines 74 to frame buffer 64. At this time, address generator 84 also stops incrementing the address. Therefore the address and the data always match in terms of where the data are intended to be placed on the screen of monitor 80 once address generator 84 and control sequencer 62 are restarted. It is thus not necessary to buffer address generator 84. The status of phase detector 56 is provided to address generator 84 on status line 94 to allow the host processor to poll this bit and get information about the incoming line video. In accordance with the invention, a user interface as discussed above is provided. This user interface is preferably implemented in software in the form of a C-language program running on the processor of a host computer system. An example of a C-language program in accordance with the invention is shown in Figures 3a-3i. The purpose of this software user interface is to make the converter user friendly, by allowing access to the scaling and window location registers as discussed below. This picture manipulation feature enhances the usability of the converter. The following describes operation of the software in accordance with the invention. All internal registers have a 32-bit format. The register data is in the eight least significant bits, i.e., bits D07-D00. The control register (see Figure 4) in control sequencer 62 allows software associated with the processor to select video source 44 of Figure 2, start and stop the operation of the converter, and enable window scaling by use of processor address/data bus 86 and processor control signals 87. The control register is a read/write register. The SDA bit refers to serial data. The SCL bit refers to the serial clock. The SCL bit is the communication bit to DACs in block 46 that allow the brightness, contrast, hue, and saturation of the picture to be digitally controlled. The two bits indicated by X are not used in this embodiment of the invention. The FS bit is the full size bit for enabling the size registers in block 84 to become active (i.e., scaling). The value one in the FS bit enables the size register to provide a full size picture. The start bit is used so that a one in this register will start the converter operation. Zero in the start bit stops the converter operation. The VS1 and VS0 bits indicate video source 44 (see Figure 2) selection. As shown in the table below, VS and V0 are used to indicate a composite or S-video or RGB inputs. VS1 VS0 0 0 Composite 0 1 S-Video 1 X RGB Thus, in accordance with the preferred embodiment, the type of video input is automatically detected by use of the VS1 and VS0 bits and compared to the IDC (input devices connected) bit. The status register in Figure 5 (part of mode control in block 84) includes the bits as shown. If the IDC bit is one, this indicates that the video source selected by the VS1 and VS0 bits in the control register is active. A zero value for the IDC bit may indicate a bad connection to the video source or no power to the video source. The IDC bit is polled to determine if a valid video source has been selected. The SCLO bit is described above. When the VSYNC bit is zero this indicates that the selected video source is in vertical retrace. The VSYNC bit is used typically during freeze frame to ensure that a full field is complete. Another register provided in block 84 in this embodiment is the x-y size register (shown in Figure 6) which allows dynamic window sizing. One of 16 values can be loaded into each x and y nibble. X and y are the rate at which pixels are discarded in the x and y direction when an image is reduced in size so as to become a window on the screen of monitor 80. Dynamic window scaling allows the user to scale the video window to various sizes. A pixel dropping method is employed to accomplish this task. In this method, incoming digitized pixel data is selectively not written into the pixel buffer 52. In the x direction pixels are selectively not written. In the y direction, entire lines are selectively not written. Control signals X_SIZE and Y_SIZE (which are outputs from the sizing circuitry in block 84 which provides selective pixel dropping as described below) go directly into control sequencer 62. When X_SIZE or Y-SIZE are in an active state, control sequencer 62 disables writing to pixel buffer 52 for as long as X_SIZE or Y_SIZE are active, hence pixels and lines can be dropped to accommodate the selected window size. There are several algorithms that can accomplish the selective pixel dropping. One is the n/m algorithm, where m is the total number of incoming visible digital pixels (pre-pixel buffer 52), and n is the number of pixels desired in the scan converted video window. In the x direction m is equal to 640. In the y direction the same formula applies, except that horizontal lines are counted instead of pixels, and in the y direction m is equal to 480. In one embodiment of the present invention, a simplified version of this algorithm is implemented. This implementation allows for 16 fixed window sizes. For this reason the two x-y size registers in block 84 need be only four bits each (i.e., one eight-bit register partitioned into two nibbles as in Figure 6). The formula implemented for the fixed scale sizes is: (15-d)/(16-d), where d controls the window scale ratio. The d value is loaded into the x-y size register from processor address/data bus 86 and processor control signal bus 87. The x-y size register may be loaded with different values to give a "stretched" look in the window. Full size (640 × 480) is accomplished by setting the FS bit in the Control/Status register. The x-y size register and the Control/Status register are both located in block 84. The linear start address register in address generator 84 of Figure 2 contains a value of the upper left corner of the window (i.e., window location) to be displayed minus a value which preferably is 1,152 as discussed above. Figure 7 shows the bit arrangement of this register, shown as being three 8-bit registers. In accordance with the preferred embodiment of the invention, the linear start address register is in the form of a linear frame buffer scheme where the addresses are all in one stream line-by-line. Alternately, a binary-type stream where the left edge is always at an even binary break point (i.e., 256, 512, 768, 1024, etc.) may be used. Thus, a linear address structure is used in accordance with the invention. Figures 8a to 8i show schematically the embodiment of Figure 2 implemented in commercially available integrated circuits. Figures 8a and 8b show the circuitry of block 46. Figure 8c shows the circuitry of block 56. Figure 8d shows the circuitry of blocks 52 and 62. Figures 8e, 8f, 8g show the circuitry of block 84. Figures 8h and 8i show interface circuitry relating to Figures 8a to 8h. In accordance with another embodiment of the invention, the above described circuitry is implemented in an ASIC format. Each of the blocks 56, 62, and 84 shown in Figure 2 is implemented in one or more ASIC chips which are manufactured conventionally. The ASIC chips include the same logic circuitry as is included in the integrated circuits and other components shown in Figures 8a to 8i. The circuitry may differ in other embodiments. The ASIC chips plus the frame buffer 64 circuitry are preferably surface mounted on a printed circuit board and interconnected. The printed circuit board is preferably of the well-known type which conveniently interconnects with the bus of the host computer. One embodiment of the invention includes a method of controlling, on a per-pixel basis, the data written into frame buffer memory. This method supports clipping of multiple overlapping windows of arbitrary shapes, superimposing graphics or text, and animation. Window clipping allows multiple graphic windows to overlap and one of them to be the front-most window even when the live video window is active (i.e., the system is converting video in real time). These windows can be of arbitrary shape and size. Window clipping is accomplished by dedicating a memory bit in RAM for each pixel. This dedicated memory bit is in another embodiment one of the frame buffer bits. Thus a plane measuring X bits in width and Y bits in height (referring to screen pixel width and height) and one bit deep is established. The dedicated bit determines whether or not the incoming digitized video for a particular pixel is to be written into frame buffer 8. The dedicated memory bits thus represent a one-bit deep plane whose array characteristics match that of frame buffer 8. This one-bit deep plane is hereinafter called the Mask plane and is shown as MASK RAM 90 in Figure 9. Figure 9 is otherwise similar to Figure 1, except for the additional input and output bundled hardware control lines to MASK plane 90. When the system updates the windows in frame buffer 8, Mask plane 90 will contain information regarding the location of the windows (i.e., a "shadow" of all the windows). The front-most window is usually the last window to be updated. The live video window must clear out any existing shadow if the line video window is to become the front-most window. When live video is active, the data from MASK RAM 90 is read and evaluated by control sequencer 14. On a per-pixel basis, live video via frame buffer control 28 is inhibited when a shadow of the existing windows is encountered. This method allows other windows to be the front-most window and allows the live video window to be clipped. Since Mask plane 90 can be accessed by a system application through processor Address/Data/Control bus 23 and frame buffer control line 96, Mask plane 90 can also be used for special effects such as superimposing graphics or text into the live video window. This is accomplished by a system application that writes a shadow of the arbitrary shapes, i.e., windows, characters, and symbols, into Mask plane 90. The date written into Mask plane 90 determines which regions of the live video window will be clipped. Mask plane 90 is connected to three control lines 92, 94, 96. Control line 92 allows control sequencer 14 to control Mask plane 90 to allow Mask plane 90 data (i.e., shadows) to be evaluated by control sequencer 14 so control sequencer 14 can inhibit writing. Control line 94 (part of frame buffer control 28) allows Mask plane 90 to provide a write-inhibit control signal to frame buffer 8. Control line 96 allows frame buffer 8 to control Mask plane 96. Mask plane 90 needs to have the "shadow" data to be written into it so that the shadow data can be evaluated by control sequencer 14. Then control sequencer 14 can inhibit writing to frame buffer 8 by use of frame buffer control 28. Mask plane 90 is also connected to frame buffer data bus 98 to receive data from frame buffer 8. This disclosure includes copyrighted material. Permission is given for facsimile reproduction of this material, but the copyright owner reserves all other rights. The above description of the invention is illustrative and not limiting. Further modification to the described embodiments of the invention will be apparent to one of ordinary skill in the art in light of the disclosure and claims appended hereto.

Claims: A converter for converting a video signal from a first to a second scan rate comprising: a pixel buffer (4) for receiving a portion of the video signal at a first scan rate; an address generator (36) for determining a display location for the video signal; a non-interlaced raster frame buffer (8) operatively connected to the pixel buffer (4) for receiving the video signal at a second scan rate differing from the first scan rate; a control sequencer operatively connected to the pixel buffer and to the address generator (36) for providing the portion of the video signal stored in the pixel buffer (4) and the display location to the frame buffer (8) upon occurrence of a predetermined event; characterised by means (90) for controlling video data written into the frame buffer on a per-pixel basis so as to provide clipping of multiple overlapping windows, wherein the windows are of any size and shape. The device of claim 1, wherein the means (90) for controlling comprises memory including a bit associated with each pixel. The device of claim 2, wherein the bit associated with each pixel is a portion of the frame buffer (8). A method of converting a scan rate of a video signal comprising the steps of: storing a portion of the video signal which is received at a first scan rate; providing the stored portion of the video signal to a frame buffer at a second scan rate differing from the first scan rate, upon occurrence of a predetermined event; determining a location on a display for the video signal; providing the portion of the video signal stored in the frame buffer to the display at the determined location; characterised by controlling video data written into the frame buffer on a per-pixel basis so as to provide clipping of multiple overlapping windows, wherein the windows are of any size and shape. The device of claim 4, further comprising, to control the data written into the frame buffer on a per-pixel basis, providing a dedicated bit in memory for each pixel.

Inventors: MAIETTA MICHAEL, SMITH DAVID M, MAIETTA, MICHAEL, SMITH, DAVID M.
Applicants: RASTEROPS CORP, TRUEVISION, INC.
Application year: 1990
Main classification: G09G1
IPC codes: G09G5, H04N9, G09G1, H04N5, H04N11