2. Description of the Prior Art Various systems are known for transmitting video signals. The easiest way to transmit an RGB signal is to send three separate signals; one for red, one for green and one for blue. While simple, this method is costly and requires much bandwidth, which is hard to come by in the frequency spectrum. An alternate method is to IQ modulate two of the color signals and send the third unmodified. This method saves somewhat on bandwidth but is still expensive both bandwidth and component-wise. As such, because of the relatively wide bandwidth required for transmission of RGB color video signals, systems are known for converting signals in RGB format to NTSC/PAL format and transmitting the signals in NTSC/PAL format. Examples of such systems are disclosed in U. S. Patent Nos. 4,631,692; 4,985,754; 5,402,180 ; 5,703,993 and 5,896,178, hereby incorporated by reference.

Even though such systems are able to convert from high resolution RGB color signals to a NTSC video transmission system while being frugal with bandwidth, the conversion is known to cause substantial loss of resolution. Thus, there is a need for encoding and transmitting RGB signals with improved resolution.

SUMMARY OF THE INVENTION The system and method in accordance with the present invention relates to encoding an RGB signal, generated by a computer, for display on either a CRT or LCD. In accordance with an important aspect of the invention, the system and method converts RGB signals for transmission with little resolution loss, while keeping bandwidth to a minimum and is easily transmitted, for example, by way of a quadrature modulator.

DESCRIPTION OF THE DRAWING These and other advantages of the present invention will be readily understood with reference to the following specification and attached drawing wherein: FIG. 1 is a schematic diagram of system for encoding RGB video signals in accordance with the present invention.

FIG. 2 is a schematic diagram of an alternative embodiment of a system for encoding RGB video signals.

FIG. 3 is an exemplary diagram illustrating the output of the system illustrated in FIG. 2.

DETAILED DESCRIPTION In accordance with an important aspect of the invention, an RGB signal is encoded and transmitted with as little resolution loss as possible while keeping bandwidth to a minimum. Two embodiments of the invention are disclosed. In one embodiment, color code scaling is used in which two or more colors are scaled and combined to create a composite voltage signal. In an alternative embodiment, line color multiplexing is used in which two or more of the color video signals are multiplexed in the time domain. In embodiments in which only two color video signals are combined, the remaining color video signal is unmodified. As used herein, the unmodified signal is to be understood to mean only that is not combined with any other color video signals in the sense of the type of combinations discussed herein. With both embodiments, at any one instant in time, a maximum of only two signals need to be sent over a RF link, instead of three. As such, a simple IQ RF modulator can be used to transmit these two signals as one signal having a phase and magnitude component. As such, component cost are reduced significantly along with the bandwidth use and while allowing for greater range in RF.

COLOR CODE SCALING In this embodiment, one video color signal is transmitted unmodified while the other two color video signals are combined as discussed below. Since the eye is believed to be most sensitive to green shades, the green color signal may be sent unmodified in one embodiment. The red and blue signals are scaled in voltage to create one signal. In particular, the red and blue color video signals, for example, from a video card may be summed together to form a composite signal in analog form. A receiver receives the composite analog signal and converts it to a digital signal by way of an analog to a digital converter (A/D). In order to distinguish the red and blue bits at the output of the A/D, color code scaling of the signals is used in accordance with an important aspect of the invention. More particularly, most known video cards employ an 8-bit digital to analog converter DAC (not shown) for generating each of the red, green and blue color video signals; INGREEN, INBLUE and INRED. As such, for a typical 0.7V video signal, there are 256 discrete voltage levels output by the DAC to simulate a true analog color. The invention is based on the fact that the RGB colors were originally digital signals. As such, the unmodified signal will have 1 of 256 voltage levels to represent green. In accordance with an important aspect of the invention, an 8-bit signal is formed from two 4-bit sources without any overlap. In particular, the four most significant bits are assigned to one color and the four least significant bits to the other color. For example, the upper four bits (D7, D6, D5, D4 x x x x) can be assigned to red. As such, the red signal can range, in the digital world, from 16 to 240 in steps of 16, providing 16 possible values of red. If the 4 LSBs are used to represent the blue signal (x x x x D3, D2, DI, Do), the blue signal will range from 0 to 15 in steps of one, giving 16 possible values of blue.

In the analog world, the voltages representing the colors red and blue may be scaled in such a way that they do not overlap. It will be understood that it is possible to adjust the coding scheme to implement any number of ways of encoding three colors into two signals, for example, 4 bits of green on top of 4 bits of blue and 4 bits of red on top of 4 bits of blue, and so on. In particular, assuming that red is scaled from 0 to 5V range, instead of 0 to 0.7V, the 0 to 5V range output is actually 256 discrete steps due to the DAC in the video controller. Each step is thus 5V/256 or 19.5mV in amplitude.

In the present invention, scaling may be done by circuitry or alternatively in software, for example, the red signal may be scaled to a 5V maximum to (5V/16) or. 3125V minimum. Therefore, for a maximum saturation of red to be displayed on the screen, the red output circuitry outputs 5V, and if there were no red, the output is. 3125V. This range leaves room between 0 and. 3125V to insert the blue signal, for example, through appropriate scaling in circuitry, a maximum blue saturation of. 3125V and a minimum blue saturation of 0V can be assigned. These red and blue signals can be summed together to form a composite signal with minimal interaction and transmitted.

On the recovery side, the composite signal is scaled appropriately for input to an 8-bit A/D converter. Since the initial scaling was based on 256 possible levels with the division between red and blue occurring at 16, the output of the A/D will be as follows: The 4 most significant bits will represent the red portion of the signal, and the 4 least significant bits will represent the blue portion. These digital bits may be applied to a ladder network to convert them back to analog signals to drive the red and blue inputs of a CRT.

A system for implementing the invention is illustrated in Fig. 1 and generally identified with the reference numeral 20. Referring to FIG. 1, an RGB video signal from a RGB video controller card (not shown) suitable for an RGB monitor is intercepted through a connector P 1, a standard 15 pin VGA connector that interfaces to the output of any video card (not shown) from a computer or video source. The signals available at the connector P1 are standard RGB color video signals: red, INRED (pin 1), green INGREEN (pin 2) and blue IN BLUE (pin 3); vertical sync INVSYNC (pin 14), horizontal sync INHSYNC (pin 13) and ground (pins 5,6,7,8 and 10). Pins 4,9,11 and 15 are unused.

The RED, GREEN and BLUE color video signals are terminated by way of terminating resistors RI, R2 and R3, for example, 75 Q resistors, connected between pins 1-3 of the connector P, and ground for impedance matching. As set forth in exemplary form below, the RED and BLUE color signals are summed or combined and scaled while the GREEN color is passed through unmodified. A voltage divider network, formed from the resistors R8, R, 3 and Rl,, is used to scale the RED and BLUE color video signals. Exemplary values of these resistors are 4.7kQ, 150kQ and 150kQ, respectively. In particular, the resistor R8 is coupled to the RED color video signal (pin 1) to generate a scaled RED color video signal, while the parallel combination of the resistors R13 and Ru, are coupled to the BLUE color video signal (pin 3) to generate a scaled BLUE color video signal. The scaled red and blue color video signals are summed or combined at a node 22 to form a composite signal and applied to a negative input of op amp 24. A feedback resistor R22, for example, 1 OkQ, is connected between the output of the op 24 and its inverting input. The non-inverting input of the op amp 24 is grounded. The output voltage V, at the output of the op amp 24 will thus represent the sum of the red INRED and blue INBLUE color video signals.

The output voltage V. from the op amp 24 is applied to a variable scaling circuit, formed from an op amp 26, a plurality of resistors R4, Ru and R20 and a potentiometer R5. The scaling circuit allows the sum of the red and blue color video signals to be appropriately scaled. The scaling is a function of the resistor values for R4, Ril and R20 and the potentiometer R5. In particular, the output of the op amp 24 is applied to an inverting input of the op amp 26 by way of the input resistor Roll, for example lOkQ. The resistor R20, for example, 10kQ, is configured as a feedback resistor. The resistor R4, for example 50kQ, is connected between the non-inverting input of the op amp 26 and a positive 12 volt power supply for the op amp 26. The potentiometer R5 is connected between ground and the non-inverting input of the op amp 26.

The output of the op amp 26 represents a scaled value of the sum of the composite red and blue color video signals INRED, INBLUE. The composite signal may be filtered by a filter consisting of a resistor Rlo, for example 1 OkQ, and a capacitor Cl, for example 1 u F, modulated and transmitted wirelessly, for example, by conventional RF circuitry. Alternatively, the signals can be transmitted digitally. In embodiments where the signals are transmitted in analog form, the received signals are converted on the receiver side to digital signals. In particular, the signals are demodulated by conventional demodulation circuitry (not shown) and applied to an input IN of an analog to digital converter (A/D), an eight (8) bit A/D, for example, a model no ADS 830 by Analog Devices, which reconstructs the summed red and blue color video signals INRED, INBLUE. An oscillator 30, a pair of resistors Rl,, R21, as well as a pair of capacitors C2 and C3 are known support components required to operate the A/D internally.

In the exemplary embodiment, since the red color video signal INRED was scaled and assigned to the upper four bits and the blue color video signal INBLUE was assigned to the lower four bits, a plurality of resistors R (2kQ), R7 (4kQ), R9 (8kQ) and R12 (16kQ) are summed together at a node 32 to form an analog voltage PRED, representative of the red color video signal. Similarly, the lower four bits are summed at a node 34 by way of a plurality of resistor R, 4 (2kQ), R, 6 (4kQ), R, 7 (8kQ) and Rlg (16kQ) to form an analog voltage PBLUE, representative of the blue color video signal.

The PRED and PBLUE signals are applied to a pair of buffer amps 36 and 38, respectively. In particular, the signals PBLUE and PRED are applied to inverting inputs of a pair of op amps 40 and 42, respectively, whose non-inverting inputs are grounded. Feedback resistors R4, (lkQ) and R5, (lkQ) determine the output voltages of the buffer amps 36 and 38 respectively. The outputs of the buffer amps 36 and 38 are applied to a pair of variable output amps 44 and 46, respectively, to enable the red and green color video signals to be appropriately scaled for output to a CRT or LCD. The variable output amp 44 includes an op amp 48 and a plurality of resistors R37 (50kQ), R39 (6.6kQ), R40 (lkQ) and a potentiometer R3g. Similarly the variable output amp 46 includes an op amp 50, a plurality of resistors R42 (50kQ), R48 (6.6kQ), R50 (lkQ) and a potentiometer R43, The scaled red and color video signals, available at the output of the op amps 48 and 50, are applied to pins 1 and 3, respectively of a connector P2, a standard 15 pin VGA connector for connection to a CRT (not shown) or an LCD projector (not shown). The vertical and horizontal sync signals INVSYNC, INHSYNC are applied to pins 13 and 14. The green color video signal from the connector PI is not modified in the process and simply modulated and transmitted at the transmitter end and demodulated at the receiver end. In particular, the received green color video signal is demodulated and applied directly to pin 2 of the connector P2. Since the green color video signal was not modified, it may be slightly ahead of the red and blue color video signals. As such, the green color video signal INGREEN may be passed through a delay line 54 to synchronize it with the red and blue color video signals. Such delay lines are known in the art. An exemplary delay for this application is a model no. A0805, manufactured by RCD Components, having a delay of 400-500ns.

The circuitry within the dashed boxes 52 and 56 represents test circuitry that does not form a part of the present invention. Similarly the capacitors C4-Clg within the box 58 are for power supply bypassing which are well known in the art. Similarly, the connectors JP l, JP2 and JP3 are for test purposes and does not form part of the present invention.

Alternatively, the three color video signals can be combined together in a similar manner as discussed above. In this embodiment, the three color video signals are combined as discussed above. Instead of assigning 8 bits to one color video signal and 8 bits to the combination of the other color video signals, the 16 total bits may be distributed among the three color video signals in various ways. For example, bits can be assigned as follows: green-6, blue-6 and red-4.

LINE COLOR MULTIPLEXING In accordance with an alternative embodiment of present invention, two or more of the color video signals are multiplexed in the time domain. Thus, at any one instant in time, a maximum of only two signals need to be sent over RF, instead of three, similar to the other embodiment discussed above.

In an embodiment where only two signals are combined, similar to the embodiment above, green color signals, for example, may be sent unmodified while the red and blue signals are time multiplexed. Thus any one line on the screen of a reconstructed signal, seen over three full frames of video will look, for example, as illustrated in FIG. 3. Since there are normally from 60 to 90 frames of video displayed on a CRT or LCD per second, a human eye cannot see the switching of red and blue on and off, but rather averages the results. In essence, most humans end up seeing a picture that looks somewhat greenish, but otherwise has the right color characteristics. The reason the screen may look somewhat greenish is that green is available all the time, while blue and red are alternatively switched on and off, for example, 50% of the time.

To compensate, video amplifiers are used to amplify the red and blue color video signals to raise their average levels back up to a point that the display of a system showing this kind of video is virtually indistinguishable from one showing full RGB signals. As such, a video signal is reproduced by one or two colors at one time, which makes it very easy to modulate and send by RF.

Although switching red and blue on a line by line basis created the most pleasing results, there are at least two other ways the same idea can be expanded on. 1) Pixel per pixel multiplexing. The same scheme can be used to switch red and blue (or any other combination) on and off alternatively per pixel instead of per line. 2) Frame by frame multiplexing. The same scheme can be used to switch red and blue (or any other combination) on and off alternatively per frame instead of line.

FIG. 2 represents an exemplary embodiment in which the red and green color video signals are time multiplexed and the green color video signal is sent unmodified. In this embodiment, a video signal destined for an RGB monitor through a connector PI, a standard VGA connector as discussed above, is intercepted. The output to a CRT or LCD monitor after modification is through another standard VGA connector P2. The green signal (or any of the other color video signals) may be passed through unmodified. As discussed above, some delay may need to be added to the unmodified signal to compensate for the delays encountered by the combined signals as they pass through the circuitry. In order to account for the loss of signal strength, for example, 50%, due to the multiplexing, the red and blue color video signals, available on pins 1 and 3, respectively, of the connector P 1, are applied to a pair of op amps 60 and 62, respectively. In particular, these signals are applied to the non-inverting inputs of the op amps 60 and 62. The op amp 60 includes a feedback resistor R, and an input potentiometer R3. The input potentiometer R3 is connected to the inverting input of the op amp 60. Similarly, the op amp 62 includes a feedback resistor R2. A potentiometer R4 is connected to the inverting input of the op amp 62. The combinations of R,/R3and R2/R4form an adjustable gain network for the op amps 60 and 62.

The output of the op amps 60 and 62, identified as the signals REDOUT and BLUEOUT, are coupled to the collector terminals of a pair of NPN bipolar transistors 64 and 66, respectively. These transistors 64 and 66 are driven by a pair of flip-flops 68 and 70. In particular, the output of the flip-flop 68 is applied to the base terminal of the transistor 66 by way of an input resistor R,. The/Q output of the flip- flop 70 is applied to the base terminal of the transistor 64 by way of an input resistor R6.

The emitter terminals of both of the transistors 64 and 66 are grounded. The transistors 64 and 66 pull the red and blue outputs to ground alternatively on a line by line basis to emulate a multiplexer. Alternatively a commercially available multiplexer could be used.

Therefore the output of the circuit switches on a line by line basis between sending a line of red or a line of blue. Thus, at any one instant in time, there are only two colors being sent, green and (red or blue).

The VGA sync signals HSYNC and VSYNC are used to provide the timing for the circuit. In particular, the vertical sync and horizontal sync signals VSYNC, HSYNC, available on pins 13 and 14 of the connector PI, are applied to an AND gate 70. The output of the AND gate 70 is connected to one terminal of a single pole double throw switch 72. The horizontal sync signal HSYNC is connected to the other pole. The common pole defines a clock signal CLK. The clock circuit allows for control of the color switching regardless of whether the total number of horizontal sync pulses in one frame is even or odd by adding in an extra pulse from the vertical sync. In operation, the switch 72 is used to select whether the pulse gets added to the clocking circuit. In the position shown in Fig. 2, a vertical sync signal VSYNC is added to the horizontal SYNC signal HSYNC. In the alternative position of the switch, the clock signal CLK consists solely of the horizontal sync pulses HSYNC.

This clock signal CLK is used to drive the flip-flops 68 and 70, for example, type 74 flip-flops. As shown, the/Q outputs of the flip-flops 68 and 70 are applied to their D inputs. In operation, as the horizontal sync pulses HSYNC go high, the outputs of the flip-flops 68 and 70 flip from high to low every time a rising edge from the horizontal sync signal HSYNC is detected. Thus, at any given line in the frame, the transistor 66 is either on or off, while the transistor 64 is an opposite state. Since the REDOUT and the BLUEOUT signals are connected to the collectors of the transistors 66 and 68, on any given line, and alternating frame by frame, the REDOUT signal will be pulled to ground and the BLUEOUT signal will be allowed to pass through to the output connector P2 and vice versa. This alternation, line by line, of having REDON, BLUEOFF and then REDOFF, BLUEON saves bandwidth, which is critical for RF transmission, and, along with the amplification provided by the op amps 60 and 62 of the original RED and BLUE color video signals, allows for virtually no noticeable difference from a full bandwidth RGB signal.

The connectors JP 1 and JP2 are used for the power supply for the op amps 60 and 62.

Alternatively, as discussed above, all three color video signals may be time multiplexed.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. For example, the signals may be transmitted in digital form. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described above.

What is claimed and desired to be covered by a Letters Patent is as follows: