Inventors:

Paolo L. SICCARDO Michael C. CHAN

BACKGROUND OF THE INVENTION

1) Field of the Invention: This invention relates to video systems for the vision impaired and, more particularly, to schemes for enhancing visibility of video information.

2) Prior Art: Color CCTV's and LPCA's with a "Reverse video mode" (polarity reversal) switch or other form of control, can reverse the brightness distribution of an image, from 'dark on light or white' to 'white (or light) on dark' or vice-versa, for easier and more comfortable reading.

The function is performed by reversing the polarity of the video signal (composite or RGB) .

If the image is monochromatic (black and white) , a correct picture polarity reversal is obtained. However, for a multi-colored (polychromatic) image, the operation changes the

colors to their respective complements (red becomes green, blue becomes yellow, etc.) as an undesired side effect, therefore preventing true color operation in reverse video mode.

Monochromatic CCTVs are used by low vision people to read printed matter. A useful feature of almost all such devices is a "Positive/Negative" switch or "Polarity" switch, the function of which is to change black portions of the image into white and vice-versa. Reading with reversed polarity (white letters on a black background) offers distinct advantages of reading speed and less eye fatigue for many low vision people.

Color CCTVs and color Large Print Computer Access devices for low vision reading typically offer a similar "Positive/Negative" switch or polarity reversal control. In such cases, though, reversing the polarity changes not only the brightness relationships (black and white) , but also creates complementary colors (yellow becomes blue and so on) .

SUMMARY OF THE INVENTION

Object of the Invention: A new approach to reverse video mode in video systems, CCTV's and LPCA's, etc. allows for user selectable polarity reversal of the image's brightness distribution for comfortable viewing and reading, without distorting, complementing or modifying the displayed image colors. This is particularly important in video systems designed for the vision impaired.

Alternatively, in one particular embodiment of the invention the image's colors can be reversed without affecting the brightness distribution.

The system processes the chrominance and the luminance components of the color video signal separately.

If the chrominance and luminance informatipn are not available in separate form, they are extracted from the source signal (Composite Video, RGB signal or data, or pixel index signal or data) , and eventually they are recombined in the original form after processing.

The polarity reversal function is applied only to the luminance component, where the chrominance is left unmodified. Therefore all the color information in the original image remains unaltered, while the relative brightness of the image gets reversed as required for comfortable viewing.

Alternatively, the luminance and chrominance components of the signal are processed simultaneously and without separation, but the color reference burst is subject to selectable phase rotation, thus allowing for independent color reversal.

In this invention the polarities of the chrominance and luminance components of a color video signal are each independently selectable. This means that 1) the luminances (gray scale values) and colors may each be preserved, 2) the luminances can be inverted without changing colors, 3) the colors can be complemented without changing the gray scale values, and 4) the colors can be complemented and luminances can be inverted together. The ability to reverse the gray scale while keeping colors true allows a low vision user to obtain the preferred visual appearance for text (light letters on a darker background) while maintaining true colors. The

ability to complement colors without affecting gray scales allows a partially-color-blind low vision user to change the hue of colored text to one that may be more easily visible. Effecting both reversals together (luminance and chrominance) produces the usual "polarity reversal" function; effecting neither reversal produces a normal video picture.

According to the invention, what is provided is a method of processing an image signal designed to control at least in part operation of the visual display of a low vision system. The image signal has an aspect .-defining a hue for display by said device and an aspect defining brightness of the display at such point. The invention includes using such signal with one of said hue and brightness aspects changed without the other also being changed to control operation of a visual display device.

The method includes changing one aspect of the signal to cause said display device to display the complement of the hue or brightness defined by said aspect of said signal prior to said processing. In certain embodiments, the signal is a composite video signal having chrominance and color burst components whose phase relationship defines the hue. A luminance component has a value which defines the brightness. The step of using the signal includes changing the value of the luminance component without changing the phase relationship between the chrominance and color burst components. The signal in such embodiment is a composite video signal having chrominance and color burst components whose phase relationship defines the hue. A luminance component value defines the brightness. The step of using the signal includes changing the phase relationship between the chrominance and color burst components

without changing the gain of the luminance component. The phase relationship is changed by shifting the phase of the color burst component by 180" without making a corresponding change in the phase of the chrominance component.

Apparatus is provided for processing an image signal designed to control at least in part operation of the display of a visual display device. The image signal has an aspect defining a hue for display by said device and an aspect defining brightness of the display. Means are provided for receiving the signal. Means are provided for altering the signal to provide a modified image signal which will control operation of the display with one of the aspects changed without the other of the aspects also being changed.

A system is provided for processing a synchronization signal, a luminance signal, and a chroma signal for producing a video image. The system includes means for processing the synchronization signal, means for processing the luminance signal, and means for processing the chroma signal. The means for processing the chroma signal includes means for selectively and independently altering the phase of a chroma reference subcarrier for the chroma signal. The means for processing said luminance component includes means for selectively and independently reversing the polarity of the luminance signal.

The system further includes means for combining a processed synchronization signal, a processed luminance signal, and a processed chroma reference subcarrier signal into a processed composite output video signal. The means for processing the chroma

component includes means for selectively altering the phase of a chroma reference-subcarrier to provide independent variation of the chrominance component of the output video signal. Means are provided for separating the synchronization signal, the video signal, and the chroma signal from a composite video input signal.

A system according to the invention is provided for enhancing the visibility of a display of a composite video input signal having a synchronization component, a video component including a luminance component rand a chrominance component, and a chroma component. The system includes various means for processing: the synchronization component of the composite video signal, the luminance component of the composite signal, and the chroma component of the composite signal. All of these processed components are combined into a processed composite output video signal. The means for processing the chroma signal includes means for selectively reversing the polarity (relative phase) of the chroma signal. The means for processing the luminance component includes means for selectively reversing the polarity (relative amplitude) of the luminance signal. In addition, the means for processing the chroma component includes means for selectively altering the phase of a chroma reference-subcarrier to provide independent variation of the chrominance component of the output video signal. The system further includes means for separating the synchronization, video, and chroma reference-subcarrier from the composite video input signal. The means for processing the chroma signal includes gating means for selectively passing either a color reference subcarrier having a first phase or a color reference subcarrier having a phase opposite the first phase.

A method is provided according to the invention for processing a synchronization signal, a luminance signal, and a chroma signal for a video image. The method includes the steps of processing the synchronization signal, processing the luminance signal; and processing the chroma signal. The step of processing the chroma signal includes selectively and independently altering the phase of a chroma reference subcarrier; and the step of processing the luminance component includes selectively and independently reversing the polarity of the luminance signal. The method further includes combining a processed .synchronization signal, a processed luminance signal, and a processed chroma reference subcarrier signal into a processed composite output video signal. The step of processing the chroma component includes selectively altering the phase of a chroma reference-subcarrier to provide independent variation of the chrominance component of the output video signal.

A method is also provided for enhancing the visibility of a display of a composite video input signal having a synchronization component, a video component including a luminance component and a chrominance component, and a chroma component. The method includes processing the synchronization component of the composite video signal, processing the luminance component of the composite signal; and processing the chroma component of the composite signal. These components are then combined into a processed composite output video signal. The step of processing the chroma signal includes selectively reversing the polarity of the chroma signal and the step of processing the luminance component includes selectively reversing the polarity of the luminance signal. The method further includes the step of

processing said chroma component by selectively altering the phase of a chroma reference-subcarrier to provide independent variation of the chrominance component of the output video signal. The method additionally includes the step of processing the chroma signal, including the step of selectively gating a color reference subcarrier having a certain phase or a color reference subcarrier having a first phase opposite the first phase.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention:

FIG. 1 is a block diagram of an image processing system according to the invention for low vision CCTV or LPCA systems that interface with separate chrominance and luminance component signals.

FIG. 2 is a block diagram of an image processing system according to the invention for low vision CCTV and LPCA systems that interface with composite chrominance and luminance signals, for example, composite NTSC, PAL, SECAM.

FIG. 3 is a block diagram of an image processing system according to the invention for low vision LPCA systems that operate in a digital or analog pixel-index or RGB mode and can be implemented in hardware, in software, in firmware, or by reprogramming a color look-up table with a properly generated translation.

FIG. 4 is a functional block diagram of an exemplary image processing system according to the invention.

FIG. 5 is a timing diagram for signals at various points in the image processing system of FIG. 4.

FIG. 6 is a detailed circuit diagram of the image processing system of FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to those embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims.

Notes on Implementation: Several versions of the system can be created, referring to FIGS. 1, 2, and 3. Version "A" (with reference to FIG. 1) is dedicated for CCTV or LPCA systems that interface with separate chrominance and luminance component signals (as an example S-VHS or ED-Beta type or digital or analog Y-U-V or others) . Version "B" (with reference to FIG. 2) is dedicated for CCTV and LPCA systems that interface with composite chrominance and luminance signals (as an example composite NTSC, PAL, SECAM, or others) . In this case a chrominance separation/recombination circuit or

alternatively a color burst phase rotation circuit can be employed.

All versions can be retrofitted to an existing EM-CCTV or LPCA systems (insertion between source and display device) or they can be used as a base for specific designs.

Version "C" (with reference to FIG. 3) is specific for LPCA systems that operate in digital as well as analog pixel index or RGB modes, and can be implemented in hardware, in software, in firmware, or by reprogramming the color look-up table with a properly generated translation.

Implementations of the concept.

A) FIG. 1 shows that separate luminance and chrominance processing can be implemented very simply in an environment that uses S-VHS, ED-BETA, or similar devices, that interface with separate chrominance and luminance (Y/C) component signals from an input device 10, such as a composite video camera or other source. Luminance and chrominance processing are handled by a processing unit 12. In this case the luminance component can be treated exactly like a regular RS-170 monochrome composite signal, complete with sync. If the desired function consists of polarity reversal as performed in a video inverter 14, a sync separator is required to reverse only the polarity of the active video signal, leaving the synchronization portion unchanged. The chrominance component, complete with color burst, will ride on a separate wire 16 and will not be typically processed. Luminance and chrominance signals are each fed to an output device 18, which is, for example, a component video CRT monitor.

-11-

Alternatively, the chrominance information may be subject to processing as well.

B) FIG. 2 shows that, if the separate chroma and luma signals are not available at the source input device 20 and/or not recognized by the display system, or output device 22, it may be desirable or necessary to perform three steps as suggested by FIG. 2: i) separation of chroma and luma from composite signal; ii) processing in a processing unit 12 identical to case A) as above described; and iii) recombination of chroma and luma components in a composite .signal.

If the source or the display are compatible with separate chroma/luma signals, it is possible that step i) or step iii) are not necessary.

Alternatively, one can avoid the separation and recombination of chroma and luma components and obtain the same result through phase rotation of the color reference burst. This alternate embodiment is described later in B4) .

Bl) Separation of Chroma and Luma:

In FIG. 2 the Color Filter block 24 performs the function of separating the image information of a composite signal into distinct luminance and chrominance components. There are several standard ways of accomplishing this task, and a brief description of three methods Bl.l, B1.2, B1.3 follows. The chrominance portion of the signal is typically modulated on the luminance signal at the frequency of the color subcarrier (NTSC: 3.58 MHz.

PAL: 4.43 MHz., etc.). Therefore it can be separated by one of the following three methods (NTSC case) :

Bl.l) DIGITAL COMB FILTER with A/D conversion of the composite signal, digital storage of one or more video lines, comparison and combination of the different line samples to derive digital luma and chroma. If two consecutive video lines out of a single field are stored, as Linel and Line2, then due to the phase alternation of the chrominance carrier, the simplest approach is:

Chroma = (Line 2 - Line l)/2, and Luma = (Line 2 - Line l)/2.

Finally, D/A conversion to analog Y and C outputs is performed on the signal. This solution is expensive but offers excellent quality and signal control options. No adjustments are needed. Picture reversal can be performed digitally on the sampled luminance.

B1.2) ANALOG COMB FILTER with a 1H analog delay line: the phase alternation concept is similar to the Bl.l method above, in the analog domain. A simple summing circuit performs the same subtraction and addition. This is a lower cost solution, suitable to many real-life applications.

B1.3) An ANALOG NOTCH FILTER is used to block or let go the chroma modulation: the analog composite video signal is fed to two resonant L-C filters, both centered around the subcarrier frequency (3.58 or 4.43 MHz). One filter is parallel-resonant, and will let the chroma component through; the other is series-resonant, and will clean up the luminance of the chroma part. No power is required for this method.

B2) VIDEO INVERTER

The luminance component of the video signal, as available at the source (Case A) or after proper separation (Case B) is reversed in polarity by a standard circuit that is shown in FIGS. 1 and 2 as a Video Inverter block 14. This circuit consists of a sync separation stage and a selectable polarity reversal amplifier. The sync information from the sync separator is reconstructed after the optional polarity reversal, to avoid the undesirable case of reversed sync polarity in the generated signal.

B3) :RECOMBINATION OF CHROMA AND LUMA

The recombination of chrominance and luminance in a composite video signal is a common procedure that can be performed by mixing the two signals together. In FIG. 2 the Color Mixer block 26 performs this function. The simplest approach is the use of two 75 Ohm mixing resistors.

B4) ALTERNATE EMBODIMENT: Independent Color and Intensity Reversal.

Refer to the FIG. 4, discussed hereinbelow, for a system for independently selectable luminance and color inversion.

C) Image Processing, Computer Graphics, and SW/HW-LPCA IMPLEMENTATION.

FIG. 3 shows a block diagram of an image processing system according to the invention for LPCA systems that operate in a digital or analog pixel- index or RGB mode and can be implemented in hardware, in software, in firmware, or by reprogramming a color look-up table with a properly generated translation. The input system 30 of many computer graphics systems is either analog or digital RGB-based or pixel index based, and therefore does not explicitly or

implicitly make use of separate or composite chrominance and luminance signals. However, one can perform a conversion from the RGB domain to a different color space, through a transformation matrix 32, Matrix A. This different color space utilizes signals in separate chroma and luma form; at this stage one can perform luma inversion through an inversion algorithm if desired. The last step consists of transformation of the signals to the original RGB color space in a matrix 34 prior to transmitting the pixel data to an appropriate output system 38.

After this step of processing through the transformation Matrix A, one can use the processed image to control a display device with user- selectable polarity reversal that does not affect the colors.

The three steps of the algorithm can be represented as follows: Λfe,

R G B -> (Color Space Transform. " Matrix A") -> Luma+Chroma

Luma+Chroma -> (Luma Inversion Algorithm) -> Luma+Chroma

Luma+Chroma -> (Color Space Transform. "Matrix B") -> R G B

Cl) COLOR SPACE TRANSFORMATION ALGORITHM. The algorithm is described here in general form. The algorithm can be implemented in software (C2) , system firmware (C3) , or with specialized hardware (C4) , or as a special case in a look-up table (C5) .

In each case the Color Space Transformation "Matrix A" or "Matrix B", when required, consists of multiplication of the original image value by a proper matrix of coefficients. Depending on the choice of coefficients, the original image signal is transformed into another signal, perfectly equivalent to the first, but expressed in a different set of coordinates: pixel index to HSI, RGB to YUV, etc.

For this application it is desirable to choose a transformed color space that contains explicit reference to luminance or intensity, separately from chrominance or hue/saturation information.

As an example standard matrices exist to transform RGB signals into YUV signals, and back; the YUV color space contains a reference to luminance ("Y") that is available separately from the chrominance ("U", "V") information. The transformation is applied as:

[Y U V] = [R G B].M and [R G B] = [Y U V].M ^"1 where M is a standard 3x3 matrix of coefficients, M ^*1 denotes the matrix inversion operation, and "." denotes the standard rows by columns matrix multiplication. In this case Matrix A=M, and Matrix B=M ^'1 .

The polarity reversal algorithm for the Luma component, if required as indicated by the block 36 of FIG. 3, can consist of an algebraic complement operation: Reverse(Y) = Max(Y) + Min(Y) - Y where Max(Y) and Min(Y) denote the maximum and minimum possible values for the Y component, respectively.

C2) Software implementation.

In general, in a programmable image processing system, computer graphics system, or Large Print Computer Access Device, there may be a memory buffer that contains image data information. A programmable processor is commonly available to perform operations on the image data.

One can use a proper sequence of processor instructions to apply the "Matrix A" transformation to the image data, and therefore obtain intermediate luma/chroma information. As a second step optional luma inversion through algebraic complement can be performed. Eventually a proper sequence of instructions can apply the transformation matrix "B" to reconstruct the image data in the original form, with optional luminance reversal and unaltered colors.

The three steps and sequences of processor instructions combined together constitute an application program that, when executed on the processor, performs the desired function.

C3) System Firmware Implementation.

In an image processing system, computer graphics system, or Large Print Computer Access Device, a set of coded instructions (firmware) can perform the task of controlling the transfer of information from the image data memory to the display or output or storage device (visualization) .

In this case the color space transformation, optional polarity reversal, and final reconstruction of the image signal in its original format, or an equivalent set of operations such as look-up table loading as described later in C5) , can all be

-17- performed by the system firmware during the visualization process, in a similar manner as described for the software approach in C2) .

C4) Hardware Implementation. If the image signal is available as an electrical (digital or analog) signal, the above described algorithm Cl) can be implemented in hardware.

In this case hardware-based multiplier circuits are employed to perform the transformations through multiplication by the matrixes "A" and "B" (FIG. 3) as described in the algorithm Cl) . This operation can be performed in an equivalent manner in the digital or analog domain. After the first transformation a signal is available that carries the same image information as the original one, but with luminance and chrominance information available in a separate form. At this stage an optional polarity reversal of the luminance component can be performed. After the final transformation to the original color space, the signal can be fed to the output device (display, hard copy, storage, etc.).

C5) Look-Up Table Implementation. If a color look-up table is used, a general "RGB-to-RGB" or "Pixel Index-to-RGB" programmable translation system is available.

By loading the look-up table entries with properly chosen parameters it is possible to implement any given transformation; specifically the one described in the three steps above. The parameters are calculated first according to the above given general algorithm Cl) , and when loaded in

the look-up table they act as a one-step implementation.

For a 16-color look-up table based RGB system, the Pixel Index Information can consist of a stream of 4-bit words, each defining one of 16 possible values for a pixel (Pixel Index) . The color look-up table contains 16 triplets of RGB values corresponding specifically to each of the 16 possible Pixel Index values, therefore allowing for selection of any arbitrary transformation from the 16 Pixel Index values space to the RGB space. One can load the look-up table with new RGB values calculated according to the above described algorithm Cl) , therefore obtaining a reversal of the image luminance distribution without affecting the image chrominance distribution.

In this invention the polarities of the chrominance and luminance components of a color video signal are each independently selectable. This means that (l) the luminances (gray scale values) and colors may each be preserved, (2) the luminances can be inverted without changing colors, (3) the colors can be complemented without changing the gray scale values, and (4) the colors can be complemented and luminances can be inverted together. The ability to reverse the gray scale while keeping colors true allows a low vision user to obtain the preferred visual appearance for text (light letters on a darker background) while maintaining true colors. The ability to complement colors without affecting gray scales allows a partially-color-blind low vision user to change the hue of colored text to one that may be more easily visible. Effecting both reversals together (luminance and chrominance) produces the

-19- usual "polarity reversal" function; effecting neither reversal produces a normal video picture.

FIG. 4 shows an implementation which is based on the following standard method of encoding chrominance in a video signal by a phase angle: To establish true colors, a composite video signal contains not only the luminance (encoded as amplitude variations) and chrominance (encoded as a sub- carrier) information, but also a color reference signal, the color burst. The color produced depends on the relative instantaneous phase between the sub- carrier and the Color Burst signal. Colors may be changed by changing this phase relationship. The phase relationship may change if the chrominance phase changes as a result of scene colors changing, or it may change if the Color Burst phase is deliberately altered. Normally the Color Burst phase is kept fixed to guarantee true colors. The Color Burst signal occurs only during the Back Porch Interval of the horizontal blanking period of the composite video signal, during which time video information is not present. The Color Burst signal is typically used to keep a local oscillator precisely synchronized, so the reference phase information is available during the time video information is present.

Refer to FIG. 4, which shows a specific embodiment of a system for separately switchable luminance and color inversion and to the timing diagram of FIG. 5, for an understanding of the invention. In FIG. 4 the Composite Video Input signal is applied to three signal processors, the Chroma Reference Processor 40, the Video Processor 42, and the Sync Processor 44. Operation of the Sync Processor 44 is standard and is not further

discussed. Output signals of the Sync Processor 44 are used to control the time-division multiplexing of the Chroma Reference and Video Processors 40,42 described below as well as to provide the sync component of the final Composite Video Output.

Operation of the Video Processor 42 is typical of polarity controls on monochromatic systems and on other color systems having a polarity capability. A composite video input signal is applied to a phase splitter VI that produces positive and negative polarity video signals. Pedestal bias stages V2 and V3 allow DC level adjustments of the phase-split outputs. Such adjustment is desirable to maintain appropriate gray scales when either signal is selected by Video Selection Logic V4. The Video

Selection Logic V4 allows either the positive or the negative video signal to pass to the Video Amplifier V5. A control signal SWV operates a video selection gate which provides signals which gate the inverted Composite Blanking signal to either a POS Video Gate or a NEG Video Gate. The enabled gate in turn allows either the Positive Video signal from the phase- splitter or the Negative Video Signal from the phase- splitter to be applied to the input terminal of a video amplifier V5. The output of the video amplifier V5 is then applied to a Summing Matrix Ml.

The Video Selection Logic stage V4 gates the selected video through only during active video time, inhibiting the signal during the Composite Blanking time. This is necessary because the phase splitter negative output contains inverted sync components as well as inverted video components, but the sync components in the final output signal must always be of the correct polarity.

FIG. 5 is a timing diagram for signals at various points in the image processing system of FIG. 4. The Timing Diagram, on lines 1, 4, and 5, shows the relationship between the Composite Video signal, the Selected Gated Video signal when the Positive signal is selected (solid line) , and the Selected Gated Video signal when the Negative signal is selected (dashed line) .

Because the Chroma Burst reference signal occurs during a portion of the Composite Blanking period and the Selected Gated Video signal is gated off during composite blanking, the Selected Gated Video signal applied to the video amplifier V5 contains no Color Burst component.

Note that if the Color Burst signal is simply preserved along with the sync components during the Composite Blanking period and reapplied to the Negative Selected Video Signal to produce the Composite video Output Signal, the phase relationship between the Color Burst and the chroma components is inverted with respect to the original relationship in the Composite Video Signal. This is the source of the color reversal produced on other color systems when the "Polarity" is reversed.

Operation of the Chroma Reference Processor 40 is unique to this invention and is similar to that of the Video Processor 42. In contrast to the Video Processor 42, which gates signals ON during active video time and OFF during Composite Blanking time, the Chroma Reference Processor 40 gates signals OFF during the active video time and ON during that specific portion of the Back Porch interval of the Composite Blanking time when the Color Burst Signal is active. An Input Reference Gate Stage Cl performs

this gating function. A Gated Chroma Reference Signal is applied to a phase splitter C2, which produces Positive- and Negative-Phase Gated Chroma Reference Signals. The Positive-Phase Gated Chroma Reference Signal has Zero (0) degrees phase shift with respect to the Color Burst signal. The Negative-Phase Gated Chroma Reference Signal has 180 degrees phase shift with respect to the Color Burst signal. Adjustable gain stages, C3 and C4, allow signal amplitude adjustment of the two signals in order to obtain the standard reference amplitude.

Chroma Reference Selection Logic C5 allows either the Positive- or the Negative-Phase Gated Chroma Reference Signal to pass to the Summing Matrix Stage Ml. The operation of the Chroma Reference

Selection Logic C5 is similar to that of the Video Selection Logic stage V4, described above. Selection is controlled by a signal SWC. In FIG. 5, Timing Diagram lines 1 and 3 show the relationship between the Chroma Reference portion of the Composite Video

Input signal, which consists of several cycles of the reference waveform, and the Selected Gated Chroma Reference when either the Positive-Phase (solid line in the magnified view of a portion of line 3) or Negative-Phase (dashed line) is selected. As illustrated, the Selected Gated Chroma Reference has the same phase (0 degrees) as the Chroma Burst of the Composite Video Input Signal when the Positive-Phase signal is selected; the Selected Gated Chroma Reference has the opposite phase (180 degrees) to the Chroma Burst of the Composite Video Input signal when the Negative-Phase signal is selected.

The Summing Matrix Stage Ml produces the Composite Video Output Signal by combining the Amplified Selected Gated Video signal (FIG. 5, Line

5) , the Selected Gated Chroma Reference signal (Line 3) , and the Composite Sync signal (Line 6) . The relationship between the Chroma Burst component and the chroma sub-carrier phase component in this Composite Video Output signal is the same as in the Composite Video Input signal under either of the following conditions:

1) The Video Selection Logic V4 selects the Positive Video Signal and the Chroma Reference Selection Logic C5 selects the

Positive-Phase Gated Chroma Reference Signal.

2) The Video Selection Logic V4 selects the Negative Video Signal and the Chroma Reference Selection Logic C5 selects the Negative-Phase Gated Chroma Reference Signal.

Under either of these conditions the original colors are preserved. Under condition 1, the original luminance relationship are preserved; under condition 2, the luminance relationships are reversed.

Alternatively, the relationship between the Chroma Burst component and the chroma sub-carrier phase component in the Composite Video Out signal is opposite to that of the Composite Video Input signal under either of the following conditions:

3) The Video Selection Logic V4 selects the Positive Video Signal and the Chroma Reference Selection Logic C5 selects the Negative-Phase Gated Chroma Reference Signal. 4) The Video Selection Logic selects the

Negative Video Signal and the Chroma Reference Selection Logic C5 selects the Positive-Phase Gated Chroma Reference Signal.

Under either of these conditions the original colors are complemented. Under condition 3, the original luminance relationships are preserved; under condition 4, the luminance relationships are reversed.

Because the Video Selection Logic V4 is independent of the Chroma Reference Selection Logic C5, the luminance and chrominance of the Composite Video Output signal may each be independently inverted. Comparison of conditions 1) and 2) indicates that one may change the polarity of the luminance -component of the Composite Video Output signal without changing its chrominance component by reversing both the SWV and SWC signals. In practice it may be desirable to have a single switch that controls both SWV and SWC appropriately using standard techniques.

FIG. 6 is a detailed schematic of the system shown in block diagram form in FIG. 4.

Alternate Phase Control

The description above indicates means for selecting one of two phase relationships between the Selected Gated Chroma Reference signal and the original Color Burst signal: Zero degrees or 180 degrees. Although these two phase relationship values provide the most obviously useful form of the Composite Video Output for use in low vision reading systems, there is in principle no requirement that there be only two selections. By use of a phase- shifting circuit, any angular phase relationship may be obtained. Such means would provide a form of arbitrary control of the colors in the Composite Video Output.

It should be noted that the terminology "polarity reversal" as used herein means obtaining the complement of the original hue and/or brightness.

The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use con¬ templated. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents.