Abstract:
When viewing a 4:3 aspect ratio image on a widescreen display, disturbing black bands appear to the left and right of the displayed image. Similarly, when viewing a letterbox image on a 4:3 aspect ratio display, disturbing black bands appear above and below the displayed image. These black bands result in uneven ageing of the phosphors in the display screen. An apparatus is provided which detects these black bands and automatically expands the image in the appropriate direction such that these black bands are automatically eliminated.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of The Invention 
     The subject invention relates to displaying images having a certain aspect ratio on display screens having a different aspect ratio. 
     2. Description of The Related Art 
     With the advent of widescreen displays in which the display screen has an aspect ratio of 16:9, a problem has arisen when images having an aspect ratio of 4:3 are displayed on the screen. In particular, as shown in FIG. 1A, the 4:3 aspect ratio image  1  is displayed on the screen with vertical black bands  2  and  3 . Since at the present time, most video signals have the 4:3 aspect ratio, “burn in” may occur on the display screen where the screen phosphors are aged only in the area where the image is displayed. This then leads to distortions when viewing a true 16:9 image on the display. 
     Similarly, motion pictures are generally shot in a 16:9 aspect ratio. However, when that picture is reformatted for the standard 4:3 aspect ratio, information is lost. Now many motion picture studios release these motion pictures also in “letterbox” format in which the 16:9 image is compressed such that it fits the 4:3 aspect ratio. This is shown in FIG. 1B where the 16:9 image  4  appears between two horizontal black bands  5  and  6 . 
     In addition to being detrimental to the display, the appearance of these vertical or horizontal black bands is disturbing to the user of the display and detracts from the viewing experience. 
     Conversion circuitry is know that is capable of expanding a displayed image both horizontally and vertically in order to eliminate these black bands. However, it is up to the user to decide when and which of these conversions is to be used. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a method and apparatus for automatically eliminating both horizontal and vertical black bands from the borders of displayed images. 
     This object is achieved in a method for automatically eliminating horizontal and vertical black bands from the borders of a displayed video image, the method comprising the steps of performing a first detection of the occurrence of a black level in an input video signal for at least n lines at the beginning and end of a frame in the input video signal; performing a second detection of the occurrence of a black level in said input video signal for at least m pixels at the beginning and end of each line in a frame of the input video signal; vertically up-converting the lines in the input video signal in dependence on said first detection; and horizontally expanding the lines in the input video signal in dependence on said detection. 
     An apparatus for automatically eliminating horizontal and vertical black bands from the borders of a displayed video image, comprises first means for performing a first detection of the occurrence of a black level in an input video signal for at least n lines at the beginning and end of a frame in the video signal; second means for performing a second detection of the occurrence of a black level in an input video signal for at least m pixels at the beginning and end of each line in the video signal; means for vertically up-converting the lines in the video signal in dependence on said first detection; and means for horizontally expanding the lines in the video signal in dependence on said detection. 
     In a preferred embodiment of the invention, the first and second detections are performed for a predetermined number of fields (or frames) to insure that the black bands consistently persist. It should be understood that with this preferred embodiment, the black bands will appear on the display at least temporarily. 
     In order to prevent the black bands from being temporarily visible, the above method may alternatively include delaying the input video signal for at least one field (or frame) while the black level detection is being performed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     With the above and additional objects and advantages in mind as will hereinafter appear, the invention will be described with reference to the accompanying drawings, in which: 
     FIGS. 1A and 1B show illustrations of video displays with black bands appearing vertically on opposite ends and appearing horizontally at the top and bottom of respective video displays; 
     FIG. 2 shows a block diagram of an embodiment of the invention; 
     FIGS. 3A and 3B show an embodiment of the black level detector of FIG. 2; and 
     FIGS. 4A and 4B show embodiments of the vertical and horizontal black band detectors of FIG.  3 B. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 2, the apparatus is shown having an input for receiving an input video signal which is applied to an analog-to-digital (A/D) converter  10 . The digitized video signal is then applied to a field (or frame) delay  12  and then to an input of a demultiplexer  14 . The demultiplexer  14  has a first output connected to a vertical scan converter  16 , a second output connected to a horizontal line expander  18 , and a third output. A multiplexer  20  is also provided and includes a first input connected to an output of the vertical scan converter  16 , a second input connected to an output of the horizontal line expander  18 , and a third input connected to the third output of the demultiplexer  14 . The output from the multiplexer  20  is connected to a digital-to-analog (D/A) converter  22 , the output therefrom forming the output of the apparatus. 
     A black level detector  24  is provided for detecting a black level in the input video signal. To that end, an input of the black level detector  24  is connected to the output of the A/D converter  10 . A synchronization signal separator  26  is connected to the input to receive the input video signal and supplies horizontal (H) and vertical (V) synchronization signals to the black level detector  24 . If the black level detector  24  detects the black level for the first and last, for example,  20  lines in a frame (or the first and last  10  lines in a field) of the input video signal, the black level detector  24  causes the demultiplexer  14  to apply the delayed video signal to the vertical scan converter  16 , and the multiplexer  20  to apply the output from the vertical scan converter  16  to the D/A converter  22 . Similarly, if the black level detector  24  detects the black level for the first and last, for example,  20  pixels in each line in a frame (or field) of the input video signal, the black level detector  24  causes the demultiplexer  14  to apply the delayed video signal to the horizonal line expander  18 , and the multiplexer  20  to apply the output from the horizontal line expander to the D/A converter  22 . Of course, if the black level detector  24  does not detect the black level as such, the demultiplexer  14  applies the delayed video signal directly to the multiplexer  20  which, in turn, applies the delayed video signal to the D/A converter  22 . 
     In an alternative embodiment, the field/frame delay  12  is omitted while the black level detector  24  examines the input video signal and does not switch the demultiplexer  14  or the multiplexer  20  until it detects the appropriate condition for several consecutive frames (or fields). 
     FIGS. 3A and 3B show an embodiment of the black level detector  24 . As shown in FIG. 3A, the digitized video signal is filtered in a median horizontal low-pass filter  30 , and a median vertical low-pass filter  31  to remove burst noise. A threshold detector  32  then provides a “0” output for all video pixel samples less than a threshold level, e.g.,  5 , and a “1” output for all video pixel samples greater than or equal to the threshold level. This reduces the incoming video image to a series of “0” and “1” values. It is expected that in the blank parts of the screen, these values would be “0”.Of course, there may be “0” values naturally occurring within the actual video signal. The output from the threshold detector  32  is further smoothed by another series of median horizontal and vertical low-pass filters  33  and  34 , to remove any spurious transitions. 
     FIG. 3B shows, in block diagram form, circuitry for detecting the left and right black bands as well as the top and bottom black bands. In particular, the output A from the median vertical low-pass filter  34  and the horizontal and vertical synchronization signals H and V are applied to a vertical black band detector  35 , for detecting the black bands  2  and  3  shown in FIG. 1A, and to a horizontal black band detector  36 , for detecting the black bands  5  and  6  in Fig.  1 B. The outputs from the vertical and horizontal black band detectors  35  and  36  are applied to a logic circuit  37  which, in turn, applies a switching signal to the demultiplexer  14  and the multiplexer  20 . In particular, if the output from the vertical black band detector  35  is “1”, the logic circuit  37  switches the demultiplexer  14  and the multiplexer  20  to the horizontal scan converter  18 . Similarly, if the output from the horizontal black band detector  36  is “1”,the logic circuit  37  switches the demultiplexer  14  and the multiplexer  20  to the vertical scan converter  16 . If the outputs from the vertical and horizontal black band detectors  35  and  36  are both “0”,the logic circuit  37  directly connects the demultiplexer  14  to the multiplexer  20 . 
     FIG. 4A shows an embodiment of the vertical black band detector  35 . The output A from the median vertical low-pass filter  34  is applied the set input of a set-reset flip-flop  40 , while the horizontal synchronization H is applied to the reset (R) input. The horizontal synchronization signal H is also applied to a reset input of a pixel counter  41  which counts a pixel clock signal supplied by a pixel clock  42  locked to the horizontal synchronization signal H. The pixel counter  41  thereby counts the pixels in a line of the input video signal. Assuming a black band on the left edge of the picture, the signal A is initially “0”. At the time of a transition from “0” to “1”, the set-reset flip-flop  40  applies a signal to a first latch  43  which then captures the current pixel count value n1 in the pixel counter  41 . This count value n1 then is applied to a first input of a logic circuit  44 . The output from the pixel counter  41  is also applied to a second latch  45  which receives, as a latch signal, the output A having first been inverted in an inverter  46 . The second latch  47  captures the pixel count value at each occurrence of a “1” to “0” transition. However, since only the location of the last transition is desired, the output from this second latch  45  is applied to a third latch  47  which is latched by the horizontal synchronization signal H. The output n2 from this third latch  47  is applied to a second input of the logic circuit  44 . The logic circuit  44  determines when the count value n1 from the first latch  43  exceeds a given number, e.g.,  20 , which would indicate that a left black band is present in a particular line, and determines when the count value n2 from the third latch  47  is smaller than a given number indicating the presence of a right black band. If both black bands are present in a given line, the logic circuit  44  outputs a “1” value. The output from the logic circuit  44  is applied to a counter  48  which is reset by the vertical synchronization signal V. The counter  48  counts the number of lines containing left and right black bands in a field. If this number exceeds a given number of lines, e.g., 260 lines, the threshold detector  48  applies a “1” value to its output, which is applied to the logic circuit  37 . 
     In a modification (not shown) of this embodiment, the output values from the first and third latches  43  and  47  may be stored and averaged over all of the lines in any given field to determine average values of n1 and n2 for the field. Then these average field values may be further averaged over several fields to determine, with a high degree of accuracy, the locations of the left and right black borders. This averaging, in conjunction with the median filters and threshold detection, will mitigate potential false occurrences of the left and right black borders. 
     FIG. 4B shows an embodiment of the horizontal black band detector  36 . In particular, The output A from the median vertical low-pass filter  34  is applied the set input of a set-reset flip-flop  40 ′, while the vertical synchronization signal V is applied to the reset (R) input. The vertical synchronization signal V is also applied to a reset input of a line counter  41 ′ which receives the horizontal synchronization signal H as a count input. The line counter  41 ′ thereby counts the lines in the input video signal. Assuming a black band at the top of the picture, the signal A is initially “ 0 ”. At the time of a transition from “0” to “1”, the set-reset flip-flop  40 ′ applies a signal to a first latch  43 ′ which then captures the current line count value in the line counter  41 . This count value is then applied to a first input of a logic circuit  50 . The output from the line counter  41 ′ is also applied to a second latch  45 ′ which receives, as a latch signal, the output A having first been inverted in an inverter  46 ′. The output from this second latch  45 ′ is applied to a third latch  47 ′ which is latched by the vertical synchronization signal V. The output from this third latch  47 ′ is then applied to a second input of the logic circuit  50 . The logic circuit  50  determines when the count value from the first latch  43 ′ exceeds a given number, e.g.,  10 , which would indicate that a top black band is present in a particular field, and determines when the count value from the third latch  47 ′ is smaller than a given number, e.g.,  255 , indicating the presence of a bottom black band. If both bands are present in a given field, the logic circuit  44  outputs a “1” value, which is applied to the logic circuit  37 . 
     Numerous alterations and modifications of the structure herein disclosed will present themselves to those skilled in the art. However, it is to be understood that the above described embodiment is for purposes of illustration only and not to be construed as a limitation of the invention. All such modifications which do not depart from the spirit of the invention are intended to be included within the scope of the appended claims.