Enhancement of a video signal produced from information stored in a digital memory

#This invention generally relates to video signal processing and is specifically directed to enhancement of an analog video signal produced for display from information stored in digital form.

#For various applications, such as in a slow-scan television system, it is desirable to perform a scan conversion process on pictorial information such that the rate of scanning the information is changed from a typical broadcast television scan rate to a slower rate for transmission of the information and then back again to the original rate for display on a television monitor. To accomplish this scan rate conversion, a memory for storing the picture information that is scanned or transmitted at the slower rate is required so that an entire picture may be viewed at one time. This is because the picture information is scanned and/or transmitted at too slow a rate for the entire picture to be displayed immediately upon receipt of the transmission within the persistence time of phosphors used on the face of television tubes. A digital memory is preferred because of its precise retention and accurate reproduction. A slow-scan television apparatus including a digital memory is described in an article by Steber, "SSTV to Fast Scan Converter", Parts I and II, QST, March 1975, pp. 33-40 and May 1975, pp.28-46.

#In digital storage, the picture information is represented in discrete memory cells. This means that the picture is "quantized" both in space (two dimensions, x and y), and in grey shades. Because it is desirable to minimize the cost of the memory, and to minimize information transmission time, it is desirable to use as few memory cells as possible to represent the picture. While the possible arrangements can vary widely, in the following discussion, an example is considered, wherein the information stored in the memory represents a picture composed of 128 horizontal scan lines, each of which contains 128 picture element locations and 16 grey shades for each picture element intensity value. Since it takes 4 bits to represent 16 grey shades, the required memory capacity is 128 times 128 times 4 memory cells.

#FIG. 1 shows a series of horizontal scan lines having picture element locations in a prior art television picture display.

#FIG. 2 shows a series of scan lines wherein each scan line as shown in FIG. 1 is repeated and shown a second time. This also is prior art. FIG. 3 is an illustration of a portion of a prior art television picture produced when using the scan line repetition scheme shown in FIG. 2.

#FIG. 4 is an illustration of a portion of a television picture produced from the same digitally stored information as the picture of FIG. 3, as enhanced in accordance with the present invention.

#Referring to FIG. 5, an analog video signal produced from the digitally stored video information is received on line 12. This analog video signal has a voltage waveform as shown in FIG. 6A. Each successive pair of scan lines M and N in this analog signal contains identical video signal intensity information for correspondingly positoned picture element locations. The video signal has a sync level 14 and a blanking level 16.

#Input lines 18 and 20 receive oppositely valued digital clocking signals CLK and CLK, as shown in FIGS 6B and 6C respectively. The transition rate of the clocking signals CLK and CLK is twice the frequency of the picture element locations in each scan line of the analog video signal.

#Input lines 22 and 24 receive oppositively valued digital signals K and K as shown in FIGS. 6D and 6E respectively. The transition rate of digital signals K and K corresponds to the horizontal scan rate of the analog video signal.