Clock recovery circuit for a video signal

Abstract: A medical imaging system includes a video interface (18) connecting different medical imaging modalities (21), (22), (23), (24) to a laser printer (20). The video interface (18) includes a clock recovery phase-lock-loop (PLL) (28) as a frequency synthesizer video signal from a modality. A second PLL (42) inserts a pulse train equal to the horizontal frequency into an analog video signal having a vertical sync interval with no serrating pulses. This assures the clock recovery PLL will maintain lock during the vertical sync interval.

Description: This invention relates in general to medical imaging systems and more particularly to an interface between a medical imaging modality and a radiographic printer. In traditional film/screen medical diagnostic imaging, an anatomical part of a patient is positioned between an x-ray source and an unexposed film/screen, an x-ray exposure of the anatomical part produces a latent x-ray image in the film and the film is developed. The developed x-ray film is then viewed on a light box by a diagnostician (radiologist/physician). More recently, medical diagnostic imaging modalities, such as CT and MRI scanners, have produced video x-ray images which are viewed on a video monitor. Frequently, a permanent x-ray image of a video x-ray image is made by photographing the video monitor to produce an x-ray film image. The x-ray film image can also be produced by a laser printer (such as the KODAK EKTASCAN LASER PRINTER sold by the Eastman Kodak Company, Rochester, New York). When using a laser printer to produce a permanent x-ray film image, problems arise from the different video formats used by different medical imaging modalities. Typically, the analog video signal produced by the modality is converted to a digital image signal which is used to drive the laser printer. Because some incoming video formats lack serrating pulses during the vertical sync time interval, when a phase-locked-loop (PLL) is used as a frequency synthesizer to recover a pixel clock from a composite video signal, the clock recovery circuitry will lose lock if no serrating pulses are present. The signal compensating and PLL circuits disclosed in the following patents are not entirely suitable for such applications: US-A-4,251,833, US-A-4,905,085, US-A-4,520,394, US-A-4,851,910, US-A-3,469,032, US-A-3,991,270, US-A-4,709,268, US-A-4,945,413, US-A-4,379,309, and U.K. Patent Application GB 2086177A. According to the present invention, there is provided a circuit for generating a pulse train equal to the horizontal scan frequency during the vertical sync interval of a video signal which has no serrating pulses during such interval. The circuit includes a phase-lock-loop clock recovery circuit for providing a pixel clock synchronized to the horizontal frequency of an analog video signal to be digitized and a multiplexer having first and second inputs. A second phase-lock-loop circuit is connected to the second input and the analog video signal composite sync is applied directly to the first input and to the second phase-lock-loop circuit. Control means controls the multiplexer as follows: When an analog video signal is received which has serrating pulses during the vertical sync interval, the multiplexer is controlled to pass it directly from the first input to the clock recovery phase-lock-loop circuit. When an analog video signal is received which has no serrating pulses during the vertical sync interval, the multiplexer is controlled to insert serrating pulses produced by the second phase-lock-loop circuit at the horizontal frequency into the vertical sync interval of the analog video signal. The multiplexer is controlled to pass the enhanced analog video signal from the second input to the clock recovery PLL. In this manner, the clock recovery PLL is prevented from losing lock no matter which kind of analog video signal is received. FIG. 1 is a block diagram of a medical imaging system including an embodiment of the present invention. FIG. 2 is a schematic diagram of the embodiment of FIG. 1. FIGS. 3 and 4 are waveform diagrams useful in describing operation of the present invention. Referring now to the Figures, there will be described an embodiment of the present invention as used in a medical imaging system. It will be understood that the present invention can also be used in other imaging systems in which an analog video signal is converted to a digital image signal. The medical imaging system in FIG. 1 includes medical imaging modalities 10, 12, 14, 16 (such as CT, MRI, US, PET), video interface 18 and laser printer 20. Modalities 10, 12, 14, 16 produce x-ray analog video images which are applied to inputs 21, 22, 23, 24 of video multiplexer (MUX) 26 of video interface 18. Video interface 18 includes clock recovery PLL 28, digitizer 30, frame store 32 and control processor unit (CPU) 34. CPU 34 enables MUX 26 to pass a video signal from only one of modalities 10, 12, 14, 16 to digitizer 30 where the active video is converted from an analog image to a digital image. The horizontal sync is applied to PLL 28 which produces the pixel clock for digitizer 30. The digital image is stored in frame store 32 before it is applied to laser printer 20 to produce a hardcopy x-ray film image. Other types of printers, such as thermal printers or cathode ray tube printers, can also be used to produce a hard copy x-ray image. Referring now to FIGS. 2-4, there will be described an embodiment of the present invention. As shown, MUX 26 includes a multiplexer circuit 36 having first and second inputs 38 and 40. A second phase-lock-loop (PLL) 42 is connected to input 40 and generates a pulse train at a frequency equal to the horizontal frequency of a received analog video signal having no serrated pulses during the vertical sync interval. When a modality 10-16 provides an analog video signal to MUX 26 (FIG. 1) which has a vertical sync interval with serrating pulses at horizontal frequency (FIG. 3), CPU 34 controls multiplexer circuit 36 of MUX 26 to pass the analog video signal at input 38 directly to clock recovery PLL 28. PLL 28 will stay in lock during the vertical sync interval due to the serrated pulses present during this interval. When a modality 10-16 provides an analog video signal to MUX 26 (FIG. 1) which has a vertical sync interval with no serrated pulses, CPU 34 controls MUX 36 to insert a pulse train at horizontal frequency into the vertical sync interval of the analog video signal. CPU 34 controls multiplexer circuit 36 to pass the enhanced analog video signal at input 40 to clock recovery PLL 28. This assures that the PLL 28 will remain in lock condition during the vertical time interval. The invention has applicability in medical imaging systems in which analog video signals with different formats from medical imaging modalities are provided to a radiographic laser printer to produce radiographic film.

Claims: A clock recovery circuit comprising: means (26) for receiving, as a received analog video signal, either a first analog video signal having a vertical sync interval with serrated pulses at horizontal frequency or a second analog video signal having a vertical sync interval with no serrated pulses at horizontal frequency; a clock recovery phase-lock-loop, referred to as clock recovery PLL (28), for producing a pulse train synchronized with the horizontal frequency of said received analog video signal. a multiplexer (36) having first and second inputs (38,40) wherein said first input directly receives said received analog video signal, and wherein a second phase-lock-loop (PLL 42) circuit (42) comprised in said clock recovery circuit produces a pulse train at the horizontal frequency of said received analog video signal and is connected to said second input of said multiplexer; control means (34) for controlling said multiplexer, such that when an analog video signal is received having a vertical sync interval with serrated pulses at horizontal frequency, said signal is passed from said first input directly to said clock recovery PLL, and when an analog video signal is received having a vertical sync interval with no serrated pulses at horizontal frequency, said multiplexer (36) inserts said pulse train at said horizontal frequency into said vertical sync interval of said received analog video signal, and the thus enhanced analog video signal is passed from said second input to said clock recovery PLL, to assure synchronization of said clock recovery PLL with said received analog video signal during the vertical sync interval. The circuit of claim 1 wherein said first and second analog video signals are received from different medical image modalities. The circuit of claim 1 wherein said clock recovery PLL effects recovery of the pixel clock of said received analog video signal and wherein an analog-to-digital converter (30) is included, which is synchronized with said recovered pixel clock and which converts said received analog video signal into a digital video signal. The circuit of claim 3 wherein said digital video signal is applied to a laser printer which produces a film image of said video signal.

Inventors: MACOMBER ELLIOT DEFOREST, MAUNE JOSEPH PATRICK, MCNEILLY PETER JOSEPH, MACOMBER, ELLIOT DEFOREST, MAUNE, JOSEPH PATRICK, MCNEILLY, PETER JOSEPH, Macomber, Elliot Deforest, c/o Eastman Kodak Co., Maune, Joseph Patrick, c/o Eastman Kodak Co., McNeilly, Peter Joseph, c/o Eastman Kodak Co.
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1992
Main classification: H04N1
IPC codes: H04N1, G06T1, H03L7