Abstract:
A reliable, low cost system for generating precisely aligned pixel clock and index signals for a digital scanning or printing system is based on a novel single channel encoder pattern detected with a single sensor. The signal output of the sensor representing the encoder pattern is processed with phase locked loop based circuit to generate pixel clock and index signals of known and guaranteed phase.

Description:
FIELD OF THE INVENTION 
     The invention relates in general to the field of electronic printing of photographic images. More specifically, the invention relates to encoders and pixel clock circuits that control the timing of line writing operations in electronic photographic printers. 
     BACKGROUND OF THE INVENTION 
     Photographic printing of digitized images is accomplished by modulating the intensity of a light beam as the beam moves relative to a photosensitive media. The most common motion pattern has the beam move rapidly along a single line as a single row of pixels is modulated or &#34;clocked out&#34; starting from a fixed margin location. The beam is then indexed to the next line of the page and returned to the margin before writing the next row of pixels. 
     A high degree of precision is required in identifying the margin location and pixel boundaries since misalignment of the pixels by as little as ten percent of a single pixel width degrades the perceptible quality of the printed image. Conventional printers have employed a two channel position encoding scheme to prevent misalignment. One channel of relatively high resolution is used to generate a pixel clock while a second channel generates one index pulse per revolution at a fixed position relative to signal line start. Synchronization of the pixel clock and the index to assure line to line uniformity is not an easy matter, and systems employ precise alignment of the two encoder channels and associated sensors to insure non-coincidence of pixel clock and line start signal transitions. 
     In view of the above, it is an object of the invention to provide a reliable low cost means for generating precisely aligned pixel clock and index signals for a digital scanning or printing system. 
     SUMMARY OF THE INVENTION 
     The invention provides a reliable, low cost system for generating precisely aligned pixel clock and index signals for a digital scanning or printing system. The system is based on a novel single channel encoder pattern detected with a single sensor. The signal output of the sensor representing the encoder pattern is processed with phase locked loop based circuit to generate pixel clock and index signals of known and guaranteed phase. 
     Specifically, an encoder track having M total cycles is provided, wherein M-1 cycles of the encoder track have a duty ratio less than 50 percent and one cycle of the encoder track has a duty ratio greater than 50 percent. The encoder track is read by a sensor that generates an encoder signal that is supplied to a pixel clock circuit. The pixel clock circuit includes a phase lock loop circuit having an encoder signal input, a feedback signal input, and a pixel clock output coupled to an input of a divider circuit, wherein the divider circuit generates a feedback output signal that is supplied to the feedback signal input of the phase lock loop circuit. A flip-flop is used to generate an index signal in response to the encoder signal generated by the sensor and the feedback signal generated by the divider. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Reference should now be made to the following detailed description of the preferred embodiments of the invention and the accompanying drawings, wherein: 
     FIG. 1 is a schematic representation of a side view of a print engine; 
     FIG. 2 is a schematic representation of a encoder wheel assembly attached to a rotating drum of the print engine shown in FIG. 1; 
     FIG. 3 illustrates a phase-lock-loop pixel clock generating circuit employing a voltage controlled oscillator and a divider; 
     FIG. 4 illustrates the relationships among the physical encoder track, the PLL clock circuit signals and a generated pixel clock; 
     FIG. 5 illustrates the relationship between the generated pixel clock and an index pulse generated by the line start sensor shown in FIG. 2; 
     FIG. 6 illustrates the impact of small variations in index signal timing relative to the pixel clock; 
     FIG. 7 illustrates the displacement of all pixels in a line due to the misplacement of the first pixel relative to previous lines; 
     FIG. 8 is a schematic diagram of a pixel clock circuit in accordance with the invention; 
     FIG. 9 illustrates the relationship between the modified encoder track and the signals generated by the clock circuit; and 
     FIG. 10 illustrates an encoder assembly in accordance with the invention that utilizes a single encoder sensor and an encoder wheel with the modified encoder track illustrated in FIG. 9. 
    
    
     DESCRIPTION OF THE INVENTION 
     A side view of a print engine is schematically represented in FIG. 1. Photographic media 10 is supported as on the inner surface of a write media support cylinder 12, such that a write beam emitted from a rotating print drum 14 and modulated so as to represent information stored in successive pixels, writes a single image line on the photographic media 10 for each revolution of the rotating print drum 14. The print drum 14 is advanced one pixel width through the write support cylinder 12 for each revolution, so that the next line of pixels writes to the adjacent physical line on the photographic media 10. 
     Referring now to FIG. 2, the print drum 14 is attached to an encoder assembly that includes an encoder wheel 16, a motion encoder sensor 18 and a line start index sensor 20. The motion encoder sensor 18 is used to generate a multiple pulse per revolution encoder signal, based on an encoder track located on the encoder wheel 16, which is used to measure the location of the print drum 14. The line start index sensor 20 generates a single output per line index signal, based on an index track located on the encoder wheel 16, which is used to identify the image margin location. The line start index sensor 20 is usually mounted in the same assembly as the motion encoder sensor 18, as shown in FIG. 2, although other physical embodiments are sometimes employed. 
     In order to reduce the line count (lines per revolution) required for the encoder assembly, phase locked loop (PLL) multiplication of the encoder output frequency is commonly employed in the art. A pixel clock circuit employing a phase lock loop (PLL) circuit 22 with an internal voltage controlled oscillator (not shown) and a divider 24 is illustrated in FIG. 3. The relationships among the physical encoder track, the pixel clock circuit signals and the generated pixel clock are displayed in FIG. 4. The duty cycle of the encoder output is arbitrary in general since the PLL is responsive only to rising edges The action of the PLL pixel clock circuit is to drive the output (VCO) of the internal voltage controlled oscillator such that the divided down output pulse train (FDBK) contains coincident rising edges in one to one correspondence with those of the encoder signal. It should be noted that since the divider 24 is falling edge triggered, the recovered pixel clock contains falling edges coincident with encoder signal rising edges at a frequency N times that of the encoder. The relationship between the generated pixel clock and an index pulse generated by the line start sensor 20 is shown in FIG. 5. The first rising edge in the pixel clock stream that occurs after the rising edge of the index signal is preferably chosen to clock out the first pixel of the line. 
     The impact of small variations in index signal timing relative to the pixel clock is displayed in FIG. 6. If the rising edges of index signal and pixel clock are coincident, ambiguity exists in the choice of pixel clock interval for the first pixel of the line. Near the point of coincidence of these signals, extremely small variations in index signal or pixel clock location can result in full pixel width line displacement. If the first pixel is misplaced relative to previous lines, all succeeding pixels in that line will be displaced correspondingly as is illustrated in FIG. 7. 
     Referring now to FIG. 8, a pixel clock circuit in accordance with the invention is shown which, in conjunction with a modified encoder track, eliminates index pulse placement ambiguity. As in the circuit illustrated in FIG. 4, the pixel clock is generated via PLL multiplication of the encoder signal by means responsive to the rising edge of the encoder signal only. In the previous case, the duty cycle of the encoder track signal was irrelevant. Now, however, the track is configured such that M-1 cycles of the track have duty ratio less than 50 percent while one cycle has duty ratio greater than 50 percent, where there are M total cycles on the encoder track. 
     The value of the encoder signal is stored in a flip flop 26 on falling edges of the FDBK signal which, by using the appropriate choice of circuitry in the divider 24&#39; and the rising edge synchronizing nature of the PLL, are made to occur precisely midway between rising edges of the encoder signal. The resulting unitrack index signal is generated once per revolution and changes state only on falling edges of the pixel clock. The pixel clock signal (PIXEL CLK) is generated based only on rising edge information of the encoder signal, independent of encoder signal falling edges. The unitrack index signal is generated based only on falling edge information in the encoder signal as long as the encoder signal rising edges are sufficiently uniformly spaced to maintain proper operation of the PLL. Such uniformity is easily maintained through standard manufacturing techniques applied to the encoder pattern in conjunction with the uniform rotational speed of the drum otherwise required to maintain consistent image quality. The relationship between the modified encoder track and the signals generated by the clock circuit are shown in FIG. 9. 
     The use of the modified encoder track and the pixel clock circuit of FIG. 8 eliminates the need for a separate line start index sensor. Thus, the encoder assembly for the print engine can be simplified by using a single encoder sensor 18&#39; to read the modified encoder track which is placed on the encoder wheel 16&#39; as shown in FIG. 10. 
     The invention has been described with reference to certain preferred embodiments thereof, it will be understood, however, that modifications and variations are possible within the scope of the appended claims. For example, circuit elements other than the flip-flop 26 can be used to store the encoder signal. 
     PARTS LIST 
     10 PHOTOGRAPHIC MEDIA 
     12 WRITE MEDIA SUPPORT CYLINDER 
     14 ROTATING PRINT DRUM 
     14&#39; ROTATING PRINT DRUM 
     16 ENCODER WHEEL 
     16&#39; ENCODER WHEEL 
     18 MOTION ENCODER SENSOR 
     18&#39; MOTION ENCODER SENSOR 
     20 LINE START INDEX SENSOR 
     22 PHASE LOCK LOOP (PLL) CIRCUIT 
     24 DIVIDER 
     26 FLIP FLOP