Abstract:
Described are a method and apparatus for developing a high quality pictorial field display from transmitted facsimile data. The display, which comprises an array of two-level (&#34;on&#34;/&#34;off&#34;) energy sources such as light-emitting diodes, achieves multitone operation by digitally controlling the time duration during which the energy sources are activated. Inherent output errors of the energy sources are corrected by multiplying received data signals destined for a particular energy source with a predetermined correction factor characteristic of the particular energy source. The multiplied data is stored in a multiport memory connected to the energy sources, permitting the simultaneous activation of the energy sources in the array.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to facsimile systems. More particularly, this invention relates to imaging arrays having a plurality of energy sources and means for accessing and activating the energy sources. 
     2. Description of the Prior Art 
     In a conventional facsimile transceiver, transmission of data is performed by scanning a datacontaining document line by line and by converting the light reflected from the scanned portions of the document into a series of corresponding electrical signals. Those signals are transmitted, typically over a conventional telephone line, to a remote facsimile transceiver where the signals are processed to reproduce the information on a suitable print medium. 
     In the transmitter portion of a conventional transceiver, a light beam, generally produced by a cathode ray tube, is caused to scan along one axis across the document while the document is incrementally moved along on axis perpendicular to the direction of scanning. In that manner, the entire document is effectively traversed with parallel scans. 
     In the receiver portion of a transceiver, several electronic and mechanical techniques are in common use for the purpose of processing received data to produce an image on a print medium. One such technique employs a mechanical stylus operating in response to the received data signals to print the desired pattern on specially prepared paper. Another technique employs a print paper which contains overlaying black and white layers. Portions of the white layer are selectively burned or etched away by the use of an electrically charged stylus that operates in accordance with the received data signals, thereby developing the desired pattern. 
     Still other known facsimile receivers employ a light source, such as a cathode ray tube. The light beam is modulated in accordance with the received data signals and is scanned over a suitably treated medium to form a pattern of locations on the paper. The medium may be photograhic film which is subsequently processed to produce a permanent image. U.S. Pat. No. 3,924,061 issued to Tregay et al on Dec. 2, 1975, and U.S. Pat. No. 3,869,569 issued to Mason et al on Mar. 4, 1975, are examples of such facsimile receivers. 
     In yet other receivers, the light source employed is a laser beam. Such receivers operate in a manner similar to those employing a cathode ray tube but the beam is generally applied to a print medium other than photographic film. In &#34;An Experimental Page Facsimile System,&#34; by H. A. Watson, Bell Laboratories Record, March 1975, page 153, a laser receiver is described where the print medium is bismuth film. 
     A common thread to the above-described techniques is the use of a single light source to form a two-dimensional received image and the use of analog modulation of the light source intensity to control the gray levels of the resultant pattern. 
     In a slightly different field of art, a two-dimensional display is achieved by the use of a plurality of light sources arranged in a matrix configuration. Of common knowledge are gaseous displays and light-emitting diode (LED) arrays which are used extensively to display alphanumerics. For example, LED arrays having groups of diodes preselectively interconnected and activated as a group (e.g., seven segments) are commonly used in calculator displays. A slightly different LED array arrangement is disclosed in U.S. Pat. No. 3,800,177 issued to Russ on May 26, 1974, where the LED array is arranged in a horizontal and vertical address matrix and where the activation of a particular horizontal and vertical address line pair activates a single LED. Whatever the application, LED arrays have heretofore been used only to display a twotone image (generally red on a black background). 
     Different arrangements, such as described in U.S. Pat. No. 3,863,023 issued to Schmersal et al on Jan. 28, 1975, provide multitone displays. In the arrangement described by Schmersal, multitone operation (graduated intensity levels of a particular tone such as green, red, black, etc.) is achieved in a gaseous discharge panel having a multiple number of memory planes. In particular, a number of gray level ranges are defined and an equal number of memory planes are employed, with each memory plane having the same number of storage areas as the number of storage and discharge areas in the display panel. (For simplicity, the term &#34;gray level&#34; is employed in this disclosure regardless of the actual hue employed). In generating the data base, a pictorial field is scanned with a vidicon tube and the elements of the signal corresponding to the picture elements are digitized according to the gray level range in which they fall. In forming a replica of the image field, the signals representing the digitized picture elements are fed in sequence to the various memory planes corresponding to each gray level. The brightness level of each plane is controlled by the storage characteristics of each memory plane and the duration of excitation. 
     The Schmersal et al apparatus is cumbersome because it requires the use of hardware that is both bulky and expensive. Additionally, the memory planes of Schmersal must be accurately aligned and separately driven with high voltages. 
     Another method for effecting different gray levels is described in U.S. Pat. No. 3,604,846 issued to Behane et al on Sept. 14, 1971. In accordance with the teachings of Behane et al, gray level graduations can be achieved by subdividing the area of each picture element (pel) into a plurality of subareas, e.g., a 3 × 3 matrix having nine subareas, and by marking black a number of the subareas in accordance with the gray level desired. Thus, white pels are obtained by marking black none of the subareas, progressively darker gray pels are obtained by marking black greater numbers of subareas, and black pels are obtained by marking black all nine subareas. 
     The Behane method is useful in situations where high receiver resolution is inherent in the system and is, therefore, obtained at low cost. Where high resolution is not inexpensively available, this method becomes too costly because for each macroscopic resolution element (pel), a large plurality of microscopic resolution elements (the subareas) must be employed. 
     To circumvent the difficulties and disadvantages of  pg,6 prior art facsimile systems, a new facsimile receiver has been invented and disclosed in a patent application filed concurrently with this disclosure (Goldschmidt et al Ser. No. 750,273, filed on Dec. 13, 1976. The new facsimile receiver employs a plurality of energy sources arranged in an array. The energy sources employed are two-level (ON/OFF) sources, such as light-emitting diodes. The sources are connected to a multiport memory which activates the plurality of energy sources simultaneously. 
     In some applications, the energy sources are formed in a line array which represents a single scan of the transmitted facsimile data. To generate a two-dimensional pictorial field, the print medium is passed along an axis perpendicular to the line array as successive scans are illuminated. In such applications, it is extremely important that all of the energy sources should provide exactly the same energy outputs in response to the same stimuli. 
     It is an object of this invention, therefore, to provide means for effectively controlling and equalizing the output of the energy sources in the array. 
     It is another object of this invention to provide means for equalizing the output of the energy sources in real time. 
     SUMMARY OF THE INVENTION 
     These and other objectives are achieved with the disclosed method and apparatus for developing a high quality pictorial field display from transmitted facsimile data. The display, which comprises an array of two-level energy sources, such as light-emitting diodes, achieves multitone operation by digitally controlling the time duration during which the energy sources are activated. Inherent output errors of the energy sources are corrected by multiplying received data signals destined for a particular energy source with a predetermined correction factor characteristic of the particular energy source. The multiplied data is stored in a multiport memory connected to the energy sources, permitting the simultaneous illumination of the energy sources in the array. 
     Although the energy sources contemplated in this invention may be any energy sources (such as X-ray sources), throughout the remainder of this disclosure reference shall be made only to light sources and, in particular, only to light-emitting diodes (LEDs). 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 illustrates a general schematic diagram of a facsimile receiver embodying the principles of this invention; 
     FIGS. 2 and 3 represent clock timing diagrams for different intensity control methods of the FIG. 1 receiver; 
     FIG. 4 depicts a block diagram of a signal modifier 10 suitable for use in the FIG. 1 receiver; and 
     FIG. 5 depicts the schematic diagram of control element 40 in the receiver of FIG. 1. 
    
    
     DETAILED DESCRIPTION 
     The pictorial information destined for transmission in a facsimile transmitter is obtained by sampling and coding the scanned information. The sampling process effectively divides each line scan into a plurality of picture elements (pels), and the coding process formats the pel data into binary coded words representing the gray levels of the pels. The coded words are transmitted sequentially, with the least significant bit (LSB) first. Binary coded words, in the context of this invention, are multibit fields where the value of each bit, A n , is 2 n-1 , where n is the position of the bit within the field. 
     FIG. 1 depicts a schematic diagram of facsimile receiver embodying the principles of this invention. In FIG. 1, received data is applied to memory 20 through modifying element 10. Memory 20, which is shown in FIG. 1 as a shift register, is a memory having a plurality of output ports 20 1 , 20 2 , 20 3 , . . . 20 i , arranged at regular storage intervals. The number of ports and the storage interval between ports are selected by the user. In the embodiment disclosed herein, it is advantageous for the storage interval between adjacent ports to be at least equal to the size of the data words (number of bits) applied to memory 20. For purposes of this disclosure, therefore, words entering memory 20 are chosen to have 7 bits each and the parallel outputs of register 20 are selected to occur every 7 bits. Thus selected, corresponding bits of each of the words stored in shift register 20 appear at their respective output ports simultaneously. 
     Corresponding to each output port 20 i , there is a light emitting diode (LED) 30 i  having a preselected one of its terminals connected to its respective memory output port. The diode terminals not connected to memory output ports are connected to line 41 extending from control element 40. Diodes 30 1 , 30 2 , 30 3 , . . . 30 i  are arranged in a diode array 30. In accordance with the principles of this invention, diode array 30 is enabled by applying an appropriate voltage on line 41, causing each and every diode in array 30 to be enabled simultaneously. With the depicted polarity of diodes 30 i , the enabling voltage on line 41 is a low voltage (logic level &#34;0&#34;) which permits current to flow from memory 20, through diodes 30 i , and into control element 40. The output signal of memory 20 is controlled with line 44, which provides a shifting clock to register 20 from control element 40. 
     Also in accordance with the principles of this invention, control of the gray level developed by each diode 30 i  is obtained through lines 44 and 41, in combination with modifier element 10, as described below. 
     GRAY LEVEL CONTROL 
     In accordance with one method for obtaining multitone operation, element 10 converts received binary coded words into a nonpositional binary format and stores the converted words in memory 20. A nonpositional format is one where each bit has an equal weight. For example, a three-bit binary coded input of decimal value 5 (101) is converted to a 7-bit field having five &#34;1s&#34; interspersed therein, e.g., 0011111. Corresponding to this conversion method, control element 40 is arranged to provide to line 44 seven data-display clock pulses of equal period, for shifting the 7-bit field through shift register 20. Concurrently, control element 40 provides an enable pulse to line 41 to enable the diodes in array 30 for the duration of the seven data-display clock periods. While enabled, each diode 30 i  lights up a number of times equal only to the number of &#34;1s &#34; in the word stored and shifted out of the corresponding memory 20 port to which each diode 30 i  is connected. The seven equal period data-display clock pulses of line 44 and the enabling pulse of line 41 are depicted in FIG. 2. 
     To achieve the above-described conversion process in element 10, a conventional combinatorial circuit is best employed. One such network comprises one three-bit register for storing the received binary coded signal words, seven gate arrangements connected to the three-bit register for encoding the signals words, and one seven-bit register for storing the encoded words. The Boolean equations of the seven gate arrangements are A+B+C, A+B, A+BC, A, AC+AB, AB, and ABC, where A, B and C are the outputs of the three-bit register, A is the MSB (most significant bit) and C is the LSB. 
     EXAMPLE 
     Given an input sequence of five words having the binary coded values 001, 011, 010, 000, and 110, element 10 converts the input sequence to the sequence 1000000, 1110000, 1100000, 0000000, and 1111110. The converted sequence is stored in memory 20 (with a data-load clock signal on line 44) with the least significant bits first which, in accordance with the spatial arrangement of FIG. 1, appears as follows: 
     
         ______________________________________11111100000000110000011100001000000ports   20.sub.1 20.sub.2 20.sub.3 20.sub.4 20.sub.5______________________________________ 
    
     During the first data-diaplay clock pulse (on line 44) all output ports present a logic &#34;0&#34; output and therefore none of the LEDs in array 30 light up. During the second pulse, shift register 20 is advanced by one bit causing port 20 1  to present a logic &#34;1&#34; output which, in turn, causes LED 30 1  to light up. Advancing shift register 20 further, during the third and fourth clock pulses only LED 30 1  lights up; during the fifth clock pulse LEDs 30 1  and 30 4  light up; during the sixth clock pulse LEDs 30 1 , 30 3 , and 30 4  light up; and during the seventh clock pulse LEDs 30 1 , 30 3 , 30 4  and 30 5  light up. 
     During each data-display clock pulse that causes an LED to light up, a fixed quantum of light is generated. During each enable cycle, each LED produces a quantum of light equal to the magnitude of the received word which corresponds to that LED. Thus, in accordance with the principles of this invention, each LED in array 30 is capable of developing within each cycle any one of 7 gray levels; the dimmest being the LED which does not light up at all and the brightest being the LED which lights up at each and every data-display clock pulse of line 44. 
     In accordance with another method for obtaining multitone operation, the binary coded format of the words received by element 10 is not changed. Because of the compactness inherent in positional encoding, memory 20 may be made smaller than in accordance with the method described above, or conversely, the same memory can accommodate a larger number of multitone variations. On the other hand, because of the positional nature of binary encoding, control element 40 can no longer deliver to line 44 clock pulses of equal duration during the enable cycle. Rather, the clock pulses produced by control element 40 and applied to line 44 must be related to the particular coding of the input data; and in the case of binary coding, must be related by a multiplicative factor of 2 to each other. That is, if the bits stored in memory 20 appear at the output ports with the LSB first followed by successively more significant bits, then the second clock pulse on line 44 must have twice the period of the first clock pulse, and each successive clock pulse must have twice the period of its preceding pulse. FIG. 3 depicts the three data-display clock pulses necessary to produce the 7 gray levels obtainable with the abovedescribed nonpositional method. 
     One advantage of placing positionally coded data in memory 20 is the avoidance of a conversion in element 10. As expected, however, the illumination enabling pulse of line 41, as depicted in FIG. 3, is of duration equal to the duration of the illumination enabling pulse of FIG. 2. No time saving is realized through the use of positionally coded data because in order to obtain the certain quantum of light output, the LEDs must be illuminated for a particular period of time regardless by which method that illumination time is obtained. 
     EQUALIZATION 
     As indicated previously, element 10 modifies the format of data entered into memory 20 in order to implement the particular multitone method selected. Element 10 also serves the additional function of equalizing the LEDs in array 30. 
     Generally, the diodes in LED array 30 are constructed from different slabs of material. It is expected, therefore, that the LEDs in array 30 do not all produce the same quanta of light in response to the same stimuli. Also, the outputs of memory 20 do not necessarily provide exactly the same stimulus when required to do so. Such irregularities produce undesired variations in the light output of array 30 but, when not extreme, the variations are not noticeable in a two-dimensional array where each LED corresponds to a particular pel in the pictorial field. In fact, variations in light output of up to 2:1 have been experienced, and such variations are noticed even in two-dimensional arrays. 
     Many facsimile receivers, however, employ a linear (one-dimensional) array rather than a two-dimensional array with a number of LED in the linear array equal to the number of pels in one scan. The pictorial field is developed in such receivers by moving a suitable print medium across the array as successive scans are illuminated. In such receivers, even very slight differences in LEDs light outputs are noticeable. These differences manifest themselves as longitudinal striations across the pictorial field, commonly referred to as artifacts. 
     Linear arrays are light sensitive in still another way. Two-dimensional LED arrays produce satisfactory multitone pictorial fields even with a low number of bits in the data words. Inasmuch as changes in gray level are expected from pel to pel and from scan to scan, no artifacts show up. Operating two-dimensional arrays with only few bits to define the gray levels is, therefore, quite feasible. With a linear array, on the other hand, the accuracy of each level must be tightly controlled to prevent artifacts even if the number of gray levels is relatively small. Therefore, the number of bits employed to effectively define each gray level in a LED facsimile system having a linear array is larger than the number of bits required to distinguish a desired number of gray levels. For example, with 15 gray levels (characterizable by five bits), it has been found that the use of seven-bit words to accurately define each of the gray levels is recommended. 
     The correction, or equalization, for the output light variations in the LEDs of array 30 is performed in element 10. Since the total light output of each LED is simply a sum of a number of light pulses having fixed quanta of light, it has been found that the light response of each LED is linear with respect to the magnitude of the stimulus and that the light output error is a multiplicative error. Forearmed with this finding, modifier element 10 is adapted to multiply the magnitude of each received signal by a multiplicative correction factor. The correction factor for each signal relates to the error characteristic of the LED in array 30 which displays the multiplied signal. If the light output of LED 30 1 , for example, is 0.75 of normal and the light output of LED 30 4  is 1.22 of normal, then signal words destined to LED 30 1  are divided by 0.75 (or multiplied in element 10 by 1.33) while the signal words destined to LED 30 4  are divided by 1.22 (or multiplied in element 10 by 0.82). 
     To implement the above-described corrections, element 10 may employ a variety of techniques. As disclosed herein and shown in FIG. 4, element 10 comprises a ROM memory 11, a conventional multiplier 12, an input register 17 and an output register 18. Register 17 stores incoming words by latching the input under control of bus 42 while the latched words are applied to one input of multiplier 12. ROM 11 stores the multiplicative factors required for each LED and applies the factors (via bus 14) to the other input of multiplier 12, also under control of bus 42. Bus 42 signals emanate from control element 40. 
     Each multiplicative factor provided by ROM 11 is a binary word characterizing the multiplicative factor associated with the LED for which the currently applied received signal, appearing on lead 15, is destined. The received signals on lead 15 and the multiplicative factors on lead 14 are multiplied in multiplier 12. The product signals developed by multiplier 12 have a number of bits equal to the number of bits in the words appearing on lead 15 plus the number of bits in the words appearing on bus 14. Since that large number of bits may not be necessary, the product signals of multiplier 12 are truncated to the number of bits suitable for memory 20 by storing only the desired bits in register 18. The truncated signals are applied to memory 20 directly (via lead 16) when binary encoded words are desired to be stored in memory 20. When words otherwise encoded are desired to be stored in memory 20, a code conversion block must be interposed between multiplier 12 and memory 20, as for example, the code conversion circuitry for nonpositional coding described infra. 
     The multiplicative correction factors stored in ROM 11 relate to the light output variations of the particular LEDs used in array 30. These factors are obtained by actually constructing array 30, by interfacing array 30 with memory 20, and by testing the light response of each LED within the array. Once obtained, the multiplicative factors are stored in ROM 11. 
     ILLUMINATION CONSIDERATIONS 
     Since the apparatus of FIG. 1 illuminates the LEDs of a full scan simultaneously, it appears that the actual illumination must occur during the time interval between scans, when the data in memory 20 is received and is stable. That is, all received signals for one scan must first be received, be multiplied in modifier element 10, and be stored in memory 20. Only then may array 30 be enabled. 
     With a receiver organization as in FIG. 1, when pel information is transmitted at a truly regular pel clock rate (corresponding to the pel sampling rate of the transmitter), then only one pel clock interval is available for illumination. If one pel clock interval is not sufficient for reliably generating the necessary quanta of light out of the LEDs of array 30, transmission efficiency can be sacrificed and a plurality of pel clock intervals may be dedicated to the illumination phase. This can be done, for instance, at the transmitter end, by scanning and sampling at a somewhat faster rate and by inserting idle time intervals between scans. 
     With a different approach, the operational speed of the FIG. 1 apparatus may be disassociated from the bandwidth of the transmission medium, capitalizing thereby on the high speed achievable in element 10 and in memory 20. Specifically, an auxiliary memory may be interposed between the received signals and element 10, or between element 10 and memory 20, and the received signals may be stored therein at the relatively low rate of the transmission medium. When the information of a full scan has been received, the auxiliary memory may be read out at a high rate into memory 20, reducing thereby the time spent by memory 20 in the loading phase and correspondingly increasing the time spent in the illumination phase. 
     With a still different approach, an auxiliary memory may be employed which has a plurality of output ports. The output ports of the auxiliary memory and of memory 20 may be combined to permit multiplexed operation, and when operating in this manner, array 30 may be illuminated constantly. The constant illumination of array 30 permits a most conservative utilization of the LEDs in array 30, increasing reliability, and eliminating the necessity for an enabling pulse on line 41, thus permitting diodes 30 i  to be permanently enabled by grounding line 41. 
     CONTROL ELEMENT 
     FIG. 5 illustrates the schematic diagram of control element 40 suitable for the FIG. 1 receiver. Bus 43 is the control input and it comprises lines 43 1  and 43 2 . Line 43 1  delivers a pulse whenever a new scan is initiated while line 43 2  delivers a pulse whenever a new word is received and applied to lead 15. An oscillator 410 is employed in element 40 to synchronize the operations of the FIG. 1 facsimile receiver. The signals on line 43 2  are synchronized to oscillator 410 in flip-flop 420 and having been synchronized, are applied to binary counter 430 and to delay element 440. The parallel outputs of counter 430 and line 43 2  are included in bus 42. Line 43 2  is used in modifier 10 to strobe the applied data signals into register 17, and the parallel outputs of counter 430 are used to address ROM 11. To account for the time consumed in modifier 10 in the correction of data, delay element 440 is adapted to delay the synchronized pulses of flip-flop 420 by an amount of time commensurate with the time consumed in element 10. The output signal of element 440 sets flip-flop 441 and thereby initials a burst of k clock pulses from oscillator 410, where k is the number of bits per words applied from modifier 10 to memory 20. The burst of clock pulses is derived from gate 442 which is responsive to flip-flop 441 and to oscillator 410. The output signal of gate 442 is the data-load clock signal which is passed to line 44 through OR gate 435 and is employed to load memory 20. The output signal of element 440 and the data-load signal are also included in bus 42 and are employed in modifier 10 to strobe the truncated product into register 18 and to shift the strobed information out of register 18 and into memory 20. Flip-flop 441 is reset by a divide-by-k counter 443 which is responsive to the clock pulses of gate 442. 
     The new-scan indicating pulse of line 43 1  is synchronized to oscillator 410 in flip-flop 450 and is applied to counter 430 to reset the counter. 
     Combinatorial network 470, responsive to counter 430, develops the data-display clock signal of lead 44, via OR gate 435. When binary coded words are stored in memory 20, the desired sequence of pulses out of network 470 must be arranged to form the binary spacing shown in FIG. 3. The basic spacing is a function of the number of bits in the words stored in memory 20, and the time available for illumination. The functional interrelationship can be seen from the following. For seven-bit words, seven intervals are required. Each interval must be twice the duration of the previous interval. If the basic (and shortest) interval is one pel clock period long, a total of 127 pel clock periods are required to obtain the seven intervals (1 + 2 + 4 . . . + 64). When the available illumination time, K, (in number of pel clock periods) is equal to a multiple of 127 (K = 127M where M is an integer) then the first and shortest interval may conveniently be set to M pel clock periods. When so chosen, the seven desired intervals, may be obtained by detecting states M, 3M, 7M, 15M, 31M and 63M in counter 430. The state detections are performed in logic network 470 with conventional gate arrangements. 
     The embodiment of modifier 10 depicted in FIG. 4 and described above is, of course, only illustrative. A simpler embodiment can be had, for example, by replacing register 17, multiplier 12 and ROM 11 with a single multiplier ROM which is accessed by the juxtaposed line 15 and bus 42. Line 15 contains the input signals and bus 42 contains the destined LED information. Together, the location of the product signals is uniquely defined, permitting the multiplier ROM to prestore the product signals. 
     Aside from the simplicity and speed of such a modifier 10 design, the use of a ROM instead of a multiplier permits the correction of nonlinear errors in the light output of the LEDs of array 30. The only penalty may be an increased cost associated with a large ROM; but that cost is compensated by the reduced hardware of modifier 10, which may even encompass the elimination of register 18.