MONOLITHIC DELTA FRAME CIRCUIT

Abstract: A monolithic delta frame circuit comprises a high speed line address circuit (110), demultiplexer (112), line shift regis­ ter (114), plurality of buffer amplifier (116), array of difference frame elements (118), reset circuit means (132), plurality of sample and hold means (138), multiplexer (139) and high speed line address circuit (140). The high speed line address (110) clocks the single line video input at a fast rate into the de­ multiplexer (112) for demultiplexing into the line shift register (114), the line shift register shifts the single line signals and noise into the elements of the array of difference frame ele­ ments (118). As the data from the previous frame which con­ sists of noise or noise minus signals is still present in the differ­ ence elements only the signal of delta signal protion feeds through reset circuit means (132) at a slower rate to the multi­ plexer (139). The rest circuit means (132) introduces offset noises into the signal which are substantially reduced by feed­ back through the reset circuit means. The high speed line ad­ dress register (110) is connected to the multiplexer (139) for controlling the video output signals through the video out ter­ minal. The video output is at the fast rate, to reduce this rate for minimizing display blind time the sample and hold circuits (138) are introduced between the reset circuit means (132) and mul­ tiplexer (139). Thus the monolithic delta frame may be used to remove reset noise and rate conversion or in tandem to re­ move reset noise and offset noise as well as rate conversion.

Description: This invention relates to delta frame circuit for eliminating substantial offset (fixed pattern noise) from low amplitude signals as defined in the precharacterizing part of claim 1 Signal detectors of the delta frame type are disclosed in US-A-4,617,593 and in US-A-4,622,587. The delta frame circuit is to be used in an imager system of the type which in the the past, has included a lens system, a detector matrix, drive and read out electronics, a video processor, a timing and control means and a display means. The detector matrix has been an array of ferroelectric elements of photoconductive elements arranged in rows and columns. With the elements in rows and columns, the detector array is X-Y addressable through X-Y address circuits. The detector matrix is coupled to drive and read out electronics. For an array of optoelectronic semiconductor elements, as each element is shielded by the chopper, each element is X and Y addressed by the drive electronics and charged to a reference voltage. As each successive element is charged the preceding charged elements are receiving photons to vary the charges thereon. Then the drive circuit X-Y addresses each element and the image voltaoes multiplexed to the video processor for processing and display. The disadvantages of the prior art imaging systems are: the video signals include reset noise, and the image resolution needs improvement. It is the object of the invention to provide a delta frame circuit for use in an imaging system which is an integrated monolithic circuit built using standard MOS processing. This object is achieved by means of a delta frame circuit which includes the features of the characterizing part of claim 1. Said delta frame circuit, when used in an imaging system of the type described above, contributes to reducing reset noise in the video signals and to improving the resolution of the image. Improvements of the delta frame circuit are described in subclaims 2 to 5. The invention itself and advantages thereof may best be understood by reference to the following detailed description of illustative embodiments when read in conjunction with the accompanying drawings in which: Figure 1 is a block diagram of a first embodiment of an imaging system and including in dashed lines the additional transversal filter block constituting the second embodiment of the imaging system in which the delta frame circuit according to the invention can be used; Figure 2 is a view, partly schematic, showing one embodiment of the imaging system; Figure 3a is a view, partly schematic, of the total subtraction amplifier chain; Figure 3b is a timing diagram for the operation of the total subtraction amplifier chain; Figure 4 is a view, partly schematic, of the integrated delta frame circuit Figure 5a is a block diagram of the first embodiment of the invention: Figure 5b is a timing diagram for the first monpolithic delta frame; Figure 5c is a timing diagram for the second monolithic delta frame; and Figure 6 is a block diagram of the tranversal filter. Referring now to Fig. 1 in which is shown for description purposes only and not by way of limitation an ac type infrared detector system 10. The infrared system 10 comprises an optical system 12, chopper 14, detector 16, video processor 18, display 26, and timing and control mechanism 28. The video processor 18 includes first and second monolithic difference (delta) frames 20 and 22 and in a second embodiment, a transversal filter 24. The optical system 12 is a combination of lenses for focusing electromagnetic wave energy emanating from a scene. The chopper 14 is a mechanical chopper for interrupting the focused energy to provide intermittent signals for an ac imager system. It will be appreciated by those skilled in the art that various types of lens systems and choppers exist capable of performing the desired optical functions. Referring now to Fig. 2, the detector 16 is, for example, a parallel amplifier array of 10⁴ elements 30 arranged in rows and colums of 100 elements each. Each element includes a solid state switch 32, and a capacitor 34. Switches 32 may be, for example, field effect transistors (FET) formed by diffusing for each switch two N-type regions called the drain 36 and the source 38, side by side into the surface of a P-type silicon slice. A layer of silcon oxide is grown over the surface of the silicon slice and metal strips are formed on the oxides to form the gates 40. The capacitors 34 are formed, for example, of a wafer of ferroelectric material having longitudinal lead patterns 42 and pads 44 formed on opposing major surfaces by evaporation/etching techniques. The pads and the crossover portions of the lead patterns form the plates of the capacitors 34 with the ferroelectric material forming the dielectric. The gates 40 of the switches 32 of each row are connected by leads 46 to a Y addressing circuit 47 and the sources and drains are connected by leads 48 and 50, respectively, to the first plates 44 of capacitors 34 which have their second plates connected by leads 52 to V-bias, and by leads 50 to the reset line 54. Sense lines 54 have opposing ends connected, respectively, to reset switches 56 and to the junction of capacitors 57 and amplifier chain circuits 58. The capacitors 57 are used to represent the inherent capacitance of the sense lines 54. The reset switches 56 are, for example, FET switches having their gates 60 connected by lead 62 to a reset voltage source (not shown). The reset circuit is to charge the capacitors 34 to a reference voltage when shielded from light (photons) by the chopper. The amplifier chain circuits 58, described in detail hereinafter, are connected to the drains of FET switches 65. Switches 65 have their gates 63 connected to an X-addressing circuit 64 and their sources connected to output lead 66. Those persons skilled in the art desiring more information concerning the details of the detector structure are referred to United States Patent US-A-4,379,232 issued April 5, 1983 to George S. Hopper, assignor to Texas Instruments Incorporated. Referring now to Figs. 2 and 3a, each of the amplifier chain circuits 58 (Fig. 2) includes a common source metal oxide semiconductor (MOS) amplifier (low noise front stages) 68 (Fig. 3a) connected to a sense lead 54 (Fig. 2). A double correlation sample filter (DCS) 70 is connected to the output of the low noise front stages 68. The DCS filter 70 includes a resistor 72 connected to the junction of capacitors 74 and 76. Capacitor 74 is grounded and capacitor 76 is connected to the junction of a DCS clamp 78 and amplifier 80. Clamp 78 is a FET switch which is turned on to store preamplifier noise on capacitor 74 and off to discharge the capacitor 74 into the load. Amplifier 80 is to restore the filtered signal. Amplifier 80 has its output connected to the junction of positive and negative sample-and-hold (S&H) circuits 82 and 84. The positive S&H circuit 82 includes a FET switch 86 connected to the junction of a capacitor 88 and amplifier 90. The capacitor 88 is grounded and the amplifier 90 has its output connected to capacitor 92. The negative S&H circuit 84 includes a FET switch 94 having its source connected to the junction of capacitor 96 and inverter amplifier 98. Inverter amplifier 98 has a gain of -C s / (C d +C s ) times that of amplifier 90 and has its output connected to capacitor 100. A secondary clamp 102, which is a FET switch, has its source connected to the junction of capacitors 92 and 100 and amplifier 104. The FET switch of the secondary clamp can be turned on to establish a voltage reference across capacitors 92 and 100. The multiplexed output of amplifier 104 is connected to the junction of off chip electronics 107 for removal of the reset noise on output lead 66. The off chip electronics 107 includes a storage means 106 connected by lead 105 to receive the multiplexed outputs of the amplifiers 104 and to the negative terminal of a summing (differencing) amplifier 108. The positive terminal of amplifier 108 is connected to receive the multiplexed outputs of amplifiers 104 of multiplexer 64. The output of amplifier 108 is connected to monolithic delta frame 22 for removal of the fixed offset noise and conversion of the pixed rate. The storage means 106 and differencing amplifier 108 of off chip electronics 107 preferable is a monolithic delta frame 20 which is capable of performing the required reset noise storage and subtraction. The total subtraction amplifier chain is to substantially cancel reset noise that exists in the detector array. Reset noise in a detector array exists owing to the uncertainty in voltage appearing across a capacitor as that capacitor is being charged by a resistive source. The root mean square (RMS) voltage uncertainties due to reset action on detector and sense node capacitances in a detector are given by (KT/C d ) 1/2 , and (KT/C s ) 1/2 , respectively, where K is Boltzman's constant, T is the time, C d is the capacitance of the detector and C s is the capacitance of the sense line 54. These noises are shared together as a detector is read such that the RMS voltage present on the sense node at signal read is [(C d /C d +C s )² KT/C d + (C s /C d +C s )² KT/C s ] 1/2 = (KT/C d +C s ) 1/2 = RMS While the RMS represents average noise the present invention utilizes the individual events of this RMS term; the individual events are determined and substracted by the above-described circuit. The value of these noise events are anticipated for each frame by measuring the values of the (KT/C d ) 1/2 and (KT/Cs) 1/2 events after the capacitances are reset by line switch 56 (Fig. 2) but before signal accumulation is begun, storing these values in the positive and negative S&H circuits 82 and 84 , summing them together in amplifier 104, storing the summed outputs for elements 30 of the first frame (N1) , as in the above equation, and subtracting the elements of the first frame (N1) from the signal read samples for the corresponding element of the next frame in difference amplifier 108. In the parallel amplifier matrix, measuring the amount of voltage error existing on the detector capacitance directly after C d has been completely reset, i.e. separated from the sense node, is impossible as no sensing apparatus (amplifier) exists at that point. Thus, and indirect means of measuring detector excursions before the accumulation of signal voltage is required for parallel amplifier reset noise subtraction. Referring now to Fig. 3b, for a description of the detector matrix timing and resulting reset noise for a parallel amplifier circuit. The detector capacitance is completely reset in two parts. In the first (1) part, the reset switch 56 (Fig. 2) resets C d and C s together and then in the second part (2) the C d is separated from C s by the opening of the address switch 32. Although (KT/C d ) 1/2 volts of RMS noise is left on the detector, this method of reset is required to sense the specific values of these noise events by indirect means. The resulting reset noises are set forth in Table 1. AFTER EVENT NOISE VOLTAGE EXCURSION ON DETECTOR ON SENSE NODE 1 N 1 N 1 2 N1 + N2 N1 - (C d /C s ) N2 3 N1 + N2 + SIG N 3 4 C d /(C d +C s )(N1+N2+SIG)+ C s N3/C d +C s C d /(C d +C s )(N1+N2+SIG)+ C s N3/C d +C s It is necessary that a whole frame time (1/2 chopper cycle time in which all detectors must be sampled and reset) exist between events 2 and 3 during which signal is accumulated on Cd. Also the - C d /C s N2 term present on the sense node after transition event 2 is correlated with the N2 term on the detector due to conservation of charge between C d and C s when the address switch is opened. At this point samples can be made only of the sense line, thus it is clear that this correlation between detector and sense node noises after event 2 is essential in indirectly anticipating the noise voltage present after event 4. After each switching transition the voltage existing on the sense node is sampled such that sample: S1 = N1 S2 = N1 - C d /C s N2 S3 = N3 S4 = C d /(C d +C s )(N1+N2+SIG) + C s /(C d +C s ) N3 The following algorithm is used to null all reset noise terms and leave only a signal term. S4 - C s /(C d +C s )(S3-S2) - S1 = C d /(C d +C s ) SIG As a whole array address cycle or 1/2 chopper cycle exists between the related S1, S2 terms and S3, S4 terms, the noise storage means 106 must be capable of storing a noise sample from every detector in the array (an entire frame of noise line data) for comparison with the next frames. Thus, storage and differencing means such as, for example, a monolithic delta (difference) frame device, is used as the off chip electronics 107. The monolithic delta frames 20 and 22 (Fig.1) are identical in construction, thus only one need be described. The monolithic delta frame (Fig. 4) includes a high speed line address circuit 110. A demultiplexer 112 is connected to the high speed address circuit and to the video output of amplifier 104 (Fig. 3a). A line shift means 114 (Fig.4) is connected to the demultiplexer 112. a plurality (100) of buffer amplifiers 116 are connected to the line shift register 114. A buffer amplifier 116 is connected to each row of elements 118. Each element 118 includes a capacitor 120 and a switch 122. The buffer amplifier 116 is connected to one plate of the capacitor 120 and a FET switch 122 has its drain connected to a second plate of the capacitor, its gate connected by lead 124 to a slow speed column (Y) address register 126 and its source connected by lead 128 to a reset circuit element 130. Again there is one element 130 for each row (100 rows). Each line reset element includes a FET switch 132 having its gate connected to line reset lead 134, its source connected to the junction of lead 128 and amplifier 136 and S&H circuits 138. S&H circuits 138 are connected to a multiplexer 139. A high speed line address register 140 is connected to the multiplexer 139 for controlling its video output signals through video out terminal 142. Monolithic delta frame 22 has it video in terminal connected to the video out terminal 142 of monolithic delta frame 20. The monolithic delta frame 20 performs the noise storage and substraction function; while, monolithic delta frame 22 accomplishes the offset suppression and 2x/1x conversion. In monolithic delta frame 20, the use of the high impedance buffer 136 on each line 128 lowers the line parasitics and the additional capacitance created by the line buffers is substantially reduced by the source follower input stage. Becausae of this parasitic reduction, the required capacitance of the storage capacitors 120 is low so that they may be built into the metal oxide semiconductor (MOS) IC using standard processing; hence, the name Monolithic or Integrated Delta Frame. High-speed timing requirements are made worse, however, by the addition of the buffers; therefore, the single-line video input is demultiplexed line by line by demultiplexer 112 and then shifted by line shift means 114 into the parallel-input delta frame matrix. The circuit element 130 is a high-gain inverting output amplifier chain with dc feedback to suppress amplifier fixed-pattern noise offsets in a dc-coupled multiplexer 139. It will be apreciated by those persons skilled in the art that the 2x to 1x speed conversion rate can be made at either monolithic delta frame 20 or 22 by including S&H circuits 138 having common gate connections. Referring now to Fig.s 5a-5c the detector 16 output is clocked into and out of the monolithic delta frame 20 and into the monolithic delta frame 22 at a 2X rate (Fig. 5b). The output of monolithic delta frame 22 (Fig.5c) is clocked out at a 1X rate to the display 26. As used in Fig.s 5a and 5c the term "column shift" refers to the shift command given to the low-speed column address shift register 126 Fig. 4). The arguments X and Y (Fig.s 5b & 5c) refer to the frame and column numbers, respectively and the S to the signal. It should be noted also that because of the line shift feature of the monolithic delta frames there is a 2X line delay through each of the frames, i.e. The effect of a signal line input is not present at the outputs until one 2X line time later. As the storage action of the monolithic delta frame takes place when the line reset command is given, the timing diagram for monolithic delta frame 20 (Fig. 5b) indicates that the storage command for monolithic delta frame 20 takes place only after a noise line has been shifted into the matrix by the line shift pulse. Thus fullfilling the noise storage requirement when an S + N line is shifted into the capacitor matrix thereof. As the noise line data from the previous frame is still present in the form of voltage on storage capacitors, only the S portion of the S + N voltage data feeds through the capacitors into the sense lines. The amplifiers on each sense line passes its signal voltage on to be read out by the output multiplexer. After the noise subtraction operaton, there is no longer an S + N and N line for each column of detectors, but only a signal line. Nevertheless, this signal line is still clockilng at the 2x pixel rate. To minimize display blank time the line data rate is converted to a 1x rate by the second monlithic delta frame which also subtracts fixed pattern noise. Thus this monolithic frame detector is an improvement over the "Delta Frame Circuit" of U.S. Patent US-A-4,178,612, issued Dec. 11, 1979. The Transversal filter 24 (Fig. 6) of the second embodiment is connected to the second monolithic delta frame circuit 22 (Fig. 1) for peak signal detection to improve display resolution. It includes, for example, a 6-stage, 1 bit free-running digital shift register 150 (Fig. 6) connected to a corresponding number of sets (6) of sample-and-hold (S&H) circuits 152. The sets of S&H circuits are connected to the output of the second monolithic delta frame 22 (Fig. 1) for receiving the video inputs and to acorresponding number of sets of line driver amplifiers 154 (Fig. 6) to a corresponding number of sets of output switches 156. Each set of S&H circuits 152 contains five S&H circuits connected to five consecutive bits of the shift register 150. Each set of line drivers 154 includes five amplifiers connected to the five S&H circuits of each corresponding set thereof. Whilst, each set of output switches 156 has five transistor switches connected to the five amplifiers of each corresponding set thereof. The sixth bit of the shift register 150 is selectively connected to each set of output switches 156 for sequentially operating each set of output switches. Five voltage weighting circuits 158 are connected to the five output switches of each set of output switches, and the outputs of the weighting circuits are connected to a summer 160. The output of the summer 160 is connected to the display 26 (Fig. 1). The transversal filter 24 has its sample-and-hold circuits connected to the single line video output of monolithic delta frame 22 and performs a peaking function thereon. The six stage, 1 bit shift register 150 addresses sequentially the six sets of sample-and-hold circuits 152 to store each pixel of information in five places simultaneously (See Table 2). The sixth bit is used to read out a set of sample-and-hold circuits when the correct sequence of pixels is stored in that set. The five pixels of the set read out are voltage weighted by the weighting circuits 158 and applied to the summer 160. Pixel tracing is shown in Table 3. S REG. OUPUT STAGE #1 S&H NODE STAGE #2 S&H NODE STAGE #3 S&H NODE STAGE #4 S&H NODE STAGE #5 S&H NODE STAGE #6 S&H NODE 1 1 SW OUT 5 4 3 2 2 2 1 SW OUT 5 4 3 3 3 2 1 SW OUT 5 4 4 4 3 2 1 SW OUT 5 5 5 4 3 2 1 SW OUT 6 SW OUT 5 4 3 2 1 PIXEL STAGE #1 S&H NODE STAGE #2 S&H NODE STAGE #3 S&H NODE STAGE #4 S&H NODE STAGE #5 S&H NODE STAGE #6 S&H NODE 1 1 SW OUT 5 4 3 2 2 2 1 SW OUT 5 4 3 3 3 2 1 SW OUT 5 4 4 4 3 2 1 SW OUT 5 5 5 4 3 2 1 SW OUT 5 5 4 3 2 1 SW OUT 6 SW OUT 5 4 3 2 1 7 1 SW OUT 5 4 3 2 8 2 1 SW OUT 5 4 3 9 3 2 1 SW OUT 5 4 10 4 3 2 1 SW OUT 5 11 5 4 3 2 1 SW OUT 12 SW OUT 5 4 3 2 1 13 1 SW OUT 5 4 3 2 14 2 1 SW OUT 5 4 3 15 3 2 1 SW OUT 5 4 16 4 3 2 1 SW OUT 5 17 5 4 3 2 1 SW OUT In operation of the transversal filter (Fig. 6 & Table 3), pixel 1 is stored simultaneously in S&H circuits 152, stages 1,3,4,5 and 6 at pin locations 1,5,4,3, and 2, respectively; the switch out bit is connected to stage 2, pin 6. The remaining pixels 2-5 for one frame together with the switch out bit are sequentially stored at succeeding pin locations of the stages for the first cycle. The number of stages determines the length of the filter. With the sixth bit of shift register, switch out of stage 1 occurs, pixels 1-5 are weighted in weighting circuits 158 and summed by summer 160 for providing peak voltage for pixel 1, while, pixel 6 is stored in positions of 5-1 of stages 2-6, respectively. Next, with each new bit, stages 2-6 are read out for weighting pixels 2-6 to complete the cycle. Next, the cycle is repeated to input pixels 7-12 and to determine their peak value and thereafter the cycle repeated until completion. Assuming Beta 0 = 1.63 Beta 1 = -0.25 and Beta 2 = 0.0625 and further assuming zero values for pixels 1-5, a one for pixels 6-8 and a zero for pixels 9-13 the outputs of summer 160 are: 0,0,-0.25,+1.6,-0.25,+0,-0.19,0,0,-.25,+1.4, and +1.4. Without the filter the corresponding values are zeros for pixels 1-5, ones for pixels 6-8 and zeros for pixels 9-12. Thus, a comparison of the peak values with the digital values readily shows the improvement in resolution for the image displayed.

Claims: A delta frame circuit for receiving a video input line having a video component and an unwanted noise component thereon comprising a high speed address circuit (110), a demultiplexer (112) operatively connected to said high speed address circuit (110) and said video input, a low speed address circuit (126), a plurality of difference frame elements (118) arranged in rows and columns operatively connected by rows to said demultiplexer (112) and by columns to said low speed address circuit (126), a multiplexer (139) for producing a video signal at a video output (142), characterized by further including a line shift means (114) operatively connected between said demultiplexer (112) and said plurality of difference frame elements (118), said demultiplexer (112) storing in parallel, samples of the video signal and noise components and said line shift means (114) simultaneously shifting by line the demultiplexed signals into said difference frame elements (118), a reset means (130) for suppressing amplifier fixed-pattern noise offsets including a line buffer amplifer (136) for lowering parasitics operatively connected to said difference frame elements (118), and a line address register (140) operatively connected to said multiplexer (139) for controlling the output of the video component, the circuit elements of said delta frame circuit being integrated in a monolithic circuit. A delta frame circuit according to claim 1, characterized in that each line in the demultiplexer (112) stores a pixel of video or noise information and after all lines are sampled and stored in the demultiplexer (112), said line shift means (114) simultaneously shifts the information to said difference frame elements (118). A delta frame circuit according to claim 1 wherein the reset means (130) comprises a solid state switch (132) and an amplifier (136), said switch (132) coupled to a line reset lead (134) and positioned within a feedback loop around said amplifier (136). A delta frame circuit according to claim 1 wherein said low speed column address (126) has a clocking rate which is lower than the high speed line address rate. A delta frame circuit according to claim 1 further including a sample-and-hold circuit (138) operatively coupled to said reset means (130) to modify the speed conversion rate through the circuit.

Inventors: DUDLEY, DANA
Applicants: TEXAS INSTRUMENTS INCORPORATED
Application year: 1985
Main classification: H04N5
IPC codes: H04N5