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The grid potential of the triode 3-2 remains substan tially constant during modulation hence the limiting effect upon the high vfrequency response of the large circuit ca pacitance between the photocathodes and ground and pho tocathodes and the photoanodes is substantially eliminated. Due to the wiring ‘and paralleling of many photocathodes to the ‘common grid 34, such capacitance can be as high as 100 micromicrofarads. It can be shown that when the tube 32 is a pentode with an open loop gain of 150 and when the highest fre quency component‘ of the photo-signal modulation is 3 kilocycles, when the modulation due to a black spot of 0.001 square inch area is 6.6-10-10 amperes, the resistor 29 can be 6.8 megohms, so that the peak signal at the anode 33 is approximately 5 millivolts. Such minimum signal shown at 36 in FIG. 4 is ampli tied to a 50 volt level by a conventional ampli?er 37 in order to operate the trigger generator 39. In order to raise the 5 mi-lli-volts minimum input signal to the re quired 50 volt level, ampli?er 37 has a voltage gain of 10,000 and is conveniently an R.C. coupled A.C. ampli ?er wherein the gain is constant and is stabilized against tube changes and ageing by limiting the stage gain of each stage of a total of vfour stages to ten. This is conveniently arranged by the use of anode follower type teed-back net works ‘for each stage. The output‘signal of the ampli?er 38 shown in FIG. 4 initiates the output pulse 43 of the pulse generator 39 for which various conventional pulse producing circuit arrangements may be employed. For example, a biased off grid controlled gas-?lled triode, known in the art as a thyratron can be switched into conduction by sig ml 38 which has .a positive polarity or similarly, a Schmit-t trigger circuit may be employed. The cu-t-o? bias applied to the control grids of the said devices is such that the paper noise level is safely below the trigger voltage level so that only the defect signal will initiate the output pulse which, in turn, actu ates the grading switch. When the defect to be detected is a 3/32 x 1/32 inch square black spot or is a weaker blemish extending at right angles to the direction of travel of the paper, an amplitude sen sitive direct input channel between ampli?er 37 and pulse genera-tor 39 is e?ective. For example, defects which are l/gg inch rwide and have the following intensities and dimen sions at right angles to the paper motion will produce identical output signals. Size of defects in inches Intensity of defect In the At right expressed in direction of angles to fractions of paper paper full black motion motion 3432 is: 1 %2 Ms % $62 1/2 is %2 it Me 362 1 3412 it: 10 $620 It is pointed out however that when a weak blemish ex tends in the direction of travel of the paper and is narrow at right angles to the paper travel, integrating links are used between ampli?er 37 and pulse generator 39, one such link being shown at 41 in FIG. 4._ Section 45 of the signal 4!? shown in FIG. 4, is such an ampli?ed defect signal. Amplitude 4.6 of the unidirectional defect signal is small er than the peak amplitude '47 of the paper background noise signal. in spite of this, the level of the defect signal 49 is raised above the voltage level at which the pulse ‘gen erator 39 is triggered by an integrator network or stage 41. The simplest form of this network consists of a capacitor Ci and resistor R1‘. When the time constant Ci-Ri is of the order of the duration Ti of the defect signal, the triggering voltage level is safely reached within the time interval Ti as shown by the signal 49.
The statistical ‘average of the noise signal 44 consists of equal positive and negative rvoltage excursions so that its integrated mean level 48 is constant and is below the triggering voltage level. When the extent of the taint defect in the direction of the paper travel varies, the re quired integrating time constant Ti is changed in pro portion. Advantageously, an amplifying integrating stage is used as shown in FIG. 5. The output stage 50 of the ampli?er 37, as shown in FIG. 4, is connected by coupling capaci~ tor 51 and resistor 52 to the control grid of the integrator ampli?er pentode 65*. The cathode bias resistor 55, grid leak resistor 54, screen feed resistor 57 and plate load resistor 59 all have such values that the gain of this pen tode stage is several hundred when the feed-back resistor 53 is disconnected. By connecting the plate to the grid by means of the coupling capacitor 56 and feedback resis tor 53, the gain of the pentode stage is set at approxi mately 20. At the same time, the linear ampli?cation of any one frequency component is improved. The in tegrating time constant is de?ned by the plate load resistor 59 and capacitor 61. The integrator capacitor 61 has such value that it builds up the unidirectional defect signal amplitude 4.6 while the noise signal consisting of peaks of alternating polarities and having a higher fre quency content, is not integrated and is attenuated. The inputs of several such integrating ampli?ers with various time constants may be connected to the plate of tube 59 and the outputs connected by adder resistors and an adder ampli?er to the control grid of the pulse generator. The output pulse produced by a defect located at posi tion D, see FIG. 2, is delayed until the leading edge of the continuous sheet material 2 has approached position F where the de?ector edge 17 of grading switch 16 is lo cated. Meanwhile, the sheet 2 has been cut by blades 13 and 14 at the position indicated by the letter E. The aforementioned delay consists of a constant part and a variable part. The constant part of the delay is de?ned by the time required for the defect in sheet 2 to move from the position D to the position E. For example, when the velocity of travel of the paper is 200 inches per sec ond and the distance from D to E is twenty-?ve inches, such constant part of the delay is 25/200 which equals 0.125 part of a second. The variable part of the delay is de?ned by 3 parameters, namely the angular position of the rotating cutter member 14 at the instant the defect arrives at position E, the distance from E to F and the speed at which the cut sheet is carried by the tape 15. In what follows, it will be seen that a thyratron 64, see FIG. 6, associated with microswitches 62, 74 and 75 to gether with relays 68 and 7 0 and grading switch actuating solenoid 72 will satisfy the conditions set by the said parameters for reliable sorting. ‘Initially, mioroswitches 74 and 75 are closed, micro switch 62. is open, thyratron 64 is biased to cut-oif by having its control grid connected by resistor 66 to adjust able potential pider 67 which is connected between ground and a negative supply rail while coils 68, 7G and 72 are not energized and contacts 69, 71 and 73 are open. Thyratron 64 is ?red by a positive pulse delayed relative to the instant of the leading edge of pulse 43 by the constant pant of the delay. See FIG. 4. By using a phantastron delay generator which is well 10 15 25 30 40 50 55 60 65 70 75 known in the art, pulse 43 starts the plate voltage run down of a normally cut~oif phantastron which ‘bottoms at the end ‘of the constant ‘delay and then returns to its initial cut-off condition. The rdiiferentiated screen volt age jump- at this instant is the positive pulse 76 which ?res thyratlron 64. See FIG. 6. The variable second part of the delay is introduced ‘by synchronizing the instant of operation of the solenoid of the grading switch 16 with the rotating cutting blade 14. When the cutting blades 13 and 14 meet, microswitch 62 closes and the increased current through relay coil 68 closes contacts 69 which energizes relay coil 70 which, by closing contacts 71, connects solenoid 72 which ac tuates grading switch 16. Contacts 73 close simultane ously with contacts 71 holding coil 70 energized until microswitch 74 momentarily opens. The momentary opening of microswitch 74 is synchronized with the rotat ing cutter blade, thus, for example by placing microswitch 74 diagonally opposite microswitch 62, it initiates the cycle of operation. By this means then, the grading switch 16 is retained in the desired position for the pas sage of half a length of the cut sheet in the desired posi tion. The edge 17 of the grading switch 16 is so shaped and its movement is such that it does not interfere with the passage of the cut sheets once they are de?ected to ward either the second grade compartment 19 or the ?rst grade compartment 26.