Patent GB-1417735-A (GB)
Title: Bivalent signalling processing methods and circuit arrangements
IPC: H03K5/08
Filed: 1973-02-14
Abstract: 1417735 Pulse thresholding circuits NIXDORF COMPUTER AG 14 Feb 1973 [22 Feb 1972] 7277/73 Heading H3T [Also in Division G4] A pulse thresholder derives the threshold level for each transition from the mean of the present peak value and the stored value of the previous peak, the stored value being decremented stepwise before determination of the mean by an amount dependent upon the amplitude. The invention is described as applied to detecting marks on a tag by passing a light pen thereover, the signal so produced having the form shown in Fig. 2 in which the background level varies along the dotted line. The signal could however be derived from a magnetically or electrostatically coded device.  In Fig. 5, the signal from amplifier 11 is passed directly to one input of a computer 13 and also to maximum and minimum level stores 17, 18, the mean level being derived at 21 and passed to the other input of the comparator as a threshold level. The resulting two level data signals are applied to output terminals 25. Fig. 3 shows a portion E11 of the input signal, the stored positive and negative peak levels E17, E18 and the mean level E21. A control circuit 24 detects an output transition and applies it to adecrementing circuit 19 which causes the peak levels momentarily to move towards each other during the transition by reducing the time constant of the circuit. This brings the peak level down when the peak level of the input wave is decreasing.  Stabilization of the circuit during the rest period is effected as a result of a bi-stable circuit 23 being the set at the end of the previous signal group. The bi-stable circuit operates contacts 15 and 22 so as to earth the reference input of the comparator and provide a feed-back path through 15. The latter causes the circuit to oscillate by a small amount about a level such that the two inputs of the comparator are approximately equal and this level is stored at 16 to provide a base level for the subsequent pulse thresholding cycle.  The period t p  (Fig. 2) represents the result of a scan before reaching the data marks. The initial rise is detected in the maximum store and is used to reset the bi-stable circuit so that the switches 15 and 22 are changed over, also to reset the decrementer to a long time constant as well-as to cause an attenuator 20 to reduce to half value the output of the minimum peak store 18 applied to the mean level evaluator. Accordingly the maximum peak detector 17 slowly charges to the peak level during t p  and the minimum peak level supplied to the evaluator slowly moves towards half the peak values. The transition at the beginning of t r  switches 19 to a short time constant and removes the attenuator 20 so that signal threshold holding occurs as already described. A circuit 27, 28 acts on a feed back transistor 29 to limit the peak value of the input signal. The end of a signal sequence may be identified by logic circuits not shown connected to output terminal 25 or by a recorded indicator bit and this may be used to restore the bi-stable circuit so that switches 19 and 22 are reset.