Patent GB-896855-A (GB)
Title: Improvements in spurious signal suppression in automatic symbol reader
IPC: G06V30/192
Filed: 1959-12-23
Abstract: 896,855. Automatic character reading. GENERAL ELECTRIC CO. Dec. 23, 1959 [Dec. 29, 1958], No. 43683/59. Addition to 796,579. Class 106 (1). In the parent Specification characters are sensed magnetically to produce characteristic waveform signals which are passed into a tapped delay line having tappings connected to each of a number of character recognizing correlation networks. The network giving the highest response identifies the character. According to this invention, an additional network is provided to recognize signal waveforms derived from spurious magnetic matter on the carrier. A magnetic speck makes a signal waveform which, after elimination of high frequencies, is as shown in Fig. 2, a positive excursion being followed by an equal negative excursion. Such a signal constitutes an " improper " waveform, i.e. none of the characters for which the apparatus is designed would produce such a signal and the apparatus of the parent Specification in the absence of a clear threshold level is likely to respond by giving an output on two or more output leads thereby producing an error signal and causing the whole record to be rejected. It is inconvenient to reject records merely because they have spurious specks and the additional network which is provided to recognize the speck waveform and provide an appropriate threshold level. No error signal is therefore produced and the record is not rejected. The speck recognizing network consists merely of potential dividers 127, 128, Fig. 3, connected between earth and tappings G and H of delay line 103. At the instant the reading gates open the speck waveform is registered with the G and H tappings as shown in Fig. 2, so that a positive signal from divider 128 passes to the grid of triode 131 and a negative signal from divider 127 to the grid of triode 132. The triodes have a common cathode load 133 and an amplified signal is produced on terminal 135. This is used to set the threshold level of the comparators, one for each character recognition network, which determine which lead has the highest output. The presence of a speck waveform could be used to give a signal on a corresponding output lead but there is no point in providing one, the purpose of the recognition network being only to provide a threshold level such that none of the character recognition circuits respond. Timing signals.-The sampling gates at the outputs of the comparators open at the instant a character or speck waveform is correctly located in the delay line. The sampling pulse is derived from the dividers 151-153 connected to the first three tappings A, B, C, of the delay line. As the leading edge of the waveform enters the delay line a signal appears on tapping A. After a small interval this signal is balanced by the combined signals on tappings B and C and the circuit 158 detects the balance to give a timing pulse. This is delayed by an amount which causes the sampling gates to open when the waveform is correctly located in the delay line. Sensing required area of record.-To avoid sensing areas of the record outside that where the magnetic characters are to be expected, a photo-electric detector detects the leading edge of the record. The derived signal is delayed in a monostable multivibrator and used to set a second monostable multivibrator at approximately the instant the data areas can be expected to appear under the reading head. The " set " output is used to enable the recognition circuits, the multivibrator resetting after a predetermined time equal to the time taken for the desired data to pass under the reading head, to again disable the recognition circuits. This arrangement also disables the apparatus when no record is present. Specification 896,854 also is referred to.