Patent GB-875694-A (GB)
Title: Improvements in or relating to magnetic reproducing apparatus
IPC: G11B5/00
Filed: 1958-07-11
Abstract: 875,694. Digital-data storage apparatus. INTERNATIONAL BUSINESS MACHINES CORPORATION. July 11, 1958 [July 15, 1959], No. 22290/58. Class 106 (1). Apparatus for reproducing magnetic data recordings includes a single transducer arranged to sense data-bearing recordings on a magnetic storage medium and to supply two signals corresponding to said data to a duality of sensing units, said signals being of different amplitudes, and each of said sensing units having different discriminating characteristics and comparing means connected to receive and compare the sensed signals from the sensing units and to initiate an error signal whenever said signals are not identical. The magnetic tape digital data recording system described is designed to reject spurious noise pulses but to accept low amplitude " dropout " pulses (a dropout being produced either when a properly recorded signal is read with the tape not in intimate contact with the head, for example because of dirt, or when a badly recorded signal is read). This is effected by passing the signals produced by each reading head through two amplifying channels of different gain, integrating the signals so produced in each channel, using the integrated outputs to control two voltage discrimination triggers, and comparing the output signals produced by the triggers, and is based on the observed fact that in such an arrangement the low gain channel (referred to as the " lo channel ") will tend to reject noise (and also dropouts) whereas the high gain channel ("hi channel ") will tend to reproduce dropouts (and also noise). In the actual system described, Fig. 3, this process is combined with conventional horizontal and vertical parity checking to produce a substantially error-free read out of a tape and is also used to check a tape as it is being recorded by immediately reading each character before the next is recorded. Reading amplifier.-A two-gap write/read head, Fig. 1B, is employed which permits the reading of characters very soon after they are written. Read signals which always represent 1 bits and can be either positive or negative going, pass via a pre-amplifier PA, Fig. 4A, having overall negative feed-back from the third stage to the first stage, an inverter I, a phase splitter PS to a mixer M so arranged that each 1 bit pulse, whether it be positive or negative, causes a rise in its cathode current. From the mixer two outputs, on lines D1 and D3 are taken, each of which passes via an AND gate A, Fig. 4B, an R.C. integrating circuit to a Schmitt trigger which only goes "on" for those periods when its input voltage exceeds a certain threshold level. The outputs from these triggers constitute the lo and hi outputs and pass to lo and hi registers respectively. Fig. 5A shows in solid lines the signals produced by the mixer M in respect of a 1-bit and also in respect of a small amount of noise, together, in dotted lines, with the respective integrated signals applied to the triggers. Fig. 5B shows how a moderate amount of noise is rejected by the lo channel but passed by the hi channel. An excessive amount of noise is passed by both channels. Figs. 6A and 6B show how very small dropouts are rejected by both channels whereas a moderate dropout, while being rejected by the lo channel is passed by the hi channel. As dropouts have a relatively long rise time their integrated outputs follow them. Read/write checking system.-The system can function either to check the recording of a tape as it is being recorded or to read a tape, selecting the more correct of the hi and lo channels for onward transmission to a computer for example. On the tape data is recorded serially by character and each character is recorded by seven " vertical " parallel bits (including a parity check bit) arranged across the tape, one in each of seven "horizontal" tracks. A " record " consists of a variable number of characters and ends with a check character which is such as to make the number of 1-bits in each track even. Thus a "horizontal" parity check in each track can be made for each record. The system also detects errors due to excessive " skew " of the tape, that is if the tape is so skew that some bits are read after the normal sampling time (which allows for a certain acceptable amount of skew) then a skew error indication is produced. Recording.-Immediately after each character is recorded on the tape it is read (the seven bits in parallel) and transmitted by lo and hi sensing units, Fig. 3, to lo and hi registers respectively, the triggers in the registers actually being set on by the going off of the Schmitt triggers in the lo and hi channels. The lo register applies true and inverse signals to a vertical redundancy check (VRC) unit which provides a VRC output when the parity is not even. Soon after this, at the end of the tolerance period allowed for acceptable skew, a skew sample pulse is applied to a VRC gate which tests the output of the vertical redundancy check unit and produces an output signal, which is passed to an error indicator, if an odd parity is detected. The trailing edge of the skew sample pulse is used to reset the lo registers and simultaneously transfer its contents to a horizontal redundancy check (HRC) register and to a character match register. The skew sample pulse is followed by an intermediate sample pulse which transfers the contents of the hi register via a hi register write gate to the character match register in such a way that if the data already stored therein is the same as the data being transferred thereto (as it should be if no errors have arisen) then the character match register is restored to its reset condition. If it is not restored to its reset condition then it produces a signal which is applied to a character O gate. After the intermediate sample pulse a later sample pulse is applied to the VRC gate to retest the output of the VRC unit. This output should be zero because no further bits should have been read into the lo register since the skew sample pulse. It a bit has been entered into the lo register since the skew sample pulse; because of excessive skew, this will now be detected. After the late sample pulse a match sample pulse is applied to the character O gate to test the output of the character match register. Finally, at the end of the record, a HRCH pulse tests the outputs of a horizontal redundancy check register and if parity error does exist causes a neon lamp to be lit to indicate in which track the error lies. Reading.-Each character is read and entered in the hi and lo registers as before. If no vertical redundancy error is detected (in respect of the lo register) then the lo and hi registers are reset simultaneously, the contents of the lo register being transferred to both the HRC register and the character match register from where it passes via a character recognition gate to a computer for example. If a vertical redundancy is detected in respect of the lo register then the contents of the hi register are transferred to both the HRC register and the character match register and thence to the computer, the contents of the lo register being disregarded. In these circumstances an indication that the hi output has been transferred to the computer as if this occurs frequently it could be indicative of faulty equipment. Testing.-The control unit can be set to perform various tests such as parity checking the hi register.