1,054,135. Digital data storage apparatus. E. BREUNINGER KOMM. GES. AUF AKTIEN. March 20, 1964 [April 3, 1963; June 20, 1963; Aug. 17, 1963], No. 11961/64. Heading G4C. Information to be recorded on a magnetic medium is checked and only correct information is transmitted to the writing head, a further check being made on the magnetic field produced by the writing head in response to the signals transmitted. Means for correcting single errors due to drop-out may be provided. In a further arrangement, information set up on a keyboard is recorded and the recorded information checked by being read back and compared with the information set up in the keyboard, a fresh recording being made if an error is detected. Checking data to be recorded.-A data item to be recorded is supplied from an input device SE, Fig. 1 (not shown), to a code checking circuit CK1, and thence if the item is an allowable coded representation, to a writing device G connected to the writing head SK, the magnetic field of which is monitored by a second check circuit CK2. If an allowable code signal is sensed, the input device SE is activated to supply the next input data item. In a modification (Fig. 2, not shown) the same check circuit is used to check both the input supplied by the device SE and the field at the writing head SK. Decimal data is supplied in a 2-out-of-5 code, Fig. 3 (not shown), but is actually recorded by applying two different recording frequencies f1, f2 to two out of four recording tracks Sp1- Sp4. As shown in detail in Fig. 4 (not shown), a multi-decimal-digit number set up on keys T1-Tn is recorded digit-by-digit by a head SK under the control of a stepping switch drive W. A digit to be recorded energizes two of five relays I-V, and a relay Z which closes a contact to supply potential to a tree of contacts (top left of Fig. 4) arranged so that only if two relays I-V are energized is a circuit closed to a terminal Za thereby enabling generator G1, G2 for the two recording frequencies f1, f2. In all other cases, a circuit is closed to a terminal Fa to indicate an error. The generators G1, G2 are connected via an array of contacts controlled by the relays I-V so that the digit is recorded on four tracks Sp1-Sp4 as indicated by the code at the right of Fig. 3. Second windings in the writing head are connected to a detector Ton E, bottom right of Fig. 4, which if both recording frequencies are present, causes relays K1, K2 to be energized thereby closing a circuit to the stepping drive W to cause the next digit to be recorded. Single error correction.-In one form of this arrangement each digit is recorded in a 2-outof-5 code in five tracks Sp1-Sp5, so that as shown in Fig. 6 (not shown), each digit is represented by two " information " markings M1 and three " filler " markings M2, the markings M1, M2 being different audio frequencies. The data on the recording tape is sensed by recoding heads (Fig. 7, not shown) each connected to a pair of relays E1-E5, F1-F5 responsive to the reactive markings M1, M2, relays E1-E5 controlling contacts sets e1-e5, Fig. 8 (not shown), so that if an allowable code representation is sensed, just two of these sets e1-e5 will be operated, thereby energizing a relay E. A relay F is energized if only one set of contacts e1-e5 is operated, indicating that only one " information " marking M1 has been sensed. By means of the circuit shown in Fig. 9 (not shown), a single error due to drop-out can be corrected, since if only one " information " marking M1 is sensed, the relay F, Fig. 8, is energized thereby closing contact f, Fig. 9, and only the relays F1-F5 which are not energized either due to the drop-out or due to marking M1 being present will retain their contacts f1-f5 closed to supply the required two markings on the five output lines A1-A5. In a modification, the magnetic surface is provided initially with marks of a third frequency, overwritten by the first and second frequency markings, M1, M2 for data recording the data may be recorded by marking two out of four tracks (Fig. 10, not shown). Checking keyboard entries by playback.-A 5-decimal digit number to be recorded is set up in a 5-column keyboard (Fig. 11, not shown) in key sets TS1-TS5, Fig. 12, and assuming that a circuit K detects a complete 5-digit setting, on depression of a start key, a signal from a key TSt actuates a circuit St to initiate recording of the set-up number under the control of a stepping circuit KS, followed by playback of the recorded data during a second cycle of the stepping circuit KS, a comparator VS detecting any equality between the set and the recorded data. Decimal digits are stored in a 2-out-of-5 code on five tape tracks (Fig. 13, not shown). As shown in detail in Fig. 14 (not shown) and Fig. 15 (not shown), when a complete 5-digit entry has been set up, a relay K is energized so that on depression of the key TSt, the tape drive motor M is started and relays A, B, are energized to initiate stepping of the stepping circuit comprising stepping relays KS and contacts ks1-ks5, thereby causing the setup data to be recorded digit-by-digit by the head SK. When the recording is complete A relay U is energized to enable a read head LK to read out the stored data, each stored digit actuating two of five relays E1-E5, the digit thus read out being compared with the corresponding digit set up in the keyboard TS1-TS5 by relays F1-F5, the stepping switch control KS being again cycled during this reading operation. If the data has been correctly recorded, a relay V is energized to allow new data to be entered in the keyboard. If, however, any discrepancy between the set data and the recorded data is detected by relays F1-F5 an alarm relay A5 is energized thereby causing an error-indicating signal to be recorded via contacts as2, ks6 and energizing a relay W to initiate re-recording of the set data. If this second recording is also found to be incorrect, a main alarm HA is operated and the tape drive is stopped. In a modification (Figs. 16, 17, not shown), the data to be written on the tape is checked before being applied to the write head, and the data read back from the tape is checked by detecting that both of the two frequencies required for recording are in fact present, and operating a counter each time both frequencies are detected so that it can be determined whether the correct number of correct individual digits has been recorded.