Patent GB-1287220-A (GB)
Title: Improvements in magnetic recording and reproducing systems
Inventor: IRWIN JOHN WILLIAM
IPC: G11B20/14
Filed: 1970-12-17
Abstract: 1287220 Magnetic data stores INTERNATIONAL BUSINESS MACHINES CORP 17 Dec 1970 [29 Dec 1969] 59903/70 Heading G4C In a parallel-bit magnetic recording system groups of data signals alternate with groups of synchronizing signals along each track. In the event of dead-tracking (i.e. if any read head is not providing error-free data, e.g. due to tape lift-off) the sync. group subsequently read from the dead track is used to resynchronize the signals read from all the tracks. As described each block of signals, Fig. 1, comprises a preamble 10 and postamble 24 bounding alternate data groups 12, 16, 22a and sync. groups 14, 20. Each data group is preceded by start marker bits 0.1 (11, 15, 21) and followed by end marker bits 1.0(13, 17,23). The number of bits between the beginning of successive groups is constant. As the final data group 22a may not contain as many bits as do earlier groups padding bits P are added to make the number of bits in the group marked 22 divisible by the capacity (e.g. 16) of a counter associated with the read/write circuitry. The bits are recorded in self-clocking form, a byte being recorded in parallel tracks and including a parity bit. The tape may be read in either direction of motion. Write mode, Figs. 3A, 3B (not shown).-When a data byte is present in the input/output register the tape motion is started. A predetermined number (e.g. 28) of preamble sync. bits 10 are recorded, followed by a first marker 11. The byte is then recorded and a byte counter, set to the number of bytes in a data group (e.g. 1024), is decremented by one. Further bytes are recorded until the data group is complete. The byte counter now being at zero a sync. group preceded by end marker bits is recorded under the control of a sync. bit counter. If now there are further bytes available the byte counter is reset (to 1024) and the recording of a further data group occurs. Should there be no further bytes available when the byte counter has not reached zero, padding bits are added, preceded by end marker bits. Postamble sync. bits are now recorded under the control of the sync. bit counter. Read mode, Fig. 4, 5 (not shown).-After reading the preamble bits 10 and the marker bits 11, data bytes are transferred to the output register via deskewing circuitry and error detection/correction circuitry. As each byte is transferred the count in the byte counter is stepped. When the counter indicates that the complete byte has been read, or when an end marker is detected prior to this (e.g. marker 23 following short data group 22a) a sync. cycle takes place as described below. If the tape is moving backwards it is necessary to ignore the padding signals P. This is done by sensing the end marker 23 and by altering the byte counter by the number of padding bits read. Data group 22a is then read and the byte counter adjusted at each byte transfer as described above. If the tape is moving forward and the check bit which immediately follows each data group has been transferred to the output register the byte counter should be at a preset count. If it is not reading is stopped, but if it is at this count, or if the tape is moving backwards, reading continues until the counter reaches a second preset count at which each of the read head should be reading a sync. group. At this point a check is made to determine whether any track which had been dead is by now resynchronized, and then the dead track is requeued in the skew buffers by placing the dead track at the maximum leading position relative to the most lagging active track. Provided that at least a preset number of tracks are giving satisfactory signals a test is made to determine that the maximum skew in the tracks does not exceed the correction capability of the circuitry. If a byte has been assembled in the skew circuitry successful readout of the dead track has been achieved and reading of the next data group proceeds as before.