Patent GB-1439551-A (GB)
Title: Data recording apparatus
IPC: G11B5/596
Filed: 1974-03-15
Abstract: 1439551 Position control in disc recording and playback; servopositioning BURROUGHS CORP 15 March 1974 11610/74 Headings G3N and G3R [Also in Division G5] Data recording apparatus has equally spaced tracks on an upper and a lower face of a disc 2, a transducer head assembly 8 with an upper head 12 for the upper face and a lower head 14 for the lower face, head 14 being offset radially relative to the disc centre from head 12 by onehalf the distance between the centre of immediately adjacent tracks on the upper or lower face of the disc, and means for utilizing signals picked up by one of said heads from the corresponding disc face to position heads 12, 14 so that said one head is exactly centred between two adjacent tracks on said corresponding disc face, whereby the other head is exactly centred on a track on the other face of the disc for selectively recording, reading or erasing data on such track. For recording digital data, the tracks on each disc face are divided into groups of 16 successive tracks each, recording in the first, second, third .... sixteenth track of each group being at bit rates or clock frequencies of about 3MH z  respectively + 0.6%, -2.0%, + 0.2%, -2.4%, -0.2%, -2.8%, -0.6%, -3.2%, -1.0%, + 1.6%, -0.6%, + 2.0%, -0.2%, + 2.4%, + 0.2% and +2.8% of their nominal frequency of 3MH z , the clock frequency difference between adjacent tracks optionally being about 2.8% of 3MH z  and the maximum difference between any track clock frequency and 3MH z  being about 3.15% of 3MH z . The thirteen clock frequencies thus required are provided by a circuit 40, the data to be recorded passing thereto from a central processor 18 via a buffer store 32, processor 18 giving a track address signal into store 36 having an output 42 controlling circuit 40 to give the clock frequency of the selected track. Data at this clock frequency passes to one or the other head 12, 14 via a head changeover switch 44 and a read/write amplifier 46 or 48. Positional information signals from one head pass to network 56 and data replayed by the other head passes to a data channel 52. Network 56 includes filters for passing signals at each of the thirteen clock frequencies, a signal on line 58 activating the pair of filters corresponding to the clock frequencies of the two adjacent tracks which would be scanned by that head 12 or 14 currently acting as the position sensor when the other head is centred on the chosen track, the filtered signals from the position sensing head passing via respective lines 60, 62 to a comparator 64 giving an output of zero if the record/ playback head is exactly centred on the chosen track but otherwise giving an error signal to energize linear actuator 16 to move the head assembly 8 in the sense to reduce the error signal to zero. Because of the division of the tracks into groups, an ambiguity would arise every 16  tracks and a coarse position control potentiometer 80 is therefore provided giving an input on line 82 to amplifier 74, this input being compared in amplifier 74 with an input on line 76 fed, via a digital-to-analogue converter 78, from track address store 36, the amplifier 74 giving a zero output if the difference between the signals on lines 76, 82 corresponds to less than a predetermined number less than seven, e.g. three, tracks, but otherwise giving an error signal output proportional to this difference, circuit 70 passing the sum of the error signals from circuits 64, 74 to actuator 16. Methods of accurately recording tracks on a virgin disc using the apparatus of Fig. 5 are also described.