Patent GB-1494675-A (GB)
Title: Audio record and playback unit therefor
IPC: G11B27/10
Filed: 1975-02-12
Abstract: 1494675 Sound recording and playback PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 12 Feb 1975 [15 Feb 1974] 5939/75 Heading G5R An information carrier has tracks formed as the successive turns of a spiral, Fig. 2 (not shown), or as concentric circles, Fig. 1, in which sound is recorded in a time division multiplex code which takes the form of m groups of pits 1, 2, 3, ... in each of the tracks, the groups not being spatially interlaced with one another on the carrier, at least the first (m-1) groups in each track each consisting of n said pits, and a starting pit S 1 , S 2  &c. being provided in each track at the start thereof, which starting pit has a feature distinguishing it from all other pits in the same track. The pits are elementary grooves about 1 Ám. wide and each extending along the track, the starting grooves being distinguished by being in the form of a given code word, e.g. used for track identification, or by being shorter or, preferably, longer than the other pits. In one embodiment the mth group in each track has (n-1) grooves. In an alternative embodiment, the mth groups each have n grooves and the start of each group is indicated by a group groove (G 1 ), (G 2 ) &c., Fig. 6 (not shown) which are used to enhance playback synchronization. Information for controlling dynamic compression and decompression circuits may be recorded as compression or decompression grooves immediately after the group grooves. The sound information may be recorded in pulse duration, pulse code, or delta modulation format, and n may be 120, for example. A disc record carrying 40,000 to 60,000 tracks can give a playing time, at 1500 r.p.m., of over 50 to 70 hours when pulse duration modulation is used. A playback unit for such records has a photoelectric detector 7, Fig. 7, converting the reflected and modulated read beam from an He-Ne laser into electrical pulses corresponding to the various grooves. The sound signal pulses are fed to a loud-speaker 29 via a sampling gate 9 the opening of which is controlled by a logic circuit responding directly to the sound pulses 1, 2 ... n and starting pulses S, separated by circuit 8, or to the outputs of oscillators 40, 41 synchronized to the starting and sound pulses respectively by phase discriminators 42, 46. For a record having no group grooves and (n-1) sound grooves in the mth group of each track, the logic circuit comprises a counter 13 counting the starting pulses, counters 18 and (32), Figs. 4, 5 (not shown), counting the sound pulses, flip-flops (15), (33), (38) and blocking gates (30), (31) whereby gate 9 passes only the 1st sound groove of each group, during a first scan of a given track, then only the 2nd groove of each group during a second scan of the same track and so on to the nth scan of that track at the end of which an output of counter 13 controls a driver 27 to tilt a read-beam deflecting mirror (2) to start scanning the next track. An embodiment, Fig. 7 for a record having the group grooves and n sound grooves in all groups, plays back the record in a similar fashion but uses a different form of counter 18 to control sampling gate 9 allowing elimination of circuits (15), (30), (31), (32), (33) and (38) of the previous embodiment, an oscillator 51 synchronized to the group pulses G, separated by circuit 8, supplying a control input 56 of counter 18 to reset it at each group pulse. To bridge over gaps in the replayed sound, particularly those due to the track starting groove and first group groove being juxtaposed together between adjacent sound grooves, the output of gate 9 may be passed to loud-speaker 29 via charge transfer devices in the form of bucket brigade or charge-coupled devices 60, 61 arranged so that the signal is read from gate 9 into one device simultaneously with readout to loud-speaker 29 from the other device and then vice versa. Stereo playback is possible with a record in which the odd and even numbered grooves of each group respectively correspond to the left and right signals, the record being played by apparatus similar to that of Fig. 7 but having two sampling gates (91), (92), Fig. 8 (not shown), passing only the left and right signals respectively and controlled by respective sound pulse counter (181), (182). A delay line (80) in one channel compensates for the sequential, rather than simultaneous, arrangement of the left and right signals on the record. Quadraphonic playback is similarly possible using four such sampling gates, four such counters and three delay lines with a record, in which for each groove group, grooves 1, 5, 9 ...; 2, 6, 10 ...; 3, 7, 11 ...; 4, 8, 12 ... represent channels 1, 2, 3, 4 respectively. A similarly extended arrangement may provide for reproducing k different instruments through respective loud-speakers. Track changing and following by the read beam 1, Fig. 9, may be accomplished with the aid of a mirror 2 on a bar 82 tilted by piezoelectric bars 83, 84. Such an arrangement permits track changing in about 30 Ásec. and is particularly useful with a spiral track record, the capacitor formed between electrodes applied to bars 83, 84 being charged via a resistor so that beam 1 follows a given turn of the spiral and then the capacitor being discharged by a transistor activated via line 81, Fig. 7, each time a starting groove is scanned so that beam 1 is rapidly returned to the start of the same turn, the output of counter 13 inhibiting the discharge when the requisite number of scans of the track have been effected so that scanning of the next track begins, during the first scan of which a carriage carrying the optical scanner is moved to its new position, and the sawtooth voltage applied to bars 83, 84 thereafter again becoming operational to tilt mirror 2.