Patent GB-1204527-A (GB)
Title: Recording and playback apparatus
IPC: A61B5/336
Filed: 1967-10-23
Abstract: 1,204,527. Recording and playback; electric digital data storage. INTERNATIONAL BUSINESS MACHINES CORP. 23 Oct., 1967 [7 Nov., 1966], No. 48063/67. Headings G4C and G5R. [Also in Division H4] To compensate for wow and flutter defects on playback of recorded F.M. signals, a tone signal is recorded simultaneously with the main information and used on playback to derive a compensation signal which is modified by a factor dependent on the instantaneous frequency of the F.M. carrier then subtracted from the main information played back. The system may be used in magnetic, optical or electromechanical recording of seismological or physiological data. The tone may be recorded in the main or a separate track and may also be employed to distinguish analogue and digital data and to indicate dropouts.  Recording (Fig. 6).-ECG data is mixed with a 400 c/s. tone from oscillator 46 and the composite signal frequency-modulated in unit 49 on a 2KC carrier then recorded by a conventional tape dictation recorder 63. The 2KC carrier generates a negative pulse in shaper 52 at each zero-crossing to trigger single-shot 53 whose output is clipped 54 to be symmetrical when the carrier is at 2KC. At this frequency, there is no feedback voltage at junction SJ via line 41. The feedback arrangement ensures that the oscillator 49 is maintained at the proper frequency according to the input. Similarly, the 400 c/s. tone is demodulated 59, 34 to give a symmetrical output which does not affect the junction SJ.  Playback (Fig. 8).-The circuitry is similar except that the impedances Z 1 , Z 2  are transposed and the oscillator 49 is replaced by the tape recorder in playback mode. The signal reproduced is demodulated 64-67 and the nominal 400 c/s. tone filtered out 73, demodulated and fed to a multiplier and shaper 78. The shaper is controlled by the demodulated tone to variably amplify the signal fed back via ,lines 80, 81 from the output, this signal representing the instantaneous carrier frequency. By arranging for the voltage amplitudes of the shapers 67, 76 to be related as the ratio of the actual carrier frequency (subject to distortion) to the carrier centre-frequency, the output of resistance summer 68 provides the proportionally compensated output signal. The Specification includes calculations to demonstrate this. The shaper 78 is formed by a transistor circuit (Fig. 7, not shown) (see Division H4) which amplitude-modulates a pulse train according to the input signal (i.e. the data on lines 80, 81) to an extent proportional to the difference between the actual and original tone frequencies, the pulse train repetition rate being generated equal to the tone frequency. Distinguishing data.-The tone may be used to clock data samples when going analogue to digital. The amplitude of the demodulated tone indicates real time on playback.  The tone is coded to be 400 c/s. for analogue and 350 c/s for digital data so that a computer may distinguish the data-types on playback. Dropouts are indicated by spikes in the reproduced tone on line 85. Data coding different voltage levels or sets of consecutive levels may be assigned to different characters.