Patent GB-979496-A (GB)
Title: Improvements in magnetic recording and reproducing systems
IPC: H04N5/95
Filed: 1962-09-04
Abstract: 979,496. Automatic phase control systems; delay lines. AMPEX CORPORATION. Sept. 4, 1962 [Sept. 11, 1961], No. 33911/62. Drawings to Specification. Headings H3A and H3U. A 500 kc./s. pilot sine wave is additively combined with a frequency modulated carrier of different frequency range before recording transversely on magnetic tape, and the reproduced signals from a plurality of magnetic heads successively scanning the tape have the pilot signals filtered from them and phase compared with an equal frequency standard to derive D.C. phase error voltages each applied to correct the phase of the reproduced carrier signal portions from each magnetic head. The phase correctors may be balanced delay lines with pairs of oppositely poled semi-conductor capacitors shunting a series of inductors, the error D.C. signal being applied in push-pull to the capacitor ends unconnected with the inductors so that the delay is varied but the FM carrier applied to the inductors has no delay varying effect. The successively reproduced and time corrected carrier portions are then combined before demodulation. The carrier may be frequency modulated by television video or single polarity radar pulses. The carrier signals from each of four rotating magnetic heads are amplified and, after the pilot frequency has been separated, each channel given an adjustable delay, before the pilot frequency is recombined again with each channel. Each channel is then processed by a phase equalizer (to compensate resonance in each of the tuned heads) and gated so that the 180 degrees time sequence carrier portions are combined in a pair of adders, the two adders then feeding two pilot frequency filters and the delay line timing correctors. The corrected outputs (in alternate time sequence) are then each applied to gates opened alternately by trapezoidal pulses at twice the first gating frequency, to feed a signal adder producing a continuous FM carrier, as in Specification 942,887, which, after suppressing the pilot frequency, is then demodulated. Interference compensator.-Cross talk from the 500 kc./s. pilot frequency may be balanced out from the FM demodulator output by adding an attenuated and phase-adjusted output from the frequency standard (of pilot frequency) so that the interference shown on an oscilloscope disappears. Rotary head synchronizing.-The filtered pilot frequencies used for correcting the delay line timing correctors are also applied as in Specification 942,887 to gates actuated by 15625 c.p.s. trapezoidal gate pulses to add the time sequence pilot frequencies from the two channels and produce a continuous pilot frequency having the same time error as the FM carrier before timing correction in the delay lines. This pilot frequency, as in Specification 961,634, after limiting to a rectangular waveform, feeds a blocking oscillator producing pulses gated through an AND gate at a rate 15625 c.p.s. (derived from the frequency standard) to a monostable circuit generating pulses having the same phase error as the pilot frequency and used for controlling in frequency and phase the motor driving the rotary magnetic heads assembly. Switching waveform (gate pulse) generator.- Pulses from a photo-cell sensing the position of the rotary magnetic head assembly are generated at 244 per second and pass through a filter to waveform squaring amplifiers, one preceded by a 90 degree R.C. phase shifter, with outputs each applied to a pair of trapezoidal gate pulse generators. These each comprise a PNP and a NPN transistor with collectors connected together and to a storage capacitor so that, the transistors being rendered conductive alternately by the square waveform input, sawtooth waves would be formed across the capacitor but the latter is shunted by a pair of oppositely-poled back-biased rectifiers so that trapezoidal pulses are formed and derived through an emitter follower. The filter output, at 244 c.p.s., is also rectified to form through a filter a 488 c/s. wave which through a compensating R.C. phase shifter and squaring amplifier operates the trapezoidal gate pulse generators for producing the final combined FM carrier, and the final combined pilot frequency. Time base monitor.-The residual time or phase error of the system after correction may be monitored on an oscilloscope by an AND gate acting as a phase detector between the frequency standard and the pilot frequency by producing width modulated pulses. Specifications 858,145 and 915,254 also are referred to.