Patent GB-940449-A (GB)
Title: Electrical systems for reproducing recorded signals
IPC: G11B15/61
Filed: 1960-03-08
Abstract: 940,449. Automatic speed control. RADIO CORPORATION OF AMERICA. March 8, 1960 [March 26, 1959 ], No. 8211/60. Heading G3R. [Also in Division H3] Relates to a system for detecting and compensating for errors in the relative time of occurrence of recorded television signals as they are extracted from a magnetic tape. As shown, successive transverse tracks on a magnetic tape 14 are scanned by a rotating head-wheel 20 carrying four magnetic heads, a tone-wheel 32 generating a pulse in pick-up 34 for each revolution of the head-wheel. Each scan produces a group of repetitive pulse signals, and these groups are repeated at a rate related to the speed of the head-wheel. Switching pulses from a tone-wheel amplifier and multiplier 72 are applied to a circuit 70 to signal when each head comes into scanning relation with the tape, horizontal synchronizing pulses also being separated from the television signal and supplied to the circuit whereby successive heads are connected to F.M. demodulator 74 when the same horizontal pulse occurs on adjacent heads. The output from circuit 74 is applied to sync. separator 76 which strips the horizontal and vertical components, an amplifier 78 eliminating the vertical components. The horizontal synchronizing pulses are then fed to a phase detector 80 which senses any repetitive variation in the interval between successive pulses. The detector 80 comprises an oscillator 84 producing a sawtooth waveform which is transmitted through delay circuit 90 to control-tube circuit 86, the pulses from the amplifier 78 being added to the sawtooth waveform. The current flowing through the controltube is in the form of constant amplitude pulses whose width depends upon the relative phase between the pulses and sawtooth waves. The oscillator 84 is controlled from the circuit 86 through low-pass filter 88 whereby the frequency of the sawtooth oscillations is related to the average frequency of the horizontal synchronizing pulses and is not affected by timing errors in the pulses. Alternatively, the reference pulses may be derived from a local generator. A voltage corresponding to the plate current in the circuit 86 is amplified in amplifier 92 and then applied to bandpass filter 94 and phase-shifter 96. The output of the phaseshifter is applied to amplifier 98 which amplifies the component whose frequency corresponds with the scanning rate. A phase detector 100 has inputs derived from the amplifiers 72 and 98 and its output is a sine wave with a D.C. component which varies in accordance with the phase relationship between the input pulses. A low-pass filter 102 extracts the D.C. component which is applied to servo amplifier 104 providing a signal whose phase displacement is determined by the D.C. error voltage. The servo amplifier feeds motor 106 for adjusting the pressure of the tape against the head-wheel so as to compensate for any tape-pressure variations during playback. Synchronizing pulses for operating the switching circuit 70 are derived from the oscillator 84 through differentiator 112 and clipper-amplifier 114. These pulses are effectively advanced with respect to the horizontal synchronizing pulses from the tape due to the delay circuit 90. If there are no timing errors in the horizontal synchronizing pulses, no output is derived from the phase detector 100, the servo motor remaining inactive until a timing error occurs.