Patent GB-830262-A (GB)
Title: Improvements in or relating to phasing system for multiple track recording
IPC: H04N5/919
Filed: 1956-04-18
Abstract: 830,262. Magnetic recording and reproducing apparatus. MINNESOTA MINING & MANUFACTURING CO. April 18, 1956 [May. 9, 1955], No. 11741/56. Class 40 (2). Apparatus for simultaneously reproducing from a number of recorded tracks each of said tracks including a common cyclicly recurring reference signal for accurately phasing and synchronizing the signals from each track to eliminate effects of flutter and misalignment of the reproducing heads, comprises a delay line in each channel, means for developing from the reference signal from the output of the delay line in one channel a mean constant comparison frequency, means in each of the channels for comparing the phase of the reference frequency with said comparison frequency to develop an error signal corresponding to the phase difference and means connected to each channel and responsive to the error signal for varying the delay of the corresponding delay line to bring the phases of the signals from the outputs of the respective delay lines in coincidence with the phase of said reference signal in the output of the delay line in said one channel. Fig. 1 shows an embodiment in which 3 represents the transducer connected to one channel selected as a master channel, while 31 represents one of any number of slave heads connected to complementary channels, all of which channels have been recorded with a cyclically repeating reference signal in addition to, but different in character from, the message signals also recorded thereon. Each line from the heads, after pre-amplification, is fed through pre-amplifiers 5, 5 1  having 100% feedback to produce a gain of unity and zero output impedance, so that the output voltage is independent of the impedance into which it is working. Such impedance. comprises delay lines 9, 91, delay and impedance of which are variable by electrical bias. The delay lines 9, 91 are terminated by impedances 11, 11 1 , the value of which is also varied by bias to match the line impedance. The message outputs are taken via amplifiers 13, 13 1  and lines 15, 15 1  from the junctions of delay lines 9, 9 1  and the impedances 11, 11 1 . A branch circuit 151, peculiar only to the master channel, is connected via a gate 17 (not normally required, as when the reference signal is continuous and differentiated from the message signals) to a discriminator 21 which in known manner develops an error signal in dependence on the polarity and magnitude of the phase difference between the reference frequency and a comparison signal from a bias-controlled oscillator 29, the frequency of which is controlled by the said error signal. Rapid fluctuations due to flutter are damped out by a low-pass filter circuit 23 having a long time constant so that the oscillator produces a comparison frequency which is the mean of the reference frequency carried by the master channel. A second feedback loop 15<SP>11</SP>, which is similarly duplicated in the other channels is taken from the output 15 and fed to another phase-discriminator 33 and produces an error signal which is fed through a network 35 similar to network 23 but having a much shorter time constant (i.e. shorter than the period of the maximum flutter frequency), and applied to the control tube 37 to vary the delay of line 9, and also applied to the terminal impedance 11 to maintain its value equal to that of line 9. The polarity of the signal applied to the delay line 9 is such that when the reference signal is instantaneously leading, the delay is increased, and vice versa. The feed-back loops in the other channels accomplish the same purpose so that variations on all channels are corrected in accordance with the comparison frequency obtained from the master channel. Two forms of bias-controlled delay lines are described with reference to Figs. 2 and 3 (not shown), in which the bias on control tubes vary the saturation of inductance cores to vary the time delay up to a maximum of about 2.5 micro-seconds. Figs. 4 and 5 (also not shown), illustrate modifications of the apparatus of Fig. 1, specifically for television recordings in which the reference frequency is obtained at the instants of the interline blanking pulses when the signal drops to the " black level ".