Patent GB-1159987-A (GB)
Title: Improvements in or relating to Magnetic Transducer Systems and Apparatus
IPC: G11B5/03
Filed: 1966-09-13
Abstract: 1,159,987. Magnetic heads; simultaneous recording and playback. WINSTON RESEARCH CORP. 13 Sept., 1966 [27 Sept., 1965; 23 Feb., 1966], No. 40943/66. Heading G5R. An electromagnetic transducer comprises an electromagnetic member provided with information signal terminals and a gap normally positioned adjacent to a moving magnetic medium, there being an electrical conductor located in the gap to which a bias signal is applied for independently varying the magnetization of the medium and for sensing the varying magnetization of the medium, the frequency of the bias signal being substantially more than five times the highest significant frequency component of the information signal. A system for the simultaneous recording and playback of an information signal on and from a magnetic medium driven along a predetermined path comprises an electromagnetic transducer as above and playback means coupled to the electrical conductor. As shown in Fig. 1, an information signal is supplied from a source 12 to coil 14 on transducer 10 and a H.F. bias signal is supplied from a source 36 only to a conductive member 22 located in the gap between polepieces 26 and 30 of the transducer. Non-conductive gap spacers 32 may be provided. Fig. 4 is a cross-section through one of the transducers in a head stack, the complete stack including non-magnetic metal side pieces 59, 60, fillings of epoxy resin 62, and screening (not shown) between individual transducers. Each transducer has coils 14 but the conductive member 22 is common to all the transducers. In Fig. 7 the H.F. bias carrier signal is supplied to conductive member 114 through a filter 140 and transformer 142 while the information signal is supplied to coil 113 from source 148 through resistor 192 and input terminal 150. With both signals applied simultaneously to the magnetic recording tape 112, the latter is said to act as a third power modulator, recording the bias as the carrier and the information signal as the modulating signal; the application of the H.F. signal to the tape with the information signal causes the magnetic field due to the H.F. signal to vary in accordance with the information signal. Sensing means 160 is accordingly provided, and may perform one or more of the following functions: playback, simultaneous playback, feedback, of which the simultaneous playback and feedback functions are shown. As shown, the sensing means includes a tuned circuit 162 coupled to transformer 142, inductor coil 164 serving also as a transformer coil. The circuit passes, e.g. the " suppressed carrier side-band " components, e.g. of frequency 2WcŒWm, of the spectrum of the recorded signals (where Wc is the bias signal frequency and Wm is the information signal frequency). Tuned circuit 162 is followed by trap circuit 172 connected in series to an open-ended coaxial cable 177 to eliminate any bias oscillator frequency which may have passed through circuit 162. The "s.c.s." components are then passed through a demodulator 182; in this demodulator the carrier is re-inserted by a certain amount of feedthrough of the second harmonic (2Wc) signal generated by the bias oscillator 138 which is not completely eliminated by filter 140. The demodulator 182 is connected to input terminal 150 through a feedback gain amplifier 191 and resistor 194. The resistors 192 and 194 function as a summing junction or error detector which compares the signal from an input means 148 with that recorded (supplied by sensing means 160) and provides a signal relating to the difference between these signals. The signal supplied by sensing means 160, because of the transformer coupling via transformer 142, is 180 degrees out of phase with that supplied by input means 148; this facilitates generation of an error signal by resistors 192 and 194. The frequency of the bias signal is such that the recorded signal is sampled, and playback and error detection are accomplished, before there is any substantial change in a significant characteristic (e.g. frequency, phase, amplitude) of the input signal. Thus the effect of tape noise may be minimized, dropouts may be compensated for, and signal distortion minimized (or record level increased while maintaining the same distortion). An error signal such as that supplied to coil 113, may be supplied to bias means 136 to adjust the amplitude of the bias signal, and thereby adjust the recording process, to correct for distortion, dropouts or noise. The head stack of Fig. 4 may be used in instrumentation recorders with many tracks for recording wideband signal information (approximately D.C. to 1À5 megacycles).