Patent GB-729647-A (GB)
Title: Improvements in and relating to electrical recording and reproduction
Inventor: STYLES PETER ROBERT
IPC: G01R13/04
Filed: 1951-12-10
Abstract: 729,647. Electric control systems; recording. NATIONAL RESEARCH DEVELOPMENT CORPORATION. March 10, 1953 [Dec. 10, 1951], No. 28880/51. Classes 40 (1) and 40 (2). Two carrier waves of the same high frequency have their phase difference adjusted in accordance with the value of a variable, and are simultaneously recorded, e.g. on a magnetic tape. The phase of .one carrier only may be adjustable in this manner but preferably the phase of both carriers is simultaneously varied in opposite senses. Recording, Figs. 1, 3. A relaxation oscillator A produces a saw-toothed carrier wave 2 which is fed through an amplifier B 1 directly to one grid of a double triode D1  (channel 1) and through an amplifying phase-inversion stage B2 to the corresponding grid of a double triode D2 (channel 2). A signal voltage 1, proportional to the variable to be recorded, is fed through a low-pass filter E to the other triode grids of the tubes D1, D2. These tubes are connected as Schmitt trigger circuits and produce square-topped outputs whose leading edges 4 coincide in time with the vertical edges of the respective saw-tooth waves 2, 3. The pulse trailing edges 5 coincide with the intersections 5<SP>1</SP> of the saw-tooth and signal voltages so that the pulse widths in the two channels vary differentially with the signal voltage amplitude. In each channel the pulses are differentiated at C1, C2, and rectified, the negative pulses 8, 9 corresponding in timing to the trailing edges 5 producing the square-topped waveforms 10, 11 in flip-flop circuits associated with valves F1, F2. Filters G1, G2 filter out the harmonics from the waveforms 10, 11, passing the fundamental sinusoidal outputs through respective amplifier stages H1-I1, H2-I2 to transformers Jl, J2. The outputs from the transformers Jl, J2 of the respective channels are thus sinusoids, 12, 13, both of which are phase-modulated, but in opposite sense, in accordance with the signal voltage amplitude. These are passed to known recording heads which record the two carriers side-by-side on a magnetic tape. Reproducing, Fig. 4. The two recording heads may be used as pick-offs, their outputs being fed over respective transformers K1, K2 and amplifier stages L1-M1-Q1-P1, L2- M2-Q2-P2 including high-pass filters N1, N2 to the two respective control grids of a nineelectrode valve Q. The circuit constants are such as to clip the output of Q to substantially rectangular form, the ratio of the widths of successive half-waves being a function of the phase difference of the two input signals. This output is rectified at R and passed through a low-pass filter S via output terminals TE to an oscilloscope, loud-speaker or other device. Modifications. If only one carrier is to be modulated, a sinusoidal carrier wave generated by a valve oscillator may be fed directly to one recording-head. The carrier is also applied directly to the grid of one valve and through a condenser to the grid of a similar valve so that the anode current of the second valve is in phase quadrature with that of the first valve. The two anodes are in parallel and the grid circuits balanced so that the phase of the resultant anode current, which is fed to the second recording head, is 45 degrees relative to the unmodulated carrier. The signal to be recorded is applied in push-pull across the valve grids so varying the phase of the resultant anode current accordingly. For determining the time of transient phenomena, a magnetic tape forming a closed loop may pass continuously by the recording heads preceded immediately by an erasing head. Then if the erasing and recording heads are put out of action immediately after the transient has been recorded, the time thereof is determined by counting the number of circuits of the record tape loop.  Reference has been directed by the Comptroller to Specification 920,480.