568,053. Automatic control systems. PATELHOLD PATENTVERWERTUNGS- & ELEKTRO-HOLDING AKT.-GES, July 12, 1943, No. 11290. Convention date, July 10, 1942. [Class 38 (iv)] [Also in Group XL] Oscillation frequency.-A device for automatically influencing oscillatory circuits in dependence on an applied alternating voltage, is characterized by means for producing a tuning regulating value, the magnitude and sign of which are dependent on the detuning of the tuned circuits as compared with the alternating voltages, by means of phase comparison of the alternating voltages present before and beyond the oscillatory circuits, means for diminishing the detuning by this regulating value, means for producing a second regulating value dependent on the detuning by means of phase comparison of the alternating voltages present before and beyond the oscillatory circuits, and means for influencing the dependence of the output voltage on the input voltage supplied by the second regulating value, in the direction of a decrease in this dependence with a decrease in the detuning. Fig. 2 shows an arrangement in which an input oscillation el is applied through a variable coupling N1 to synchronize an oscillator G, to give an output e2. Ring modulator M1 is fed with e1 (after a 90 degrees phase shift at P1) and with e2 to produce a voltage v dependent on the detuning between e1 and e2. This voltage is applied to vary the saturation of a choke in the oscillator circuit until synchronism is effected. Also a voltage u derived from e1 and e2 in ring modulator M2 is applied to reduce the coupling in N1 so that when synchronism is effected, it is maintained by the control voltage v, there being substantially no drive from e1. In this arrangement and in that of Fig. 1 (not shown), the phase or amplitude modulation components may be taken from the ring modulators M1 and M2 as at U3, V3. Fig. 1 (not shown) shows an arrangement for reducing the detuning between a filter circuit and an input oscillation. In this case, the tuning of the filter circuit remains fixed and the detuning is connected by applying a control voltage, derived as before from e1 and e2, to vary the tuning of a local oscillator which is used to heterodyne with a received oscillation to provide the input oscillation e1. Also the second derived control voltage is applied to a potential variable resistance to reduce the band width of the filter when the tuning is correct. In Fig. 3, the invention is applied to the control of a saw-tooth oscillator G2 in which a condenser C2 is charged through resistance R2 and discharged through a gasfilled valve V2. The control oscillation e1 is fed through a coupling network N2 including an amplifier V3 and combination C5, R5 to give pulses which are applied to the grid of V2 to synchronize the saw-tooth oscillations. As in previous embodiments, a ring modulator M1 fed from the input and output, produces a control voltage V which depends on the phase difference, and this is fed to the grid of V2 to bring the oscillator into synchronism. When this has been effected, the detected output from M1, after smoothing at B2, is applied to reduce the coupling in N2, the synchronism being maintained by voltage V. This arrangement is suitable for producing synchronized picture or line deflection voltage in television receiving apparatus. The embodiments of Figs. 1 and 2 are stated to be useful in homodyne receiving systems. Specification 525,342 is referred to. The Specification as open to inspection under Sect. 91 also comprises the following subjectmatter. In Fig. 1 (Cancelled) (not shown), control voltages derived according to the invention are applied to alter the tuning of a bandpass filter by means of potential variable condensers and the output voltage alone is used for contracting the band width when the tuning is correct. In Fig. 2 (Cancelled) (not shown), the automatic tuning of a filter circuit is effected by the control voltages acting through electromagnetic instruments. Fig. 5 (Cancelled) (not shown) shows an arrangement in which the frequency of an oscillator is unchanged, but the frequency of the drive oscillation is altered to reduce detuning by applying a control potential to vary the tuning of a local oscillator which heterodynes with a received oscillation to provide - the drive oscillation. The tuning control voltage is derived from the output oscillation by means of a discriminator of the off-tune type. A second control voltage is taken from the discriminator and, after detection, is used to increase the oscillator output when in tune, by increasing the feed-back coupling or by increasing the amplification. This subject-matter does not appear in the Specification as accepted.