GB759568A - Improvements in or relating to level-control circuits for level-control in accordance with a pilot signal - Google Patents

Abstract

759,568. Automatic volume control. PHILIPS ELECTRICAL INDUSTRIES, Ltd. July 26, 1954 [July 24, 1953], No. 21821/64. Class 40 (4). [Also in Group XL (b)] For a pilot controlled gain regulating system wherein a line balance impedance comprises a number of shunt attenuator sections including a resistor and a variable capacitor in series for dealing with different parts of the frequency band, the rotors of the variable capacitors are ganged to a common shaft so that when the shaft is driven by a mechanism controlled in accordance with the level of the pilot signal the values of the capacitors are varied in the same sense. The auto-transformer 21 ensures a minimum of self-inductance in the circuits dealing with the higher frequencies, and transformer 22 besides including an anti-parasitic circuit 23 across its primary is capacitatively shielded to minimize the effect of parasitic transformer capacity at the low frequencies. A pilot current detected and amplified in pilot receiver 6 controls maximum and minimum relays 29, 30, in the control circuit of a motor 20 which is coupled to drive the ganged condenser. Matching of the network to the cable 1 is facilitated by equalizer 2. A modified arrangement including a bridge " T " type network having a variable inductance also coupled to the motor shaft to provide the more accurate matching necessary for television transmission systems, is described with reference to Fig. 2 (not shown). In this arrangement the attenuator section and the bridging arm are reciprocal impedances, and the matching is improved by a correcting network in one of the series arms. The remaining attenuator sections may also be incorporated in filter sections. Line and attenuator characteristics are described with reference to Fig. 3 (not shown) and component values are given. Control of the motor over an auxiliary line is also referred to.