GB349140A - Improvements in or relating to radio transmitting systems - Google Patents

Abstract

349,140. Wireless transmitting systems. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., Marconi House, Strand, London.-(Assignees of Roberts, W. van B. ; 155, Hodge Road, Princeton, New Jersey, U.S.A.) Feb. 28, 1930, No. 6734. Convention date, April 2, 1929. [Class 40 (v).] In broadcasting and other radio transmitting systems, the signals are maintained at a uniform volume level before modulation so as to secure a uniform depth of modulation, the necessary variations in low-frequency amplification before modulation being accompanied by variations in opposite sense in the amplification of the modulated wave. The Figure shows an arrangement for securing this effect automatically. Signals arising in a microphone 1 are fed through transformers 3, 21 and amplifier 4 to a modulator 5 receiving its carrier wave from a line 6. The modulated wave is transferred through tuned circuits 42, 43 and amplifier 8 to an aerial 10. Part of the energy of the microphone circuit is diverted to an amplifier 38, rectifier 37 and smoothing circuit 32 .. 36, giving a direct current through a coil 31 proportional to the signal volume. If this volume is low, the armature 30 moves in a direction to increase the shunting resistance 23 across the microphone transformers, thus keeping the signal amplitude constant at the point where it is applied to the modulator 5. In order that receiving stations may respond to the signal volume variations, the armature 30 also controls the coupling of the post-modulation circuits, e.g. by regulating the capacity coupling 27, 28 between the tuned circuits 42, 43. Hand control may be substituted for the automatic control shown. The system reduces the effect of " strays " during periods of low signal volume.