Patent GB-445431-A (GB)
Title: Improvements relating to radio broadcast systems
IPC: H03C1/52
Filed: 1934-10-17
Abstract: 445,431. Wireless transmitting and receiving systems. ECKERSLEY, P. P., 82, Swan Court, Chelsea, London. Oct. 17, 1934, Nos. 29741/34 and 28711/35. [Class 40 (v)] In a modulated carrier-wave communication system, the carrier wave and one side-band are fully transmitted, the components of the other side-band near the carrier are not greatly attenuated, but the middle and outer parts of the latter side-band are attenuated as much as possible consistently with the avoidance of audible harmonic distortion at the receiver. The attenuation may be effected at the transmitter or the receiver or partly at both. A tendency to reduce the amplitudes of the higher audio-frequency components is counteracted by networks in the audio-frequency circuits at the transmitter or receiver. The object is to reduce interference by signals on carriers adjacent to the desired carrier. Fig. 9 shows a suitable transmitting circuit. The carrier from a source O is amplified in a valve V1 and is modulated by a valve V2 which has audiofrequency voltages applied to its plate circuit from a microphone p acting through amplifiers V3, V4. The modulated carrier passing to the aerial or to further amplifiers along a lead w is attenuated by a filter F the frequencycharacteristic of which is shown in Fig. 4. The carrier-wave fo is situated at the shoulder of the curve, or somewhat to the right thereof, so that one side-band is fully transmitted, whilst attenuation of the other side-band increases the further the frequency is from the carrier f0. This involves a reduction in the amplitude of the side-band components representing the higher audio-frequencies, and a network r, s, k is therefore interposed between the audiofrequency amplifiers V3, V4 to increase the relative amplitude of the higher notes. The Specification explains that when one of the side-bands is totally suppressed, audible harmonic distortion results unless the percentage modulation is low. The present system, by attenuating the components of one side-band in proportion to their remoteness from the carrier, applies the most drastic attenuation to those components representing the notes of higher frequency, and these notes are least likely to give rise to harmonic distortion because they only occur in weak amplitude and therefore at low percentage modulation. The Specification gives formulµ for calculating the allowable attenuation of the filter at each frequency component of the side-band, in terms of the percentage harmonic distortion that is to be permitted, and of the modulation percentage that is found to occur in practice at the frequency in question. For receiving signals sent by the transmitting system described above, the receiver response should be flat over the full range of the side-bands. The system may however be applied to a receiver and may then be used to receive normal transmissions comprising both side bands in full. For this purpose, the receiver, Fig. 10, comprises a frequency-changing stage V5 with a local oscillator LC feeding the intermediate frequency to a detector V6 through a filter F. The filter operates as described with reference to Fig. 4, in which f0 now represents the intermediate frequency. The detected signal is fed to audio-frequency amplifiers which include a network r, s, k designed to emphasize the higher audio-frequencies.