Patent GB-346394-A (GB)
Title: Interference prevention in signalling conductors
IPC: H04B3/04
Filed: 1930-01-07
Abstract: 346,394. Correcting distortion. STANDARD TELEPHONES & CABLES, Ltd., and SANDEMAN, E. K., Columbia House, Aldwych, London. Jan. 7, 1930, No. 613. Drawings to Specification. [Class 40 (iv).] In a telephone transmission system in which interference or noise gives rise to distortion, various adjustments are effected so that the system may be used with a maximum energy level for a given quality of transmission. In one system the transmitting amplifier has a gain-frequency characteristic such that the average number of overloads occurring per unit time and due to an elemental band of frequencies, is constant over the whole of the speech range. If the tolerable number of overloads per minute for the entire range is L and the range is divided into n bands, the amplifier is adjusted to give L/n overloads per minute on each band. To determine the required gain for any particular band, speech is fed to the amplifier through a pass filter and the gain is adjusted so that overloading, detected by a valve of known volume-handling capacity, a peak voltmeter or other form of indicator, conforms to requirements. An overload spectrum for the whole range may thus be obtained and by insertion of a corresponding equalizing network at the input to the amplifier, its output has an " equal overload spectrum." The receiving amplifier has gain characteristics such that the overall attenuation is independent of frequency or conforms to any predetermined curve. In a modified system, account is taken of the fact that overloading is more tolerable on the higher than on the lower frequencies. To obtain data for the equalizing network, a band in the speech range is amplified more than the rest so that any overloading is due to that band only. After amplification (and overloading) the whole range is passed through an equalizer to restore it to normal, and articulation tests are made from which a gain-frequency curve for constant intelligibility is plotted which is used to construct an equalizer for the input of the amplifier. In a further modification, account is taken of the interfering effect of received noise at various frequencies. A transmitting amplifier is made up with a preliminary gain-frequency characteristic taking into account maximum input with equal overload output, attenuation-frequency curve of the cable, and received-noise spectrum. The noise spectrum is first adjusted by multiplying the amplitudes at various frequencies by the interference factor which represents the relative interfering effect of equal noise amplitudes at various frequencies. A curve is then plotted showing the interfering effect, at various frequencies, in decibels relative to the effect at 1000 cycles, and the attenuation frequency curve is added to it. If the resultant curve shows a maximum gain at or near the maximum frequency of the input, it may be advantageous to reduce the gain by observing the rule that the difference between maximum and minimum gain shall not exceed (30-S) decibels when S is the difference between maximum and minimum of the equal overload spectrum. A transmitting amplifier having this characteristic is then set up and also a receiving amplifier giving, with the cable, any pre-assigned attenuation characteristic, and articulation tests are made for a series of complementary changes in gain at the transmitter and receiver. Thus a particular elemental band is chosen and, with a particular energy level for this band, a series of tests are made with varying levels for the rest of the speech range. A further series of tests are made with different energy levels of the elemental band so that the conditions giving maximum intelligibility are determined. Starting from this point, the conditions for maximum intelligibility are determined in respect of another elemental band, and the process is preferably continued until the entire range has been covered. The tests are then recommenced with the first elemental band and as many cycles of tests are carried out as are required by the quality of transmission desired. Carried to the limit, the system is such that any complementary change in gain in either direction will result in decrease of intelligibility.