Patent GB-495815-A (GB)
Title: Improvements in or relating to electric signal transmission systems
IPC: H04B3/00
Filed: 1937-02-18
Abstract: 495,815. Loading. COLLARD, J., and MANIFOLD, M. BOWMAN-. Feb. 18, 1937, Nos. 31865/37 (divided out of 476,799, [Group XL]), and 464/38. [Class 40 (iv)] [Also in Group XL] Relates to networks for either correcting or imitating, over a wide band of frequencies in television and like transmission systems, distortion of amplitude and phase due to variation with frequency of the resistance and inductance of a cable or other transmission line, or of its capacity and leakance, or of all four of these quantities. For each section of the equalizer an individual reference frequency p is assigned ; it is the frequency at which the transmission loss due to that section is a specified fraction of the maximum value of such loss, and in an example is the frequency at which the reactance and resistance of each arm of the section in question are equal. According to the invention the values p of the reference frequencies for successive sections are so spaced that the transmission loss or phase delay or both at the reference frequencies increase by equal increments from section to section. The equalizers may be located at the end of a cable or may be distributed as loading at intervals short compared with the wave-lengths employed. The invention is applicable to any cable or other transmission line in which the loss is represented by #An#<n> and the phase-displacement by #An#<n>tan¢# where the pulsatance #<1. The Specification gives a mathematical analysis showing that if the loss due to the cable is F(#) and if the reference frequencies p1, P2 ... are chosen so that the quantities F(pn) are spaced at equal intervals x, while the nth section of the equalizer causes a loss of f(qn), where qn=pn/#, then the equalizers cause a total loss equal to that caused by the cable if F(#) is of the form A#n; it is found, moreover, that in this case the phase-displacement ##(qn) is proportional to #<n>tan¢n#. The theory is then applied to the particular case of constant-resistance sections, for instance those shown in Fig. 4, where L/C = R1 R2=Z0<SP>2>, Z0 being the characteristic impedance. A series of such sections can be made to represent a cable. The Specification shows that the required conditions can be satisfied by means of arms built up of reactances in series or parallel with resistances, the propagation constant k of the resulting ladder section being given by the logarithm of IIsr(q+jt)/IIgr (q +ja,) where the s, t, g and a are constants. When the values of the constants for a network giving the same attenuation and phase-displacement as the cable have been found, interchange of the constants tr and ar leads to the design of a correcting network which yields attenuation and phase-displacement that compensate those yielded by the cable. It is shown that if n < 1 the spacing interval x is finite and that for every term An#<n> in the attenuation characteristic there is a corresponding term An#<n>tan¢n# in the phase characteristic. A more powerfully attenuating section may have the form shown in Fig. 5, or a tee-section, Fig. 6, may be adopted. A chain of attenuators having different spacing factors x may be used provided that these factors satisfy a condition given in the Specification. The invention may be applied to correct the distortion in speaking-tubes, or in delay devices in which mechanical waves are transmitted through a fluid, the acoustic vibrations being first converted into their electrical equivalents. Specification 476,799 is referred to.