Patent GB-523434-A (GB)
Title: Systems of transmission of electric signals
IPC: H03H7/32
Filed: 1939-01-06
Abstract: 523,434. Transmission systems. STANDARD TELEPHONES & CABLES, Ltd. Jan. 6, 1939, No. 524. Convention date, Jan. 25, 1928. [Class 40 (iv)] [Also in Group XL] Distortion of signals due to reflections in the transmission medium (echoes) is minimized by producing in one or more bye-paths each including a delay device, a sign inverting device and an attenuator, neutralizing signals of the same amplitude as but opposite in sign to the echo components of the signals in the direct path. The reflections may be caused by mis-matching of line impedances or in the case of wireless transmission, by ground or atmospheric layers. The principle of the invention is shown in Fig. 1 in which the signal with echoes is transmitted over a direct path 2 and over a path 1 in which it is delayed at 3, inverted at 4 and suitably attenuated at 5 so that when mixed with the direct signal at 6, the echoes are balanced out. In the arrangement shown in Fig. 4 for automatically adjusting the amplitude of the correcting signals, a periodic pilot signal after traversing the delay network 3 is separated from other signals at 10 and used to unblock a measuring device 9 at moments when pilot signal echoes arrive directly at 9, and the determinations 'of 9 as regards the amplitude of the echo are used to control the attenuator 5 to adjust the amplitude of the neutralizing signals. Alternatively, Fig. 5 (not shown) the measuring device may determine the amplitude of the residual echo at the output of mixer 6. When the disturbance consists of a trail of echoes of differing time lags, a number of parallel correcting arrangements with correspondingly different delay periods are provided as in Figs. 7 and 8 (not shown). Each correcting arrangement is suitably tapped off a common artificial line used as the delay network, and separate attentuating devices are used for each corrector, but phase reversal is effected after the outputs of the correctors have been mixed. When measurement occurs before correction (Fig. 7) a single separator suffices, but when the residue of the echo is measured (Fig. 8) each corrector has its own separator. The measurement of echo amplitude in a number of associated correcting circuits may be effected by a cathode ray tube with a number of output electrodes as shown in Figs. 9 and 11. The beam is swept across the output electrodes 9 at suitable speed by a control circuit 14 governed by the output of pilot signal separator 10 and the incoming signal to be corrected is applied to the control electrode of the tube. The trail of echoes of the pilot give rise to voltages across resistances RL, Fig. 11, which are used to control the respective attenuators 5. When residual echo is measured the final output signal is applied to the control electrode. A simple delay and attenuating network for producing from a signal and an exponential trail of echoes, a trail approximating to the existing trail, is shown in Fig. 2 the values of the circuit elements being determined in accordance with an exponential equation appearing in the Specification. Fig. 3 (not shown) comprises a valve circuit for a similar purpose. In a more detailed embodiment shown in Fig. 6, the direct path is shown in the lower rectangle 11 and the delay path consists of an artificial line LA, an inverting and attenuating valve LPV, the outputs of the two paths being mixed over two parallel valves LS1. LS2. The delay introduced by LA is adjusted by a switch CP and the connection to the pilot separator 10 is taken from a point on LA slightly in advance of its output so that the measuring device 9 is prepared for the arrival of the echo which in this case is the residual echo from the output of the mixer 6. The separator 10 is of the type used to separate synchronizing signals in television systems and may be preceded by a direct current component restorer 12, The measuring device 9 comprises a screen-grid valve conditioned by the pilot impulses, and the voltage built up across resistance CE due to the effect of the residual pilot echo is used to bias the attenuating valve LPV in the delay path. In a case where echoes are introduced by a transmission line from a wireless transmitter to the antenna, pre-correction may be effected in circuits preceding the modulator but measurement of echo is in this case effected on signals derived from a local wireless receiver.