Patent GB-518976-A (GB)
Title: Improvements in or relating to telephone and like systems
IPC: H04Q1/446
Filed: 1938-09-09
Abstract: 518,976. Distinguishing speech from voicefrequency signals. JONES, C. E. P., and BIRSS, R. R. Sept. 9, 1938, No. 26306. [Class 40 (iv)] [Also in Group XL] A receiver for calling or .supervisory signals transmitted as voice frequency currents or as voice frequency modulation of a carrier current comprises means whereby actuation of a relay responsive to incoming signals brings a guarding arrangement into a preparatory or testing condition from which it is brought to a condition for preventing response of the receiver if the received currents vary in amplitude as in the case of speech. This latter condition may be made to persist during the discharge time of a condenser so as to prevent repeated impulses from speech from having a cumulative effect. Five arrangements are described and all of them may comprise an automatic volume controlling arrangement such as that in the system shown in Fig. 3. First arrangement, Fig. 1. Currents of signal frequency after selection if desired by a filter (not shown), amplification by a 'valve VA with a current limiting resistance in its grid circuit, further selection by a tuned transformer TRB and rectification by a voltage doubler UA are applied to the signal grid of a pentode valve VB to operate quick-acting relays A, AA in its anode circuit. These relays charge a condenser QA and if they remain operated long enough release a relay BA to give a signal. In the case of speech, however, their operation will only be temporary and on falling back they prevent the release of BA and connect up the condenser QA which paralyses the valve VA for a period depending on the dimensions of its discharge current through RQ and RV. Second arrangement, Fig. 2 (not shown). This is substantially the same as that just described except that the valves VA, VB are replaced by a single pentode the signal and screen grids of which play the parts of the grid and anode of VA and the suppressor grid and anode those of the signal grid and anode of VB. A limiting resistance is included in the suppressor grid circuit. Third arrangement, Fig. 3. In this case relay A is controlled as in the previous arrangement but is arranged to pull up somewhat sluggishly with the aid of a normally shortcircuited winding. The alternating current component of the anode current of VA is passed to a transformer TRC, the secondary of which is connected via a delay network DN to a transformer TRY and directly to a transformer TRX. The voltage produced by the secondaries of these transformers jointly depends on the difference in amplitude at successive. instants of the received currents. Consequently if signals are being received there is no such difference and the relapse of BA follows the operation of A to make the signal effective. In the case of speech, however, a voltage is produced which causes the operation of G, GG and the valve VA is disabled for a period by the condenser QA, and A falls and prevents the release of BA. The release of G, GG after the release of A is delayed by a. condenser QB. In this system an additional winding on the input transformer feeds a rectifier US and a resistance-condenser combination QS, RL so as to give VB a normal bias which depends on the signal strength. By this means the response of G, GG depends on the relative rather.than the absolute differences of amplitude of the signal at successive instants. Fourth arrangement, Fig. 4. The relay A is controlled as before but on the operation of A a relay GA in the anode circuit of a further valve VB is energized by received currents of signal frequency selected by a tuned transformer TRB and rectified at UA. Discrimination is given by comparing the signal voltages passed on to another winding of TRB and those appearing in the secondary of an untuned transformer TRC. In the case of a steady signal these voltages are equal and the operation of A is followed by the relapse of BA. In the case of speech, however, the rectified resultant voltage is applied to the suppressor grid of the receiving pentode VA, A is released and BA is held. The disabling of VA further increases the discrepancy between the voltages passed on by TRB and TRC and so tends to increase the disabling voltage and A cannot re-operate until the relapse of GA which remains locked while an appreciable component of signal frequency is being received. Fifth system, Fig. 5. Received currents after selection if desired by a filter (not shown) and rectification at UR cause the operation of a relay A in the anode circuit of a valve VA and this is followed by a relay AA which opens the circuit of the signal relays BA as in other arrangements. A condenser QX which has been charged by rectified received current is connected by AA in the grid circuit of a second valve VB. In the case of signals this will prevent the operation of a relay G in the anode circuit of VB in response to voltages subsequently derived from the lowest winding of the input transformer TRA. In the case of speech, however, the peaks of the voltage from TRA exceed the reverse voltage of condenser QX and G is then operated and locks up, causing a normally charged condenser QA to discharge slowly and paralyse VA. Relay A falls at once and relay AA after a delay owing to its shortcircuited winding. The release of BA is prevented and G is unlocked but remains operated via the valve VB owing to the circuit over g2 until QA is partially discharged, the relapse of G taking place, however, while VA is still disabled.