Patent GB-630098-A (GB)
Title: Improvements in or relating to signalling systems
IPC: H04B14/04
Filed: 1947-06-06
Abstract: 630,098. Secret transmission. WESTERN ELECTRIC CO., Inc. June 6, 1947, No. 15024. Convention date, July 9, 1945. Addition to 630,094. [Class 40 (iv)] [Also in Group XL (c)] In a signal transmission system the amplitude of a signal wave is determined at predetermined intervals and also a reference current is produced which is variable in steps, means being provided for transmitting a series of pulses in accordance with the result of a comparison between these two quantities. Transmitter.-At the speech transmitter, in the embodiment described, the sampling, comparing, coding and radio transmitting circuits, Fig. 4, are controlled by a pulse generator, Fig. 5. The latter comprises a relaxation oscillator 510, 511, 512, the short pulses from the cathode resistance 514 of which have a recurrence frequency of 6000 cycles per second, for example, and are fed to a delay network 516 giving a succession of positive pulses in leads 1 ... 4 for each input pulse. These positive pulses are repeated in leads P1 ... P3 by valves 520, 521, 531, 541 and leads D1 ... D3 convey a negative pulse at the beginning of the cycle from valves 520, 530, 540 followed by a further negative pulse of smaller amplitude from valves 521, 531, 541 respectively fed from leads 2, 3, 4 respectively. These pulses are illustrated diagrammatically in Fig. 3. Also at the beginning of the cycle a positive pulse in lead 1 is applied to valve 420, Fig. 4, to produce in the secondary of its output transformer 422 a positive pulse followed immediately by a negative pulse. The source 410, Fig. 4, of speech currents feeds a transformer 413 connected to a sampling circuit comprising condenser 415 and series and shunt diodes 417, 416 respectively. The pulses from transformer 422 render the diodes momentarily and successively conducting to discharge the condenser 415 and then recharge it to a potential representative of the instantaneous amplitude of the speech current. The coding circuit compares the voltage across condenser 415 with that across resistance R0, the current in which is adjusted in steps until they are as nearly equal as possible. This current is that flowing through the left hand sections of double triodes V1, V2, V3, which each have a common cathode resistance R1, R2, R3 respectively. The current through the latter tends to remain substantially constant and normally the right hand sections are conducting, the others being cut off, so that no current flows through R0. The current controlled by V2 is preferably twice that controlled by V3, while V1 controls a current twice that controlled by V2. The left hand triodes are controlled by pulses applied through double diodes V4, V5, V6. Positive pulses from transformers P1, P2, P3 render the left hand triodes conducting to pass current through resistance RO and charge condensers 433, 443, 453 respectively to maintain conduction until negative pulses arrive. Small negative pulses from transformers D1, D2, D3 cut off the left hand triodes only if the potential drop across R0 exceeds the sample potential on condenser 415, the diodes 435, 445, 455 being biassed by the cathode resistor 470 of a triode V7, the grid potential of which depends on the difference between the potentials developed by condenser 415 and resistance R0. At the beginning of each cycle large negative pulses are applied to D1, D2; D3 to cut off triodes 432, 442, 452. The immediately following positive pulse in P1, timed by delay network S1, sends the current of triode 432 through R0, and if the resultant R0 potential exceeds that across condenser 415, the next small negative pulse in D1 will cut off 432 and the discharge of 433 through transformer N1 sends a pulse to the radio transmitter 480. Subsequently, triodes 442 and 452 are rendered conducting in succession and then non-conducting if the test potential across R0 exceeds that across 415. If a triode remains conducting, no pulse is transmitted via transformers N2, N3 to the radio circuit. The transmitter 480 is blocked during the presence of the large negative pulses which are applied to V2, V3 at the beginning of each cycle by a pulse from transformer 424, the primary of which is energized by valve 420 and the secondary of which is in series with transformers N1, N2, N3. Receiver.-At the receiver, Fig. 6, the pulse output of the radio detector 610, after amplification in valve 612, is fed through inverter 621 to synchronize a relaxation oscillator 623, 624, 625, which generates pulses in resistance 627 at the cycle recurrence frequency of the incoming code pulses. In order that the oscillator triggering may be brought into coincidence with the first pulse in a cycle the triggering time may be delayed by about one-third of a cycle period by operating a key 693. This produces a positive pulse across inductance 694 to render valve 691 conducting for the appropriate period, and thus partially discharge the oscillator condenser 625. A further key operation will further delay the triggering instant. The decoder comprises the double triode V9 having a common cathode resistance 634. The left hand triode is normally cut off and a positive code pulse applied to its grid causes a charging pulse to flow into its anode condenser 642. The magnitude of this charge depends on the charge on the cathode condenser 652 which is charged through valve 651 at the beginning of the cycle when a synchronizing pulse is fed to its grid condenser 631. Condenser 652 discharges exponentially during the cycle, so that the code pulses cause increments of charge of correct magnitude to be fed to condenser 642 to build up a potential corresponding to one of the signal samples derived at the transmitter. At the end of each cycle the condenser 642 is discharged through valve 661, a synchronizing pulse being applied to the grid of the latter, and the resultant pulses are passed to the signal reproducer 681 through a low-pass filter 670.