1,167,822. Digital transmission systems. EVERSHED & VIGNOLES Ltd. 21 Aug., 1968 [21 Aug., 1967], No. 38376/67. Heading H4P. Apparatus for transmitting signals frequency-modulated with pulsed data of the mark-space kind comprises three oscillators tuned to different frequencies and representing mark, centre and space conditions of the data, and means responsive to a change in input signal, indicating a required change in frequency, to stop transmission of the existing signal at the next zero point in its waveform and to start transmission of the required frequency at the next zero point, following the cessation of transmission of the earlier signal, in the waveform generated by the oscillator operating at the required frequency. Oscillators 10, 12, 14 respectively generate space, centre and mark frequencies which are transmitted to output terminal 28 via respective gates 20, 22, 24 and a summation circuit 26. Gates 20, 24 are controlled by bi-stable circuits 30, 38 respectively while gate 22 is controlled by centre bi-stable circuits 34, 36 via a gate 32 functioning as an AND gate. Each bi-stable circuit has two free steering gates and an A.C. input common to both sides of the bi-stable circuit; the A.C. inputs of bi-stable circuits 30, 38 are fed from oscillators 10, 14 respectively via squaring circuits 40, 42 while A.C. inputs for bi-stable circuits 34, 36 are derived from oscillator 12 via squaring circuit 44. Each bi-stable circuit reverses on receipt of the next pulse at the A.C. input following the reversal of potentials applied to the steering inputs. The left-hand side of bi-stable circuit 30 receives steering signals from input connection 16 via line 46 and the right-hand side via invertor gate 48. The left-hand side of bi-stable circuit 34 also receives steering signals from input 16 via invertor gate 48 and the right-hand side via further invertor gate 52. Similarly, steering signals from input connection 18 are applied in opposite polarities to the two sides of bistable circuits 36 and 38. Bi-stable circuit 30 has a second output which provides a control for invertor gate 52 while a second output from bi-stable circuit 34 provides a gate control signal for invertor gate 48. Similar interlocks exist for bi-stable circuits 36, 38. Assume both inputs 16, 18 are at centre frequency levels; then bi-stable circuits 34, 36 together maintain gate 22 in a signal passing condition, through gate 32. If the input signal at 16 now changes to " space " level steering signals are applied to bi-stable circuits 30, 34, of such polarity that the signal to the left-hand side of circuit 34 is effective on receipt of the next pulse at the A.C. input to turn the left-hand side off and right-hand side on. The signals at the steering inputs of bi-stable circuit 30 are of such polarity that the signal at the right-hand steering input, if it passes through invertor gate 48, will be effective to switch the right-hand side off and left-hand side on, on receipt of the next pulse at the A.C. input; bi-stable circuit 34 through inhibiting connection 54 prevents invertor gate 48 from passing the signal from input 16 until bi-stable circuit 34 has reversed, thereby shutting gate 22. Switching on of the left-hand side of bi-stable circuit 30 causes gate 20 to become conductive at the instant the space frequency waveform is passing through zero. When the signal at input 16 reverts to centre frequency level, the space bi-stable circuit 30 applies an inhibiting bias to gate 52 to prevent bi-stable circuit 34 from reversing until bistable circuit 30 has switched to close gate 20. Similar operation of the circuit occurs on " mark " signals. Gate 32 performs an "AND" function in that centre frequency is only transmitted by gate 22 if bi-stable circuits 34, 36 indicate that centre frequency levels exist at both inputs 16, 18, but since inverted signals are derived an OR gate is used to achieve this result.