787,251. Secret transmission. IWASAKI TSUSHINKI KABUSHIKI KAISHA. Oct. 25, 1954, No. 30693/54. Class 40 (4)] In a system in which speech signals from a number of channels are transmitted simultaneously over a common line, the frequency band of each channel is divided into a number of sub-bands and a complete audio-frequency spectrum is transmitted which is made up of one or more different sub-bands from each channel, the arrangement including a periodically - operated switch which applies various combinations of the sub-bands from each channel in sequence to the line. At the receiving end of the line, the subbands are distributed to their appropriate channels by a further switch which operates in synchronism with the transmission switch. Since a portion only of each channel is received at any instant, the missing sub-bands are made good by the delayed insertion of a signal corresponding to these bands as they were last received, the receiver including a delay arrangement comprising a magnetic disc recorder. The basic system is described with reference to Fig. 1, in which complete speech signals from channels 1 and 2 are transmitted alternately. When these signals are separated on a time basis, the missing alternate time periods are made good by inserting delayed signals 1R, 2R from the last appropriate speech period. It is suggested that alternation of signals could occur every 40 milliseconds. Some portion of the signal in each channel is received continuously in the arrangement shown in Fig. 2, the lower and upper subbands of channel 1 being transmitted alternately with the upper and lower sub-bands of channel 2. In Figs. 2 to 7 the ordinates represent frequency and the abscissµ time. Each channel 1 and 2 may be divided into five sub-bands, as shown in Fig. 3. Five channels 1 to 5 are employed in the Fig. 4 arrangement, a sub-band from each channel being transmitted sequentially. To improve secrecy these sub-bands may be transmitted irregularly, Fig. 5 (not shown). Frequency compression may also be used, Fig. 6, in which sub-bands band care converted to b<SP>1</SP> and c<SP>1</SP> in the same frequency range as a sub-band a. Alternatively the subbands band c, Fig. 7, may be delayed by magnetic recording so that they are transmitted in the same time interval as a, the intervening time periods being silent. Details of a 2- channel transmitter and receiver associated with a line 5 are shown in Figs. 8 and 9 respectively, amplifiers in the system being represented by symbols 111, low-pass filters by 112 and band-pass filters by 122. In this arrangement, two sub-bands of each channel 1, 2 are alternately transmitted as in Fig. 2 under the control of a switch 4 which may comprise a thermionic or mechanical relay or a ring modulator operating at a frequency of 20 c/s., for example, from a square-wave generator 3. At the receiver, Fig. 9, a synchronizing signal controls a switch 6 by way of a selective circuit 8 and a square-wave generator 7, the synchronism of switches 4 and 6 causing the sub-bands of channels 1 and 2 to be directed through filters to corresponding receivers 10 and 20. The incoming signal is also applied by the synchronously-operated switch 6 to the recording heads 13, 23 of a pair of magnetic disc recorders 14, 24 rotated together by a motor 9, the arrangement causing recordings of alternately transmitted sub-bands to be obtained on each disc. After a rotational delay of 50 milliseconds corresponding to the period of one signal alternation, the recorded signals are returned by reproducing heads 15, 25 to their appropriate circuits and make good the missing sub-bands which arise from the manner of transmission. The recorded signals are subsequently erased by magnets 16 and 26. An alternative arrangement is shown in Fig. 10, in which the components 15 are equalizers and the components 111 amplifiers. In this arrangement two switches 4, 14 are provided at the transmitter so that a single low-pass filter 112 and a single highpass filter 122 only are provided. At the receiver, the sub-bands of the incoming signal are initially segregated by low and high pass filters 114 and 125 and recorded on disc recorders 11 and 21 respectively. Each recorder has two reproducing heads 12, 13 and 22, 23 which are spaced apart in rotational distance by one signal alternation and are associated with switches 6, 16, 26 and 36 operating in synchronism with switches 4 and 14. In operation, each disc records successively a sub-band for each receiver 10 and 20 and the reproduced signals are directed by the switches from heads 12, 22 to the correct receiver. The missing subbands are made good at the same time by reproduction at head 13 and 23 of the sub-bands recorded during the preceding alternation. As before, synchronizing is effected by a current in the line which controls pulse generators 7, 17 over a selective circuit 8, while erasion of the recordings is effected by magnets 19, 29.