US2406790A - System for the electric transmission of sound and other waves - Google Patents

Description

Sept. 3, 1946- w. A. BEATTY ETAL SYSTEM'FOR THE ELECTRIC TRANSMISSION OF SOUND AND OTHER WAVES Filed Jan. 16, 1941 2 Sheets-Sheet 1 Fig. 1.

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SYSTEM FOR TI IE ELECTRIC TRANSMISSION OF SOUND AND OTHER WAVES Filed Jan. 16, 1941 2 Sheets-Sheet 2 Fig. 5.

Patented Sept. 3, 1946 UNITED STATES PATENT OFFICE SYSTEM FOR THE ELECTRIC TRAN SIVHS- SIGN OF SOUND AND OTHER WAVES York, N. Y.

Application January 16, 1941, Serial No. 374,660 In Great Britain January 16, 1940 Claims.

This invention relates to a transmission system which for the purpose of conveying intelligence utilises pulses which are time modulated in such a manner that their durations or times of occurrence are characteristic of the amplitudes of sound or like waves. The system has particular application to secret or camouflaged communication methods, and although the description which follows relates to such methods of communication, the system is not limited to secret transmission but can with advantage be employed in other manners.

In United States Patent 2,256,336, September 16, 1941, and U. S. application Ser. No. 375,814, filed January 24, 1941, reference is made to time modulated pulses of various types, and these different types of pulses for the purpose of convenience Were given identification code letters, and some of the same code letters for similar types of pulses will also be used in the description which follows.

The objects of the invention are achieved by modifying the time modulation of a train of pulses which in the normal unmodified condition are time modulated so that their durations are characteristic of the amplitude of a sound or like wave, said train of pulses being characterised in that in the unmodified condition one series of edges, referred to as the fixed edges, either the leading edges or the trailing edges in every case, of the pulses in the train, occurs at equal intervals of time (i. e. the pulses in the unmodified condition are rectangular trailing edge RT or rectangular leading edge RL pulses), said modification of said time modulation being the cyclic changing of the times of occurrence of said normally fixed edges, or the cyclic changing of the times of occurrence of both the normally fixed edges and the other edges of the pulse train, said cyclic time changes being characteristic of the amplitude of a sound or like wave (the first of the above stated time modulations being for the purpose of convenience referred to as the main time modulation, while the second is referred to as the modifying time modulation, the combined systems being referred to as duplex time modulation, i. e. the pulses are RFL (rectangular with fluttering leading edge the trailing edge being variable) or RFT (rectangular with fluttering trailing edge the leading edge being variable) or are RL-i-F (rectangular pulse with leading edge normally fixed and trailing edge variable the trailing and leading edges being flutter modulated in phase by similar amounts) or RT-l-F (rectangular pulses with trailing edge normally fixed and leading edge variable, the

trailing and leading edge being flutter modulated in phase by similar amounts) pulses as described in the above mentioned patent application Ser. No. 375,814, transmitting said duplex time modulated pulses by wire or wireless, and at a receiver reconstituting in any known manner, if desired, the intelligence characteristic of the main time modulation, while for the purpose of reconstituting the intelligence due to said modifying time modulation, pulses of constant duration are derived from said modified fixed edges, said derived pulses of constant duration occurring at cyclically varying intervals of time giving the intelligence characteristic of the modifying time modulation.

In a, practical example adapted for the purpose of transmitting a camouflaged communica tion, the system Works as follows, referring to accompanying drawings serving to illustrate the description in which, Fig. l is a diagram of the modulating circuit; Figs. 2, 3 and 4 are curves illustrating operation of the system, and Fig. 5 is a discriminating circuit arrangement.

RT-l-F pulses are generated according to the method disclosed in the above mentioned patent application Ser. No. 375,814, the main modulation of the RT pulses being much greater than the modifying modulation (1. e. flutter modulation) of said pulses. Referring to Fig. 1 of the drawings a signal wave characteristic of speech is taken from source II to speech scrambling apparatus I2 referred to as unintelligible scrambling apparatus, and also to speech scrambling appara tus l3 referred to as intelligible scrambling apparatus. The apparatus l2 scrambles the incoming speech in such a random fashion that intelligible speech cannot be reconstituted, while the apparatus l3 scrambles the speech in such a manner that with suitable apparatus intelligible speech can be reconstituted. The scrambled speech outputs from apparatus l2 and is are fed to the above mentioned RT+F pulse generating and time modulating apparatus it, said apparatus producing a train of RT-l-F pulses 15, part of such pulse train being shown in Fig. 2. The durations of the pulses l5 vary cyclically in accordance With the variations in amplitude of the unintelligible scrambled speech from apparatus I2, while the time of occurrence of the trailing edges iii of said pulses varies cyclically in accordance with the amplitude of the scrambled speech from apparatus l3, said cyclic tone manner described above, now has two types of 3 time modulation, and when such a, train is received by known amplitude modulated apparatus the intelligible and unintelligible scrambled speech can be made audible. Owingto the fact that the unintelligible scrambled speech has a time modulation much greater than that of the speech. The modifying modulation due to the intelligible scrambling would with ordinary methods of reception be entirely masked by the much stronger main modulation, and unless persons were familiar with a method of separating the modifying time modulation from the main time modulation the intelligence conveyed by the former time modulation would always be unintelligible and its existence would not even be suspected.

A method of separating the F type of modulation from an RT-i-F time modulation has been described in the above mentioned patent application Ser. No. 375,814, this method consisting of setting up at the end of pulse signals, damped trains of varying amplitudes and separating the resulting damped. trains from the main signal. The above mentioned method employing damped trains requires very careful adjustments at the transmitter and receiver, and one object of this invention is to disclose simpler methods of separating a modifying time modulation from a main time modulation.

The improved method of separating the modifying time modulation from the main time depends upon deriving from the pulse edges which in the unmodified condition would occur at equal time intervals, short marking pulses of constant duration but occurring at time intervals characteristic of the modifying time modulation. Such a derived pulse train is shown in Fig. 3 the pulses 18 of constant duration occurring at time intervals characteristic of the times of occurrence of the trailing edges I6 of the pulses It. The effect of such pulses occurring at unequal intervals of time is to time modulate the interval periods of time between successive pulses, and on an analysis it can be shown that such time modulation is characteristic of the changes in amplitude of a sound or like wave, this novel time modulation characteristic being diiferent to that usually ob.- tained when the time modulation is characteristic of the amplitude of a sound or like wave. It has now been found that time modulation characteristic of the changes in amplitude of a sound or like wave is as effective for the purpose of conveying.

of the pulses 20 corresponding to the trailing edges R6 of the pulses l5, while the pulses 2| .cor-

respond to the leading edges 22 of the pulses |5.,

The pulses 2| can be removed from the train l9 by means of an amplitude filter, giving the train of pulses I8 shown in Fig. 3. q

Owing to the fact that resistance capacity networks are employed for the establishment of the pulses 18, some of the intelligence due to the main time modulation also appears in the circuits giving the pulses E8. The effect of such break through of the main time modulation is obviated by feeding some of the pulse train shown in Fig. 2 in reversed phase to the break through modulation thus cancelling out the break through interfering signal. The method described is capable of giving signal discrimination such that when with normal receivers the intelligence due to the modifying modulation is entirely masked by the intelligence due to the main modulation, with special reception methods as described the intelligence due to the modifying modulation can be separated out with practically no remaining trace of the intelligence due to the main modulation.

A circuit arrangement which gives good discrimination between the intelligence due to the modifying time modulation, and the intelligence due to the main modulation Will now be described. Referring to Fig. 5 there is shown a three valve circuit arrangement.

The duplex time modulated pulse train is fed into the circuit at the two terminals 3|, earth, and 32 across which is connected a short time constant circuit consisting of condenser 33 and 7 variable resistance 34 in series, capable of differentiating the leading edges of the pulse train constituting the signal, these edges in the case under consideration being normally fixed edges and are slightly time modulated by the modifying time modulation, the resulting differentiated pulses are pulses of constant Width occurring at varying time intervals. These pulses are now fed through resistance 35 to the control grid 41 of valve 31, which also has an anode 38, suppressor grid 39, screen grid Ml, cathode 42 to which is connected the suppressor grid 39, and a heater 43.

As the incoming signal consists of a positive pulse, the diiferentiated puls will be negative so that this will appear as a positive amplified pulse, across the anode resistance 41 of valve 37. To prevent a differentiated pulse from the trailing edge of incoming signal pulse appearing here also the grid 4| is kept at a positive potential by means of resistance 36 connected from H. T.+.

to the junction of condenser 33 and resistance 34. The unwanted signal which this edge provides is also further reduced by resistance 44 connected from input terminal 32 to cathode 42, from which is also connected cathode resistance 45, and cathod condenser 46. Adjustment of variable resistance 34 gives a position at which the ratio of wanted signal to unwanted signal (1. e. modifying intelligence to main intelligence) is a maximum. The circuit for valve 31 is further completed by screen resistance 48 and associated condenser 49.

The required signal, in the form of short duration pulses occurring at varying time intervals are now detected and converted into an audio frequency signal by passing them from the anode 3B of valve 31. to the grid 54 of valve 52 via condenser 59, and resistance further amplification.

In addition to grid 54, valve 5 2 has an anode 53, cathode 5s and heater 5s, and its circuit iscompleted by connnecting cathode 5 5 to earth,

also inaddition, there is a condenser 58 from anode 53 to earth to bypass the higher unwanted frequencies.

The circuit for valve 63 which also consists of an anode 64, suppressor grid 65 connected to cathode t8, screened grid 66 and heater 69, is completed by grid leak 6i, across which is condenser (52, cathode resistance 72 and condenser l3, and an inductance 5:! as an anode load from anode 64 to H. T. The required signal which is developed across this is then fed to the output terminal 14 via condenser H and is therefore obtained between terminals i i and 15, the latter being at earth potential.

Another method of obtaining from the train of pulses I5 shown in Fig. 2 of the drawings a train of pulses substantiall similar to those shown in Fig. 3, is to feed the pulses [5 over two channels one of which has a time delay greater than the other by a time period equal to the duration of the pulses [8. The pulses l5 fed over the channel with the more delay are reversed in phase and combined with the pulses [5 fed over the other channel, giving a pulse train substantially similar to that shown in Fig. 4. Using an amplitude filter as already described the positive pulses are separated from the train having positive and negative pulses, resulting in a pulse train similar to that shown in Fig. 3 except that the trailing instead of the leading edge of the pulses coincide with the trailing edges it of the pulses i5.

If desired the camouflaging of the intelligence can be made more certain by arranging at the transmitter that the circuit between the apparatus l3 and M is closed only when an output is being obtained from th apparatus I2. This can be achieved in any known manner for instance b means of mechanical or thermionic relays.

Although the duplex time modulation system has been described having referenc to camoufiaged communication methods, it should be obvious that it can b applied in other manners. For instance, the duplex time modulation could relate to the stereophonic transmission of a. sound wave, or the modifying time modulation could relate to the degree of volume expansion or compression of the signal conveyed by the main time modulation, by continuously transmitting by means of said modifying time modulation a subaudible signal the mean amplitud of which is characteristic of said volume expansion or compression.

What is claimed is:

1. The method of transmitting two kinds of intelligence simultaneously by means of a single train of pulses of constant amplitude, comprising time modulating said pulses in one sense to transmit one of said kinds of intelligence, and time modulating said impulses in a different sense to transmit the other of said two kinds of intelligence.

2. The method according to claim 1, wherein said one of said kinds of intelligence permits one corresponding edge of each of the impulses to occur at equal time intervals.

3. The method according to claim 1, wherein the time modulation of said one kind of intelligence is greater than the time modulation of said other kind of intelligence.

4. The method according to claim 1, further comprising deriving said one kind of intelligence from a source of original intelligence by random scrambling of said original intelligence so that said original intelligenc cannot be reconstituted therefrom, and deriving said other kind of intelligenc by intelligible scrambling of said original intelligence so that said original intelligence can be reconstructed therefrom.

5. The method according to claim 1, further comprising deriving said one kind of intelligence from a first source of original intelligence by random scrambling of said original intelligence so that said original intelligence cannot be reconstituted therefrom, and deriving said other kind of intelligence by intelligible scrambling of original intelligence from a second source of original intelligence so that said second original intelligence can be reconstructed therefrom.

ii. The method of signalling with two kinds of inteliigence a single train of pulses of constant amplitude, comprising giving to said pulses a time modulation of one magnitude by a first of said two kinds of intelligence, giving to said pulses a time modulation of a lower magnitude by the second of said two kinds of intelligence, receiving said pulses after both modulations, and discriminating between said two modulations to produce a signal by said second kind of intelligence.

7. A receiver for receiving energy from a system in which a main intelligence and a modifying intelligence are transmitted by different time modulations of a single train of main pulses of constant amplitude and adapted to separate said modifying intelligence from said main intelligence, including intelligence separation means comprising apparatus to establish a train of marking pulses having constant durations which are short compared to the main pulses, said marking pulses serving to indicate the times of occurrence of the normally fixed edges of said main pulses, combined with output apparatus arranged to give an output due to the time modulation of the time intervals between said marking pulses, said time modulation being due to the cyclic variations in the times of occurrence of said normally fixed edges, and means coupled to said output apparatus for suppressing any output due to said main time modulation.

8. A receiver according to claim 7 further comprising apparatus serving .to establish said train of marking pulses, said apparatus comprising means adapted to differentiate said main pulses, and to given an output due to the normally fixed edge differential pulses and to simultaneously suppress the other edge differential pulses.

9. A receiver according to claim 7 further com prising apparatus serving to establish said train of marking pulses, said apparatus comprising means for feeding said main pulses in reversed phase over a channel having a delay equal to the duration of said marking pulses, and being further adapted to give an output due to the com-- bination of said reversed phase and delayed pulses with said main pulses, said output comprising two trains of marking pulses one positive and the other negative, and apparatus arranged to give an output due to whichever of said positive or negative marking pulses serve to mark the normally fixed edges of said main pulses, and to simultaneously suppress the other marking pulses marking the other edges of said main pulses.

10. A receiver according to claim '7 wherein said means serving to suppress an output due to said main time modulation, comprises means for reversing the phase of said main pulses and for combining said reversed phase pulses having any desired amplitude with said main ulses which may appear in said final output circuit.

WILLIAM ARNOLD BE'ATTY. LESLIE WILFRED HOUGHTON.

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