US2435879A - High-speed transmission communication apparatus - Google Patents

Description

Feb. 10, 1948. s. D. EILENBERGER ,879

HIGH SPEED TRANSMISSION COMMUNICATION APPARATUS Filed Aug. 14, 1942 5 Sheets-Sheet l m w w h. R F/l. r51? MIXER H w: M R Y O. E m m m E 0/ 56 v .,T M m B J /Y J XTEM PT xTlmm w m YT...

4a 46 8+ 5- T/ME Rid/IVS M0701? F IG- 1948. s. D. EILENBERGER 2,435,379

HIGH SPEED TRANSMISSION COMMUNICATION APPARATUS 2/ I18 MI was as? m (/666 (UMMD/V GROUND m REL/9Y5 aura! T T/warans 5- 5+ /6/ M2 g {z I M INVENTOR F IG. 2

T ORNEY Feb. 10, 1948. s. D. EILENBERGER HIGH SPEED TRANSMISSION COMMUNICATION APPARATUS Filed Aug. 14, 1942 3 Sheets-Sheet 3 FROM FIG. 3

INVENTOR BY ATTO Patented Feb. 10, 1948 UNITED STATES PATENT OFFICE HIGH-SPEED TRANSMISSION COMMUNI- CATION APPARATUS Application August 14, 1942, Serial No. 454,855

16 Claims. 1

This invention relates to a method of communication very similar to the method disclosed in my co-pending patent application, filed May 22, 1942, Serial Number 444,091, except that in the present invention a method is disclosed whereby a continuous recording may be made over a relatively long period of time and the transmission of such recorded intelligence is carried out as a series of short pulses of modulated energy. This method is particularly applicable to the transmission of intelligence by means of a modulated radio wave, but may be arranged to transmit such intelligence by means of wire lines, if so desired.

Briefly the invention consists of a method whereby the time required to transmit a message of given length is greatly reduced. This is accomplished by first recording the message to be transmitted on a magnetic steel tape at normal tape recording speed and then transmitting this recorded message at high tape speed, such as twenty times normal speed, so that the time required for transmission would in this instance be one-twentieth of that required to record the message, and for the purposes of this application such a. condition will be referred to as a recording transmission ratio of 20:1. At the receiving station this process is reversed to recover the original signal.

The foregoing paragraph briefly describes the method involved in transmitting one pulse which represents the intelligence recorded in a given length of time, on a single magnetic steel tape of definite length moving at a, definite speed. For example, a magnetic steel tape 60 feet in length and moving at a speed of 3 feet per second would be capable of recording intelligence for a period of 20 seconds. Assuming a recording/ transmission ratio of 20: 1, this recorded message would be transmitted in 1 second. The present invention discloses a method of using two such tapes in such a manner that the recording process may be carried out continuously for as long a period as desired, and the transmission will then be carried out in the form of one transmission pulse for each recording interval. For example, with two tapes 60 feet long recording at 3 feet per second and a recording/transmission ratio of 20:1, the transmission would be made as a series of pulses of modulated energy lasting 1 second each, and one such pulse .would be transmitted during each 20 second period.

Reference should be made to the aforementioned copending application for a more complete nderstanding of the basic principles of this 2 system, as fully disclosed therein. This co-pending application discloses methods whereby the basic principle is applied to military or naval .communications. The present invention deals more fully with the commercial application of this system.

Among the numerous objects of this invention are:

First, to provide a method of communication where the total transmission time is relatively short as compared to the length of the message to be transmitted.

Second, to provide a method of communication where the actual intelligence to be transmitted is recorded first on one magnetic steel tape and immediately following such first recording, recording is continued on a second magnetic steel tape, the intelligence recorded on the first tape being transmitted as a short pulse during the interval that intelligence is being recorded on the second tape, after which the process is repeated so that the recording of the intelligence to be transmitted may be carried out continuously, regardless of the length of the message to be transmitted.

Third, to provide a method of communication where the efiective power of the transmitter may be greatly increased beyond the normal limitation of plate dissipation rating of the final amplifier tubes.

Fourth, to provide a method of combining any or all of the objects mentioned above in a single system.

It is understood that a basic purpose of this invention is to provide a means of transmitting a message of specific length in a relatively short period of time, as compared to the time required to record such message and that in the practice of the present invention the transmission is always made by means of this relatively short pulse of modulated carrier energy.

To practice the invention herein disclosed proper consideration must be given to the design of certain circuit components of both transmitter and receiver.

For purposes of example the normal voice frequency range may be considered as 250 to 2500 C. P. S. and that a message covering this range is to be recorded on a magnetic steel tape within a time limitation of 20 seconds and that such 20 second recorded message is to be transmitted in one second. This would be a recording/transmission ratio of 20:1.

In the example given above where the highest frequency signal was taken as 2500 cycles the highest modulating frequency will become 50,000 cycles when reproduction of the recorded message is carried out at 20 times normal tape recording speed.

The magnetic reproducing head must therefore be able to properly reproduce at 50,000 cycles and the associated amplifiers and modulators must also function properly at this extended frequency range.

The design of such amplifiers and modulating equipment is well known to those skilled in the art, such amplifiers being commonly used for various purposes, it being well known that such amplifiers may be designed to have relatively fiat gain characteristics throughout any desired frequency range or that such amplifiers may be designed to have a rising gain characteristic at the higher frequencies if such a rising gain characteristic is desirable,

The design of magnetic tape reproducing heads to work efiiciently at 50,000 cycles is also well known to those specifically skilled in the art of magnetic tape recording. It is considered practical by those so skilled in the art of magnetic tape recording to record and reproduce by means of a magnetic steel tape frequencies as high as 100,000 cycles per second and possibly higher frequencies may be recorded and reproduced.

The present invention does not deal with magnetic tape recording as such but is concerned with disclosing a method whereby the application of such tape recording methods may be so used as to produce a new and novel result as herein disclosed.

The ability of a magnetic steel tape to record or reproduce a certain frequency depends not only on proper design of the magnetic recording and reproducing heads but also on the speed at which the tape is moving in the field of such recording or reproducing heads. It is generally considered that a tape speed of 1.5 to 2.0 feet per second is satisfactory for use in reproducing voice frequencies in the range of 250 to 2500 cycles per second.

For the purposes of this application this normal tape recording speed is taken as 3 feet per second, it being understood that this tape speed is used only for purposes of example and that any tape speed giving satisfactory results may be used to record the original message. In the example given reproduction is carried out at 20 times normal recording speed which would be equal to a tape speed of 60 feet per second. Such relatively high tape speed does not imply a high shaft speed at the revolving drums which move the tape, this factor being solely dependent on drum diameter and for purposes of example only this tape winding drum diameter is here taken as 8 inches.

This invention will be best .understood from a consideration of the following detailed description in view of the accompanying drawings forming a part of the specification; nevertheless, it is understood that the invention is not confined to the disclosure, being susceptible to such changes and modifications as define no material departure from the salient features of the invention as expressed in the appended claims.

In the drawings:

Fig. 1 shows in schematic form one arrangement of a radio transmitter to produce a series of short pulses of carrier energy modulated with the intelligence to be transmitted, in accordance with this invention.

Fig. 2 shows in schematic form one arrange- 4 ment of a receiver for use with the transmitter of Fi 1.

Fig. 3 shows circuit details of trigger unit T of Fig. 2.

Referring more particularly to Fig. 1, A is a source of original signal such as a microphone having output terminals I and 3 connected to the input terminals I03 and 4 of amplifier B, through gain control 2. Amplifier B is conventional and may be of any type desired. The function of this amplifier B is to amplify the original signal to a level high enough to energize recording head 23, which ordinarily requires approximately .25 watt. Amplifier B has output terminals 5 and 6 of which terminal 6 is connected to the common ground connection 42 and terminal 5 is connected through plate resistor I and normally closed contacts 22a of relay 2] to terminal 99 of magnetic recording head 23. The purpose of plate resistor I is to limit the D. C. current flowing through recording head 23 to the plate circuit of the finalamplifier tube of amplifier B. This D. C. current acts as a polarizing current for magnetic recording head 23 and such current is ordinarily limited to ap proximately 5 milliamperes, in which case series resistor 8 would approximate 50,000 ohms.

Magnetic recording head 23 is of the conventional type designed to record in a frequency range of 250 to 2500 C. P. S. The pole piece of magnetic recording head 23 is held in light contact with magnetic tape I0a at point 9a. It should be noted that while magnetic recording head 23 is shown as a single Winding that such a magnetic recording head having two windings and two pole pieces arranged on opposite sides of magnetic tape I0a may be used, the single Winding arrangement shown in the drawing being used only for purposes of clarity and simplicity in this example.

The opposite terminal I00 of magnetic recording head 23 is connected to terminal IOI of obliterating head 24a and also to one terminal of the normally closed contacts 22b of relay 2!. Magnetic obliterating head 24a is of the conventional type designed to magnetically saturate magnetic tape Illa to obliterate previously recorded messages. The opposite terminal I02 of obliterating head 24a is connected through resistor 33a to the common ground connection 42 which is also common to B connection 46. A slight air gap is maintained at point I la between the pole piece of magnetic obliterating head 24a and magnetic tape Illa, as is common practice. The D. C. resistance value of magnetic obliterating head 24a coil and series return resistor 33:: are so chosen as to provide sufiicient magnetic flux for magnetic tape saturation. These values will approximate 4000 ohms for magnetic obliterating head 24a coil and 8,500 ohms for resistor 33a with a B plus supply of 250 volts, in which case magnetic obliterating head 24a coil would pass 20 milliamperes and dissipate 1.6 watts.

The recording cycle is initiated by closing switch I01, which energizes clutch Ilia, through the released contacts 22c of relay 2|, and normally closed contacts of switch I05. Switch I05 is a normally closed microswitch used to index the position of magnetic tape I0a and is operated by a notch in the tape or by the additional thickness of the tape joint, both methods having been used successfully, The purpose of switch I05 is to cause magnetic tape I0a to make a sin le circuit of its length and stop, this stop always being in the same position, so that proper message continuity is always maintained during recording and transmission. It is understood that the mechanical indexing methods illustrated by switch |05are used here for purposes of example only and that numerous other indexing methods, such as photo-electric control may be used.

-Relay 2| is in the released position so that recording head 23 is directly connected to amplifier B and obliterating head 24a is also energized.

- Time motor M5 is energized simultaneously with.

the start of the recording cycle. This time motor M5 is directly connected to cams HI, H2 and H3 by shaft 44, this entire timer unit, as shown within the dotted line 43, is designated by the letter J and is of a conventional multiple cam type, Time motor M5 is normally of the synchronous motor type, energized from a source of voltage 48, and so geared as tomake one revolution of shaft 44 in 40 seconds, which equals two recording cycles.

Cams HI, H2 and H3 are indexed in respect to each other as shown in Fig. 1 but need not be indexed with their respective switches 35, 38, 4B and 4|. Assuming these cams to be in the position shown, cam extension 34 of cam HI will travel 180 degrees before it will operate switch 35.

Motor MI is arranged to drive shaft |-5a at a speed such that tape winding drum |2a will move tape" |a three feet per second, when shaft Ida is connected to shaft la through magnetic clutch l6a, the operation of which is controlled by-coil l9a. shown in Fig. 1, these shafts Ma and |5a are connected by means of magnetic clutcnlfia, coil |9a being energized from a source of voltage 5| in series with released contacts 55 of relay 53, released contacts 220 of relay 2| and normally closed contacts of switch Hi5. Therefore, magnetic tape |0a is moving at the normal recording speed of 3 feet per second during the 20 second recording interval that relay 2| remains in the released position.

Driving motor M3 is similar to driving motor MI except that the speed is such as to drive shaft |8a twenty times the speed of shaft |5a, so that when shaft |8a is connected to shaft |4a through magnetic clutch Ila, magnetic tape Illa will be moving at 2 0 times the normal recording speed or 60 feet per second. However, magnetic clutch Ila, which is energized by coil 20a, remains inoperative during the recording cycle of tape unit GI. Y After a 20 second recordin interval, cam extension 34 of cam HI will close switch 35, thus energizing relay 2| and moving contacts 22a, 22b and 220 to the operated position. thus transferring the recording and obliterating circuits from tape unit GI to tape unit G2.

Tape unit GI and G2 are identical in every respect and after the transfer of these circuits With the circuit in the position by the closure of relay 2| contacts, the signal of cams H2 and H3 so that motor M3 will be' driving magnetic tape Illa at this accelerated speed before cam H2 initiates the transmission cycle. During the period that cam H3 holds contacts 4| operated, cam extension 3'! will close switch 38-through 9.1 degrees of are which is equal to 1 second plus a small operating tolerance to allow for relay operate time in both the transmitter and receiver,

Switch 38 simultaneously energizes relays '56 and 51, Relay 5! is a fast operate type which controls carrier power in a manner explained further below and relay 55 has a copper slug 98 to provide a slight delay in operation. Relay 55 controls modulation and the delayed operation is designed to provide a short pulse of unmodulated power to operate the trigger circuit at the receiver before modulation is started, so that modulation will not start before the receiver is ready to record the transmission. This delay would be equal to the relay operate time in the receiver triggerv circuit, which would normally approximate 4 or 5 milliseconds. After this delay, relay 56 operates, closing contacts 58 which in turn closes the cathode circuits of mixer stages CI and C2 through cathode resistor 60 which is filtered by condenser 6|. Terminal 28 on mixer stage Cl and terminal 12 on mixer stage 02 represent direct connections to the cathode of the respective mixer tubes, so that modulating energy is only furnished to amplifier D at such times as relay 56 is in the operated position. High pass filter V is designed to have a low frequency cutoff at 5000 cycles, so that the original recording will not be reproduced and passed on to amplifier D as additional modulatin energy at such times as relay '58 is operated.

It is understood that this arrangement of parallel mixer circuits and high pass filter is shown only for reasons of simplicity and clarity in the example. Numerous alternate arrangements exist; for example, relay 55 might be a polarized relay with a center neutral position, so that alternately the cathode circuit was closed to one mixer circuit on one transmission cycle and to the opposite mixer circuit on the next transmission cycle. As a further example, switch 38 may be arranged in duplicate and two separate relays used in place of the single relay 56 shown, which would accomplish the same results as the polarized relay.

With both relay 56 and 51 in the operated position, modulated carrier energy is radiated for the 1 second period, this representing the complete signal recorded on tape unit GI.

, Amplifier D is designed'to provide amplification to a maximum frequency of 50 k. c. and modulator unit E, which in this example may be considered the final stage of amplifier D, is also designed to operate up to a maximum frequency of 50 k. c. Oscillator F is a conventional oscillator-buffer arrangement and may be of any type desired. For purposes of example, oscillator F is shown as operating at 40 megacycles but any other desired oscillator frequency may be used.

The final amplifier stage W, shown within the dotted line rectangle Hi4, may be of any type desired, the arrangement of a single tetrode as shown being used merely for the sake of simplicity and clarity in this example.

Carrier power is not normally radiated by final amplifier W due to the fact that cathode 11, in series with cathode resistor 52, which is filtered by condenser 63, is held open by the normally open releasedcontacts 59 of relay 5'1, Magnetic reproducing head 25 is directly connected to input terminals. 26 and 21 of mixer C but mixer C is held inoperative by holding open the cathode circuit of the first stage amplifier tube.

Magnetic reproducing head 49 is designed t reproduce efficiently in the frequency range of 5,000 to 50,000 C. P. S. and the pole piece is in light contact with magnetic tape Illa at point 9a, As previously explained in the description of recording head 23, reproducing head. 25 may be in two sections, having two coilsin. series and two pole pieces, if desired.

Relay 51' operates in .004 second thus closing? contacts 59- and establishing an operating circuit for cathode ll of vacuum tube 13, all other operating voltages being already present. Filament i8 is energized at points yy. Plate 14 is energized by high voltage terminal 50 in series with final tank coil 84. Plate tank coil 84 is tuned to resonance by condenser 85 and is inductively coupled to antenna coil 82 and also capacity coupled through condenser 33, the entire final tank circuit being relatively low inductance and high capacity, and designed to provide efilcient operation with a radiated carrier band width of 100 k. c. as will be the case with a maximum modulating fre-- quency of 50 k. c. and double side band modulation, The final output of antenna coil 82 is connected to antenna 86 and common ground connection 42.

The high voltage plate supply is filtered by condenser 8|. Voltage is supplied to screen grid" from the same high voltage source 50, in series with resistor 79, the screen grid voltage being filtered by condenser 80.

The output of oscillator-buffer unit F is available at terminals 91 and 98 where terminal 98 is the common ground connection and terminal 91 is capacity coupled to control grid 76' of final amplifier tube 73 through coupling condenser 6T. Control grid 15 is biased by grid resistor 64 in series with radio frequency choke coil 66.

The output of amplifier D is available at terminals 90 and SI' and is connected to the input terminals 92 and 94 of modulator E, in series with gain control 93. The output of modulator E is available at terminals 95 and 96 where terminal 96 is the common ground connection and terminal 95 is directly connected to grid bias re"- sistor 6'4 of final amplifier tube 13, this arrangement being a form of grid modulation relatively independent of modulating frequency. Grid bias resistor 64 would approximate 100,000 ohms in the arrangement shown and condenser 65 would approximate .00005 t. and is designed to effectively bypass radio frequency voltage at the'carrier frequency without materially affecting the 50 k. c. modulating frequency.

It is understood that other modulation methods may be used and that while in the arrange ment shown a form of grid-amplitude modulation is used for purposes of example that any other form of amplitude modulation, as well as frequency and phase modulation, may be used. The only requirement is that such modulation arrangement be effective at the final modulating frequency and that any lump inductance used in the final plate circuit be used insuch a Way that the stored energy of such lump inductance does not alter the desired time interval that carrier power is radiated.

After a transmission time cycle of 1.0M seconds time, cam H2 will open contacts 38 thus. breaking the operating circuit of relays 56 and 51, both of which will release. This will cut ofi carrier power by opening the cathode circuit of final amplifier tube i3 and will instantly stop reproduction by opening the cathodecircuitsv of mixer unit C. Of the transmission time cycle of 1.004 seconds, one second only will be: occupied in actual 8" transmission, and during this second, the mar netic tape III will make one complete circuit. of its length.

Summarizing the operation of. the first record ing interval. represented by tape unit GI and associated. apparatus, an: original message covering a frequency range of 250 to: 2500 cycles and requiring 20 seconds to record has been transmitted: in 1.004. seconds under conditions where the modulating. frequency was 50 k. c. and the frequency band occupied by the modulated carrier was 100- k. c. Cami extension 39 of cam III will hold relay 53 operated for a short period after the transmission of tape unit GI is completed, after which contacts 4| will be open thus releas ing relay 53, de-energizing clutch lila, energizing clutch l'5a and restoring tape unit GI to its original recording speed. of 3 feet per second.

It' is understood that the timing arrangement illustrated by unit J, as enclosed within the dotted rectangle 43, is used here merely for purposes of example, numerous other timing arrangements being possible which would accomplish the desired result of switching the various circuits.

The entire process just described in connection with tape unit GI is now being repeated with tape unit G2; all units of which are identical to tape unit GI.

Summarizing the operation of the complete transmitter, cam H l divides the recording time equally between magnetic tape I (la and Nb, by controlling the operation of relay 2]. Cam H3 controls tape speed in such a manner that the tape which has a complete recording will operate at accelerated speed for a portion of the time that the opposite tape unit is recording. Cam H2 initiates the transmission cycle during the time that cam H3 holds the circuits closed for high speed operation of the tape which has the complete recording.

The entire process is fully automatic, so that the signal originating at unit A may be continuous, regardless of time, and the system as described may be used to transmit intelligence over a period of minutes or hours, which would be impossible with a single tape unit.

Summarizing the operation of the transmitter as shown by Fig. 1, a method is shown whereby an original message covering the frequency range of 250 to 2500 C; P. S. and requiring 20 seconds to record, this being approximately equalv to 65 average words spoken at an average rate, has been transmitted in approximately one second under conditions where the modulating frequency was 50 k. c. and the frequency band occupied by the modulated carrier covered a range of 100 k. c.

A'. minimum of interference would be created with established service due. tothe extremely short timethat carrier power is radiated. Numerous transmitters of this typemay be operated on the. same carrier mid-frequency with the actual transmission periods arranged so as not to interfere with each other.

In the description of Fig. 1 and the example given in connection therewith, no mention was made as to output power of the carrier. In any existing system, the maximum output power for a particular transmitter is usually determined by'the maximum rating of the final R. F. amplifier tube or: tubes. In a transmitter of the type shown in connection with Fig. 1, the plate dissipation rating of the. final amplifier tube or tubes would no longer be. the limiting factor in determining maximum carrier output power for any 9. particular tube or tubes. This is so due to the extremely short time such tubes are required to deliver carrier power and therefore the limiting factors become:

First, the peak filament emission of the tube.

Second, the internal insulation of the tube.

Third, the external plate circuit insulation as awhole.

By choosing proper tubes having high internal insulation and by the use of properly designed external plate circuit arrangements. the plate input may be raised to a value limited by the peak filament emission of the tube, regardless of what the actual plate dissipation rating of the tube may be. By this method it is possible to radiate modulated carrier energy considerably in excess of the amount indicated by the size and rating of any particular combination of tubes.

Referring now to Fig. 2 which illustrates a receiver for use with the transmitter of Fig. 1.

Antenna I08 and round I I19 are connected to the input of radio frequency unit K, which is designed to have a normal amplification band width of 100 k. c. at a mid-fre uenc of 40 m. c. P represents the converter sect on in a superhetrodyne receiver and L the I. F. section. which is also des gned with a 100 k. c. amplification band wid h. N represents a detector c rcuit which performs the usual dual purpose of detection and developing automatic volume control volta e. T represents a trigger c rcu t usin a gas filled tetrode wh ch is control ed by received carrier power. It is understood that trig er nit T; as shown in detail by Fi 3. is used here merely for purposes of example. and that any other form of carrier o erated tr ger circ it may be used to control the receiver, various forms of such trigger circuits b in in common use. Unit is a wide band amplifier designed to amplify the output of detector u it N. Am l fier O is similar to amplifier D of the ransmitter in Fig. 1 and is desi ned to w k efii ent y in the freq encv range of to 50 k. c. Mag etic tape sections G3 and G4 ar very similar to the taco s ctions GI and G2 described in connection with F g. 1. Amplifier Q is a conventional audio amplifier designed to work at ordinary speech frequencies and R is a reproducer which may b of anv type desired.

Design of wide band units K. P. L and N is well known to those sk lled in the art. The output of I. F. unit L is connected to input term nals H0 and III of detector N. The AVC voltage developed by one diode section of detector N is available at terminals H2 and H3 where terminal I I2 is negative and terminal H3 is positive, in respect to each other without re ard to chassis ground. represented by common ground terminal I89 and B minus terminal 4. This AVC volta e may be used in a conventional manner as AVC voltage. and it is also used to control trigger unit T, as fully explained further below.

The arrangement of magnetic tape unit G3 is mechanically the same as the similar unit GI shown in Fig. 1. except that magnetic tane I M has a length of 65 it. The additional five feet is provided as tolerance for the release of trigger circuit P as hereinafter explained. With the circuit in standby position ready to receive a transmission, all relays are released as shown and indexing cam H4 has stopped cam shaft I16 in the position shown. When a transmission is received, trigger circuit T causes relay I48 to operate thus closing contacts I52 and initiating the following sequence of operation:

Relay I49 operates and locks up through its own contacts, remaining locked up during the entire time transmission is being received; timing motor MID is instantly energized; cam extension I13 of cam H5 immediately closes switch I55 thus operating relay I46 and closing all of contacts I50; clutch I891: operates thus connecting tape drum driving shaft IBM to high speed motor M8 so that magnetic tape I28a is moving at the 20 times normal speed.

The above sequence of operation takes place simultaneously. Recording is accomplished in a manner identical to that previously described in connection with Fig. 1, the plate circuit of amplifier 0 being connected to recording head I23a through closed contacts I58 of relay I46. amplifier O and obliterating head I25a being connected to a source of plate voltageJBO through this same set of contacts.

After the transmission cycle is completed and carrier power cut off at the transmitter, trigger circuit T will release relay I48, thus stopping the recording and obliterating cycle of tape unit G3. Simultaneously with the release of relay I48 cam H5 will release contacts I55 and relay I48 which also releases clutch two additional revolutions, magnetic tape I 2811 will come to a stop at the point controlled by indexing switch "I.

Magnetic tape I 28?) was moved from the atrest position controlled by indexing switch I12 during the transmission cycle, while contacts I52 of relay I48 were closed, so that magnetic tape I282), and tape unit G4 as a whole is in the reproducing position, the cathode circuit of mixer stage C4, represented by terminal I38b, being closed through the released contacts of switch I51 and cathode resistor I59 which is filtered by condenser I58. However, on the first reproduction cycle of a new transmission, no message will be reproduced.

Twenty seconds after the first transmission is received, a second transmission will be received, again operating relay I48; on this second trans mission, cam I14 will be in position'to close switch I56 thus causing the operation of relay I41. The recording process will now be repeated with tape unit G4 in a manner identical to that previous-- ly described for tape unit G3. I

Simultaneously with the start of the second transmission, reproduction of the first transmission will start with tape unit G3. Magnetic pick-up I33a is directly connected to mixer unit C3, the cathode circuit I38a being closed through the now operated contacts of switch I51, cam extension I15 of cam H1 being indexed to operate switch I51 simultaneously with the operation of switch I56,

Reproduction takes place through amplifier Q, which is a conventional audio frequency amplifier covering the voice frequency range, and reproducer R, which may be of any type desired.

The arrangement of magnetic recording, reproducing and obliterating heads of tape units G3 and G4 is similar to that previously described in connection with tape units GI and G2 of Fig. 1 except that in the receiver of Fig. 2, magnetic recording heads I24a and I241) are designed to record in the range of 10 to 50 k. 0., while magnetic reproducing heads I33a and I33b are designed to reproduce in the ordinary speech range of 250 to 2500 cycles. obliterating heads I25a I a. After one or' 11 and I 25b are identical with those used in the transmitter of Fig. l.

The recording and reproducing process will go on continuously as long as a transmission is being received, and reproduction wll be constant, as was recording at the transmitter. The position of tape units G3 and. G4 is under control of indexing switches Ill and I12 so that recording and reproduction always start. at the same point, thus preserving the continuity of the original message.

Timing motor MI!) will operate as long as relay I49 remains locked up. The circuit may be restored to a standry position, ready to receive additional transmiss ons, by momentarily depressing push button switch I54, thus releasing relay I49 and stopping timing motor MW at the position controlled by indexing cam H4. The entire arrangement of timing unit 8, as shown within the broken line I II, is similar to timing unit J of the transmitter of Fig. 1 except that in this instance an indexing cam is used to control the standby position of the timer. It is understood that other timing arrangements may be used although the synchronous motor driven units shown in Figures 1 and 2 are of a type in common use and these may be designed to provide a high degree of precision in the timing interval.

Since the gas filled tetrode used in trigger unit T may remain conductive for approximately one quarter cycle of plate voltage after removal of the positive grid voltage and. since the release time of relay I48 may approximate .004. second, a tolerance of approximately .008 second is required to prevent obliteration of the initial part of the recorded signal. Sufficient tolerance is provided by the additional length of five feet in magnetic tapes H811 and I281).

Fig. 2 does not show any method of indicating that a message is being received and that a recording is being made. It is understood that additional contacts may be used on relay I48 so that the operator will receive visible or audible indication that a message is being received and recorded.

In the example given above, a complete signaling method has been described where an original message to be transmitted was recorded continuously in 20 second time intervals and transmitted as a series of one second pulses of modulated carrier energy. In the example given, the circuits shown by Figures 1 and 2 were simplified for reasons of clarity, it being understood that full automatic operation of both transmitter and receiver may be arranged.

In the transmitter of Fig. 1, the transmission time cycle is controlled by contacts 59 of relay operating as break-in contacts in the cathode circuit of final R. F. amplifier tube T3, it being understood that numerous alternate methods exist for exact timing of the period that carrier power is radiated. For example, the break-in may be in the screen grid voltage supply. As a further example, the break-in may be between oscillator F and final amplifier W so that no energy is available at the input to amplifier W until proper control circuits are closed. As a further example, this break-in control may be directly in the oscillator circuit with all butter and amplifier stages in full operating condition so that carrier power will be radiated for such time period as initial carrier energy is supplied by said oscillator. Other alternate methods will be apparent to those skilled in the art.

Referring now to Fig. 3, which shows in detail the schematic of unit T of Fig. 2, all that part shown inside the broken line I93 of Fig. 3 representing unit T of Fig. 2, all parts outside this broken line being a sectional view of part of Fig. 2, and the parts shown are numbered identical to those in Fig. 2.

As previously mentioned, the AVG voltage developed by detector unit N when a carrier is received, is available at output terminals '2 and H3, where H2 is negative and H3 positive, in regard to each other, without respect to chassis ground.

Vacuumv tube H1 is a gas filled tetrode having a plate I18 energized in series with D. C. relay I48 coil and variable resistor I89 by alternating current supplied by isolation transformer I90, which is connected to a. source of alternating urrent ISI. This transformer is used to make the trigger circuit ground free, and inthe example given, the A. C. plate voltage supplied by transformer I90 secondary would approximate 100 volts. Variable resistor I89 is designed to limit the current flowing through relay I48 coil, which is of the D. C. type and in this example may have a resistance approximating 2500 ohms. Condenser I88 would then approximate 2 ,uf. and is designed to prevent relay chatter'. Suppressor grid I19 is biased negative in respect to cathode I82 by bias cell I8I, the voltage of which would approximate 4 volts in this example. Filament I83 is connected to a source or filament supply mm. Control grid I88 is connected to slider I86 of potentiometer I84, the outside terminals of this potentiometer I85 and I8! being connected as shown, so that terminal I85 is positive and I8I is negative.

It is characteristic of certain gas filled. tetrodes that where the suppressor grid is biased negative the control of the control grid rests entirely in the positive region. Slider I86 is adjusted so that control grid I is normally neutral or slightly negative, so that with no signal being received and therefore no AVC voltage available at detector N output terminals H2 and I I 3, tube I'II' remains deionized and draws no plate current, so that relay I48 remains released. When carrier power is received and AVC voltage is available at output terminals III and H3 or detector N, then terminal I becomes positive in respect to terminal I81 and therefore control grid I80 also becomes positive in respect to cathode I82, which causes tube IT! to ionize immediately, thus drawing plate current and operating relay I48.

In the examples given, the recording/transmission ratio was taken as 20 to 1, it being understood that any other practical recording/transmission ratio may be used. The maximum possible recording/transmission ratio is a function of equipment design and it is understood that the recording/transmission ratios used by way of example are in no way limiting in scope as to minimum or maximum recordlug/transmission ratios which may be used in the practice of this invention.

It is understood that the maximum possible carrier output power which may be obtained by methods herein disclosed is also a function of equipment design and that these specifications are in no way limiting in scope as to maximum possible carrier output power which may be obtained by the methods disclosed by this invention.

The above examples are for the purpose of illustrating some of the methods and means by which the broad purposes of the invention may 'be carried out and are not to be deemed as restrictive in any manner. Other modifications and alternatives will occur to those skilled in the art without departing from the scope of the invention as defined by the following claims.

ducing said recorded signals with a magnetic tape speed higher than said speed of recording, modulating a high frequency carrier with said reproduced signals, transmitting such modulated high frequency carrier to a receiver, demodulating the received modulated high frequency carrier, recording the product of demodulation and obliterating the previous recording alternately on two magnetic tapes moving at a speed approximating said reproducing tape speed at the transmitter and continuously reproducing the signals so recorded at a tape speed approximating said recording tape speed at the transmitter to recover the original signal.

2. The method of signaling, which comprises the steps of recording the intelligence to be transmitted and obliterating the previous recording alternately on two magnetic tapes within a limited time period, alternately reproducing said recorded signals in a fraction of said recording time, mod ulating a high frequency carrier with said reproduced signals, transmitting such modulated high frequency carrier to a receiver, demodulating the "received modulated high frequency carrier, re-

cording the product of demodulation and obliterating the previous recording alternately on two 'magnetic tapes within a time period identical to said reproducing time period at the transmitter and continuously reproducing the signals so recorded within a time period approximating said recording time period at the transmitter to recover the original signal.

3. The method of signaling, which comprises the steps of recording within a limited time period the intelligence to be transmitted and oblitcrating the previous recording alternately On two magnetic tapes moving at relatively slow recording speed, reproducing said recorded signals at relatively high magnetic tape speed to produce a series of modulating pulses, modulating a high frequency carrier with said modulating pulses, transmitting said pulses of modulated high frequency carrier to a receiver, demodulating .the received pulses of modulated high frequency carrier, recording the product of demodulation and obliterating the previous recording alternately on two magnetic tapes moving at a speed approximating said reproducing tape speed at the transmitter and continuously reproducing the signals so recorded at a tape speed approximating said recording tape speed at the transmitter to recover the original signal.

4. In a'signaling system including a transmit ter, a receiver,'and a channel between said transmitter and receiver, the method of reducing the total time required for transmitting a signal which comprises the steps of recording the signal to be transmitted and obliterating the previous recording alternately on two magnetic tapes, alternately reproducing said recorded signals in a fraction of the recording time, transmitting said reproduced signals to a receiver,

recording such received signals and obliterating previous recordings alternately on two magnetic tapes within a time period identical to said reproducing time period at the transmitter and continuously reproducing said recorded signals within a total time period approximating said total recording time period at the transmitter.

5. In a speech transmission system including a transmitter, a receiver, and a channel between *said transmitter and receiver, the method of reducing the time required for transmitting a signal which comprises the steps of recording the signal to be transmitted and obliterating the previous recording sequentially on a plurality of magnetic tapes, reproducing saidrecorded signals in the recorded order on each tape and alternately from the two tapes in a fraction of the recording time, transmitting said reproduced signals to a receiver, recording such received signals and obliterating the previous recording sequentially on a pluraltiy of magnetic tapes within a time period identical to said reproducing time period at the transmitter and reproducing said recorded "signals in the recorded order on each tape and alternately from the two tapes within a time period approximating said recording time period at the transmitter.

6. In a signaling system, means for converting an original signal occupying a given time interval into a second signal of a series of relatively short time intervals comprising a plurality of magnetic tapes, driving means for each tape having two speeds, a recording head for each tape. an obliterating head disposed adjacent to each recording head, means for sequentiallyrendering each of said obliterating heads operative to condition each said tape to receive said signals from each of said recording heads, means for simultaneously rendering the adjacent recording head operative to record a part of said original signal on its associated tape while said driving iii the lower of the two speeds, a reproducing head for each of said tapes, means for sequentially rendering each of said reproducing heads operative to reproduce said recorded signal while said driving means causes the associated one of said tapes to describe one circuit at the higher of the two speeds, and means for causing said signals to be reproduced in the order of their recording.

7. In a signaling system, means for converting an original signal occupying a series of relatively short time intervals into a second signal occupying a, relatively long time interval comprising a plurality of magnetic tapes, driving means for each tape having two speeds, a recording head for each tape. an obliterating head disposed adjacent to each recording head, means for sequentially rendering each of said obliterating heads operative to condition each said tape to receive said signal from each of said recording heads, means for simultaneously rendering the adjacent recording head operative to record a part of said original signal on its associated tape while said driving means cause said tape to describe one circuit at the higher of the two speeds, a reproducing head for each of said tapes, means for sequentially rendering each of said reproducing heads operative to reproduce said recorded signal while said driving means causes the associated one of said tapes to describe one circuit at the lower or the two speeds, and means for causing said signals to be reproduced in the order of their "recording.

obliterating-a previously recorded message at slow speed in alternation on a pair of magnetic tapes,

reproducing the message of each tape in a fraction of the time required for recording, the recording and obliteration on one tape being effected while the message is reproduced on the other tape, modulating a high frequency carrier with said reproduced signals, transmitting such modulated high frequency carrier to a receiver, demodulating the received modulated high frequency carrier, recording the product of demodulation and obliterating the previous recording on one of two magnetic tapesmoving at a speed approximating said reproducing tape speed at the transmitter and continuously reproducing the signals so recorded at a tape speed approximating said recording tape speed at the transmitter to recover the original signal.

9. In a signaling system including a transmitter, a receiver, and a channel between said-transmitter and receiver, the method of reducing the total time required for transmitting a signal which comprises the steps of recording the signal to be transmitted and obliterating any previous recording on apair of magnetic tapes in sequence, reproducing said recorded signals in the order of ,recordation in a fraction of the time required for each recordation, transmitting said reproduced signals to a receiver, recording such received signals and obliterating previously recorded signals successively on a pair :of tapes within a time period identical to the reproduction time period at the transmitter, and-continuously reproducing said-recorded signals in the order of recordation within a total time period approximating said total recording time period at the transmitter.

10. In a signaling system including a transmitter, a receiver, and a channel between said transmitter and receiver, the method of reducing the total time required for transmitting a signal which comprises the steps of recording the signal to be transmitted and obliterating the previous recording alternately on two magnetic tapes, reproducing said recorded signals in a fract on of therecording time, in opposite alternation, transmitting said reproduced signals to a receiver, recording such received signals andobliterating the previous recordings alternately on two magnetic tapes withina time period identical to said reproducing time period at the transmitter and continuously reproducing said recorded signals, in opposite alternation, within a total time period approximating said total recording time period at the transmitter.

11. In a signaling system including a transmitter. a receiver, and a channel between said transmitter and receiver, the method of reducing the total t me required for transmitting a signal which comprises the steps of recording the signal to be transmitted and obliterating the previous recording sequentially on a plurality of magnetic tapes, reproducing said recorded signals in their recorded order in a fraction of the recording time, transmitting said reproduced signals to a receiver, sequentia ly recording such received signa s and obliterating previous recordings on a plurality of magnetic tapes within a time period identical to said reproducing time period at the transmitter and continuously reproducing said recorded signals in their recorded order within a total time period approximating said total recording time period at the transmitter.

12. The method of signaling, which comprises the steps of recording the intelligence to be transmitted and obliterating the previous recording alternately on two magnetic tapes moving at relatively slow recording speed, reproducing said recorded signals in the recorded order on each tape and alternately from the two tapes with a magnetic tape speed higher than said speed of record? ing, modulating a high frequency carrier with said reproduced signals, transmitting such modulated high frequency carrier to a receiver, demodulating the received modulated .hlgh frequency carrier, recording the product of demodulation and obliterating the previous recording alternately on two magnetic tapes moving at a speed approximating said reproducing tape speed at the transmitter and continuously reproducing the signals so recorded in the recorded order on each tape and alternately from the two tapes at a tape speed approximating said recording tape speed at the transmitter to recover the original signal.

13. The method of signaling, which comprises the steps of recording the intelligence to be transmitted and obliterating the previous recording alternately On two magnetic tapes within a limited time eriod, reproducing said recorded signals in the recorded order on each tape and alternately from the two tapes in a fraction of said recording time, modulating a high frequency carrier with said reproduced signals, transmitting such modulated high frequency carrier to a receiver, demodulating the received modulated high frequency carrier, recording the product of .demodulation and obliterating the previous recording alternately on two magnetic tapes within a time period identical to said reproducing time period at the transmitter andc-ontinuously reproducing the signals so recorded in the recorded order on each tape and alternately from the two tapes within a time period approximating said recording time period at the transmitter to recover the original signal.

14. The method of signaling, which comprises the steps of recording within a limited time period the intelligence to be transmitted and obliterating the previous recording alternately on two magnetic tapes moving at relatively slow recording speed, reproducing said recorded signals at relatively high magnetic tape speed in the recorded order on each tape and alternately from the two tapes to produce a series of modulating pulses, modulating a high frequency carrier with said modulating pulses, transmitting said pulses of modu ated high frequency carrier to a receiver, demodulating the received pulses of modulated high frequency carrier, recording the product of demodulation and obliterating the previous recording alternately on two magnetic tapes moving at a speed approximating said reproducing tape speed at the transmitter and continuously reproducing the signals so recorded in the recorded order on each tape and alternately from the two tapes at a tape speed approximating said recording tape speed at the transmitter to recover the original signal.

15. In a signaling system including a transmitter, a receiver, and a channel between said transmitter and receiver, the method of reducing the total time required for transmitting a signal which comprises the steps of recording the signal to be transmitted and obliterating the previous recording alternately on two magnetic tapes, alternately reproducing said recorded signals in the recorded order on each tape and alternately from the two tapes, in a fraction of the recording time, transmitting said reproduced signals to a receiver, recording such received signals and ob- 17 literating previous recordings alternately on two magnetic tapes Within a time period identical to said reproducing time period at the transmitter and continuously reproducing said recorded signals in the recorded order on each tape and alternately from the two tapes within a total time period approximating said total recording time period at the transmitter.

16. The method of signaling which comprises the steps of continuously recording a signal and obliterating a previously recorded signal at slow speed in alternation on a pair of magnetic tapes, reproducing the recording of each tape in a fraction of the time required for recording, the recording and obliteration on one tape being effected while the signal is reproduced in the recorded order on the other tape, modulating a high frequency carrier with said reproduced signals, transmitting such modulated high frequency carrier to a receiver, demodulating the received modulated high frequency carrier, recording the product of demodulation and obliterating the previous recording in alternation on two magnetic 18 tapes moving at a speed approximating said reproducing tape speed at the transmitter and continuously reproducing the signals so recorded in the recorded order at a tape speed approximating said recording tape speed at the transmitter to recover the original signal.

STANLEY D. EILENBERGER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS

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