US1798066A - Method of and means for receiving, recording, and reproducing broadcast signals - Google Patents

Description

March 24 1931.

METHOD OF E. E. CLEMENT 1,798,066 AND MEANS FOR RECEIVING, RECORDING,

, ii-57d I swank will) Patented Mar. 24, 1931 UNITED STATES PATENT OFFICE EDWARD E. CLEMENT, OF WASHINGTON, DISTRICT OF COLUMBIA, ASSIGNOR TO EDWARD F. COLLADAY, OF WASHINGTON, DISTRICT OF COLUMBIA METHOD OF AND MEANS FOR. RECEIVING, RECORDING, AND REPRODUCING BROADCAST SIGNALS Application filed November 27, 1926.

My invention relates to means for receiving, recording, and retrausmitting electrical signals, and particularly those transmitted as modulations on high frequency carrier waves carrier current. It is closely related to the invention described and claimed in my prior copend ng application, Serial No. 7%,357, filed October 28, 1924-, patented July 5, 1927, No. 1,635,153, the present application 10 being a continuation in part of said prior application and to read in connection therewith, especially with respect to the double modulation of carrier waves, and the transmission and reception of such doubly modulated 1Z5 waves.

My present invention has for its objects the production of an improved method of receiving modulated high frequency waves, and of recording and reproducing the same and the signals modulated thereon.

In my aforesaid prior application I have described a system wherein long or intermediate frequency waves are modulated with audio signals and are then relayed or distrihuted alternatively overwires or as secondary modulations upon short wave carriers. It was assumed in said description that with the resources of the art it was then possible to record waves above the band of frequencies usually employed in audio modulations, or in other words above the limits of audition, and to reproduce the same using either the method of cutting by means of a stylus, or photographic methods, for this purpose. In the present case however, while a photographic method is preferably employed for making the record, reproduction does not depend upon reproducing the high frequency wave component of the record, but preferably involves the reproduction of the audio envelope only. This method may be employed with methods of transmitting either by means of amplitude variations or by means of frequency variations. In the former case the amplitude varies, with or without variation of the frequency according to the nature of the signals; and in the latter case the amphtude remains constant and the frequency varies in a ratio which is itself variable according to the characteristics of the signals.

Serial 110. 151,118.

Transmitting by the former or amplitude variation method is usually practiced by Varying the current in the antenna circuit, a well known circuit for this purpose being that known as the Colpitts-I-Ieising circuit, illustrated and describe-d in the book called Elements of Radio Communication by E. IV. Stone, published by Crosby, Lockwood & Son, London, 1923, and for sale through D. Van Nostrand Co. in New York. The latter or frequency variation method is used for telegraphy in the well known Poulson are system and is described in various patents of which that to John Hays Hammond dated March 4, 1919, No. 1,296,504, is an example. The invention in the present case does not concern itself with the methods of modulation and transmission employed but only with the method of and apparatus for receivmg.

Briefly stated, I attain my objects by using a recording device such as the oscillograph, which will move a point of light or the equivalent across a rapidly moving photographic film. Where the modulation is by means of varying the amplitude of the carrier waves, the excursions of the light wave on the film will be shortened or lengthened in accordance with the variations in strength of the currents received, so as to produce a continuous picture or image of the audio envelope, which by careful adjustment, may be of uniform clarity or density in the positive or negative film as the case may be. To reproduce the audio modulations, it is only necessary to run this film over a proper aperture between a source of light and a sensitive device such as a photoelectric cell. The individual carrier waves will not be reproduced, and will preferably not appear in the record, the body of the an lio frequency envelope being sufficient without any detection or other intermediate step to demodulate. I am aware that it has been proposed in the past to record and reproduce sound waves photographically in a manner somewhat similar to this, but it is pointed out that there is a great diiference between directly recording the sound waves, and recording as in the present case the high frequency waves in such manner as to reconsider however that the method employii a produce their modulation envelope only, without a detector.

Where the frequency variation method is employed the oscillograph may be used, but in this case the intensity of the light employed, and the time of exposure per unit of travel of the light ray, are so regulated that complete reduction of the salts in the film does not take place except by repeated passage of the light Wave over substantially the same path. Thus, Where the normal frequency may produce a medium gray effect in the film the lowering of the frequency (and the consequent spreading of the carrier waves) will produce a very light gray or -hiteeifect, while the raising of the frequency (bunching the carrier Waves .closer together), will produce a dark gray or black effect on the film. lVhen this film is developed, the recorded parts will show a continuous strip of shad-ings in transverse periodic series of bands whose rate of change from dark to light and light todark again is in exact proportion to the rate of change at any given instant in the audio signals by which they were produced. By running this film over a sensitive device such as a photoelectric tube, with a suitable source of light and a'suitably arranged aperture for the same, the electric currents in the output circuit of the tube will vary in strength in exact proportion to the variations in the depths of shading in the record.

There is a short method of receiving which is possible by employment of the principle mentioned, i. e. the use of an oscillograph responding to the short carrier waves to produce variation in light in accordance with the modulations. This consists in causing the oscillograph to move a point of light across an aperture which may be covered by ground for example, beneath which a light sensitive device such as a photoelectric tube is arranged. As a modification the oscillogrz-iph light point might be caused to fall directly upon the sensitized element in the tube. in either case the amount of light would vary either as to frequency or as to amplitude of travel of the point. as the case might be. I

a photographic record surface is the most satisfactory at present.

My invention is illustrated in the accom panying drawings, in which Fig. 1 is a diagram of the circuits at a relay station in a broadcast distributing system, showing recording and reproducing means for the modulations on intermediate frequency carrier waves. 7

Fig. 2 is a schematic diagram showing symbolically an oscillograph, and a fragment of a photographic record tape traveling across theilight field thereof.

Fig. 3 is asimilar diagram showing rc producer with a source of light, a photomodulation a modulated intermediate fre quency Wave, hereinafter termed an IF wave, carried by a primary or radio carrier Wave, and retransmit the same by double modulation on a different radio wave. It comprises a receiving antenna, coupled through coupler 2 to the input terminals of a first detector 3 arranged to demodulate the radio carrier waveto detect off the IF wave and transfer the same through coupler 4 to the input terminals of an amplifier 5. The output circuit of amplifier 5 is coupled through coupler 6 to a modulator tube 7 operatively connected with the oscillator 8 to modulate the output thereof, the oscillator being arranged to generate the second or retransmitting radio wave and transmit the same through radiating antenna 9. A transmitting cut-off jack J is connected between the output circuit of first detector tube 8 and the primary of coupler 4 in the manner indicated to enable a local source of modulated IF waves to be directly connected with the terminals of the primary winding of coupler 4 independently of the first detector output circuit. A receiving cutoff jack J is connected between the secondary of the coupler 6 and the input circuit of the modulator tube 7 in the manner indicated to enable a recorder to be directly connected with the terminals of the secondary winding of coupler 6 in place of the input of the modulator tube 7 An electric signal recorder 10 is arranged with its input circuit connected to the terminals of a recording jack J A reproducing set 11 consisting of a reproducer 12, modulator 13 and oscillator 14 is arranged with its output circuit connected to the terminals of a rep'roducer jack J A plug and cord circuit comprising a pair of plugs P and P connected through a simple flexible cord conductor, as shown, is provided for connecting jack J and J or J and J accordingto whether incoming signals are to be recorded or recorded signals are to be transmitted. Jacks J and J form the terminals of receiving and transmitting trunks 15 and 16 respectively,which may be used as a transmitting medium for the IF frequency in place of the radio frequency carrier by connection of J 7 and J or J and J through plug and cord circuits such as P 'P As shown in Fig. 2, the apparatus elements comprising the recorder 10 consist of an oscillograph arrar'igement of the reflecting galvanomcter type, having a movable loop 17 carrying a mirror 18, and supported in the permanent magnetic field of a pair of magnets 19 and 20, maintained energizedby a suitable source 21. The terminals of the loop 17 are connected to the terminals of the recording jack J A source of light 22 is arranged to cast a concentrated beam 23 through a condensing lens 24 onto the mirror 18 from whence it is reflected onto the recording film 25, the several parts being so spaced and arranged that the beam Wlll be brought to a point in the plane of the film 25. The film 25 is maintained in a fixed plane by means of the rollers 2627, over which it is fed at a constant speed by suitable web feeding apparatus, not shown.

In Fig. 3 are shown the apparatus elements of the reproducing set 11, which consists of a light proof box or chamber 28, having a slot 29, over which the record film 25 to be reproduced is arranged to be passed at a constant given speed guided by rollers 43 and 44 and driven by suitable web feeding apparatus, not shown. Outside of the box 28 is a source of light 42, arranged to cast its rays through the record film 25 and slot 29 into the box 28, the slot 29 being arranged to include substantially the entire width of the record tape in its length and to be sufiiciently narrow to uncover only a very small portion of the record in the direction of the record length, less than that required to record the shortest variation. \Vithin the box 28 is a photoelectric cell 30 arranged to be acted upon by the light passing into the box through the slot 29. This photoelectric cell 30 may be of any known or other suitable type and is here shown as one consisting of an evacuated vessel 31 containing two electrodes, an anode 32 in the form of a ring of nickel or the like, and a cathode 33 in the form of a curved plate carrying a layer of metallic potassium or other sensitizing material. The electrodes of the light cell are connected as shown in series with a battery 34 and the primary of a transformer 35, the battery being connected with its poles arranged as shown to maintain the anode element 32 positive with relation to the cathode element. The secondary of transformer 35 connects with the input terminals of an amplifier 36, whose output circuit, including the plate battery 37 is connected to the output leads 38 leading to a modulator 13 and oscillator 14 to the terminals of the reproducer jack J. A pair of branch leads 39 extend the circuit from the output circuit 38 to the stationary contacts of a switch 40, whose movable contacts are connected to a suitable elephone receiver, such as the head set 41..

In the operation of the recorder, (Fig. 2), assuming the IF wave to be modulated as to amplitude, the modulated IF wave traverses the loop 17, by way of jack J causing the loop and with it the mirror 18 to oscillate with the IF carrier wave through a variable path, the variations being proportional to the modulations on the carrier wave. The effect of these oscillations of the mirror is to cause the light beam 23 to trace an envelope corresponding exactly to the modulations, as shown in Fig. 4. This trace is solid, the rapidly recurring single wave oscillations being overlapped to a degree determined by the speed of travel of the film, which is regulable, and the width of the slot 29.

In the operation of the reproducing set 11, (Fig. 3), the record film 25, produced as in 2 is caused to pass over a slot 29, be-

tween the same and the source of light 42, as

indicated by the arrow. As the record passes over the slot the total amount of light passing through the slot onto the photoelectric cell 30 varies as the variations in the width of the recorded envelope. The film so used should be the positive, printed from the negative, in which. case the variations would be directly as the variations in Width. This however is a matter of detail to be arranged as convenient. As the light varies, the current varies in the primary of transformer 35,v

and correspondingly in the secondary, and the variations are amplified by tube 36, and so pass to the modulator 13 and are modulated on the current produced by the IF oscillator 14, and thence put onto the terminals of the reproducer jack J. The frequency of the oscillator 14 may be anything desired, for either wire transmission or radiation, by single or double modulation.

A. modified form of record is shown in Fig. 5. This would be the record produced on the film 25 of the recorder of Fig. 2, where the IF waves energizing the loop 17 are mod ulated in terms of frequency and not in amplitude, the latter remaining constant. It is known that by varying the constants in a transmitter circuit, the frequency of the {ransmitted wave may be varied continuousysound Waves, the frequency varies at a rate which is the ratio otvariation of the sound waves, (assuming a pure tone) or at a rate which constantly changes as the several C0111- poncnt sound waves change, being thus an aggregate or band of frequencies constantly changing.

lVith such a record, the amplitude of the modulation is represented by variations in the extent of ch ange in frequency at any given instant, and the pitch would be represented by the rate of change. Thus a rate of change in frequency of sixty-four per second would represent an organ note of that pitch. If the note be loud, the degree of change might be from the normal frequency say of 20,000 to 25,000 or more, which note not so loud might be produced by a degree of change from 20,000 to 21,000.

In making such a record, the oscillograph lVhere the varying agency consists of tion and not-of limitation.

would. swing through a substantially constant path, but vary as stated in degree and frequency of change'i. e, in speed. The result is a continuous trace varying in depth, or shading,.which in the positive film need only be passed across the slot 29 as already described to'vary directly the amount of light falling in the photoelectric cell 30, in accord ance with the frequencyvariations of the IF waves received. It will be apparent as before, that by properly adjusting the speed of travel of the film, the audio modulations will be reproduced directly without the usual intermediate stage of detection.

It to be understood that the foregoing specific description is for purposes of defini- The photoelectric cell, and the selenium cell, and other devices, have been widely used for reproducing photographic traces of sound waves, and any of theseknown or other suitable devices may be used in my system to practice my method without departing from the invention, a main feature of which is to record modulated superaudio waves, and reproduce the modulations only, without other detection. This may be called graphic detection, and I shall claim the same herein broadly as my own.

lVhat I claim is:

1. The method of receiving and reproducing signals modulated upon a wave train of carrier frequency, which consists in causing the individual waves of said train to act successively at constant intensity upon a photographic record surface so as to produce thereon a record having light transmitting characteristics varying in exact accordance with the variations in said waves due to the signals modulated thereon, and causing such modulated variations of the record to corre- :pondingly vary the current flow in a signal reproducing circuit, for reproduction of the modulated signals only.

The method of receiving and reproducing low frequency signals modulated upon a wave train of intermediate frequency which in turn is modulated upon a carrier wave train of high frequency, which consists in first detectin off the intermediate freouenc carrier wave, then causing the individual waves of said intermediate frequency to act successively upon a photographic record surface so as to produce thereon a record having I light transmitting characteristics varying in exact accordance with the variations in said waves due to the signals modulated thereon, and finally causing said variations of the record to correspondingly vary the current flow in a signal reproducing circuit for reproduction-of the modulated signals only.

3. The method of recording and repro ducing modulations on high frequency waves, which consists in recording radiant traces representing individual waves of the modulated wave train upon a traveling surface,

and regulating the time .of travel so that by the-combination of the individual traces a continuous record'willbe produced for'reproducing summatic effects due to the low frequency modulations only. 7

l. The method of recording and reproducing modulations on high frequency waves, which consists in causing radianttraces representing the modulated waves received and causing the same to produce upon a traveling record surface traces transverse to the direction of travel of constant intensity but varying geometrically in accordance with the modulations imposed thereon, the relative speed of travel of the record surface and the recording means being so adjusted that the completed record will be continuous and c'apable of reproducing the signal modulations or variations without reproducing individual waves of the carrier wave train received.

The method of recording and reproducing modulations on high frequency waves, which consists in producing an oscillograph record of radiant traces representing the modulated waves received, and timing the record surface so that the individual wave traces will come together to form a practically continuous trace thereof varying in its light transmitting characteristics according to the undulations.

6. The method of relaying signals of the class described employing a high frequency carrier doubly modulated, which consists in receiving and demodulating the high frequency carrier and causing the intermediate frequency carrier to produce a record, then causing said record to reproduce the same modulated intermediate frequency waves in a modulating circuit, modulating the same thereby upon another high frequency carrier, and finally reradiating the same.

7. The method of relaying claimed in claim 6 with the additional step of transferring the intermediate frequency modulated wave train when detected oil the incominghigh frequency carrier, either tov a modulating and a radiating circuit tuned to another high carrier frequency, or to a recorder adapted to transfer modulated intermediate, frequency wavesto said second modulating circuit, alternatively, whereby signals may he relayed without change in current or method, either as a continuous act, or after a time interval.

8. The method of relaying claimed in claim 6 with the additional step of receiving and detecting the modulated intermediate fro-- tuned to receive doubly modulated high frequency waves, a detector, an amplifier tuned to receive the intermediate frequency waves only, means adapted to form a. continuous record of said intermediate frequency waves, varying in its light transmitting characteristics, according to the low frequency signals modulated thereon, means cooperating with said record to reproduce the same intermediate frequency carrier wave with its modulations, a secondar r tuned input circuit for the same, and a mo ulator-transmitter circuit adapted to receive and modulate the same upon another high frequency carrier wave, and radiate the latter.

10. Relay and reproducing apparatus for radio broadcast systems employing a doubly modulated carrier wave, which consists of a detector, an amplifier, a modulator-transmitter and a radiator, with cut-in switching means interposed between the first detector and the amplifier, and other cut-in switching means interposed between the amplifier and the modultutor-transmitter, means for record ing modulated intermediate frequency waves only, and means for reproducing said modulated intermediate frequency waves, together with means for connecting said recorder and said reproducer through said cut-in switching means to receive and record the inc0ming waves or to transmit the same modulations through the outgoing modulator-transmitter circuit.

In testimony whereof I hereunto affix my signature.

EDWARD E. CLEMENT.

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