US2207620A - Wave signaling method and apparatus - Google Patents

Description

July 9, 1940- R. A. HILFERTY WAVE SIGNALING METHOD AND APPARATUS Filed July 1, 1939 INVENTOR 22%/ RNY Patented July 9, 94

UNITED STATES WAVE SIGNALING METHOD AND APPARATUS Richard A. Hilferty, Oceanside, N. Y., assignor to Press Wireless, Inc., Chicago, Ill., a corporation of Delaware Application July 1 1939,v Serial No. 282,356v

12 Claims.

This invention relates to wave signaling systems and more especially to methods and apparatus for compensating for level variations in a transmitting medium of the wire or wireless type.

A principal object of the invention is `to provide an improved method of transmitting signals, such for example as tele-facsimile signals, telegraph signals and the like, so that the selective y fading characteristic of the transmitting me- 10 dium are compensated for.

Another object is to provide a system of telefacsimile transmission wherein the facsimile signals are simultaneously transmitted on a band vof heterogeneous carrier or sub-carrier frequen- Cil l5 cies having predetermined lower and upper frequency limits.

Another object is to provide a system of auto- `matic gain control for a receiving amplifier which utilizes the average amplitude of a received heterogeneous frequency band for level control, thus avoiding the drawbacks incident to the use of a single level control frequency. In accordance with this object, instead of transmitting the signals as a modulation of a single fundamental frequency carrier or as a modulation of a plurality of simultaneous carriers of one or more dominant or fundamental frequencies, the signals are transmitted as modulations of a noise band carrier, that is one wherein a large number of frequencies are generated but without any dominant frequency. An example of such a heterogeneous frequency band may be mentioned the noise currents'which are generated when an oscillating circuit is suddenly quenched at a lower rfrequency, of which a super-regenerative oscillao tor of any known type is typical. I have found that this ordinarily undesirable noise or hissing can be practically utilized as a carrier or subcarrier for signals such as facsimile signalsor the like, and that it has highly advantageous results when used for automatic level control at a receiving station.

Accordingly a feature of the invention relates to a signaling system wherein a noise gen-l erator of a heterogeneous frequency band is employed as a carrier or sub-carrier of message signals such as telegraph signals, facsimile signals and the like.

Another feature relates to a signal transmis- -0 sion system wherein a heterogeneous carrier frequency band is employed so as to produce a smooth noise or hiss over a relatively wide portion of the audio-frequency spectrum, the amplitude ofthe band being modulated in'accord- ,55 ance with signals to be transmitted. I

(Cl. Z50-6) tion of the following detailed descriptions andl the appended claims. 10

inasmuch as the invention in its broad aspects is concerned with the method and apparatus of generating and utilizing a carrier or sub-carrier of the heterogeneous frequency band type, only those parts of a signaling system are shown l5 as will enable the inventive concept to be understood and practised. Accordingly, apparatus land connections which are old or well-known in the art will be illustrated in the drawing in simpliiied or diagrammatic form. 20

Fig. l shows in schematic form a transmission system embodying features of the invention.

Fig. 2 is `a modification of Fig. l.

i Fig. 3 isa more detailed diagram of` connections of the system of Fig. 2 showing the hetero- '25 geneous frequency generator that may also be used in the system of Fig. l.

ylt can be shown by mathematical analysis that noise generated by the excitation or quenching 'of anoscillating circuit by means of other 30 waves of steep wave front, consists of innumerable frequency components. Thus where the excitation or quenching is by means of a square wave pulse, the frequencies .are theoretically .infinite in number. It is known that by using a grid-controlled vacuum tube and exciting it with square iront waves, that .-a series of separate frequencies are produced in the output f circuit, however these frequencies are generated on a regurla'rly continuous time basis'and successively higher frequencies are regularly recurrent as far..A as the time axis is concerned. Such an arrangement is quite commonly used in carrier telephone systems. In oontradistinction to this known type of harmonic or homogeneous frequency generator, the present invention contemplates the generation and utilization of the innumerable component frequencies which are generated ona c. random time axis and willbe referred to herein as a heterogeneous frequency band generator. 50 It is possible by proper excitation of a grid-controlled vacuum tube yto generate such hetero- `geneous frequency rbands so that the recurrence of the individual frequencies of the band is not on a harmonic basis, and ,the frequency recurrence of any given Wave or waves is fortuitous. When such heterogeneous frequencies are generated in the audio frequency band they are evident to the ear as a smooth hissing in which no particular frequency is outstanding or dominant. In accordance with the present invention, a predetermined portion ofthe spectrum of such a heterogeneous frequency generator is selected for example by band filters or the like, and this band is used Yas a carrier or sub-carrier of signals to be transmitted over a wire channel or a wireless channel. Consequently since at any given instant at the receiver a signal is represented by an innumerable number of individual heterogeneous carrier or sub-carrier frequencies, the effects of selective fading can be materially reduced at the receiver. One manner of utilizing this heterogeneous frequency band is to apply it to control the generationof an automatic volume control signal for application to a receiving amplifier. Thus where the signals are transmitted over a radio channel, it is well-known that individual frequencies are very often subjected to selective or non-uniform fading. Consequently two frequencies separated by a small fraction of a percent will not always fade simultaneously and to the same extent. In the usual system of amplitude-modulated carrier transmission Where the transmission is carried out by means of a subcarrier usually of audio frequency, only a single sub-carrier is employed for the signals. Consequenly at any given instant at the receiver, there will be present the sub-carrier frequency and its upper and lower side-band frequencies. It is the usual practice in applying A. V. C. signals at the receiver, to generate such A. V. C. signals in accordance with the received carrier or sub-carrier frequency alone, and the side-band fequencies may fade in and out irregularly with respect to the carrier or subi-carrier although the latter may be held quite constant. In accordance with the present invention, by using sub-v carrier frequencies of the heterogeneous frequency band type and utilizing these hetero-- geneous frequencies for A. V. C. control purposes there is assured at all times a considerable number of frequencies for the A. V. C. control. Consequently even if some frequencies are actually missing as a result of selective fading, there will be sufficient remaining to prevent a complete loss of the receiver output.

vReferring to Fig. l, the numeral I represents a device for generating a heterogeneous frequency wave spectrum wherein the individual'frequencies are generated fortuitously or at random rso far as timing is concerned. For example there is shown in Fig. 3, one typical generator comprising a grid-controlled vacuum tube 2 having its input and output circuits connected in feedback relation through the tuned oscillatory circuit 3, 4, so as to generate super-audible oscillations which are interrupted at a super-audible rate under control of the grid condenser 5 and grid leak resistance 6, it being understood that the D. C'. voltage for plate 'l is applied through the output transformer winding 3 in series with the high frequency choke coil 9. This generator is therefore of the well-known super-regenerative type and any other well-known form of such generator may be employed. As is well-known in the art, the output of such a generator produces a steady hissing or smooth noise which is free from any one dominant or regularly recurrent frequency component.

The audio-frequency or hissing output of device I (Fig. 1) is then passed through a suitable` band-pass filter l!) of known design whereby a predetermined section of the audio-frequency spectrum of the noise currents is passed to Device Il may means for optically scanning a representation to convert it into corresponding electric signals. The output of filter lll is applied to the facsimile scanning equipment so that the output of device il consists of the heterogeneous frequency sub-carrier of audio-frequency modu-V lated in accordance with the instantaneous facsimile signals. The modulated heterogeneous frequency sub-carrier can then be amplified in any suitable amplier l2 and impressed upon the transmission link i3 which may be a wire chan nel or a radio channel.

At the receiving station, to whichl the channel i3 is connected, the received waves are amplified-inv a suitable amplier itl includingpreferably a stage employing a grid-controlled arnplifier tube (not shown) which is arranged in the usual way to have its gain controlled b-y an A. V. C. signal. After amplification the heterogeneous frequency sub-carrier is. detected in any Well-known detector whereby the original signals are also detected and applied to the audio-frequency amplier I5 the output of which is applied to any well-known form vof signal repro- A portion of the detected sub-carrier ducer lll. heterogeneous frequencies from device l5 are applied to any well-known form` of A. V. C.l circuit !8 whereby there is produced a D. C. signal representative of the average amplitude of the heterogeneous carrier frequencies as actually reamplified in amplifier 2l and are applied to any" well-known form of modulator network 22 which is fed with a heterogeneous frequency yband sub-carrier from the generator 23 through the band-pass filter 2f. After amplification in the audio-frequency amplifier 25 the signal modulated sub-carrier is applied to the modulationnetwork 26 of any Well-known form of carrier transmitter the carrier generator of which is represented by the device 2l. The doubly modulated carrier wave is then transmitted to the receiving station Where after suitable amplification it is subjected to double demodulation to detect the heterogeneous frequency sub-carrier in order to generate the local A. V. C. signal as described in connection with Figi, and to detect from the sub-carrier the original signals to be reproduced.

Referring to Fig. 3, there `is shown. in more detail, one modulating .arrangement that is represented by the blocks 22, 23, 25 of' Fig. 2. The heterogeneous frequency generator 23 ris represented by the grid-controlled vacuum tube l which as above described is provided with known circuit connections whereby it acts as a superregenerative oscillator.

Connected in the gridv cathode circuit of the generator is a grid-biasw resistor to which is applied the output of the signal amplifier 2| so that the noise current in the output transformer 29 is modulated in accordance with signals from source 20. It will be understood of course that any other suitable way of modulating the noise currents of the super-regenerative oscillator to produce the signal-modulated heterogeneous frequency sub-carrier may be employed, and other changes and modifications may be made in the disclosed embodiments without departing from the spirit and scope of the invention. While in the foregoing description reference has been made to the production of a heterogeneous frequency in the audio frequency band, it will be understood of course that the heterogeneous frequencies thatare used as the carrier or sub-carrier may be in the superaudible spectrum or in the radio frequency spectum.

What I claim is:

l. The method of signaling which includes the steps of generating a heterogeneous frequency band wherein the frequency recurrence of any given wave or waves is fortuitous, modulating said band by a signal to be transmitted, and applying the modulated band to a transmission channel.

2. The method of signaling which includes the steps of generating a heterogeneous frequency band whereinthe frequency recurrence of any given wave or waves is fortuitous, modulating said band by a signal to be transmitted, receiving the modulated band, and detecting from the received band the signal to be transmitted.

3. The method of signaling which includes the steps of generating a noise band consisting of heterogeneous frequencies wherein the frequency recurrence of any given wave or waves is fortuitous, modulating said band by a signal to be transmitted, receiving the modulated band to Ade-v tect the signal therefrom, and deriving from the received heterogeneous frequencies a level con-` trol signal.

4. The method of signaling which includes the steps of generating a carrier in the form of a band of heterogeneous audio frequencies wherein the frequency recurrence of any given wave or waves is fortuitous, modulating said band by a signal to be transmitted, receiving and detecting the lsignal from the band, and controlling the level of the detected signal by generating a levelv ing a portion of the received band to control the level of the amplification to compensate for selective fading in the transmission medium.

6. The method of radio signaling which includes the steps of generating a plurality of subcarrier frequencies of heterogeneous frequencies wherein the frequency recurrence of any given wave or waves is fortuitous, modulating said frequencies by a signal to be transmitted, modulating a radio carrier by said modulated frequencies, receiving the modulated radio carrier and detecting therefrom the sub-carriers, and detecting from the sub-carriers said signal.

7. The method of signaling employing a superregenerative oscillator which includes the steps of generating a signal to be transmitted, exciting the oscillator to produce heterogeneous waves wherein the frequency recurrence of any given wave or waves is fortuitous, applying said signal to the oscillator to modulate the normal noise currents in the output circuit thereof, and transmitting said modulated noise currents to a distant receiving station.

8. In a signaling system, means to generate a band of heterogeneous frequencies wherein the frequency recurrence of any given wave or Waves is fortuitous, means to modulate said band by a signal to be transmitted, and means to apply said modulated band to a transmission channel.

9. In a. signaling system, means to generate a series of heterogeneous frequencies wherein the frequency recurrence of any given Wave or waves is fortuitous, the series being free from any dominant or fundamental frequency, means to modulate all said frequencies simultaneously by a sig-` nal, and means to transmit said modulated frequencies to a distant receiving station.

10. In a signaling system, a super-regenerative oscillator for generating noise currents composed of heterogeneous frequencies wherein the frequency recurrence of any given wave or waves is fortuitous, a band-pass lter for selecting a. predetermined band of said frequencies, and means to modulate said selected band by a signal to be transmitted.

11. A signaling system according to claim 10, in which means are provided to receive said modulated band and to detect therefrom the signal being transmitted, and means to derive from the received -noise currents a level control signal.

12. In combination a grid-controlled vacuum tube, a feed-back circuit between the input and outputof said tube for generating super-audible oscillations, means associated with said tube to interrupt said oscillations to generate in the output circuit noise currents wherein the frequency recurrence of any given wave or waves is fortuitous, means to apply a signal to be transmitted to said tube to modulate said noise currents, and means to apply said modulated noise 00 currents as a carrier to a transmission channel.

RICHARD A. I-IILFERTY.

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