US1737843A - Wave-transferring system - Google Patents

Description

Patented Dec. 3, 1929 UNITED STATES PATENT OFFICE NEW YORK RALPH V. L. HARTLEY, OF SOUTH ORANGE, NEW JERSEY, ASSIGNOR TO WESTERN ELECTRIC COMPANY, INCORPORATED, OF NET/V YORK, N. Y.,

A CORIORATION OF VJAVE-TRANSFERRING SYSTEM Application fi1ed December 29, 1924, Serial No. 758,531. Renewed January 10, 1929.

.This'invention relates to wave transmis' sion, especially in communication systems, such for instance as telephonesystems subect to lnterference' or disturbances, for example, interference of the type frequently exthe disturbances, and, if desired, to also im-' prove the form of the received signals, for instance by compensating for a non-uniform line attenuation.

The effect of any external disturbances can be made negligible if sufficient energy is used at the transmitting end of the system and 'jsuflicient uniform or distortionless attenuation at the receiving end, but the permissible amplification at the sending end is limited by the restrictions which are imposed on the sending energy, since the apparatus, such for instance as the amplifying means, at that end must not be unduly overloaded. In accordance with the invention, the effect of a given amount of interference or disturbance is minimized with a given sending energy, or the sendin ener necessar to secure a iven by b effected in the energy spectrum of the signal (that is, in the frequency distribution of the signal energy) and the noise energy which are finally produced at the receiver; This may be accomplished, for instance, by the introduction of distortion at the receiving end and a compensating distortion at the sending end, the sending end distortions preferably being complementary to the receiving end distortion (and any line distortion), so that there will be no resultant or overall distortion of the signals transmitted. The com plementary distortions are effected by networks, one at the receiving end of the system and one at the sending end; and an amplifier at the sending end preferably amplifies the signals delivered from the sending end network to compensate for a partor all of the attenuation produced by the two networks. The attenuation frequency characteristics or" the networks should be such that the reduction of apparent loudness ofthe noise caused by the receiving end network will exceed the difference between the combined attenuations (of signals) introduced by the networks and the decrease in equivalent load on the sending end of the system which results from passing undistorted signals through the sending network. The equivalent load referred to here is the energy level of the undistorted signals of which the sending energy is an equivalent as regards the load limit of the apparatus carrying the sending energy.

The frequency spectrum of signal energy usually (as for instance is the case with the frequency spectrum of the energy of normal speech) contains frequencies of comparative-V 1y high energy level and frequencies of comparatively low energy level. By means of this invention, not only can the effect of noise currents be reduced without increasing the total power at the sending end of the system, but the part of the noise represented by j currents of the latter frequencies can be reduced or substantially eliminated by attenuating or substantially extinguishing those noise currents in the receiving end network. This can be done without necessitating unduly increasing the likelihood of overloading the amplifier or other apparatus at the sending end, even the load capacity of such amplifier or such other apparatus be more dependent upon peak values, or single frequency, high values of energy than upon the energy integral over the whole frequency spectrum of the currents transmitted; for the complementary network at the sending end can be given high attenuating power at the frequencies at which the signal energy is high, and will therefore prevent the gain introduced by the amplifier to overcome the signal energy losses of the two networks from increasing the energy levels at the last mentioned frequencies to values materially higher than obtained before the insertion of the networks and before the increase of the amplifier gain. For example, in the hypothetical case in which the frequency spectrum of the noise energy incoming into the receiving end of a system is complementary to that of the signal energy, it would be possible to employ a receiving end network having at every frequency an attenuating power just sufficient to substantially extinguish the ultimately received noise current of that frequency (an infinite attenuating power would of course be necessary were total extinction required), without necessitating any substantial alteration of the highest energy level delivered at any frequency at the output of the amplifier at the sending end; for there could be employed at the sending end a complementary network having at each frequency an attenuating power just sufficient to almost extinguish the signal current fed to the amplifier at that frequency, so that upon increase of the gain of the amplifier by an amount equal to the maximum attenuation of either network (the maxima would be the same for networks complementary to each other) ,the energy delivered at each frequency at the output of the amplifier would have a level not exceeding the maximum level which obtained before the insertion of the networks and the increase of the amplifier gain.

Although the invention is set forth hereinafter with especial reference to a telephone system in which thedisturbing effects of noise currents upon signals in the speech frequency range are reduced, the broad features of the invention are of general application in wavetransmission, as, for example,-in the reduction of the effects of interference upon transmission in carrier and radio frequency ranges.

In the accompanying drawings, Fig. 1 is a diagram of a radio telephone system embodying one form of the invention; and Figs. 2 to 5 are curves for facilitating explanation of the invention.

In Fig. 1 two-way transmission, for instance two-way telephonic transmission, is carried on between signaling stations 10 and 10, through lines 11 and 11 and a wire and radio link between those lines, the link comprising a line 12, radio transmitter 13, radio receiver 1 1, and line for transmitting from line 11 to line 11, and the link further comprising a line 12, radio transmitter 13, radio receiver 1 1 and line 15 7 for transmitting from line 11 to line 11. The connection of line 11 to lines 12 and 15,and the connection of line 11 to lines 12 and 15 is accomplished by means of three-winding transformers or hybrid coils 16 and 16 and line balancing networks 17 and 17 as usual. The carrier wave generated at the radio transmitter 13 and received through the radio receiver 14 is preferably of a frequency different from that of the carrier wave generated at the radio transmitter 13' and received through the radio receiver 1 1. As is indicated by the clashes in lines 11, 12, 15, 11', 12 and 15, these lines may be of any length.

In a system such as that of Fig. 1, considerable interference is likely to be experienced, due especially to the reception, by the radio receiver 1 1 of energy other thanthat radiated from the transmitter 13, and to the reception by the radio receiver 1 1 of energy other than that radiated from the transmitter 13. To reduce the effects of such interference without necessitating increase of the sending energy transmitted into line 12 or 12, distorting networks 20, 21, 20" and 21, and amplifiers 30 and 30, are provided The attenuating network 20 is att-he sending end of the channel for transmitting fromline 11 to line 11', which for convenience may be termedthe east channel; the attenuating net work 21 is at the receiving end of that channel; the attenuating network 20 is at the sending end of the channel for transmitting from line 11 to line 11, which may be called the west channel; and the attenuatingnetwork 21 is at the receiving end of that channel. In explaining the operation of the system to reduce interference effects it will be assumed that frequencies "outside of a predetermined desired frequency range are so highly attenuated before reaching amplifier 80 or 30 that they may be neglected; and that, within the limits of their load capacity, the amplifiers transmit waves ef the pre'determined desired frequency range without distortion. These conditions can easily be approximated with sufficient closeness, in practice. V h i It will now be explained how, in trans mitting from line 11 to line 11, by having the network 21 at the receiving end of the channel east introduce distortion of a suitable character, and having the network 20 intro: duce a compensating idlStOIliiOIl at the sending end, the disturbing effect of a given amount of interference can be reduced without necessitating increase of sending energy. The manner in which the channel wes topera'tes to reduce the disturbing effect of interference when the system is transmitting from line 11 to line 11 is similar, and will then be apparent without further explanation.

Consider first the case where the portion of the channel east between amplifier '30 and network 21 is free from distortion. It will be assumed that, for speech frequencies in the predetermined desired frequency range menitoned above and extending from frequency f to frequency f in Figs. 2 to 5, the apparatus to the west of network 20 is free from distortion, a condition which can easily be pr imat ,s fieent ydQSdy pra lit tice. It will also be'assumed that the magnitude of the received sound is not varied over so wide a range but what the reduction in intelligibility due to noise is unaffected by uniformly attenuating or amplifying both speech and noise by the same amount. As a method of expressing the magnitude and the frequency distribution of speech and noise energy, the energy (that is, the energy per unit of time, or the power associated with any frequency) will be expressed on a log arithmic scale the zero of which is the energy of that same frequency in undistorted speech which has been attenuated so as to be just at the threshold of audition. The absolute value of this zero is immaterial for present purposes. Consider the conditions in the channel east before the introduction of its distorting networks 20 and 21. Since the speech arriving at the east end of the channel after transmission through the channel is assumed undistorted in the frequency range from f to f and below audibility outside that range, it may be represented by the horizontal line a in Fig. 2. The distribution of the noise arriving at the east end of the channel being in general different from that of speech let it be represented by the curved line b. This is placed below a to indicate that the noise is smaller in magnitude than the speech.

Now it has been found to be at least approximately true that the effect of any disturbing noise on the intelligibility of speech is the same as that of a sound having the energy distribution of undistorted speech and the same apparent loudness as the noise. Let the curve 0 of Fig. 2 represent the mag nitude of undistorted speech which is of the same apparent loudness as the noise represented by curve I). (This magnitude, or in other words the height of curve 0, can be calculated by empirical methods indicated in the article by Fletcher and Steinberg in the Physical Review, September, 1924, vol. 24, second series, No. 3, page 306.) The distance between a and 0 or a0, expressed in transmission units is a measure of the dis turbing effect. (The transmission unit here referred to may be, for instance, that discussed in my article on The transmission unit, in Electrical Communication, July,-

1924, published by the International WVestern Electric Company, Incorporated, New York. This unit is also discussed in the following articles: phone transmission systems, by W. H. Martin, and Practical application of the transmission unit, by C. W. Smith, both in The BellSystem Technical Journal, July, 1924. published by the American Telephone and Telegraph Company, New York.)

Now let there be inserted at the east or receiving end of the channel the network 21, with loss vs. frequency characteristic such, say, as is given by curve (Z, Fig. 3. The re- The transmission unit and telee ceived speech and noise are then roughly as shown by a 6 and 0 of Fig. 4, where'the curve 0 is again the undistorted speech of the same loudness as the noise. Curves a and c of Fig. 1 are reproduced in dotted lines;

In order to restore the speech let there be inserted at the sending or west end of the channel the network 20 having the loss characteristic 6, Fig. 3, and introducing distortion complementary to that of network 21. This has no effect on the noise, but changes the received speech from (L to ta If the speech energy (or power) at the sending end of the channel was originally 9, Fig. 5, it is changed to 9 by the introduction of the sending end or west network. Since we have decreased the sending energy, it will now be possible to introduce, by amplifier 30, more sending amplification without increasing the total amplifier output power to a value higher than it originally had, and moreover, in general without exceeding the load limit even though the transmitting line of the channel originally operated at its load limit. From the standpoint of load, then the energy represented by the curve 9 is, in general, equivalent to that of undistorted speech of some value less than g; say it. We may then, by means of amplifier 80, introduce sending amplification equal to g-]i. When this is done the received speech, or speech at the east end of the channel, is raised to (Z where The difference in level between the curves a and 0 is then a measure of the relative magnitudes of speech and noise with the networks inserted. It follows then that the reduction in the effect of the interference obtained by inserting the networks is measured by c0 is the reduction in apparent loudness of the noise caused by the receiving network. It depends on the distribution of the interfering energy as well as on the characteristic of the network.

wa is the difference between the combined attenuations introduced by the networks and the decrease in equivalent load which results from passing undistorted speech through the sending network. This decrease depends upon the distribution of the speech energy, the shape of the network characteristic and the manner in which overloading occurs at the sending end.

It is interesting to consider the case where the receiving network is such as to give the noise at the receiver the distribution of speech; that is, to make Z2 a straight line. Then the speech energy at the sending end 9 will have the same shape as the interference 6. Such a pair of networks will give an imto the-manner in which overloading occurs with the particular apparatus employed at thesending end. 7

In case the attenuation in the channel, between network 20 and network 21, varies with the frequency we may treat the receiving end exactly as before. To render the overall ransmission of the channel distortionless we must, however make the distortion. of the sending network the complement of that of the receiving network plus the variable part of the attenuation frequency characteristic of the channel, between the networks. The distribution of sending power will then dif fer from {/2 by the distortion of the channel between network 20 and network 21. The value of the load equivalent it will then in general be different from that for a distortionlesschannel. Hence the optimum distortin network will depend on the attenuation requency characteristic of the channel as well as the factors already discussed.

It will be understood that the showing of the attenuating networks in the drawing is merely diagrammatic, and that any suitable networks may be employed. I

The broad features disclosed herein may be embodied in forms widely different from that specifically shown and described above, without departing from the spirit of the invention defined in the following claims.

hat is claimed is:

1. The method of reducing the disturbing effect upon signaling waves of interfering waves, which comprises changing the frequency distribution of the energy of only the signaling waves, while maintaining the total energy of said altered waves at least as low as that of said signaling waves before said alteration, and subjecting the altered signaling waves and the intefering waves to distortion of such character as to give said altered signaling waves their original form and to reduce the energy level of the undistorted signals of which the interfering Waves are an equivalent as regards apparent volume, by an amount greater than the reduction caused by the two changes in the energy level of said signals.

2. The method of reducing the disturbing effect of interfering waves upon signaling waves for transmitting speech, which coinprises changing the frequency distribution of the energy of only the signaling waves, and subjecting the altered signaling waves and the interfering waves to distortion of such character as to compensate for the dis tortion suffered by said signaling waves in the first mentioned change, and to reduce the energy levelof the undistorted speech of which the interfering waves are an equivalent as regards apparent loudness,an amount greater than the reduction caused by the two changes of said signaling waves in the energy level of said signaling waves.

3. The method of transferring signaling waves through a signaling channel traversing a reglon of interfering waves, which comprises changing the frequency distribution of the energy of said signaling waves in said channel before they enter said interference region and while maintaining the load imposed on said channel by said altered signaling waves at least as low as that imposed on said channel by said signaling waves before said change, subjecting the signaling waves and the interfering waves, after they leave the interference region, to distortion complementary to the distortion occasioned by the first mentioned change and of each character as to reduce the apparent volume of said interfering waves in said channel by an amount greater than the resultant decrease of signal volume occasioned by the changes suffered by said signal waves before entering and after leaving said interference region. p

4. A signaling wave transmitting channel having a sending end, a receiving end, a portion between said ends traversing a region of interference, distorting means at said receiving end, means introducing complementary distortion at said sending end, and an amplifier at said sending end and between the two distorting means, the characteristics of said two distorting means being such that said receiving end distorting means reduces the apparent volume of received interference waves an amount greater than the difference between the combined apparent signal volume decreases caused by said two distorting means and the apparent signal volume gain increase which the introduction of said sending end distorting means renders possible in said amplifier without increased tendency to overload the channel.

5. A duplex radio telephone system comprising two terminal circuits and two unidirectional, oppositely directed signaling channels connected to said circuits in conjugate re.ation to each other, each of said channels having means for reducing the disturbing effect of radio interference waves upon the articulation of signals transmitted through that channel and each of said means comprising a non-uniformly attenuating network at the outgoing end of one of said channels, a complementary network at the incoming end of said one channel, for predistorting the'signals in a manner opposite to that in which they are distorted by said attenuating network at said outgoing end of said one channel and means at the outgoing end of said channel for amplifying the signals to compensate for the attenuation produced therein by the networks at the incomingand outgoing ends of said channel, said networks being designed with respect to the relative energy level of said signal and said interfering waves so that a minimum amount of amplification by said amplifying means will be required to secure a given standard of operation for said system.

6. A signaling system for transferring signals over a region of interference, said system comprising means for reducing the disturbing effect of saidinterference upon the signals received over the system, while maintaining the load on the sending end of the system, with respect to the load limit of said end, at least as low as would obtain in the absence of said means, said means comprising stationary means at the sending end of the system for distorting the signals and stationary means at the receiving end of the system for restorin the form of the signals and changing the frequency distribution of the interfering energy arriving at the receiving end of the system.

7 A signaling system for transferring sig nals over a region of interference, said sys tem comprising a transmitting station, a receiving station, and means for reducing the disturbing effect of said interference upon the signals received over said system, said means comprising means at said transmitting station for changing the frequency distribution of the energy of said signals, and means at said receiving station for restoring the form of the signals and changing the energy level of the interfering waves arriving at said receiving station with respect to the change in the energy level of the signals transmitted over said system by an amount large in comparison with the resultant change in energy level of the signals produced by the means for changing the frequency energy distributions of the signals at the transmitting and receiving stations.

8. The method ofreducing the disturbing effect of interfering waves on signals transmitted from a sending station to a receiving station through an interference zone, which comprises changing the frequency distribution of the energy of said signals at said transmitting station and, at said receiving station, restoring the. form of the signals and changing the energy level of the interfering waves by an amount large in comparison with the resultant change in the energy level of the signals 9. The method of transmitting signaling waves through interfering waves, which comprises relatively changing the energy of only the signaling waves at different frequencies, and subjecting the altered signaling waves and the interfering waves to distortions of such character with respect to said change in the energy of the signaling waves as to render the disturbing effect of the interfering waves and said change and said distortions less than the disturbing effect of the interfering waves in the absence of said change and said distortions.

10. The method of reducing the disturbing effect of interfering waves upon intelligence transmitting waves in an intelligence transmission system, which comprises so distorting the intelligence transmitting waves before they encounter the interfering waves as to cause the relation between the energy levels of the intelligence transmitting waves at their different frequencies to substantially more closely approach the relation between the maximum permissible energy levels in the system at those frequencies, and compensating for at least a portion of said distortion.

11. The method of reducing the disturbing effect of interfering waves upon signaling waves in an intelligence transmission system, which comprises so distorting the signaling waves before they encounter the interfering waves as to substantially increase the amount that the signaling waves can be amplified without exceeding a given degree of overloading of the system, amplifying the altered signaling waves before they encounter the interfering waves, and compensating for at least a portion of the distortion after the signaling waves have encountered the interfering waves.

12. The method of reducing the disturbing effect of interfering waves upon telephone signaling waves in. a telephone transmission system, which comprises so distorting the telephone signaling waves before they encounter the interfering waves as to substantially increase the amount that the telephone signaling waves can be amplified without exceeding a given degree of overloading of the system, after such change subjecting the telephone signaling waves to the action of said interfering waves, and so changing the resulting waves with respect to their relative values of energy at different frequencies as to make their frequency distribution of energy more nearly approach a given standard.

13. The method of reducing the disturbing effect of interfering waves upon intelligence transmitting waves, which comprises rendering the frequency distribution of the energy of the intelligence transmitting waves much more nearly uniform before they encounter said interfering waves, after such change subjecting the intelligence transmitting waves to the action of said interfering waves, and so changing the resulting waves with respect to their relative value of energy at different frequencies as to make their frequency distribution of energy more nearly approach a given standard.

14. The method of transferring signaling. waves through a signaling channel traversing a region of interfering waves, which comprises rendering the energy of the signaling waves substantially less variable with frequency before they enter said interference region, restoring the shape of their energy frequency characteristic after they leave said lnterference region, and attenuating said 1nterfering waves in said channel.

15. A wave transferring system having a passive network of lumped impedances at one end thereof for so distorting said waves as to substantially increase the amount that the signaling waves can be amplified without exceeding a given degree of overloading of the system, means at said end of said system for amplifying said distorted waves, and a passive network of lumped impedances at the other end thereof effecting a complementary change in said waves. 7

16. The method of reducing disturbing effects of interfering waves on signaling waves, which comprises subjecting the signaling Waves before they encounter the interfering waves to distortion of such character as to reduce by a given amountthe load equivalent of the signaling waves with regard to the manner in which they tend to cause overloading in the system, and subjecting the interfering waves and the signaling waves after they encounter the interfering waves to distortions of such character as to compensate for the distortion suffered by the signaling waves in said first mentioned distortion and to reduce the apparent energy level of interfering waves as measured by their disturbing effect by an amount exceeding the com bined energy level decreases suffered by said signaling waves in said distortions before and after encountering said interfering waves minus said reduction-in load eqnivalent.

In witness whereof, I hereunto subscribe my name this 26th day of December A. D.,

RALPH v. L. HARTLEY.

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