US2439293A

US2439293A - Speech transmission system

Info

Publication number: US2439293A
Application number: US596704A
Authority: US
Inventors: Guanella Gustav
Original assignee: Radio Patents Corp
Current assignee: Radio Patents Corp
Priority date: 1944-06-01
Filing date: 1945-05-30
Publication date: 1948-04-06
Anticipated expiration: 1965-04-06
Also published as: CH244727A; DE964876C; FR913084A; GB597340A

Abstract

597,340. Transmission systems. PATELHOLD PATENTVERWERTUNGS- & ELEKTRO-HOLDING AKT.-GES. June 1, 1945, No. 13829. Convention date, June 1, 1944. [Class 40 (iv)] Speech is transmitted by first separating the signal into a number of partial frequency ranges, then dividing the frequency of each of these ranges, combining these results to form a signal for transmission, separating out the components at the receiving end, multiplying their frequency separately to form the original components, and then combining them to form the original speech. Auxiliary signals may be mixed with the components before transmission and these signals then have to be removed at the receiving end. The frequencies for transmission can thus be adjusted to obtain a narrower wave band. Apparatus for carrying out this method of transmission may also be combined with means for scrambling the speech. In one method of carrying out the invention, shown in Fig. 5, each frequency range is separated by a band-pass filter BP, the amplitude is limited in AB and the frequency divided in TE. The resulting signal is fed to a modulator M controlled by a voltage proportional to the amplitude of the original signal and produced by a rectifier G and a low-pass filter TP. The resulting signal therefore has the amplitude of the original signal, but a fraction of its frequency. In the receiving apparatus shown in Fig. 6, the bands are reproduced by filters BP and a signal of constant amplitude but multiplied to the original frequency is produced by the limiter AB and multiplier VE. The modulation product of this and the signal from BP pass through a high-pass filter HP and the original partial band signal is produced. The signals indicating amplitude and frequency may be transmitted separately instead of combined as above.

Description

April 1948; s. GUANELLA 2,439,293

SPEECH TRANSMISSION SYSTEM Filed May 30, 1945 4 Sheets-Sheet 2 w v INVENTOR. fiualalr mmem .BY

' ATTORNEY 7 April 6, 1948.

e. GUANELLA 2,439,293

SPEECH TRANSMISSION SYSTEM Filed May 30, 1945 4 sheets-sheet s INVENTOR. fimsiaw uanella BY hr m ATTORNEY 4 Sheets-Sheet 4 G. GUANELLA Filed May 30, 1945 I INVENTOR.

SPEECH TRANSMISSION SYSTEM Him ' B Zu/ fla ATTORNEY April 6, 1948,

, Patented Apr. 6, 1948 SPEECH TRANSIVIISSION SYSTEM Gustav Guanella,

to Radio Patents Zurich, Switzerland, assignor Corporation, New York, N. Y.,

a corporation of New York Application May 30, 1945, Serial No. 596,704 In Switzerland June 1, 1944 20 Claims.

The present invention relates to wave transmission or signalling systems, more particularly to the transmission of speech signals, and has for its object to provide a novel method of and system for reducing the frequency range or band width required for the transmission of signals of this type.

For the transmission of speech signals, a transmission channel is generally required which passes the required speech frequencies with their relative amplitudes in order to enable a message to be understood clearly. For this purpose, a frequency band of about 300-3,000 cycles is required in practice. Various proposals have already been made to convert or modify the signals in such a-manner that transmission is possible with a considerably smaller frequency range or band width of the transmission channel.

Thus, it has been proposed to transmit variable characteristic values or signals in place of the actual speech signals, these characteristic signals being representative of the fundamental frequency of the oscillations as well as the energy distribution of the various harmonics in the frequency spectrum, in such a manner that intelligible speech signals may be reconstructed at the receiving end by correspondingly controlled oscillating devices. Deviations from the fundamental frequency of the thus-obtained synthetic speech can, however, not be avoided in most cases and the variable energy distribution cannot be separately characterized for each speech component. This makes it impossible to avoid considerable changes from the original signals, with the result that it is difiicult to understand the message and that the characteristic voice of the speaker is lost.

Other proposals relate to methods by means of which the frequency of the speech oscillations is reduced by subjecting the entire speech frequency spectrum to a irequency'division process. It is found, however, that an oscillation process or complex signal wave containing several frequency components cannot be resolved in this way into a new signal wave of corresponding components being subdivided equally such as to one half their original frequency, from which the original process may again be obtained by frequency multiplication; for if an oscillation spectrum is subjected to a frequency division with known means, a new spectrum is obtained whose components have the same mutual spacing as in the original spectrum, so that no reduction in the band width can be achieved in this manner.

According to the present invention, the disadvantages of the previous methods of frequency division are overcome by subdividing the speech spectrum before transmission into several partial frequency ranges or sub-bands each of which contains not more than one vowel component of the speech being transmitted, and by, in turn, subjecting these components to a frequency division, whereby the signal to be transmitted will be reduced to a narrower band width. The original speech signals are again obtained at the receiver from the converted signal by individual frequency multiplication of the partial frequency ran es or sub-bands.

More specifically, the invention is based on the following considerations. Let it be assumed that an original speech signal voltage u is composed of two components having amplitudes a and frequencies W1 and W2, respectively, the latter differing from each other by the fundamental frequency W, as is the case in a complex signal wave, such as a speech signal. Such a complex wave is expressed by the following equation:

u=a sin W t+a sin W t =A sin g 1 Equation 1 represents an oscillation or wave having an average frequency By frequency division, there is obtained an alternating voltage 22 whose amplitude B, depending upon the frequency divider used, is either con-- stant or corresponds to the amplitude A, while its frequency isreduced, such as by one half, to re-f sult in an output wave as follows:

v=B.sin mi With B being constant, this oscillation process or signal wave consists of a single component and it is possible to again obtain both original .3 components by frequency multiplication. If, however, B is equal to the original amplitude, then the following equation results:

v=2a cos 2 t sin are obtained'from theorigln'al components asin Wt. By adding these components, anew corrected signal is obtained as follows:

.v=a .sin +osin (5) In thiscase, the frequency difference of both components. amounts only .to

i. e., one half the original band width,'so that .a channel with a correspondingly reduced passband-characteristic may be used for thetransmission.

In practice, the amplitudes a of the individual speech components are unequal and variable. These Variations occur, however, more slowly than those of the fundamental frequency so that transmission means having a small band pass characteristic will be adequate for transmitting the amplitude changes. In order to transmit the varying amplitudes, it is possible, for instance, to cause the amplitudes of the corresponding frequency-divided components to vary in the same manner as the original components.

Normally, speech consists of more than two components. By subdivision into an adequate number of partial frequency ranges or sub-bands, the invention may be applied to a multiplicity of components without difficulty.

Speech signals, besides containing vowels with pronounced frequency components, also include consonants Whose frequencies are more or less evenly distributed over the whole frequency spectrum. Experience has shown that .a faithful transmission of these consonants is not necessary, that is to say, that an approximately con rect characterization of the energy distribution over theindividual partial bandsissufflcient for practical purposes. This condition may be readily comp-lied with by means of the arrangements according to the invention.

The invention both as tolts objects and novel aspects will become more apparent from the followingfdetailed description taken in reference to the accompanying drawings forming part ofthis specification, and wherein:

invention embodying modified means for amplitude transmission.

Figure 6 showsa receiver including one form of frequency multiplying means for restoring boththefrequencies and amplitudes of the original signal waves.

Figure '7 illustrates a modification of Figure 6.

Figure 8 shows a further modification of a limited bandwidth transmission system, utilizing sep'aratepilot signals for transmitting amplitude characteristics.

Figure 9 and 10 illustrate systems according to the invention combined with speech scrambling arrangements for obtaining secrecy of transmission.

Figure 11 shows a more detailed circuit diagram of a system of the type shown in Figure 3.

Like reference characters identify like parts throughout the different views of the drawings.

In the arrangement shown in Figure 1, an original speech signal Voltage u is divided into the sub-bands or partial frequency ranges ui, uz un by means of band-pass filters (shown at BP]. only for the first sub-channel). The first sub-channel may, for instance, cover the frequencies from 300-400 cycles and accordingly contains at the most one speech component, it being well known that even for a low male voice the fundamental frequency is above cycles per second. By frequency division in a device TEl, the corresponding partial signals 121, '02 on are obtained, the first of which may fall within the range between 1 50 and 200 cycles, assuming a division by one'half. The total number of partial signals are then combined to produce the signal v to be transmitted and whose band width in the example s'hown'has been halved compared with the original signal u. The band width may be still further reduced by reducing the frequencies m, m an to one-third or smaller fractions.

In orderto be able to utilize a favorable frequency range of the band-pass filters, the original signal before being filtered may be modulated with auxiliary frequencies so as to be shifted to-a range falling within the passing range of the filter. After dividing the frequency subban-ds, a further frequency shift is generally necessary by modulation with corresponding auxiliary frequencies, depending on the particular type of transmission or signalling channel used.

At the receiving end, the received signal '0 is again subdivided by the filters BP1 shown in Fig. 2, into the'sub-bands v1, v2 Un- The original frequency sub-bands "1L1, u'z U11. are then obtained by correspondingly multiplying their frequency in the multiplier VE1 and are then combined to reproduce the original signal wave a.

Since the amplitudes of the speech components are variable, it is necessary to transmit the amplitudes in addition to the varying frequencies. This is possible, for instance, by causing the amplitude of the frequency-divided components to be equal By forming the modulation product of signals 81 and in in the modulator M and suppressing the sum frequencies in the low-pass filter TP, a final oscillation of results whose amplitude is proportional to the amplitude of signal in and whose frequency is equal to one half the original 7 If the frequency is divided signal frequency ii.

in TE into of the original frequency, components having frequencies are obtained from the output of modulator M.

As is understood, arrangements similar to those shown and simply indicated by dash lines for the sake of simplicity in the drawing, are provided for each frequency sub-band, to result in a complete final signal 12 or u by combination of the individual signal components in, v2 on or m, uz un, respectively. A modified arrangement according to the invention utilizing a so-called regenerative frequency divider is shown in Fig. 4. An alternating voltage (.1 hav' g a fractional frequency such as is obtained from the frequency-divided component in after limiting in the limiter AB. By forming the modulation product of in and ii in modulator M, there will again be obtained the component in having a frequency is .obtained as follows:

fo=f1fs=f14fv (6) Accordingly,

Any other frequency ratio may be obtained with a different multiplication factor for VE. Thus, the arrangement according to Fig. 4 produces a frequency division by inverse or regenerative feedback and the use of a frequency multiplier (VE) for ratios greater than one half.

In the arrangement shown in Fig. 5, a control or pilot signal'pi is obtained from each sub-band by rectifying in G and filtering in low-pass filter 'I'P, respectively, this signal corresponding to the instantaneous amplitude of the respective subband. The divided partial signal .51 of constant amplitude after passing through the limiter AB, is then controlled by 101 in. the modulator M so as amplitude corresponds to the original amplitude of in.

Dynamic compression may also be employed in connection with this type of amplitude control, if the control signal m is given a corresponding amplitude relative to the signal in by means of a suitable characteristic of the rectifier G. Thus, for instance, 101 may correspond to the root or logarithm of the amplitude of w so as to be variable over a relatively small range, in which case the amplitude of 111 varies over a range which is smaller than the amplitude range of m. The main object of this arrangement is that, when 1n has a small amplitude, there are already adequate amplitude values of 121, and when the amplitude ofui is large, the amplitude of or increases only, comparatively slightly. A corresponding dynamic compression with the arrangements shown in Figs. 1, 3 and 4 is, of course, also possible by the use of amplitude limiters with automatic amplification control in the individual subchannels. Transmission means having non-linear transmission characteristics, such as amplifier tubes with curved operating characteristics, may also be used for limiting the amplitude. Existing harmonics may easily be suppressed by additional low-pass and band-pass filters, as they do not fall within the frequency range of the individual partial bands. Amplitude compression is also possible by controlling the amplitude of the sub-' divided frequency bands by means of non-linear rectification of the frequency bands to be divided or by means of a control voltage obtained from the undivided sub-bands by non-linear transmission. At the receiving end, the original amplitude differences may be restored again by corresponding dynamic expansion means inserted in the individual sub-channels.

A suitable receiving arrangement for use in connection with the transmitter according to Fig. 5 is shown in Fig. 6. The partial bands in, v2 on are segregated from the received signal '0 by the band-pass filters BP. The frequencies may be multiplied by means of any known multiplying device. In the arrangement shown in the drawing, an auxiliary signal hi having a constant amplitude is obtained from in 'by limiting its amplitude in AB and multiplying its frequency in VE, the frequency of hl, e. g., being equal to three times the frequency of '01. A modulation product of hi and 01 comprising sum and difference frequencies is produced in the modulator M. The high-pass filter HP suppresses the difference frequencies so that an output partial signal 161 is obtained having a frequency fu=4fm The frequency is thus multiplied fourfold, while the amplitude of a1 corresponds to the amplitude of 111, thus obtaining the original signal frequency, as assumed. If dynamic compression is used at the transmitting end, corresponding dynamic expansion means will be required at the receiver.

A modified receiving arrangement is shown in Fig. 7. In the latter, the partial bands in, v2 'Un obtained from the band filters BP, after passing through the amplitude limiter AB, are frequency multiplied in VE so that a signal in is formed, the amplitude of which is constant, while its frequency is a whole number multiple of the frequency of in. The amplitude of hi is controlled in the modulator M by a control signal qi. Control signal qiis obtained by rectifying a portion of signal 121 in Gand suppressing the higher freaesaaoa quencies by means of low-pass filter TP; in other words, qr varies with the amplitude of 01 and acordingly the amplitude of ur will be proportional to the amplitude of '01. If dynamic expansion is required due to a corresponding compression at the transmitter, in a simple manner by means of a corresponding non-linear characteristic of the rectifier G or an auxiliary non-linear transmission element in the path of the control voltage qr.

The frequency-divided signals may also be transmitted with constant amplitude, if supplementary characteristic signals are transmitted by special means, which signals are characteristic of the amplitude fluctuations. If the frequencydivided signals have a constant amplitude, the side bands due to the amplitude fluctuations disappear, so that a considerably narrower band width may be used; that is, the frequency of the individual sub-bands may be divided to larger fractions of the original frequencies. Such an arrangement is illustrated in Fig. 8. Similar to the arrangement shown in Fig. 5, a partial signal 31 with constant amplitude is obtained by frequency division. Furthermore, a signal 101 characteristic of the variable amplitude is obtained by rectification of a portion of ur in G and by low-pass filtering in TP. Since the frequency of 121 is very low and not suitable for transmission, it is shifted by modulation in N, to produce a characteristic signal For, the frequency of which falls within a range suitable for transmission together with the speech signals.

In order to obtain secret transmission, the partial signals or sub-bands may be interchanged or scrambled in a known manner before transmission according to a prescribed program so that only a person knowing the proper decoding key may obtain intelligible signals from the scrambled message. Such an arangement is shown in Fig. 9. By modulation in the modulator M with an auxiliary frequency b1, the partial frequency band ur is at first shifted into a fixed or constant frequency range to which are also shifted all the remaining bands uz, us, etc., by means of corresponding auxiliary frequencies and modulators. The passing frequencies of bandpass filters BP are thus the same for all partial channels. The partial frequency bands v1, v2 'Dn formed by subdividing the frequencies in T? are then scrambled by the permutator or scrambling device P. The scrambling process may be varied according to a definite plan in a manner well known in the art. After a further modulation in M with the auxiliary frequency b1, the transposed bands are shifted back again by unequal amounts so that after the undesirable sum frequencies have been suppressed in the low-pass filter TP, there is obtained a final signal 1: suitable for transmission and having a frequency band width equal to the original signal u, but with the partial bands 01, 122, etc., mutually interchanged, in a manner well known in frequency substitution scrambling systems of this type.

The arrangement shown in Fig. 9, wherein the partial frequency bands are shifted into a common frequency range by means of modulator M, possesses the advantage that the filters BP and also the dividing devices TE may be designed for the same frequency range. Especially, the frequency division of the lowest speech frequency bands may cause difficulties which are avoided by the aforementioned frequency shift.

In arrangements utilizing characteristic signals this may beachieved' i all the sub-bands s1, s2

V4 is shown in the drawing,

forth'e amplitudes as in Fig. 8, it is alsopossible to scramble both the partial signals in order to increase the secrecy, as is illustrated in Figure 10. In the latter, thepermutator P serves to scramble Sn as 'well as the characteristic pilot signals k1, lcz kn. The interchanged characteristic values are into a frequency range suitable for transmission by a further modulation with suitable auxiliary frequencies (1, while the interchanged sub-bands are shifted back by unequal amounts by means of a further modulation with suitable auxiliary frequencies e1.

Still greater secrecy may be achieved by an inversion of at least part of the various subbands by modulation with suitable auxiliary frequencies.

Figure 11 shows a more detailed constructional form of. the arrangement illustrated in Figure 3. The amplitude limiter AB is shown to comprise two triodes V1, V2 each of which has a limiting effect for negative input signals due to the lower bend in the characteristic curve. On account of the phase-reversing effect of V1, positive input signals of tube V1 produce corresponding negative input signals for tube'vz, so that the latter serves to limit positive input voltages and the former limits the negative input voltages. For dividing the frequency, a multi-vibrator with tubes V3,

the natural frequency of which has a whole number relationship with the average passing frequency of BP, so that the vibrator frequency is carried along or synchronized by the frequency of the sub band in and represents a whole fraction of the latter in a manner well known in frequency dividers of this type. A low-pass filter having a series inductance L and parallel condensers C is provided for suppressing the harmonics of the multi-vibrator oscillations. Modulator M is shown in the form of a push-pull circuit comprising tubes V5 and V6. The sum frequencies resulting from the modulation are suppressed by the low-pass filter TP, while BP1, BPz, etc., are band-pass filters for subdividing the speech spectrum into frequency ranges.

The multi-vibrator shown in Figure 11 comprising tubes Va and V4 is of the standard harmonic feedback type comprising a pair of resonant circuits in the anode circuits of the tubes. Resonant circuit R1 from which is derived the divided output frequency is tuned to the desired fractional input frequency impressed upon. the tube V1 while the resonant circuit R2 is tuned to the diiference between the input frequency and the fractional frequency in a manner well known in the art. It is understood any other type of frequency divider circuit may be employed for the purposes of the invention.

When several messages'are transmitted simultaneously, the frequency-divided partial signals of the various messages may be interchanged in order to obtain a high degree of secrecy. In this case, a special permutating device is used by which, for instance, the second partial signal of the first message is interchanged with the second partial signal of the third message ac cording to a prescribed program, and so on. The separately transmitted characteristic amplitude values may, if necessary, also be scrambled in an analogous manner. If arrangements such as are shown in Figures 9 or 10 are used, it is also possible to interchange the third partial signal of the-first message with the fifth partial signal of the second message, and so on, since the paragain shiftedtial signals before being interchanged are shifted into the same frequency range by modulating them with auxiliar frequencies b.

The arrangements according to the invention may also be employed for the recording of messages. When the messages are recorded on photograph records, the well-knownneedle noise which occurs in the higher. speech frequency ranges is avoided, because the higher frequencies are no longer required for the recording.

For technical reasons it may be advantageous to divide the speech frequencies into partial frequency bands of different widths", that is, with. the lower speech frequencies divided into narrower partial frequency ranges or sub-bands than the higher speech frequencies, so as to make sure that two components will not occur in a lower partial frequency channel.

When using the arrangements described above in multi-channel transmission systems, a suitable frequency shift may be employed to insure that the output signal u always falls within the desired transmission frequency range. With systems as shown in Figures 9 or 10, this is possible by a suitable choice of the demodulation or auxiliary frequencies e1, d1.

I claim:

1. A method of transmitting a complex signal wave comprising fundamental and harmonic frequency components, which consists in subdividin a signal wave to be transmitted into a plurality of frequency sub-bands each having a narrow width of an order so as to substantially include not more than one of said frequency components, equally dividing the frequency of each of said sub-bands, recombining and transmitting the divided-frequency sub-bands as a signal wave of reduced band width, again subdividing the received signal wave into its corresponding subbands, and equally multiplying the frequencies of said last-mentioned sub-bands and recombining the frequency-multiplied sub-bands to restore the original signal wave.

2. A method of transmitting speech signals which comprises subdividing a speech signal wave into a plurality of frequency sub-bands each having a narrow width of an order so as to substantially include not more than one vowel component, equally dividing the frequency of each sub-band, recombining and transmitting the divided-frequency sub bands as a signal wave of reduced band width, again subdividing the received signal wave into its corresponding subbands, and equally frequency-multiplying said last-mentioned sub-bands and recombining the frequency-multiplied sub-bands to restore the original speech signals.

3. A method of transmitting speech signals which comprises subdividing a speech signal wave into a plurality of frequency sub-bands each having a width of an order of 100 cycles per second, equally dividing the frequency of each subband, recombining and transmitting the dividedfrequency sub-bands as a signal wave of reduced band width, again subdividing the received signal wave into its corresponding sub-bands, and equally frequency-multiplying said last-mentioned sub-bands and recombining the frequencymultiplied sub-bands to restore the original speech signals. v 4

4. In a system for transmitting a complex signal wave comprising fundamental and harmonic frequency components, means at the transmitter for subdividing a signal wave to be transmitted into a plurality of frequency sub- 1 original signal wave.

5. In a speech signalling system, means at the transmitter for subdividing a speech signal wave to be transmitted into a plurality of frequency sub-bands each having a narrow width of an order so as to substantially include not more than one vowel component, means for equally dividing the frequency of each sub-band, further means for combining the divided-frequency sub-bands for transmission to areceiver as a signal wave of reduced band Width, means at the receiver for again subdividing the received signal wave into its corresponding sub-bands, and further means for equally multiplying the frequencies of each received suSb-band and recombining the frequency-multiplied sub-bands to restore the original speech signals.

6. In a speech signalling system; means at the transmitter for subdividing a speech signal wave to be transmitted into :a plurality of frequency sub-bands each having a width of an order of' cycles per second, means for equally dividing the frequency of each sub-band, further means for combining the divided-frequency subbands for transmission to a receiver as a signal Wave of reduced band width, means at the receiver for again subdividing the received signal wave into its corresponding sub-bands, and further means for equally multiplying the received sub-bands'and recombining the frequency-multiplied sub-bands to restore the original speech signals.

7. In a system for transmitting speech signals, means at the transmitter for subdividing a speech signal Wave to be transmitted into a. plurality of original frequency sub-bands each having a narrow width of an order so as to substantially include not more than one vowel frequency, means for equally dividing the frequency of an amplitude-limited component of each subband and for combining the divided-frequency components with an unlimited component of the same sub-band to produce converted sub-bands of divided frequency and amplitude proportional to the amplitude of the respective original subb-ands, means for recombining and transmitting the converted sub-bands as a signal wave of limited band width, means at the receiver for again subdividing the received signal wave into its corresponding sub-bands, further means for equally frequency-multiplying amplitude-limited components of each received sub-band with unlimited components of the same sub-band to roduce final sub-bands corresponding in both frequency and amplitude to said original sub-bands, and means for recombining said final sub-bands to restore the original signal wave.

8. In a system for transmitting speech signals, means at the transmitter for dividing a speech signal wave into a plurality of original frequency sub-bands each having a narrow width of an order so as to substantially include not more than one vowel frequency, means for equally dividing aneaaee means for equally frequency-multiplying an am plitude-limited component of the received subbands to restore the original sub-band, frequencies, further means responsive to the amplitude of each received sub-band for controlling the amplitude of the respective frequency-multiplied subebands to produce final sub-bands corresponding both in frequency and amplitude to said original sub-bands, and means for combining said final sub-bands speech signals.

9. In a system for transmitting speech signals, means at the transmitter for subdividing a speech signal wave to be transmitted into a plurality of-original frequency sub-bands each having a narrow-widthof an order so as to substantially include not more than one vowel frequency, means for equally dividing the frequency of an amplitude-limited component of each subband to produce sub-bands of lowerfrequency, modulating means for combining the dividedfrequency sub-bands with non-limited components of the same sub-bands to produce converted sub-bands of lower frequency and amplitude proportional to the amplitude of the original sub-bands, means for combining and transmitting theiconverted sub-bands as a signal wave of limited bandwidth, further means at the receiver for again subdividing a received signal wave into its corresponding sub-bands, means for frequency-multiplying amplitude-limited components of the received sub-bands to restore the original sub-band frequencies, further modulating means for combining the frequency-multiplied sub-bands with non-limited components of the respective sub-bands to produce final subbands corresponding in frequency and amplitude to said original sub-bands, and means for combining said final sub-bands to restore the original speech signals.

10. In a system for transmitting speech signals, means at thetransmitter for subdividing a speech-signal wave to be transmitted. into a plurality of original frequency sub-bands each having a narrow width of an order so as to substantially include not more than one vowel frequency, means for equally dividing the frequency of amplitude-limited components of each sub-band to produce sub-bands of lower frequency, rectifyingand filtering means to producesignals proportional to the amplitude of the original subbands, means for controlling the amplitude of the divided-frequency sub-bands by their respective control signals to produce converted sub-bands of lower frequency and amplitude proportional to the original sub-band amplitudes, means for combining and transmitting the converted subbands as a signal wave of limited band width, further means at the receiver for again subdividing the received signal wave into=its corresponding sub-bands, means for equally frequency-multiplying amplitude-limited components of the received sub-bands to restore the original subband' frequencies, .means to restore the amplitude ofithermultiplied sub-bands-to correspond with to restore the original 112 the original sub-band amplitudes, and means for combining the'resultant sub-bands to reconstruct the original speech signal.

11. The method of transmitting a complex signal wave comprising fundamental and harmonic frequency components which consists in subdividing a signal wave to be transmitted into a plurality of equal frequency sub-bands having a width of an order so as to substantially include not more than one of said frequency components, equally dividing the frequency of each of said sub-bands, recombining and transmitting the divided-frequency sub-bands as a signal wave of reduced band width, again subdividing the received signal wave into its corresponding subbands, and equally multiplying the frequencies of said last-mentioned sub-bands and recombining the frequency-multiplied sub bands to restore the original signal wave.

12. The method of transmitting a complex signal Wave comprising fundamental and harmonic frequnecy components, which consists in subdividing a signal wave to be transmitted into a plurality of frequency sub-bands of widths increasing from the lower toward the upper end of the frequency spectrum of said wave and being narrow enough so as to substantially include not more than one of said componenta'equally dividing the frequency'of each of said sub-bands, recombining and transmitting the divided-frequency sub-bands as a signal wave of reduced band width, again subdividing the received signal wave into its corresponding sub-bands, and equally multiplying the frequencies of said lastmentioned sub-bands and recombining the frequency-multiplied sub-bands to restore the original signal wave.

13. In a signalling system for transmitting a complex signal wave comprising fundamental and harmonic frequency components, means at the transmitter for subdividing a signal Wave to be transmitted into a plurality of frequency subbands of equal width being narrow enough so as to substantially include not more than one of said frequency components, means for equally dividing the frequency of all of said sub-bands, further means 'for combining the divided-frequency sub-bands for transmission to a receiver as a wave of reduced band width, means at the receiver for again subdividing the received signal wave into its corresponding sub-bands, and further means for equally multiplying the frequencies of the received sub-bands and combining the frequency-multiplied sub-bands to restore the original signal wave.

14. In a signalling system for transmitting a complex signal Wave comprising fundamental and harmonic frequency components, means at the transmitter for subdividing a signal wave to be transmitted intoa plurality of frequency subbands of unequal widths increasing from the lower to the upper end of the frequency spectrum of said wave and being narrow enough so as to substantially include not more than one of said frequency components, means for equally dividing the frequency of all of said sub-bands, further means for combining the divided sub-bands for transmission to a receiver as a wave of reduced band width, means at the receiver for again subdividing the received signal wave into its corresponding sub-bands, and further means for equally multiplying the frequencies of the received sub-bands and combining the frequencymultiplied sub-bands torestore the original signal wave.

15. In a system for transmitting a complex signal wave comprising fundamental and harmonic frequency components, means at the transmitter for subdividing a signal wave to be transmitted into a plurality of frequency sub-bands each having a narrow band width of an order so as to substantially include not more than one of said frequency components, means for equally dividing the frequency of amplitude-limited components of each of said sub-bands, means for restoring the amplitude variations of the divided-frequency sub-bands and combining the same into a signal wave of reduced band Width for transmission to a receiver, means at the recelver for again subdividing the received signal wave into its corresponding sub-bands, further means for frequency-multiplying amplitudelimited components of the ,received sub-bands to restore their original frequencies, and means for restoring the amplitude variations of the frequency-multiplied sub-bands and combining the latter to reconstruct the original signal wave.

16. In a system for transmitting speech signals, means at the transmitter for subdividing a speed signal wave into a plurality of frequency sub-bands each having a band Width of the order of 100 cycles per second, means for equally dividing the frequency of amplitude-limited components of each of said sub-bands, means for restoring the amplitude variations of the dividedfrequency sub-bands and combining the same into a signal Wave of reduced band width for transmission to a receiver, means at the receiver for again subdividing the received signal Wave into its corresponding sub-bands, further means for equally frequency-multiplying amplitudelimited components of the received sub-bands to restore their original frequencies, and means for restoring the amplitude variations of the frequency-multiplied sub-bands and combining the latter to reconstruct the original signal wave.

17. In a system for transmitting a complex signal wave comprising fundamental and harmonic frequency components, means at the transmitter for subdividing a signal Wave to be transmitted into a plurality of frequency sub-bands each having a narrow band width of an order so as to substantially include not more than one of said frequency components, means for equally dividing the frequency of amplitude-limited components of each of said sub-bands, means for restoring the amplitude variations of the dividedfrequency sub-bands and combining the same into a signal wave of reduced band width for transmission to a receiver, means at the receiver for again subdividing the received signal wave into its corresponding sub-bands, further means for frequency multiplying amplitude limited components of the received sub-bands to restore their original frequencies, means for restoring the amplitude variations of the frequency-multiplied sub-bands and combining the latter to reconstruct the original signal wave, means for subjecting said sub-bands to a frequency-substitution scrambling process before transmission, and further means for unscrambling the received sub-bands prior to reconstructing the original signal wave.

18. In a system for transmitting a complex signal Wave comprising fundamental and harmonic frequency components, means at the transmitter for subdividing a signal wave to be transmitted into a plurality of frequency sub-bands each having a narrow band width of an order so as to substantially include not more than one of said frequency components, means for equally dividing the frequency of amplitude-limited components of each of said sub-bands, means for producing control signals representative of the amplitude of each sub-band, means for combinedly transmitting the divided frequency sub-bands and control signals as a composite signal wave of reduced band width, means for again subdividing the received signal wave into its corresponding sub-bands, frequency-multiplying the received sub-bands to restore their original signal frequencies, further means to restore the amplitudes of the multiplied sub-bands in accordance with their respective control signals, and means for combining the resultant sub-bands to reconstruct the original signal wave.

19. In a system for transmitting speech signals, means at the transmitter for subdividing a speech signal wave to be transmitted into a plurality of frequency sub-bands each having a band width of an order of 100 cycles per second, means for equally dividing the frequency of amplitude-limited components of each of said sub-bands, means for producing control signals representative of the amplitude of each sub-band, means for combinedly transmitting the divided frequency subbands and controlsignals as a composite signal Wave of reduced band width, means for again subdividing the received signal wave into its corresponding sub-bands, means for frequency-multiplying the received sub-bands to restore their original signal frequencies, further means for restoring the amplitudes of the multiplied subbands in accordance with their respective control signals, and means for combining the resultant sub-bands to reconstruct the original signal wave.

20. In a system for transmitting speech signals, means at the transmitter for subdividing a speech signal wave to be transmitted into a plurality of frequency sub-bands each having a band width of an order of 100 cycles per second, means for equally dividing the frequency of amplitude-limited components of each of said sub-bands, means for producing control signals representative of the amplitude of each sub-band, means for combinedly transmitting the divided frequency sub-bands and control signals as a composite signal wave of reduced band width, means for again subdividing the received signal wave into its corresponding sub-bands, means for frequency-multiplying the received sub-bands to restore their original signal frequencies, further means for restoring the amplitudes of the multiplied sub-bands in accordance with their respective control signals, means for combining the resultant sub-bands to reconstruct the original signal wave, means for subjecting both the frequency sub-bands and control signals to a substitution scrambling process at the transmitter,

and further means for unscrambling the received sub-bands and control signals at the receiver prior to the reconstruction of the original signal wave.

GUSTAV GUAN ELLA.

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

UNITED STATES PATENTS Number Name Date 2,098,956 Dudley Nov. 16, 1937 2,151,091 Dudley Mar. 21, 1939