US2726283A - Method and apparatus for reducing band requirements in transmission systems - Google Patents

Description

M. J. DI TOR@ 2,726,283

2 Sheets-Sheet l METHOD AND APPARATUS FOR REDUCING BAND REQUIREMENTS IN TRANSMISSION SYSTEMS Dec. 6, 1955 Filed Dec.

Dec. 6, 39:25 M. J. Dl TORO 2,725,283

- METHOD AND APPARATUS FOR REDUCING BAND REQUIREMENTS 1N TRANSMISSION SYSTEMS Filed DSC. 5l, 1949 2 Sheets-Sheet 2 United States Patent O METHOD AND APPARATUS FOR REDUCING BAND REQUIREMENTS IN TRANSMESSIN SYSTEMS Michael J. Di Toro, Bloomfield, N. J., assignor to International Telephone and Telegraph Corporation, a corporation of Maryland Application December 31, r1949, Serial No. 136,245

Claims. .(Cl. 179-.-15)

This invention relates to a method and apparatus for reducing the frequency range required for the transmission of signals, as for instance, speech. More particularly the invention pertains `to a system for facilitating transmission of a message over a transmission medium not adapted to readily pass .the frequency vband originally occupied by the message,as forlexample, to facilitate signalling over .long lines where the higher speech frequencies cannot readily be transmitted l.because of excessive attenuation.

The invention provides for 4the reduction in band width requirements by the compression and expansion of the band width necessary to ,convey the ,Speech intelligence.

The information transmitted by speech does not necessarily require all the frequency :space allotted to it in the Vhuman voice. The speech lmessagesof any one person can be compressed in band width without deterring appreciably from the intelligence yconveyed thereby, since the band width usually available for speech transmission is not completely utilized at all times. I his lack of utilization of the .entire available spectrum becomessradily apparent when it .is Observed 4'that .a Parsons :Speech iS composed .mostly ,of a time .Sequence of voiced (quasiperiodic) .and uni/Diced (aperiodclsounds- Thcvcicsd sounds have more of their energy .at thalowsr ,frequency portion of the spectrum, while ,the Iunvoied sounds'have more of thcirenersy in the upperportiqn 0.1?` the Spectrum.

Another lcharacteristic of speech which results Vin a band width waste 'resides in the lquasi-periodic nature of the voiced sounds Although the spectrum of voiced sounds may extend to'liigh frequencies it d oesnot necessarily `fill in the entire spectrum, but only thinclusters vof it located at D. ,C. and harmonics of the fundamental pitch. The frequency width of eachsuch cluster inversely proportional to the time duration of the Yenvelope of the message.

In systems of the general class described, which are known, the amount of equipment required to obtain the desired reduction is large' and yeconomically justifiable only whena great premium is placed on band width conservation. Such a'system'has'been d escribedin-the'U. S. Patent No.2, 151,091 issued onfMarch 21, f19r39, -to Homer Dudley. 'The Vocoderas disclosed :by Dudley in the above-mentionedk patent operates on a system of 4spectral analysis and synthesis. The speech spectrum isroughly analyzed into 10 'bands of Z50-cycles -per second each, and the aggregate intensity in each such Iband is transmitted through a separate channel of cycles per second. The transmitted intensity is -used formodlating a buzzer at the'receiving end, which roughlyvreproduces the original spectrum. An eleventh channel is-used1fortrans mitting the pitc which is, Vbroadly speaking, the frequencyof thesvocal chord.

The system of this inventionis based on theidea lthat if, in a half band compression system-for example, ithe `half band available allltbe time is.used'partofthetirne to sample Vone'half ofthe original speech spectrum, and the other time to sample the other half of the original yembodiment hereinafter described.

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band spectrum, then the ear is able to reconstruct the original spectrum.

In accordance with this invention the speech frequency spectrum is separated into two approximately equal ranges, the higher range is converted to the same level as the lower one by means of product modulation and the two ranges are then transmitted on a time sharing basis.

The system of this invention provides a great saving in equipment costs over those previously `known and is readily adaptable to existing vcommunication systems.

The above-mentioned and other'features and lobjects of this invention and the manner of attaining them will become more apparent and the invention itself lwill be best understood by reference to the `following description of an embodiment of the invention taken in conjunction with the accompanying drawings wherein:

Figure 1 shows, in block-schematic form, the transmitting equipment employed in the present embodiment of the invention;

Figure 1A shows a set of waveforms useful in explaining the operation of the network 14 shown in Figure 1; and

Figure 2 shows, in block form, the receiving equipment employed in the present embodiment of the invention.

With reference to Figure 1, a wave-form as designated by the yreference numeral 1 will be considered for purposes of simplification in explaining .the operation ofthe system. The invention Iwill be described with particular reference to speech waves, although the invention Y is equally applicable to other types of waves such as music and waves used Ato produce visualleffects or the like. The speech spectrum 1 is composed of both high and low frequency components ranging in frequency from 0.210. 3.2 kilocycles per second.

The input wave 1 is applied in common to band-pass filters 2 and 3 .which are shown in block form in Figure 1. The vfunction of the bandpass filter 2 is to remove the yhigh frequency components from the input spectrum 1 and pass that bandof frequencies ranging from 0.2 tol.6 kilocycles per second, as shown by wave-form 4. Filter 3 removes the lower frequency components and passes substantially the wave-form 5 containing frequencies yin the high range from 1.6 to 3.2 Akilocycles per second.

It should be pointed out that the system is readily adaptable to .provide speech .privacy in transmission.

Since the wave-.form ,4` is transmitted directly without modulation, the upper band width from 1.6 to 3.2 kc. which remains unused in the transmission channel may Abe 4used to pass another speech channel. For this clase,

wave form 5 would be `passed directly, and wave form 4 would be `product modulated to fitV the upper band.

When this is done, it is Apossible 'to invert its spectrum, with the result that speech 4privacy during transmission is effected. v.Such modification would fall lwithin the yscope of the present inventionand the necessary changes to ,effect this result should be readily apparent to one skilled in the art from an understanding of the present In the present embodiment of the invention the outputof band-pass filter 3 is fed to a product modulator 6, which is a modulator ofthe balanced typ, where itis modulated with a 3.2 kilocycle source vsupplied by a local oscillator v7. The product modulation is such als to transpose the frequency range of the 1.6 vt0 3.2 kilocycle band yto the same frequency range as the 0.2 to 1.6 kilocycle band. The modulation product is then fed through a .bandpass filter 8, which is tuned to the 0.2 to 1.6 kilocycle b and to removeanyspurious components resulting from the modulation.

As shown `by wave-form 9, the output of the bandpass filter 8 containsy the upper portion of the spectrum transposed to the lower frequency range. v

In order to regenerate the 3.2 kilocycle frequency at the receiver it is necessary to transmit with the compressed wave form some sub-multiple of 3.2 kilocycles per second. This is accomplished by the frequency divider network whose input is supplied by local oscillator 7. The output of the frequency divider is 0.8 kilocycles and is fed directly to the output of the transmitter.

As has been previously pointed out the speech of any one person is primarily a time sequence of voiced and unvoiced sounds. The voiced sounds are characterized in that they have a pitch dependent upon the fundamental frequency of vibration. The unvoiced sounds are composed essentially of random frequency components with no true pitch.

To recreate properly the speech spectrum 1 at the receiving end itis necessary to control the proper association of the fundamental pitch with the compressed wave-form at the transmitting end. In order to determine the fundamental pitch frequency of the speech spectrum it is iirst necessary to determine whether the sound is of periodic or aperiodic nature, and if periodic to determine the periodicity in time.

To accomplish this result in the circuit of the present invention I utilize the fundamental pitch detector 11 and a voiced-unvoiced detector and pulse generator 12. The elements 11 and 12 have been shown in block form in the transmitting circuit of Figure 1 as they are well known in the art and a full understanding of their detailed operation can be obtained from a consideration of the U. S. Patent No. 1,836,824 to I. C. Steinberg or from the article entitled Re-making speech," by Homer Dudley, published in The Journal of the Acoustical Society of America, 1939, volume 11, pages 169 to 177. In the operation of the detecting circuits 11 and 12 an output wave-form 13 is obtained which consists of pulses that are proportional to pitch and are spaced in time in accordance with the fundamental period.

To provide switching at the fundamental pitch rate for the two equal spectrum ranges it is necessary to create a control wave-form whose period is that of the fundamental and to then bisect this period into two equal control portions. I accomplish this in the present invention by the use of a period bisection network 14 shown within the dotted rectangle in Figure l. The circuit elements comprising the network 14 have been individually shown in block form in order to provide an easily understandable breakdown of the various operations performed therein, which operation will be explained with reference to the wave-forms eo thru es, Figure 1A.

In order to correlate the functions of the various components in the bisection network in terms of inputoutput wave-forms, the voltage wave 13 will be referred to as eo within the network 14, since such designation allows for an easy time related sequence study, relative to the other wave functions of network 14. Each of the wave-forms to be considered in the discussion of network 14 have been carried out to an equivalent of three fundamental periods as is shown for the voltage e0 in Figure 1 where ti thru t4. designate the defining pulses of three fundamental periods. The voltage eo is first applied to a short circuiting switch controlling an elementary linear-sweep generator which has been shown in simplitied form but which may be of any of the varied types of generators available for this purpose. Such generators are considered in detail in the text Wave-forms (M. I. T. Radiation Series, vol. 19), by Britton Chance et al., on pages 259 thru 288.

The short circuiting switch is triggered by the fundamental period pulses t1t4 and operates alternately to charge and discharge the condenser c thru the resistor r with a resulting sawtooth wave-form er. The voltage e1 in combination with the voltage eo is then applied to a clamping circuit which functions to provide a clamped voltage at epochs of eo. Thru the control of voltage eo the clamping circuit, at every epoch in time t1-t4, operates to sample the amplitude of e1, take one-half of this value, and clamp or retain the sample until the next sampling takes place. The output wave-form from the clamping circuit is shown at e2 where at epochs ti, t2 t. Volume 19 of the M. I. T. Radiation Laboratory Series mentioned above, discloses a plurality of circuit arrangements for performing the operation of the clamping circuit which is known per se.

As is apparent from the graph of the voltages eo-ea the first fundamental period, namely that between the time epochs t1 and tz is more or less wasted in that the equipment utilizes this time to adjust or become responsive to the following pitch pulses. However, the time necessary to arrive at a steady state condition from a time to, is negligible and will not affect the operation of the system. In practice it might even prove desirable to sample and clamp only at alternate fundamental pitch periods.

The voltage e2 from the clamping stage and the sawtooth voltage e1 are next fed to a mixer stage which operates to subtract the two and provide the wave-form illustrated at e3. The voltage e3 is equal to ei-ez. Again it should be noted that the M. I. T. Radiation Series text mentioned above, at chapter 18, discloses a plurality of circuits for performing this subtraction.

The voltage es is then applied to a shaper circuit which functions to provide a positive pulse each time waveform e3 passes through zero. It becomes readily apparent that the pulsed wave-form e4 is characterized in that each of the fundamental periods of wave e0 has been bisected and each half period is defined by a positive pulse.

Voltages e4 and eo are next applied to a mixing and clipping circuit where they are combined such that eo is partially subtracted from e4 but not to the extent that eo is entirely lost. The circuit provides a positive pulse each time eu goes positive and a negative pulse each time e4 (only) goes positive. The pulses are clipped to provide the output voltage wave-form e5.

In the final stage of the bisection network 14 the wave-form e5 is applied to a shaper circuit whose output is the square wave es having positive and negative variations corresponding to each half fundamental period. The wave-form es has been shown for a single period in Figure l by the reference character 15.

Thus it becomes readily apparent that the wave-form 15 is ideally suited to serve as a timing medium to control the transmission of the lower frequency spectrum (waveform 4) during one half of the fundamental period and to control the transmission of the transposed higher frequency range (wave-form 9) during the remaining one half of the fundamental period.

To enable the receiver to differentiate between the range of frequencies incoming from the transmitter a portion of the output 15 of the period bisector 14 is fed to a sync generator 16 which generates a pulse of one polarity to identify that portion of the wave 15 designated to control one of the frequency bands and a pulse of opposite polarity to designate the portion of 1S assigned to the other frequency band. The output of the sync generator is illustrated by the wave-form 17.

The voltage wave-forms as designated by reference numerals 4, 9, 15 and 17 are applied to a mixer switch 18 which functions to combine the separate portions of the speech spectrum in accordance with the control waveform 15. The switch 18 is preferably of the cathode ray distributor type having three commutation segments wherein the scanning operation is controlled by the wave 15. A switch of this type which is suitable for the present invention is disclosed in the U. S. Patent 2,387,018 to R. V. L. Hartley which issued on October 16, 1945. The switch 18 could also be of the electro-mechanical type as disclosed in the U. S. Patent 2,387,906 to T. W. W. Holden wherein the multi-commutation is effected through the use of a synchronous motor type pick-up device.

For purposes of illustrating the operation of the switch 18 it will 'be assumed that the positive portion of the square Wave 15 has been designated to control the transmission of the lower frequency range of the spectrum for one half period and the negative portion of 15 is assigned to the transposed high frequency portion of the spectrum. In like manner to enable the receiver to properly respond to the incoming wave-form, a positive sync pulse will be associated with the low band and a negative sync pulse will kbe utilized to identify the vtransposed high band. Each of the Wave-forms shown in Figure l are of comomn time duration, namely one period.

With reference to wave-form 19 which illustrates the output voltage of the transmitter, it may be seen that since the square wave V15 is of positive duration for the first one half cycle of the fundamental period, the control action of the switch 18 is such as to first transmit a positive sync pulse followed by the low frequency portion of the speech spectrum, wave-form 4. On the negative swing of the wave 15 a negative sync pulse is applied to the output through the action of switch 18 and is followed by the transposed high frequency portion of the speech spectrum, wave-form 9. In like manner the transmission of the speech message is continued on a compressed frequency band from 0.2 to 1.6 kilocycles in accordance with the controlling voltage derived in the fundamental pitch detector and period bisector network. As has been previously pointed out a 0.8 kilocycle voltage, the submultiple of the local oscillator frequency of 3.2 kilocycles, is transmitted with the compression wave in order to allow for the regeneration of the oscillator frequency at the receiver.

In the receiving circuit of Figure 2 the operation is similar to that of the transmitter in working back from the compressed wave .19 to the original spectrum. The incoming wave 19 at the receiver is first applied to a sync pulse separator 20 which may operate on an amplitude clipping principle or which may be arranged to separate the sync pulses in accordance with an impulse spacing detector arrangement as is well known in the art. The

`output of the sync separator 20, which is shown by the wave-form 21 and which corresponds to the original sync .pulse Wave-form 17, is applied in common to a square wave generator 22 and a mixing stage 23. The square wave generator generates a square wave 24 having positive and negative variations corresponding to the sync pulses and is substantially the same in timed relation as the controlling wave 15 of the transmitter. However, since the single period of the input wave to the receiver contains a pair of sync pulses in addition to the speech spectrum it may readily be seen that the switching action at the receiver must be such as to eliminate these pulses from the final output. The square wave 24 could not perform this elimination as it extends over a complete fundamental period and any switching action controlled thereby would necessarily include the objectionable sync pulses.

To provide a controlling voltage for the receiver which will eliminate the sync pulses, the square wave 24 from the generator 22 is combined with the sync pulses 21 in a mixing stage 23 to obtain a gated wave-form as illustrated at 25.

The gated wave 25 is applied to a switch 26 similar to `that utilized in the transmitter, as is the input wave 19 of the receiver. The switch 26, under control of the Voltage .25, will alternately allow switching of the two spectrum ranges only after the sync portion of the controlling voltage has passed. Thus, considering the assumption previouslyl made with respect to the control functions assigned to the positive and negative portions of the period waveform, it may be seen that the first portion of the `wave 25 immediately following the `sync pulse, will control the switch 26 to allow passage of the low frequency range of the spectrum, namely wave-form 27, corresponding to wave 4 in the transmitter. The mid-portion of the gated voltage 25 will effectively block that portion of the input voltage 19 which corresponds to the syncl pulse and then allow the passage of the transposed high frequency range of the spectrum as shown by wave-form 28, which corresponds to the wave 9 of the transmitter.

The wave-form 27 after passage by the switch 26 corresponds to the low frequency range of the spectrum frorn 0.2 to 1.6 kilocycles as originally obtained by direct filtering from the input to the transmitter. This voltage is again filtered in the band-pass filter 29 and passed vdirectly to a final mixing stage 30. However, the voltage wave 28 as passed by the switch 26 was originally transposed from the 1.6 to 3.2 kilocycle range to the 0.2 to 1.6 kilocycle range in the transmitter and must now be shifted back to the high band.

This is accomplished by first isolating the 0.8 kilocycle sub-multiple of the transmitter local oscillator frequency, which is contained in the receiver input, by means of the band-pass filter 31 tuned to this sub-harmonic of 3.2 kilocycles. The 0.8 kilocycle frequency is then multiplied in the frequency multiplier 32 to reproduce the original 3.2 kilocycle frequency of the local oscillator. The transposed high frequency portion of the spectrum as passed-by the switch 26 is then product modulated in the modulator 33 with the 3.2 kilocycle oscillator frequency to obtain the 1.6 to 3.2 kilocycle band as originally contained in the transmitter input. This portion of the speech spectrum'is then filtered in the band-pass lter 34 to remove any spuriout components of the modulation andl finally passed as wave-form 35 to the final mixing stage.

In the final mixing stage 30 the components 27 and 35 of the original spectrum are recombined to obtainthe original speech spectrum as designated by the wave 36.

While 1 have described above the principles of my invention in connection with speciiic apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention.

What is claimed is: Y

l. A system for transmitting within a relatively narrow band-width signals having a wide band of frequencies, comprising a source of wide band signals, means for translating the higher frequency signals within said wide band to the bandwidth occupied by the low frequency signals within said wide band, means for deriving from said'wide band signals a wave having the fundamental frequency thereof, means responsive to the derived fundamental wave for deriving a control voltage and means controlled by said control voltage for transmitting said translated high frequency signals and said low frequency signals on a time-sharing basis.

2. A system for transmitting signals having high and low frequency components within a bandwidth corresponding to that occupied by substantially said low'frequency components comprising a source of such signals, means for translating said high frequency signal cornponents to the band width of said low frequency signal components, means for deriving from said signals a wave having the fundamental frequency thereof, means responsive to said fundamental frequency wave for generating time control voltages proportional to the fundamental frequency of said signals and means controlled by-said voltage for transmitting said translated high frequency signal components and said low frequency signal components on a time-sharing basis.

3. A system for transmitting within a relatively narrow bandwidth signals having a wide band of frequencies, a

4source-of wide band signals, .comprising means for transwithin said wide band, means for deriving from said wide band signals a wave having the fundamental frequency thereof, means responsive to the derived fundamental wave for deriving a control voltage, means controlled by said control voltage for transmitting said translated high frequency signals and said low frequency signals on a time-sharing basis and means responsive to said signal for deriving an identifying signal for identifying the original wide band frequency allocation of said transmitted signals.

4. A system for transmitting signals having high and low frequency components within a bandwidth corresponding to that occupied by substantially said low frequency components comprising a source of such signals, means for translating said high frequency signal components to the bandwidth of said low frequency signal components, means for deriving from said signals a wave having the fundamental frequency thereof, means responsive to the derived fundamental wave for generating a control voltage proportional in timed relation to the fundamental frequency of said signals, means responsive to said control voltage for generating identifying signals to designate the original frequency allocation of said translated high frequency signal components and said low frequency signal components and means controlled by said control voltage for transmitting said translated high frequency signals and said low frequency signals with respective identifying signals on a time-sharing basis.

5. In a transmission system having input signals in the audio-frequency range, means for subdividing said input signals into a plurality of individual frequency bands, means for translating the higher frequency signal bands to the bandwidth occupied by the low frequency signals within said audio-frequency range, means for deriving from said signals a wave having the fundamental frequency thereof, means responsive to the derived fundamental wave for deriving a control voltage, means responsive to said control voltage for transmitting said high frequency translated band signals and said low frequency signals on a time-sharing basis and means responsive to said control voltage for transmitting with each of said individual signal bands an identification signal designating the original frequency of said band.

6. In a transmission system having input signals in the audio-frequency range, means for subdividing said input signals into a plurality of individual frequency bands, means for translating the higher frequency signal bands to the bandwidth occupied by the low frequency signals within said audio-frequency range, means for deriving from said signals a wave having the fundamental frequency thereof, means responsive to the derived fundamental wave for deriving a control voltage from said input signals in `accordance with the fundamental pitch thereof, transmitting means controlled by said control voltage being operative to transmit said translated high frequency bands and said low frequency signals on a timesharing basis and means responsive to said control voltage for associating an identification signal with each of said individual transmitted bands to designate the original frequency of said bands,

7. In a transmission system having input signals in the audio-frequency range, means for dividing said input signals into two equal frequency bands, means for translating one of said frequency bands to the same bandwidth as the other band, means for deriving from said signals a wave having the fundamental frequency thereof, means responsive to the derived fundamental wave for deriving a control voltage from said input signals, means responsive to said control voltage for transmitting said frequency bands on a time-sharing basis, and means responsive to said control voltage for transmitting an identication signal with each of said frequency bands to designate the original frequency of said bands.

8. In a transmission system having input signals in the audio-frequency range, means for dividing said input signals into two equal frequency bands, one of said bands extending over a low audio-frequency range and the other band over a high audio-frequency range, means for translating said high frequency band to the same bandwidth as said low frequency band, means for deriving from said signals a wave having the fundamental frequency thereof, means responsive to the derived fundamental wave for deriving a control voltage, means responsive to said control voltage being operative to transmit said low frequency signal band and said translated high frequency signal band on a time-sharing basis and means responsive to said control voltage from transmitting an identification signal with each of said frequency bands to designate the original frequency of said bands.

9. In a transmission system having input signals in the voice-frequency range, filtering means for separating said input signals into two equal frequency bands, one of said bands extending over the low-frequency range and the other over the high voice-frequency range, a product modulator operative to shift said high frequency band to the same bandwidth as said low frequency band, means for deriving from said signals a wave having the fundamental frequency thereof, means responsive to the derived fundamental wave for deriving a control voltage from said input signals, means responsive to said control voltage to transmit said low frequency signal band and said shifted high frequency signal band on a time-sharing basis and means responsive to said control voltage for transmitting an identification signal integral with each of said frequency bands designating the original frequency range of said bands.

l0. In a transmission system having input signals in the voice-frequency range, filtering means for separating said input signals into two equal frequency bands, one of said bands extending over substantially the low voicefrequency range and the other over substantially the high voice-frequency range, a product modulator operative to shift said high frequency band to the same bandwidth as said low frequency band, fundamental pitch detecting means responsive to said input signals for deriving a control voltage from said input signals in accordance with the fundamental frequency thereof, switching means controlled by said control voltage to transmit said low frequency signal band and said shifted high frequency signal band on a time-sharing basis and means responsive to said control voltage for associating an identification signal with each of said frequency bands to designate the original frequency range of said bands.

l1. A system for transmitting and receiving signals having high and low frequency components within a bandwidth corresponding to that occupied by substantially said low frequency components comprising a transmitter, including; a source of such signals means for translating said high frequency signal components to the bandwidth of said low frequency signal components, means for deriving from said signals a wave having the fundamental frequency thereof, means responsive to the derived fundamental wave for generating a control voltage proportional in timed relation to the fundamental frequency of said high and low frequency signals, means controlled by said control voltage for transmitting said translated high frequency signal components and said low frequency signal components on a time-sharing basis, a receiver coupled to said transmitter including; means for receiving said translated high frequency signal components and said low frequency signal components in timed relation and means for restoring said signals to their original frequencies.

l2. A transmitting and receiving system for conveying information contained in signals extending over a wide band of frequencies, comprising a transmitter having a source of wide band signals, means for translating high frequency signals within said wide band to the bandwidth of low frequency signals within said wide band, means for deriving from said wide band signals a Wave having the fundamental frequency thereof, means responsive to the derived fundamental wave in said transmitter for generating a control voltage, means for generating identifying signals to designate the original wide band frequency allocation of said translated high frequency signals and said low frequency signals, means controlled by said control voltage for transmitting on a time-sharing basis said translated high frequency signals and said low frequency signals, means responsive to said control voltage for transmitting respective identifying signals with said last named signals, a receiver coupled to said transmitter, means at said receiver responsive to said signals to differentiate between said identifying signals, means controlled by said last named means at said receiver for regenerating said wide band signals in accordance with the received translated high frequency and low frequency signals and said identifying signals.

13. A communication system comprising, a transmitter having input signals extending over the voice-frequency range, means at said transmitter for subdividing said voice-frequency input signals into a plurality of individual frequency bands, means for shifting said individual band signals into a single narrow bandwidth in the voice-frequency range, means for deriving from said input signals a wave having the fundamental frequency thereof, means responsive to the derived fundamental wave for deriving a control voltage, means responsive to said control voltage for transmitting said narrow frequency band signals on a time-sharing basis, means responsive to said control voltage for transmitting with each of said individual narrow band signals an identification signal to designate the original frequency allocation of said band, a receiver coupled to said transmitter, means for receiving said individual narrow band signals and said identifying signals at said receiver, and means controlled by said last named means for regenerating said transmitter input signals from said narrow band signals in accordance with said identifying signals.

14. A communication system comprising a transmitter having input signals in the voice-frequency range, means in said transmitter for dividing said input signals into two equal frequency bands, one of said bands extending over substantially the low voice-frequency range and the other band over substantially the high voice-frequency range, means for translating said high frequency band to the same bandwidth as said low frequency band, means for deriving from said input signals a wave having the fundamental frequency thereof, means responsive to the derived fundamental wave for deriving a control voltage in accordance with the fundamental pitch thereof, means responsive to said control voltage to transmit said low frequency signal band and said translated high frequency signal band on a time-sharing basis, means responsive to said control voltage for transmitting an identiication signal with each of said frequency bands to designate the original frequency of said bands, a receiver coupled to said transmitter, means at said receiver responsive to said transmitted signals to differentiate between said identifcation signals and means at said re ceiver controlled by said last named means to regenerate said transmitter input signals from said transmitted bands.

15. A bandwidth reduction system comprising a transmitter, means at said transmitter for separating signals incoming to said transmitter into a plurality of individual frequency bands, means for compressing said individual frequency bands into signals in a single bandwidth, means for detecting the fundamental pitch of said incoming signals, means responsive to the detected pitch for developing a control voltage in accordance with said pitch, means responsive to said control voltage for transmitting said compressed signals on a time-sharing basis, means responsive to said control voltage for transmitting an identification pulse with each of said individual compressed signals, a receiver coupled to said transmitter, means at said receiver operative to receive said compressed signals and said identification pulses, and means at said receiver for regenerating said transmitter input signals from said compressed signals in accordance with said identification pulses.

References Cited in the file of this patent UNITED STATES PATENTS 1,836,824 Steinberg Dec. 15, 1931 2,115,803 Dudley May 3, 1938 2,401,464 Corderman June 4, 1946 2,406,825 French Sept. 3, 1946 2,408,692 Shore Oct. 1, 1946

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