System for transferring wideband sound signals
United States Patent [:91  
 Gassmann l l SYSTEM FOR TRANSFERRING WIDEBAND SOUND SIGNALS [75] Inventor: Gerhard Giinter Gassmann,  
 Berkheim, Germany [73] Assignee: International Standard Electric Corporation, New York, NY.  
  22 Filed: Feb. 22, 1973 211 Appl No.: 334,525  
 [30] Foreign Application Priority Data Feb. 24. I972 Germany 2208805 Apr. I4. 1972 Germany 2218154 Oct. I2, 1972 2250094 52] U.S. Cl 179/1555 R Int. Cl. 04b 1/66 Field of Search l79/l SA, l5.55 B, [5.55 T  
 l56| References Cited UNITED STATES PATENTS 2.840.639 6/1958 Graham l79/l SA 2.957.948 10/[960 Edson l79/l5.55 R  
 [ 1 Apr. 1, 1975 3,030,450 4/l962 Schroeder l79/lS.55 R 3,139,487 6/l964 Logan l79/l5.55 R 3,246,084 4/!966 Krytcr l79/l5.55 R 3,349,l84 l0/l967 5 R 3.674.939 7/1972 5 R 3,696,298 10/1972 5 R Primary Examiner-Ralph D. Blakeslee Attorney, Agent, or Firm-John T. O&#39;Halloran; Menotti J. Lombardi, Jr.; Peter Van Der Sluys l 57] ABSTRACT A system for transferring sound signals is described with which broad-band sound signals can be transferred over a much more narrow-band transfer path between pickup and reproduction equipment. Starting from ear-physiological findings, the system proposes to transfer only a particular lower frequency range in original form. and of the upper frequency range only amplitude information of individual subranges. At the place of reproduction, the signal of the upper frequency range is then reproduced synthetically with the aid of the amplitude information.  
 30 Claims, 12 Drawing Figures wamEnAvn H915 Fig.5a  
 PATENTEDAPR&#39; 11915 Fig. 7c  
 SYSTEM FOR TRANSFERRING WIDEBAND SOUND SIGNALS The present invention relates to a system for transferring wideband sound signals.  
  In this application. wideband sound signal&#34; means a sound signal which lies in the frequency range from 20 Hz. to l5 kHz. which is necessary for the reproduction of highquality music. If a sound signal is to be reproduced at a place or instant which is distant from the origin in space and/or time. the transfer system must admit such a bandwidth.  
  A telephone system. for example. mainly serves to transfer speech signals. it being possible to understand syllables and recognize the subscribers voice even at a very small bandwidth in the range from about 300 to 3.000 Hz. For speech. a transfer system with even narrower bandwidth has been provided in the form of a vocoder. in which. however, the bandwidth is so small that only syllable intelligibility is insured. In that system. the necessary bandwidth is about 400 to 500 Hz.  
  According to the present allocation, mediumfrequency transmitters have a transmission bandwidth of about 40 Hz to 4.5 kHz. which already permits music broadcasting. but the quality of reproduction is poor.  
  If the transfer system contains storage media such as a phonograph record. magnetic tape. sound film. etc.. the quality of reproduction depends on the useful band width of these media. too. Sound-signal reproduction meeting high standards ofquality is possible in the VHF broadcast band.  
  In addition to the technically possible bandwidths of the individual systems. it must be considered that. in each system. the number of transfer channels. with given overall bandwidth. determines the bandwidth per channel. i.e.. the bandwidth per channel is inversely proportional to the number of channels; the effect of this is particularly evident in the medium-wave broadcasting system.  
  Considering the conditions described above. the object of transferring high-quality music with the aid of a system whose bandwidth available per channel is much smaller than that of the signal to be transferred seems unattainable.  
  On the other hand. the above mentiones definite criteria for speech and music. such as syllable intelligibility and recognizability of a particular voice or quality standards. which can be summarized under the general term of information content.  
  It is therefore the object of the present invention to provide a system for transferring wideband sound signals and mainly highquality music which. with regard to reproduction. operates without any perceptable loss of information. i.c.. without any loss of information which adversely affects the quality of reproduction.  
  The system according to the invention is characterized in that. at the input of the system. the sound signal is divided into a lower and a higher frequency range. the partial signal of the lower frequency range being transferred direct. that. instead of the partial signal of the higher frequency range. the amplitude information of the partial frequency ranges obtained by splitting up this frequency range by means of bandpass filters is transferred. that. for reproducing the overall signal. the amplitude information of the partial frequency ranges serves as modulating signal for equivalent signals lying about in the middle of the individual partial frequency ranges. and that these synthetic sound signals of the partial frequency ranges of the higher frequency range are added to the directly transmitted partial signal of the lower frequency range.  
  The invention proceeds from the recognition that the human ear is capable of relating certain tones to one another according to harmonic laws only up to a definite frequency which is slightly below or above 5 kHz. depending on the subject. For example. any fairly musical person will be able to clearly determine the octave tone of4 kHz associated with a keynote of 2 kHz. However. a great number of subjects are unable to accurately determine even the 6-kHz octave tone associated with 3 kHz. Virtually no subject can safely determine the S-kHz octave tone associated with a 4-kHz tone. Higher octave tones can be mistuned up to about ll) percent before at least part of the subjects notice this mistuning.  
  One embodiment of the invention is characterized in that pilot frequencies lying above the lower frequency range and having a lower frequency spacing (eg. 50 or 100 Hz) are used to transfer said amplitude information together with the lower frequency range. and that. prior to the reproduction. said pilot signals are demodulated (e.g., by means of multiplicative dcmodulators).  
  In one experiment. a frequency range from 40 Hz to 12 kHz was divided into a lower range extending from 40 Hz to 5 kHz and an upper range from 5 to 12 kHz. with the upper frequency range being split up into l4 logarithmically graduated partial frequency ranges and the overall bandwidth needed for the transfer being only about 5.5 or 6 kHz.  
  Furthermore. it is proposed to choose the frequency spacings of the pilot frequencies to be equal. This has the advantage that. at the receiving end. the pilot frequencies for multiplicative demodulation can be pro duced by frequency multiplication of the common sub harmonic.  
  ln another embodiment. the frequency spacing is chosen to be equal to the power line frequency or. in television. to the field frequency (51] or 6U H7] or to an integral multiple thereof. so that frequencies already existing at both ends can be used.  
  Another embodiment of the system is characterized in that. for the common transfer of the partial signal of the lower frequency range and of the amplitude information of the partial frequency ranges of the higher frequency range. said amplitude information is modulated in sequential fashion on a single subcarrier lying above the lower frequency range. said subcarrier being de&#39; modulated for recovering the amplitude information. and that said amplitude information is separated again by time selection operating synchronously with the sequential modulation at the input end. In this connec tion. it has proved technically advantageous that the amplitude information of the partial frequency ranges of the higher frequency range is modulated on a subearrier cyclically in successive time channels. and that another time channel lying within the cycle is associ ated with a sync signal.  
  The advantage resides in the fact that particularly the receivers of the transferred signal can be realized with a minimum of expense. This is of special importance if. as. e.g.. in broadcasting or television. a relatively highly centralized transmitter has a great number of receivers assigned thereto.  
  Thus. in the receiver. only a demodulator but no filter is needed to achieve a selection of the channels. the synchronization of the time selection being particularly simple and in agreement with the system.  
  In practical operation. it may be that the intended overall bandwidth ofthe pilot signal is not available due to either equipment tolerances or time-dependent changes in the transmission characteristics of a path.  
  In these cases. the sequential transfer of the amplitude information offers the advantage that there is no bandwidth loss in the reproduced upper frequency range. so that crosstalk occurs only between channels adjacent as to time.  
  Another embodiment is characterized in that the time sequence of the individual amplitude information corresponding to the partial frequency ranges of the upper frequency range is chosen within a cycle from low to high frequencies.  
  Thus it is achieved that crosstalk between channels adjacent as to time takes effect only as crosstalk between equivalent signals of adjacent frequencies, which is not disturbing to the ear because there is a high degree of correlation between signals of adjacent fre quencies. anyway.  
  Another embodiment of the invention is characterized in that the sync signal, also modulated upon the subcarrier. lies outside the modulation range for the amplitude information of the higher frequency range.  
  A further embodiment of the invention is characterized in that. at the pick-up end, there is provided a second, additional pick-up channel. to which are applied sound signals having a predominantly continuous spectrum in the upper frequency range, that in said pick-up channel. too. splitting into a lower and an upper fre quency range takes place. that the sound signal corresponding to the lower frequency range is combined with the corresponding sound signal of the first pick-up channel in an adding circuit and then transferred. that the sound signal of the upper frequency range is applied to a single rectifier circuit for the formation of an amplitude information. that said amplitude information is transferred and. in the reproduction unit, serves to modulate a noise voltage whose frequency spectrum corresponds to that of the upper frequency range of the sound signal of the second pick-up channel.  
  This has the advantage that particularly the recovery of sound signals produced by the human voice is qualitatively satisfactory. which is based on the recognition that the human voice represents a sound signal with a spectrum which is continuous in the upper frequency range.  
  In another embodiment of the system in which a time channel for the transfer of a sync signal is provided within each cycle. the amplitude information corresponding to the upper frequency range of the second pickup channel is transferred immediately prior to the sync signal.  
  This has an advantage in that crosstalk from the channel in which the information for the modulation of the noise voltage is transmitted. to any of the other channels for the amplitude information of the higher frequency range is excluded.  
  A highly uniform quality of reproduction can be achieved by the fact that. at the recording end, the amplitude of the pilot signal is readjusted in a variablegain amplifier. that the voltage amplitude of the sync signal which is taken from the associated terminal of a revolving switch is used for adjustment, and that the voltage at this terminal is applied across a filter section with great time constant (a few seconds).  
  FIGS. la and lb show, respectively. the pickup end and the reproducing end of one embodiment in which the amplitude information of the partial frequency ranges is transferred simultaneously in separate frequency channels.  
  FIGS. 2a and 2b show a corresponding embodiment with sequential transfer of the amplitude information of the partial frequency ranges.  
  FIG. 3 serves to explain a division of the modulation range in the case of transfer with amplitude modulation.  
  FIG. 4 shows a corresponding division of the modulation range in the case of frequency modulation.  
  FIGS. and 5b show an expanded embodiment of the system of FIGS. 2a and 2b. respectively. in which special properties of particular sound-signal sources are taken into account.  
  FIG. 6 shows an example of an amplitude-time diagram in the case of sequential transfer.  
  FIGS. 7a to c serve to explain the signal waveform at different points of the transfer path. taking into account the relationship between the sampling frequency and the bandwidth of the pilot signal in the case of se-&#39; quential transfer.  
  In the figures and the following description, similar reference characters are used to designate corresponding circuit parts having similar functions.  
  Applied to the input terminal I of FIG. la is the wideband sound signal to be transferred. it being irrelevant by what signal source the sound signal is provided. Connected to this input terminal I are a low-pass filter 3 as well as a number of band-pass filters 4, 5, and 6 in parallel. For ease of explanation, only three band-pass filters are shown here.  
  Connected to the outputs of the band-pass filters 4, 5, and 6 are rectifiers 7, 8. and 9, respectively. which are connected to modulators 131, I32, and 133, respectively. Also connected to these modulators are oscillators 14!, 142. and 143, respectively. The low-pass filter 3 and all modulators I31, I32, and 133 are connected to an adding circuit 17; at the output of the adding circuit 17 appears the overall signal 19 to be transferred, which, depending on the peculiarities of the system used. is modulated on a carrier for transmission via a broadcasting transmitter or. for example. stored on a sound recording for reproduction at a later time.  
  FIG. 1b shows the reproducing end with the essential circuit parts.  
  The overall signal 19 is applied to the terminal 22. Connected to this terminal 22 is the parallel connection of a low-pass filter 23 and a number of demodulators 134, 135, 136 corresponding to the number of partial frequency ranges of the upper frequency range, with associated oscillators 144, 145, and 146. Connected to the output of each demodulator 134, 135, and 136 is a low-pass filter 300 with following modulators 31, 32, and 33, respectively. and oscillators 34, 35, and 36, respectively. The outputs of the modulators 31. 32. and 33 as well as of the low-pass filter 23 are connected to an adding circuit 37, in which the overall signal intended for reproduction is formed. which is applied via an amplifier 38 to the loudspeaker 42.  
  The operation of the system whose parts essential to the invention are shown in FIGS. la and lb will be described next, with assumed numerical values being purely exemplary, i.e.. qualitative.  
  The wideband sound signal of, e.g., Hz to l2 kHz, which is applied to the input terminal I, is split up into a lower frequency range according to the bandwidth of the low-pass filter of about 6 kHz and into the higher frequency range remaining thereabove. In this higher frequency range, further division takes place with the aid of the band-pass filters 4, 5, and 6. The division is preferably performed with logarithmic graduation, with the octave from 6 kHz to 14 kHz, for example, being divided into 12 partial ranges corresponding to semitone steps. In reality, therefore, 12 band-pass filters with the bandwidth of the respective semitone step must be provided instead of the three band-pass filters shown. The partial signals relating to each partial frequency range are rectified in rectifiers in FIG. la rectifiers 7, 8 and 9 i.e.. volume-dependent equivalent signals are obtained, which are referred to here as &#34;amplitude information.  
  In the modulators I3], 132, 133, this amplitude information is modulated. for transfer, on the pilot frequencies 11, f2, and f3, generated with the oscillators 14l, I42, I43 (only 3 are shown). If transferred together with the lower frequency range, the pilot frequcncies fl,fl,j3 lie above the frequency limit of the low-pass filter 3 and are spaced about 50 to I00 Hz apart. For audio transmission in television, the frequency spacing may be chosen, for example, to be equal to the field frequency or a multiple thereof because this frequency is present at the transmitting and receiving ends, anyway.  
  The output signal of the low-pass filter 3 as well as the pilot signals are combined in the adding circuit 17 and result in an overall signal 19.  
  This overall signal 19 has a bandwidth which corresponds to the sum of the bandwidth of the lower frequency range and of the bandwidth of all pilot signals.  
  lfthe overall signal 19 is to be transmitted, for example, via a radio transmitter, it is modulated on a carrier in known manner. For transfer via a storage such as magnetic tape, sound film, etc., the overall signal may be recorded direct.  
  At the reproducing end ofthe system. the transferred overall signal 19 is applied to the input terminal 22. If the overall signal 19 was transmitted in modulated form, e.g., over a transmission path, 22 is the input terminal after the demodulation.  
  The low-pass filter 23, which has the same bandwidth as the low-pass filter 3 at the pick-up end, filters the directly transmitted component, i.e., the signal component of the lower frequency range, out of the overall signal 19. With the aid of the multiplicative demodulators 134, 135, and 136, to which are connected oscillators I44, 145, and 146, respectively, with the frequenciesfl,j2, and /3 corresponding to the pilot frequencies at the pick-up end (oscillators 14], 142, and 143, respectively. FIG. la), the volume-dependent amplitude information is demodulated and passed through simple low-pass filters 300, whose bandwidth is adapted to that of the volume-dependent amplitude information of the pick-up end (eg, 50 I00 Hz).  
  Thus, the signals appearing at the outputs of the individual low-pass filters 300 correspond to the output signals of the rectifiers 7, 8, and 9, respectively. Connected in series with each low-pass filter 300 is a modu lator 31, 32, and 33, i.e.. one for each partial frequency range of the higher frequency range, with each modulator 31, 32, 33 having an oscillator 34, 35, 36 connected thereto. Each oscillator oscillates at a frequency which lies approximately in the middle of the individual partial frequency ranges divided at the pick-up end (bandpass filters 4, 5, 6, FIG. la).  
  Accordingly, at the output of each modulator 31, 32, 33, there appears an equivalent signal with the frequency of the respective oscillator 34, 35, 36 the volume of which signal is determined by the transferred amplitude information of the respective partial frequency range.  
  The outputs of the modulators 31, 32, 33 and the low-pass filter 23 are connected to an adding circuit 37, so that a sound signal is formed there which consists of the directly transferred sound signal of the lower frequency range and of the equivalent signals of the individual partial frequency ranges of the lower frequency range.  
  The frequencies f f and f, of the oscillators I44, I45, and 146, which are needed for the multiplicative demodulation, must be synchronized, by well-known means, with the corresponding frequencies at the pickup end. This can be done in various ways. It is possible to obtain these frequencies by frequency multiplication ofa fundamental frequency available at both locations, e.g., of the power line frequency or, in the case of television transmissions, of the field frequency. It is also possible, of course, to transmit additional equivalent signals for synchronizing these oscillators,  
  In the embodiments of FIGS. 20 and 2b, the same reference characters as in FIGS. la and lb are used to designate corresponding parts.  
  In the following, the sequential transfer is described, which is more important to practical applications.  
  Referring to FIG. 2a, l is the input terminal to which the wideband sound signal to be transferred is applied. The amplifier 2 is a variable-gain (AGC) amplifier for the volume compression of this signal. Connected to the output of the amplifier 2 are a low-pass filter 3, whose bandwidth may range between 4 and 7 kHz depending on the qualitative requirements, and, in parallel thereto, the band-pass filters 4, 5, and 6 and further band-pass filters (not shown) which divide the higher frequency range not passing through the low-pass filter into partial ranges, this division preferably being effected in accordance with a logarithmic graduation. For example, an octave can be divided into l2 partial ranges corresponding to the semitones of this octave. The outputs of the filters 4, 5, 6 are followed by rectifiers 7, 8, 9, which rectify the signal components falling within the respective frequency ranges and thus each generate an auxiliary voltage depending on the volume of these signals, i.e., volume-dependent amplitude in formation of small bandwidth. In practice, these rectifiers must operate down to the millivolt range. The amplitude information is sampled cyclically one after another by an electronic, revolving switch 11 and applied via a logarithmic pre-emphasis network 12 to the mod ulator 13, to which is also applied the alternating voltage of the pilot generator 14. In the case of frequency modulation, 13 is, for example, a reactance stage which influences the frequency of the oscillator I4. If amplitude modulation is used, one sideband of this modulating signal may be suppressed with the single-sideband filter 18, which must be provided in this case. In the adding circuit I7, the pilot signal, modulated in this way. is added to the base channel. The reference numeral l9 designates the overall signal to be transferred. A rectifier 10 for the complete input signal serves to generate a correcting voltage which. at the pickup end. is used for volume compression by being applied to the amplifier 2; in addition. it is used for volume expansion at the reproducing end. as will be explained be low. For synchronizing a revolving switch at the reprtv ducing end with a revolving switch at tlte pick-up end. cg. a negative signal is applied via the sync-signal source to one of the contacts ofthe revolving switch 11. while the rectifiers 7 to 9 can only deliver positive voltages to the switch 11. In the matrix circuit 16. the volume control signal from rectifier I0 is superimposed on this negative synchronizing voltage. Since the overall signal obtained in this way and consisting ofthe sync signal and the superimposed voluntc correction signal is oniy transferred during one time channel, synchrw nous volume compression at the picloup end is accomplished by applying this control signal from the rectifier It) to the amplifier 2 not direct but via a switch 21. This switch 2] is closed only while the revolving switch is connected to the matrix 16. During the rest of the time. the amplifier 2 is provided with the voltage stored in capacitor 20.  
  Fl(i. 2b shows the arrangement at the reproducing end. 12 is the input for the transferred overall signal 19. if this overall signal 19 has been transmitted in moduiated fornt. cg. in a broadcasting system. 22 is the output of the demodulator [not shown] for the overall sig nal. Designated 22 is a tow-pass filter at the reproducing end whose bandwidth is largely identical to that of the lowpass filter 3 at the pick-up end. Connected in parallel thereto is a band-pass filter 24 which only passes the frequency range of the pilot signal. (onnected in series thereto is a demodulator 25. lfthe pilot signal has been frequency-modulated, the demodulator is a frequency discriminator. A following logarithmic deemphasis network 26 cancels any logarithmic pre emphasis applied at the pickup end. From the output of the logarithmic deemphasis network 26. the signal is applied to the rcvohing switch 27. from whose contacts&#34; the volume information associated with the indhidual time channels is taken and applied to the storage capacitors 28. 29, and 30 and to further storage capacitors (not shown). Front the storage capacitors. the \olunte information of the individual channels is applied to the modulators 3|. 32. 33. etc. which. in turn. modulate the signals of the oscillators 34. 35. and 36. which generate the equivalent frequencies for the respective partial range. 37 is the adding circuit with which the volumecontrolled equivalent signals and the base hand delivered by the low-pass filter 23 are added together. A variable-gain amplifier 38 restores the original sound volume range by volume expansion at the reproducing end. This amplifier 38 is controlled by the output signal of the timechannel storage 39. which. in turn. is connected to that contact of the revolving switch which is associated with the sync signal. The output of the variable-gain amplifier 38 is connected via an output amplifier (not shown] to the loudspeaker 42. After the demodulator 25, the demodulated pilot signal is additionally applied to art amplitude filter 40. which responds to the sync signal by reacting, for ex&#39; ample. only to negative voltages. depending on the voltage delivered by the matrix circuit 16 at the pickup end. The signal appearing at the output of this antpli&#39; tude filter 40 serves to reset the revolving switch 27 to the position corresponding to the synchronization; from this position. the switch is advanced step by step with the clock generator 41. To additionally free the synchronization of the revolving switch from interference. a selective resonant circuit tuned to the frequency of the sync pulses may be used at the output of the amplitude filter. Another possibility. which is not shown in the drawing. is to divide the stepping frequency of the clock generator 4] according to the number of positions of the switch 27, and to compare this frequency with the sync pulses provided by the amplitude filter. which. in turn. provides a filtered control voltage. with which the frequency of the clock generator can be readjusted.  
  HO. 3 shows a particulariy appropriate division of the entire modulation range as used in the case of amplitude modulation The modulation range from 0 to 100 percent is divided into a modulation range I from (1 to percent. in which the volume information of the individual time channels are lying. with the modulation percentage 0 corresponding to maximum volume. and the modulation percentage 70 to volume 0. The volume preferably varies logarithmically with the modulation percentage. Provided for the sync signal is the range lll between and ercent. with the sync signal si multaneousiy transferring the volume-correction signal. The modulation percentage 80 corresponds to the greatest volume expansion (and thus to the greatest volume of the volume correction]. while the modulation percentage lUU corresponds to absence of volume expansion. The modulation range ll between 70 and 80 serves as a safety range.  
  FIG. 4 shows the corresponding conditions if the pilot signal has been frequency-modulated. The range A corresponds to the directly transferred. lower frequency range. Reserved for the frequency deviation of the pilot signal is the range B. wherein f is the frequency associated with the maximum volume. f is the frequency associated with the minimum volume. is the frequency of the sync signal at maximum volume expansion. and jg is the frequency ofthe sync signal in the absence of volume expansion.  
  At the pickup end. as shown in H6. 5a. there are two separate pick-up channels which. for simplification. are shown here as microphones 100 and 101. which are followed by amplifiers 200 and 201. respectively. The essential circuit elements associated with the first pick-up channel are a low-pass filter 3 as well as band-pass filters 4. 5. and 6. whose number corresponds to the number of desired partial ranges in the upper frequency range. Both the low-pass filter 3 and the bandpass filters 4. 5. and 6 are connected to the output of amplifier 200 in a parallel configuration.  
  The bandwidth of the low-pass filter 3 depends on the desired extent of the lower frequency range; iii the arrangements built up in practice thus far. it ranges between 4 attd 7 kHz.  
  The bandpass filters 4. 5 and 6 divide the frequency range lying above the fundamental frequency of the iowpass filter 3 into partial frequency ranges. with the bandwidth of the bandpass filters depending on the overall bandwidth of the upper frequency range and on the intended division of this frequency range. i.e., on the number ofband-pass filters required. Connected to the outputs of the band-pass filters 4. 5. and 6 are recti fier circuits 7. 8. and 9. respectively. They rectify the partial signal corresponding to the respective band-pass filter. whereby a volume-dependent auxiliary voltage. i.e.. a volumedependent amplitude information of small bandwidth. is developed at the outputs of the rectifier circuits.  
  Thus, the signal picked up via the microphone 100 has been split up into four partial signals. one of which. namely that corresponding to the lower frequency range. is preserved in its original form. while the other partial signals can only be evaluated as discrete amplitude information.  
  Via a second pick-up channel with the microphone 101 and the amplifier 201, a second sound signal is applied. which is also divided into a lower and an upper frequency range.  
  To this end. the output of amplifier 201 is connected to a low-pass filter 301, whose bandwidth is equal to or smaller than that of the low-pass filter 3, and to a highpass filter 302 for the entire upper frequency range. Here. unlike in the first pick-up channel with the microphone I and the fol owing devices, the overall signal ofthc upper frequency range is applied to a rectifier arrangement 90, whose output signal now only represents an amplitude information. too.  
  All amplitude information. i.e.. all rectified voltages developed at the outputs of the rectifiers 7, 8, 9, and 90, are applied to a sampling device shown here as a revolving switch 11. In practice. this revolving switch II is an electronic. cyclically operating sampler as used for many applications in which a transition from space division multiplex to time division multiplex takes place.  
  The sampler III of the sampling device 11 successively samples all input contacts H0, 114 at a particular frequency. As mentioned above. the input contacts III]. 114 are each connected to the output of a rectifier arrangement.  
  Thus. samples of the amplitude information applied to the input terminals at the sampling instant appear in succession at the output I12 of the revolving switch 11.  
  The entirity of these samples in the form of voltage values is applied to a modulator 13, which is known in many designs and modes of operation.  
  The modulator I3 modulates a subcarrier or pilot signal which is generated in a generator 14.  
  The output signal ofthe modulator l3 and the output signals of the low pass filters 3 and 301 are combined in an adding circuit 17 and transferred as the overall signal I9; this is indicated by the arrow. As can also be seen from FIG. 51!. one of the input terminals 110 is connected to a voltage source 15, which has, for example. a negative voltage. the latter is transferred within each cycle as a sync signal for the reproducing end, which will be explained below.  
  In FIG. b. the overall signal 19 is applied to a lowpass filter 23 and to a band-pass filter 43 in parallel. Regarding the bandwidth. the low-pass filter 23 exactly corresponds to the low-pass filter 3 at the pick-up end because it is to pass only the lower partial frequency range of the transferred signal 19, which range has been preserved in its original form. The frequency range passed by the band-pass filter 43 corresponds to the bandwidth needed for the modulated pilot signal and lies above the limit frequency of the low&#39;pass filter 23.  
  Incidentally. the signal I9 at the pick-up end and at the reproducing end is the a.f. signal; the devices neces sary for the actual transfer or transmission via the transfer or transmission path, which is left out of account here, are neither shown nor described here.  
  The output of the low-pass filter 23 is connected to an adding circuit 37.  
  The pilot signal passed by the band-pass filter 43 and modulated with the amplitude information is applied to a demodulator 44, so that the overall sequence of the amplitude information can again be taken from the out put of the demodulator 44, which sequence essentially corresponds to the signal sequence at the output I12 of the sampling device 11 (FIG. 5a).  
  This overall sequence of the amplitude information is applied via a variable-gain amplifier 38 to the input 272 of a revolving switch 27. It will be apparent from the schematic representation of FIG. 5b that this switch 27 is of the same kind as the switch 11 (FIG. 5a). but with opposite signal flow. i.e., this switch 27 performs no sampling function. but the function of a distributor as used for the transition from time division multiplex to space division multiplex.  
  This means that the distributor 271 must revolve with the same frequency as the sampler 111 of the switch II. Since the individual partial frequency ranges lie in fixed time channels in accordance with the band-pass filters 4, 5, 6, the sampler I I1 and the distributor must be in synchronism. Since. at the pick-up end. a time slot for the transfer of a sync signal is already provided in the cycle. and since this sync signal is actually transmitted. the latter must be picked out at the receiving end and suitably evaluated. To this end, the overall signal appearing at the output of the variable-gain amplifier 38 is applied. besides to the switch 27, to a selection device 40. In this case. this selection device 40 is an amplitude filter. The output signal of this amplitude filter 40 is applied to a clock generator 4, whose clock frequency determines the cycle frequency of the switch 27 and which is synchronized by the output signal of the amplitude filter 40.  
  Connected to the output terminals 274 of the revolving switch 27 are storage capacitors 28, 29, 30, 273, and 275, respectively. In the figure. these storage capacitors are only shown at the wired output terminals 274 for lack of space.  
  These storage capacitors store the amplitude information of the respective time channel. received via the distributor 271, for the duration of one cycle. As also shown in FIG. 5b, the reproduction unit contains oscillators 34, 35, 36, which are connected to modulators 31, 32, and 33, respectively. Each oscillator oscillates at the center frequency ofa partial frequency range selected by the band-pass filters 4, 5, and 6 at the pick-up end. From the storage capacitors 28, 29, 30, the voltage stored as amplitude information is applied to the respective modulator 3l, 32, 33, where it serves as a signal for controlling the volume of the equivalent tones.  
  These modulators 31, 32, and 33 with the associated oscillators 34, 35, and 36 serve to restore the upper frequency range applied via the first pick-up channel with the microphone (FIG. 5a).  
  In practice. however. it has turned out that. for certain sound signals, a division of the upper frequency range from, e.g.. 6 to l2 kHz into, e.g.. l2 partial fre quency ranges does not insure reproduction meeting high standards of quality. An examination of the frequency spectrum of various musical instruments and of the human voice has shown that such sound signals, whose upper frequency range represents a uniform or continuous spectrum, are adversely affected in their quality when restored by the sum of the abovementioned discrete signals of 1?. partial ranges of the upper frequency range.  
  The transfer system described shows the solution to this problem. For the pickup end, the second pick-up channel was already described with reference to FIG. 5a. FIG. 5b shows, among other things, an output terminal 274, whose time slot in the overall cycle corresponds to that of the input terminal 114 of FIG. 5a, i.e.. at the terminal 274 of the reproduction unit appears the amplitude information of the terminal 114 of the pick-up unit, which amplitude information is stored for one cycle by means of the storage capacitor 275. This amplitude information, like all others, is applied as a control signal to a modulator 330. In this case, however, a noise source 46 with following high-pass filter 47 is connected to the modulator 330. Thus, the output signal of the modulator 330 is a noise signal which is variable in its volume and whose frequency spectrum is equal to that of the noise signal at the pick-up end through suitable adjustment of the high-pass filter 47 to the high-pass filter 302.  
  As already described with reference to FIG. 5a, the sound signal of the second pick-up channel, which corresponds to the lower frequency range, has been added, at the pick-up end, to the corresponding sound signal of the first pick up channel. Thus, the additional expense for the improvement of the sound quality at the reproducing end is limited mainly to the noise source 330, 46. 47.  
  In an adding circuit 37, the output signals of all modulators 31, 32, 33, and 330 are combined with the sound signal of the lower frequency range, so that the output signal of the adding circuit corresponds to the sum of the sound signals picked up at the pick-up end via the two pick-up channels.  
  As will also be apparent from FIG. 5b, one of the output terminals 274 of the switch 27 is connected to a lowpass filter 45, whose output signal is applied as control signal to a variable-gain amplifier 38. The amplitude information appearing at this output terminal 274 is that ofthe sync signal which is transferred in this time slot. This amplitude information is advantageously evaluated for readjusting the amplitude of the pilot signal appearing at the output of the band-pass filter 43. Advantageously, the time constant ofthe low-pass filter 45 is chosen to be of the order of a few seconds. As regards circuit engineering, the individual circuit elements such as the variable-gain amplifier 38, the modulators 3|, 32, 33, and 330, or the amplitude filter 40 can be realized by anyone with ordinary skill in the art, or these components are even commercially available.  
  FIG. 6. in coarse representation, shows the shape of the pilot signal as a function oftime. The amplitudes of the individual stages designated by numerals corre spond to the arbitrarily chosen volumes of the individual partial frequency ranges, which are successively sampled by the revolving switch 11 (FIG. 5a). Advantageously, with regard to a good quality of reproduction, the order of the partial frequency ranges associated with the time channels is chosen so that, of these partial frequency ranges, in a cycle, first the amplitude information on the lowest frequency of the upper frequency range and finally the amplitude information on the highest frequency is sampled.  
  In FIG. 6, the level exceeding the broken lines is that of the sync signal.  
  FIG. 7c again shows schematically a transmitter S, a filter BP, which symbolizes the bandwidth of the pilot signal, and a receiver E. With the aid of this figure, the question of the optimum pilot-signal bandwidth in relation to the sampling frequency of the revolving switches will now be discussed.  
  For simplicity, it is assumed that only one channel here: the time channel I has an amplitude information with the maximum value, while all others are at the 0 level.  
  At the output of the revolving switch S at the trans mitting end, a square-wave voltage as shown in FIGS. 7a and 7b is obtained. The square-wave voltage is converted to the continuous voltage waveform under the influence of the band-pass filter BP; this applies to the waveforms of both FIG. 7a and FIG. 7b. The squarewave voltages shown as broken lines are obtained by sampling, at the receiving end, the continuous voltages at the output of the filter BP.  
  As the waveform 7a shows, the filter bandwidth and the sampling frequency are optimally matched.  
  The waveform 7b shows that the sampling frequency is too high in relation to the filter bandwidth; this means that crosstalk to the second or even third channel is produced.  
  It has turned out, however, that, in the present trans mission system, crosstalk is generally acritical because the sound signals of the upper frequency range are correlated signals, anyway.  
  Only in a special case must this crosstalk be considered, namely in the case of crosstalk from the noise channel to one of the channels with a signal which only contains a partial range of the upper frequency range. This case is disturbing because the noise influences a voiced overtone.  
  This problem can be solved by arranging the individual time slots in a favorable order. For example, the noise is placed into a time slot immediately before that of the sync signal whose amplitude value is acritical.  
  If the overall volume is to be influenced, too, e.g., to control the volume, a time slot for the transmission of a volume signal may be reserved in the cycle.  
  This time slot is advantageously placed right behind that of the sync signal because this is most favorable from the point of view of crosstalk. The reason is that the influence of the sync signal has no disturbing effect because of the constancy of the value.  
 What is claimed is:  
  l. A transmitter for transferring wideband sound signals over a narrow frequency range, comprising:  
 means for receiving the sound signals;  
 means for dividing the received sound signals into a lower frequency range and a plurality of higher frequency ranges;  
 means for providing amplitude signals corresponding to the amplitudes of the signals in each of the higher frequency ranges;  
 means for providing a pilot frequency signal;  
 means for sequentially modulating the pilot frequency signal with the amplitude signals of the higher frequency ranges; and  
 means for transferring the signals of the lower frequency range and the sequentially modulated pilot frequency signal.  
  2. A receiver for use in a system having a transmitter of the type that transfers lower frequency range sound signals directly and a pilot frequency signal sequentially modulated by amplitude signals corresponding to amplitudes of the sound signals in a predetermined number of higher frequency ranges. said receiver comprisingi means for receiving and demodulating the sequentially modulated pilot frequency signal to provide sequential amplitude signals corresponding to the amplitudes of the signals in the higher frequency ranges;  
 means for providing synthetic signals having frequencies approximately equal to the mid-range frequency of each higher frequency range;  
 means for modulating each synthetic signal with the appropriate amplitude signal; and  
 means for receiving and reproducing the modulated synthetic signals and the directly transferred lower frequency range sound signals.  
  3. A system for transferring wideband sound signals over a narrow frequency range. comprising: a transmittcr including means for receiving the sound signals. means for dividing the received sound signals into a lower frequency range and a plurality of higher frequency ranges. means for providing amplitude signals corresponding to the amplitudes of the signals in each of the higher frequency ranges, means for providing a pilot frequency signal, means for sequentially modulating the pilot frequency signal with the amplitude signals of the higher frequency ranges. and means for transferring the signals of the lower frequency range and the sequentially modulated pilot frequency signal, and a receiver including means for receiving and demodulating the sequentially modulated pilot frequency signal to provide sequential amplitude signals corresponding to the amplitudes of the signals in the higher frequency ranges. means for providing synthetic signals having frequencies approximately equal to the mid-range frequency of each higher frequency range. means for modulating each synthetic signal with the appropriate amplitude signal and means for receiving and reproducing the modulated synthetic signals and the directly transferred lower frequency range sound signals.  
  4. A transmitter as described in claim 1, wherein the means for sequentially modulating the pilot frequency signal is synchronized by a standard frequency source that is available at both the transmitting and receiving ends of the system.  
  5. A transmitter for transferring wideband sound signals over a narrow frequency range, comprising:  
 means for receiving the sound signals;  
 means for dividing the received sound signals into a lower frequency range and a plurality of higher frequency ranges;  
 means for providing amplitude signals corresponding to the amplitudes of the signals in each of the higher frequency ranges;  
 means for providing a pilot frequency signal.  
 means for sequentially modulating the pilot frequency signal with the amplitude signals of the higher frequency ranges so that the sequential modulation takes place in successive time channels that are repeated cyclically and each cycle includes an additional time channel;  
 means for providing a sync signal corresponding to the repetition rate of the cycles, said sequential modulating means being adapted to modulate the pilot frequency signal with the sync signal during the additional time channel; and  
 means for transferring the signals of the lower frequency range and the sequentially modulated pilot frequency signal.  
  6. A receiver for use in a system having a transmitter of the type that transfers lower frequency range sound signals directly, a pilot frequency signal sequentially modulated in successive time channels by amplitude signals corresponding to the amplitudes of sound signals in a predetermined number of higher frequency ranges and by a sync signal corresponding to the repetition rate of the sequential modulation of the pilot fre&#39; quency signal, said receiver comprising:  
 means for receiving and demodulating the sequentially modulated pilot frequency signal to provide sequential amplitude signals corresponding to the amplitudes of the signals in the higher frequency ranges and a sync signal;  
 means for providing synthetic signals having frequencies approximately equal to the mid-range frequency of each higher frequency range;  
 means for modulating each synthetic signal with the appropriate amplitude signal;  
 means for distributing the sequential amplitude signals to the modulating means;  
 means responsive to the sync signal for controlling and synchronizing said distributing means to assure proper distribution of the sequential amplitude signals; and  
 means for receiving and reproducing the modulated synthetic signals and the directly transferred low frequency range sound signals.  
 7. A system for transferring wideband sound signals over a narrow frequency range, comprising:  
 a transmitter including means for receiving the sound signals, means for dividing the received sound signals into a lower frequency range and a plurality of higher frequency ranges, means for providing amplitude signals corresponding to the amplitudes of the signals in each of the higher frequency ranges, means for providing a pilot frequency signal, means for sequentially modulating the pilot frequency signal with the amplitude signals of the higher fre quency ranges, so that the sequential modulation takes place in successive time channels that are repeated cyclically and each cycle includes an additional time channel. means for providing a sync sig nal corresponding to the repetition rate of the cycles. and said sequential modulating means being adapted to modulate the pilot frequency signal with the sync signal during the additional time channel. and means for transferring the signals of the lower frequency range and the sequentially modulated pilot frequency signal; and  
 a receiver including means for receiving and demodulating the sequentially modulated pilot frequency signal to provide sequential amplitude signals corresponding to the amplitudes of the signals in the higher frequency ranges and to provide a sync signal in sequence with the amplitude signals, means for providing synthetic signals having frequencies approximately equal to the mid-range frequency of each higher frequency range. means for modulat ing each synthetic signal with the appropriate amplitude signal. distributing the sequential amplitude signals to the modulating means. means responsive to the sync signal for controlling and synchroniying said distributing means to assure proper distribution of the sequential amplitude signals. and means for receiving and reproducing the modulated syn thetic signals and the directly transferred lotver frequency range sound signals.  
 8. A transmitter as described in claim 5. wherein the time sequence of the amplitude signal modulation of the pilot frequency signal is chosen to cycle from low to high frequencies.  
  9. A receiver for use in a system ha ing a transmitter of the type that transfers lower frequency range sound signals directly and a pilot frequency signal sequentially modulated in successbe time channels by amplitude signals corresponding to the amplitudes of sound signals in a predetermined number of higher frequency ranges and in an additional time channel by an upper frequency range amplitude signal corresponding to the amplitude of all the sound signals in the higher fre quency ranges. said receiver comprising:  
 means for receiving and demodulating the sequen tially modulated pilot frequency signal to provide sequential amplitude signals corresponding to the amplitudes of the signals in the higher frequency ranges and an upper frequency range amplitude signal;  
 means for providing synthetic signals having frequencies approximately equal to the mid-range frequency of each higher frequency range:  
 means for modulating each synthetic signal with the appropriate amplitude signal.  
 means for pro\ iding a noise signal having a frequency range approvimately equal to the total frequency range of the higher frequency ranges;  
 means for modulating the noise signal with the upper frequency range amplitude signal; and  
 means for receiving and reproducing the modulated synthetic signals. the modulated noise signal and the directly transferred low frequency range sound signals.  
  10. A transmitter as described in claim 5. wherein the sync signal modulated on pilot frequency signal lies outside the modulation range of the amplitude signals.  
  ii. A transmitter as described in claim 5, wherein the sync signal lies beyond the threshold of the modulation percentage which corresponds to the amplitude signal equivalent to a zero amplitude sound signal.  
  12. A transmitter as described in claim 5. additionally comprising:  
 means for providing a volume compression signal corresponding to the amplitude of the received sound signals;  
 the means for receiving the sound signals additionally includes means for receiving the volume compression signal. said means being responsive to the re ceived signals for compressing the volume of the sound signals; and  
 means for modulating the sync signal with the volume compression signal so that the sync signal amplitude varies in relation to the volume compression. whereby the pilot frequency signal during the additional time channel has an amplitude corresponding to the volume compression.  
  [3. A transmitter as described in claim 12. wherein the sync signal modulation factor directly corresponds to the amount of volume compression.  
  14. A receiver for use in a system having a transmit ter of the type that transfers lower frequency range sound signals directly. a pilot frequency signal sequentially modulated in successive time channels by amplitude signals corresponding to the amplitudes of sound signals in a predetermined number of higher frequency ranges and by a sync signal corresponding to the repetition rate of the sequential modulation of the pilot frequency signal. said sync signal being further modulated by an amount corresponding to the volume compres sion of the received sound signals. said receiver comprising:  
 means for receiving and demodulating the sequentially modulated pilot frequency signal to provide sequential amplitude signals corresponding to the amplitudes of the signals in the higher frequency ranges and a sync signal;  
 means for providing synthetic signals having frequencies approximately equal to the mid-range frequency of each higher frequency range;  
 means for modulating each synthetic signal with the appropriate amplitude signal;  
 means for distributing the sequential amplitude signals to the modulating means;  
 means responsive to the sync signal for controlling and synchronizing said distributing means to assure proper distribution of the amplitude signals;  
 means for receiving and adding the modulated synthetic signals and the directly transferred low frequency range sound signals and for providing a combined output signal;  
 volume expansion means for receiving the combined output signal and the sync signal and for providing a volume expanded output signal in response thereto; and  
 means for receiving and reproducing the volume expanded output signal.  
 15. A system as described in claim 7. wherein the transmitter additionally comprises:  
 means for providing a volume compression signal corresponding to the amplitude of the received sound signals;  
 the means for receiving the sound signals additionally includes means for receiving the volume compression signal said means being responsive to the received signals for compressing the volume of the sound signals; and  
 means for modulating the sync signal with the volume compression signal so that the sync signal amplitude varies in relation to the volume compression whereby the pilot frequency signal during the additional time channel has an amplitude corresponding to the volume compression; and  
 the receiver additionally comprises means for receiving the modulated synthetic signals and the directly transferred low frequency range sound signals and for providing in response thereto a combined output signal;  
 volume expansion means for receiving the combined output signal and the sync signal for providing in response thereto a volume expanded signal;  
 means for receiving and reproducing the volume expanded signal.  
  [6. A transmitter as described in claim 5, additionally comprising pre-emphasis means for receiving and preemphasizing the amplitude signals approximately logarithmically prior to the modulation of the pilot frequency signal.  
  17. A system as described in claim 7, wherein the transmitter includes means for pre-emphasizing approximately logarithmically the amplitude signals prior to the modulation of the pilot frequency signal and the receiver additionally comprises means for deemphasizing the demodulated amplitude signals.  
  18. A transmitter as described in claim I, wherein the pilot frequency signal is single side-band modulated.  
  19. A transmitter as described in claim I, additionally comprising: second means for receiving the sound signals;  
 means for dividing the sound signals received by the second means into a lower frequency range and an upper frequency range corresponding in bandwidth to the total frequency range of the higher fre&#39; quency ranges;  
 means providing an upper frequency range amplitude signal; and  
 means for transferring the upper frequency range amplitude signal. 20. A receiver as described in claim 2. additionally comprising:  
 means for receiving an upper frequency range amplitude signal corresponding to the amplitude of the sound signals in the higher frequency ranges;  
 means for providing a noise signal having a frequency range approximately equal to the total frequency range of the higher frequency ranges;  
 means for modulating the noise signal with the upper frequency range amplitude signal; and  
 means for adding the modulated noise signal to the modulated synthetic signals and the directly trans ferred lower frequency signal so that reproducing means reproduces the modulated noise signal also.  
  2|. A system as described in claim 3, wherein the transmitter additionally includes:  
 second means for receiving the sound signals;  
 means for dividing the sound signals received by the second means into a lower frequency range and an upper frequency range corresponding in bandwidth to the total frequency range of the higher frequency ranges;  
 means providing an upper frequency range amplitude signal;  
 means for transferring the upper frequency range amplitude signal; and the receiver additionally comprises means for receiving the upper frequency range amplitude signal;  
 means for providing a noise signal having a frequency range approximately equal to the total frequency range of the higher frequency ranges;  
 means for modulating the noise signal with the upper i frequency range amplitude signal. and  
 means for adding the modulated noise signal to the modulated synthetic signals and the directly transferred lower frequency signals so that the reproducing means reproduces the modulated noise sig nal also.  
  22. A transmitter as described in claim I. wherein the means for sequentially modulating the pilot frequency signal with the amplitude signals is arranged so that the sequential modulation takes place in successive time channels that are repeated cyclically and each cycle in cludes an additional time channel, said transmitter additionally comprising:  
 second means for receiving the sound signal: means for dividing the sound signals received by the second means into a lower frequency range and an upper frequency range corresponding in bandwidth to the total frequency range of the higher frequency ranges; means for providing an upper frequency range am plitude signal; and said sequential modulating means being adapted to [5 modulate the pilot frequency signal with the upper frequency range amplitude signal during the additional time channel whereby the upper frequency range amplitude signal is also transmitted by the pilot frequency signal.  
  23. A transmitter as described in claim 22. wherein each cycle includes a second additional time channel. said transmitter additionally comprising:  
 means for providing a sync signal corresponding to the repetition rate of the cycles:  
 means for providing a volume compression signal corresponding to the amplitude of the received sound signals;  
 the means for receiving the sound signals additionally including:  
 means for receiving the volume compression signal. said means being responsive to the received signals for compressing the volume of the sound signals;  
 means for modulating the sync signal with the volume compression signal so that the sync signal amplitude varies in relation to the volume compression. and  
 said sequential modulating means being adapted to modulate the pilot frequency signal with the modulated sync signal during the second additional time channel whereby the pilot frequency signal has an amplitude corresponding to the volume compression during the second additional time channel.  
  24. A receiver as described in claim 9, adapted for use in a system wherein the transferred pilot frequency signal is further modulated in a second additional time channel by a sync signal that has been modulated by a volume compression signal, wherein the receiving and demodulating means also provides a sync signal. said receiver additionally comprising:  
 means for distributing the sequential amplitude signals to the modulating means.  
  means responsive to the sync signal for controlling 5 and synchronizing said distributing means to assure proper distribution to the amplitude signals; and  
 volume expansion means for receiving the modulated pilot frequency signal and the sync signal and for providing a volume expanded modulated pilot frequency signal in response thereto.  
  25. In a system for transferring \vidcband sound signals over a narrow frequency range. a transmitter comprising:  
 means for receiving the sound signals;  
 means for dividing the received sound signals into a lower frequency range and a plurality of higher frequency ranges;  
 ltl  
 means for providing amplitude signals corresponding to the amplitudes of the signals in each of the higher frequency ranges;  
 second means for receiving the sound signals;  
 means for dividing the sound signals received by the second means into a lower frequency range and an upper frequency range corresponding in bandwidth to the total frequency range of the higher frequency ranges;  
 means for providing an upper frequency range amplitude signal corresponding to the amplitude of the signals in the upper frequency range; and  
 means for transmitting the lower frequency range signals. the amplitude signals and the upper frequency range amplitude signal.  
  26. In a system for transferring wideband sound signals over a narrow frequency range. wherein a transmitter transmits the lower frequency range signals di rectly along with amplitude signals corresponding to the amplitudes of sound signals in higher frequency ranges and an upper frequency range amplitude signal corresponding to the amplitude of the signals in an upper frequency range having a bandwidth approximately equal to the total bandwidth of the higher frequency ranges. a receiver comprising:  
 means for providing synthetic signals having frequen cies approximately equal to the mid-range fre quency of each higher frequency range;  
 means for receiving the transmitted signals;  
 means for modulating each synthetic signal with the appropriate amplitude signal;  
 means for providing a noise signal having a frequency range approximately equal to the total frequency range of the higher frequency ranges;  
 means for modulating the noise signal with the upper frequency range amplitude signal;  
 means for adding the modulated noise signal. the  
 modulated synthetic signals and the lower frequency range signals; and  
 means for reproducing the added signals.  
  27. In a system for transferring wideband sound signals over a narrow frequency range, comprising:  
 a transmitter including means for receiving the sound signals, means for dividing the received sound signals into a lower frequency range and a plurality of higher frequency ranges. means for providing amplitude signals corresponding to the amplitudes of the signals each of the higher frequency ranges. means for providing a plurality of pilot frequency signals. means for positively modulating the pilot frequency signals with the amplitude signals. and means for transferring the signals of the lower frequency range and the modulated pilot frequency signals; and  
 a receiver including means for providing a plurality of local oscillator signals having frequencies corresponding to the pilot frequency signals of the transmitter. means for receiving the local oscillator signals and the modulated pilot frequency signals and for providing amplitude signals corresponding to the amplitudes of the signals in the high frequency ranges. means for providing a plurality of synthetic signals each having a frequency approximately equal to the mid-range frequency of a higher frequency range. means for modulating each synthetic signal with the appropriate amplitude signal. and means for receiving and reproducing the modu- (ill lated synthetic signals and the directly transferred low frequency signals.  
  28. A transmitter as described in claim 27, wherein said pilot frequency signals have a frequency spacing equal to an integral multiple of the frequency of a standard commonly available frequency source so that a pilot frequency oscillator is not needed.  
  29. A system as described in claim 3, wherein the means for sequentially modulating the pilot frequency signal with the amplitude signals is formed and arranged so that the sequential modulation takes place in successive time channels that are repeated cyclically and each cycle includes an additional time channel, said transmitter additionally includes:  
 second means for receiving the sound signals;  
 means for dividing the sound signals received by the second means into a lower frequency range and an upper frequency range of the higher frequency ranges:  
 means providing an upper frequency range amplitude signal;  
 said sequential modulating means being adapted to modulate the pilot frequency signal with the upper frequency range amplitude signal during the additional time channel whereby the upper frequency range amplitude signal is also transmitted on the pilot frequency signal. and the receiving and de modulating means of the receiver is arranged to provide an upper frequency range amplitude signal in sequence with the amplitude signals. said receiver additionally including:  
 means for providing a noise signal having a frequency range approximately equal to the total frequency range of the higher frequency ranges.  
 means for modulating the noise signal with the upper frequency range amplitude signal, and  
 means for adding the modulated noise signal to the modulated synthetic signals and the directly transferred lower frequency signals so that the reproducing means reproduces the modulated noise signal also.  
  30. A system as described in claim 29. wherein each cycle includes a second additional time channel. said transmitter additionally comprising:  
 means for providing a sync signal corresponding to the repetition rate of the cycles;  
 means for providing a volume compression signal corresponding to the amplitude of the received sound signals; the means for receiving the sound signals additionally including means for receiving the volume compression signal. said means being responsive to the received signals for compressing the volume of the sound signals; means for modulating the sync signal with the volume compression signal so that the sync signal amplitude varies in relation to the volume compression;  
 said sequential modulating means being adapted to modulate the pilot frequency signal with the modulated sync signal during the second additional time channel whereby the pilot frequency signal has an amplitude Corresponding to the \olume compression during the second additional time channel. wherein the receiving and demodulating means also provides a sync signal. said receiver additionally comprising:  
  21 22 means for distributing the sequential amplitude sigvolume expansion means for receiving the modum the modulmmg means; lated pilot frequency signal and the sync signal means responsive to the sync signal for controlling and synchronizing said distributing means to assure proper distribution to the amplitude signals; and  
 and for providing a volume expanded modulated pilot frequency signal in response thereto.