Variable bandwidth voice and data telephone communication system
United States Patent 1191 Gruen et a1.  
 [ 1 Apr. 1,1975  
 1 VARIABLE BANDWIDTH VOICE AND 2.426.222 8/1947 Katchatouroff 179/4 3.261.922 7/1966 Edson 179/15 BY gsgp g COMMUNICATION 3.389.225 6/1968 Myers 179/15 BW [75] Inventors: Harold Gruen. Merion&#39;, Philip S.  
 Divia Richbmo. Charles Primary bxanuner-Ralph D. Blakeslee wemeh. Holland a&#34; of pa Anorney, Agent. or Firm-Caesar. Rivise, Bernstein &amp;  
  Cohen [73] Assignee: l. 1. Communications, Inc.,  
  Lionville. Pa. [22] Filed: Sept. 5, 1972 [57] ABSTRACT 1211 Appl- 286&#39;077 A switching network preferably for use with a private branch exchange telephone system which switching 52 us CL H 179/15 FD p g BY 79 DP network facilitates the simultaneous transmission of a [51] Int. Cl. H04} 1/14 Pluraliiy of mice and/0f data grade 51811315 and [58] Field of Search 119/15 FD. 15 FE. 15 R, companying control signals, P the switching signals 17915 y |5 w 2 p 3 4 needed to establish any one of the various alternative modes of operation in which the system is capable of 1 1 References Cited operatmg&#39; UNI FED STATES PATENTS 11 Claims, 18 Drawing Figures 1.984.099 12/1934 Vlard 179/3 m x/721x 056 6545 lac-M502 DATENTEGAPR 21925 MWN kvN  
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 SHEET ISBF 16 PATENTEBAPR&#39; 1 1975 SHEET IBM 16 VARIABLE BANDWIDTH VOICE AND DATA TELEPHONE COMMUNICATION SYSTEM BACKGROUND OF THE INVENTION In the earliest telephone communication systems speech was transmitted over wires at voice frequencies. It was soon realized that this was an inefficient way of utilizing the costly wire installations since the wires are capable of transmitting a much broader band of frequencies than that needed to convey the voice signals. Out of this realization there evolved a succession of carrier systems characterized in that a plurality of com munications may be placed on a single transmission medium. Each such communication occupies a separate channel. Historically, each channel has comprised an effective customer bandwidth of approximately 3.000 Hertz, this being the frequency span available to the customer for his use in conveying the information to be transmitted. Although carrier systems have been greatly improved since their inception, they still retain the basic characteristics of a 3,000 Hertz effective customer bandwidth Various attempts have been made to introduce further economies into telephone communications in ad dition to those afforded by the carrier concept. Several of these proposals involve the reduction in effective customer bandwidth to something on the order of 1,000 Hertz. thereby enabling two channels ofinformation to be conveyed on a bandwidth of approximately 4.000 Hertz. Numerous deficiencies surround such proposals. primarily among which is the fact that they are impractical and/or incomplete ideas of inventors who are either associated with the major corporate entity in the telecommunication field, or are persons who similarly believe they are in a position where they can exert sufficient influence on said corporate entity such that the latter will convert the existing system to accommodate their proposed operating standards. Thus, the aforementioned proposals are generally impractical in the sense that in order to be implemented it would first be necessary to modify the standards of basic tele phone communication by expanding the effective customer bandwidth from 3 Kilohertz to 4 Kilohertz.  
  Besides being totally lacking in practicality. these prior art proposals are generally not capable of being implemented for what of some critical feature such as allowance for signaling capability or frequency separation between adjacent information channels. The vast majority of such systems designed to further efficiencize the transmission of telephony and telegraphy-type information cannot operate with existing facilities. They are made by persons who overlook the fact that the huge investment in existing plant equipment makes very unrealistic the adoption of their idea since that would necessarily mean the abandonment of a substantial portion of the existing facilities.  
  In summary. the prior art attempts at providing an increase in the number of voice communications capable of being accommodated within a single channel of a carrier system have been characterized by one or more of the following deficiencies:  
  a. an extremely high cost in implementation so as to make the sy stem an economic impracticality&#39;.  
  b. the necessity to adopt new operating standards and specifications with respect to the system with which the proposed units are designed to operate. Usually the adoption of the proposed system of operation means conversion to a broader frequency band per channel, an impracticality because of the high capital investment with such changes entail; and  
  c. an incomplete and/or inoperative system such as one lacking in the necessary signaling functions or wherein the operating characteristics of the system are beyond the current technology or else are designed in total ignorance of the problem of interchannel crosstalk.  
 SUMMARY OF THE INVENTION The present invention is specifically designed to meet the needs of so-called private line&#34; service; i.e., lines belonging to customers whose usage rates are so high that it is economically advantageous for them to lease a line on a permanent basis from the telephone company. Although not as practical for the short haul, the present invention has particular applicability to longhaul service connections such as for transcontinental or transoceanic lines. Since the private lines comprise the same cable and radio service, the subject invention has equal applicability to all commercial applications.  
  The subject invention has various modes of operating capability. including the ability to transmit within one effective customer bandwidth of 3,000 Hertz:  
 l. a conventional voice frequency communication plus from one to three telegraph signals;  
 2. two-voice frequency communications of abbreviated but fully intelligible bandwidths. plus from one to three telegraph channels;  
 3. one-voice frequency communication of abbreviated bandwidth plus up to thirteen telegraph channels&#39;.  
 4. onevoice frequency communication plus a bandwidth of approximately 1,500 Hertz for the transmission of data grade signals;  
 5. all data in one or more channels.  
  In addition to the foregoing. the subject invention also provides means for transmitting control signals in company of the above-identified information signals, all within the specified effective customer bandwidth of 3,000 Hertz.  
  The practical advantages of an apparatus in the nature of the subject invention should be readily apparam; however, for a more complete understanding of the invention, its advantages and mode of operation, reference is made to the detailed explanation in the specification, claims and drawings.  
 IN THE DRAWINGS FIG. IA is a graphical representation of the output of the subject invention when operating to communicate two voice frequency signals. associated signaling information, plus three telegraph channels;  
  FIG. 1B is a graphical representation of the output of the subject invention when operating to communicate one voice frequency signal of abbreviated bandwidth. associated signaling information, plus [3 telegraph channels;  
  FIG. 1C is a graphical representation of the output of the subject invention when operating to communicate one voice frequency signal, associated signaling information, plus a bandwidth of approximately 1,500 Hertz for the transmission of data grade signals;  
  FIG. 1D is a graphical representation of the output of the subject invention when operating to communicate one voice frequency signal of conventional bandwidth,  
 associated signaling information. plus three telegraph channels;  
  FIG. IE is a block diagrammatic represenation of the present invention illustrated as interfacing two private branch exchanges;  
  FIG. 2 is a block diagrammatic representation of the subject invention represented generally in FIG. IE;  
  FIG. 3 is a block diagrammatic representation of the Transmit 1 portion of FIG. 2;  
  FIG. 4 is a block diagrammatic representation of the Transmit 2 portion of FIG. 2;  
  FIG. 5 is a block diagrammatic representation of the Interface and Control Circuits portion of FIG. 2;  
  FIG. 6 is a diagrammatic representation of the Receive I portion of FIG. 2;  
  FIG. 7 is a diagrammatic representation of the Receive 2 portion of FIG. 2;  
  FIG. 8 is a more detailed representation ofthe Signaling Control Circuits portion of FIG. 2;  
  FIG. 9 is a more detailed representation of the Bandwidth Control Circuit of FIG. 5&#39;.  
  FIG. 10 is a schematic representation of a Low Pass Filter used in the present invention such as Filter 37 of FIG. 3;  
  FIG. 11 is a schematic representation of the Modulators 57 of FIG. 4 and I23 of FIG. 7;  
  FIG. 12 is a schematic representation of a Low Pass Filter used in the present invention such as filter 59 of FIG. 4:  
  FIG. [3 is a schematic representation of the High Pass Filter used in the present invention such as filter 61 of FIG. 4&#39;. and  
  FIG. 14 is a schematic representation of a Variable Threshold Circuit used in the practice ofthe subject invention. including members I15 of FIG. 6 and I37 of FIG. 7.  
  Referring first to FIGS. 1A through 10. therein are disclosed graphical representations of the frequency spectrum of signals communicated through the subject invention in its various alternative modes of operation. In each instance. the frequency spectrum in Kilo-Hertz (KHz) is plotted against amplitude measured in decibels (db). The informational content of the signals being communicated is confined within the effective customer bandwidth of approximately 3000 Hertz provided for by the telephone company. As noted above. the effective customer bandwidth is slightly less than the total bandwidth ofa channel which also includes a proportionately smaller band of frequencies reserved to the use of the telephone comapny for the purpose of conveying control signals and other information used in effecting the transmission of a communication.  
  In FIGS. IA through ID. the effective customer bandwidth is indicated by a dot-dash legend extending from approximately 0.3 KHz to 3.3 KHz.  
  FIG. 1A depicts the subject invention operative to communicate two voice frequency signals of abbreviated bandwidth, associated control signals. plus three telegraph channels. With respect to the voice frequency signals. it will be noted that they occupy two relatively independent frequency bands of the effective customer bandwidth. The first ofthese (hereinafter referred to as the low-order signal) occupies a frequency bandwidth extending from approximately 03 KHz to approximately l.l KHz, while the second signal (hereinafter referred to as the high-order signal) extends from approximately l.5 KHz to approximately 2.3 KHz.  
  Two very important characteristics of the signal spectrums of abbreviated bandwidth utilized in the practice of the subject invention concerns their flatness and also their pronounced rolloff or skirt. In this respect it is normal when speaking of effective bandwidths to measure from the point where the signal has experienced a drop of 3 db from the reference level, i.e., 3 db down; however, in the p resent instance, the skirt of the filtered output signal is so pronounced that reference. for bandwidth purposes. is made with the signal still at the zero reference level.  
  The effectiveness of the specific filter design employed in the practice of the present invention is further substantiated by the lack of crosstalk generated between the two voice frequency signals of abbreviated bandwidth of FIG. 1A. In this respect, at the upper frequency limits (i.e., l.l KHz of the low-order signal of FIG. IA) the corresponding frequency component of the high-order signal is approximately 60 db. This compares with a rejection ratio of I to I000.  
  In addition to eliminating crosstalk between the two abbreviated voice frequency communications, the sharpness of the skirt of these waveforms further permits the insertion within the envelope of frequency oriented space. defined as the effective customer bandwidth. of control signals associated with each of the abbreviated voice channels.  
  The first ofthe inband control signal channels is symmetrically positioned between the two abbreviated voice channels and is represented in FIG. IA as being centered at about l.30 KHz. The control signal channel for the second abbreviated voice channel is centered about 2.6 KHz. In accordance with standard practice. the amplitude of the control signals is approximately 20 db relative to zero reference level of the abbreviated voice frequency signals. The control signal channel is designed to accommodate the transfer of from eight to fourteen dial pulses per second and as such is provided with a frequency bandwidth of approximately 30 Hertz.  
  Also shown within the envelope of frequencies allocated by the telephone company to the use of the customer in FIG. IA are three telegraph channels of limited frequency bandwidth on the order of baud.  
  It should be noted that the telegraph communication channels are commercially available in pre-packaged plugable units and that applicants inventive contributions in this respect centers about the simultaneous transmittibility of one or more voice communications of abbreviated bandwidths simultaneously with a plurality of telegraph signals.  
  Referring now to FIG. 18, therein is disclosed a graphical representation of an array of frequencies corresponding to a single abbreviated voice frequency channel with a control signal channel plus thirteen telegraph channels.  
  The thirteen telegraph channels correspond to the CCITT standard of 75 baud and as such are each spaced at I20 Hertz. As an alternative to the i3 75 baud telegraph channels. a lesser number of baud telegraph channels may be used.  
  FIG. IC depicts a further operating alternative wherein a single abbreviated voice frequency channel occupies the lower portion of the effective customer bandwidth. while the frequencies from 1.5 to 3.1 KHz are reserved to the customer for his use in accomodating the transmission of one or more channels of date. Conventional channel modem transmitter and receiver designs and/or techniques are available for such use. For example, the bandwidth still available within FIG. IC might be used to accommodate a single data modem transmitter and receiver. having an operative capability of from I200 to I800 bits per second.  
  FIG. 1D depicts the operating capability of the present invention in an alternative mode of operation which enables the transmission of a broad band voice frequency communication signal with its control signal. plus three telegraph channels. Alternatively. the entire frequency spectrum may be utilized for the transmission of voice signals or data signals. In this respect. in each of the graphical representations of FIGS. 1A through ID. provision is made for the simultaneous transmission of signals representing three telegraph channels. It is an obvious expedient of the present invention to modify the system such that the number of telegraph channels is increased or decreased. and in fact these may be eliminated altogether. thus affording a broader frequency bandwidth for each of the abbreviated voice frequency channels. Other variations in the organization and operation of the subject system may become apparent upon reference to the detailed description of the construction and operation of appliiants invention given with respect to FIGS. lE through Turning now to FIG. 1E. therein is disclosed a block diagrammatic representation of a four-wire telephone communication system embodying the subject invention. More specifically. there is disclosed a first Private Branch Exchange (PBX 1) identified in the drawings as member 2. having connected thereto a first Voice and Data Multiplexer. identified in the drawing as member 4. At a remote location is a second Private Branch Exchange (PBX 2) having associated therewith a second Voice and Data Multiplexer identified in FIG. IE as member 6. For purposes of explanation. it may be assumed that the lirst PBX and its associated Voice and Data Multiplexer is located a substantial geographical distance from the second PBX and its associated Voice and Date Multiplexer. Narrow Band Data Transmitters/Receivers 3 and 8 are provided to accommodate the transmission of coded information.  
  The private branch exchange (PBX or PABX) is a conventional device. a simple example of which is a switchboard which enables a subscriber such as a businessman having plural handsets to selectively switch a lesser number of subscriber lines to the various handsets. In this manner a limited number of lines may be shared between a larger number of telephone sets with the added advantage of enabling connections between telephones within the office being served by the private branch exchange.  
  Long-distance communications through a PBX are conventionally handled as a normal toll call; however. sometimes a group ofcircuits is provided between two private branch exchanges. Such circuits are known as tie lines and are particularly pertinent to the subject of the present invention. Thus a large corporation having offices on both the east and west coasts. may find it economically to their advantage to facilitate the telephone communication therebetween by way of a tie l ne which may be used exclusively to connect extensions at the two office locations. or possibly have the added capability of enabling the extensions at either end to complete calls to anyplace in the public sector. In any event. the telephone company imposes strict operating standards on the operation of the tie lines. This. coupled with the substantial installation and operational costs of the lines results in a fairly expensive facility.  
  Normally. in order to increase the message-carrying capability of two PBXs interconnected by way of a tie line. it is necessary to increase the number of tie lines proportionately to satisfy the increased demand in service. This in turn necessitates the cooperation of the telephone company whose tie line facilities are being used with the resultant increase in cost for the use of these facilities.  
  The telephone company in turn will probably meet the increased demand by dedicating an additional channel of the carrier system to which present facilities are tied into. By way of the present invention. it is possible to at least double the voice frequency messagecarrying capability of each tie line and/or facilitate the transfer of one or more channels of coded information. This increase in operating capability is provided without intervention on the part ofthe telephone company. and in fact there is no need and little opportunity for the telephone company to become aware of the existence of the added operating capability. since the operation is entirely within the standards established by the telephone company and the plural information signals and the associated control signals are entirely contained within the envelope heretofore designated as the effective customer bandwidth.  
  In summary. FIG. 1E depicts two private branch exchanges being interfaced to one another through associated Voice and Data Multiplexers. The Voice and Data Multiplexers are further interconnected via a conventional four-wire tie line facility represented generally in FIG. IE as members 50, 52, 54, 56. Included in the four-wire tie line facility are the components comprising the telephone plant These comprise the service normally used to connect the PBX to the local central office of the telephone company, the toll line connecting the local central office associated with the first PBX to a similar local central office associated with the second PBX. and the switching equipment at the local central offices necessary to support the toll line operation. Since. in the preferred embodiment of the present invention all signaling information is contained within the envelope defined as the effective customer bandwidth. the switching function performed at the local central offices is unaffected.  
  Before turning to FIG. 2, it should be noted with respect to FIG. IE that the Voice and Data Multiplexer is coupled to its associated PBX via a number of interconnecting leads. The leads connecting members 2, 3 and 4 are numbered and the natrue of the information carried thereon will be apparent from the detailed description which follows. However. it should be noted that a corresponding number of leads is used to interconnect the Voice and Data Multiplexer number 2 with the PBX number 2.  
  Turning now to FIG. 2. therein is disclosed a more detailed representation of the Voice and Data Multiplexer l of FIG. 1E. Included in FIG. 2 are the six specific subunits comprising the Voice and Data Multiplexer. To the extreme left of FIG. 2 are the lines carrying information and control signals into the Voice and