MODIFIED VESTIGIAL SIDE BAND TRANSMISSION SYSTEM

Abstract: A communication transmission system includes one or more modulated carrier communication channels in which a receiver has a modified vestigial side band filter (37, 59) for eliminating phase cancellation. Accurate design of the modified vestigial side band filter at the receivers, further reduces susceptibility to frequency drift caused by component aging and component response to environmental changes. The system has particular utility in multiple channel carrier telephone communication systems which utilize limited available band width for providing a maximum number of communication channels over an extended length communication line.

Description: MODIFIED VESTIGIAL SIDE BAND TRANSMISSION SYSTEM Background of the Invention Multiple channel analog frequency division multiplexing for use in communicating multiple simultaneous information channels is well known in the art. The two most common modes of carrier modulation used in such systems are amplitude modulation and frequency modulation. Normal amplitude modulation is double side band, which results in. the-.creation of two identical side bands, frequency displaced on each side of the carrier frequency by an amount identical to the modulating frequency. The strength of the side bands, in such a system, is directly proportional to the amplitude of the modulating frequency. While double side band amplitude modulation is..simple and inexpensive to implement, it also inherently generates ■certain disadvantages. The transmission occupies twice the band width of the highest modulating frequency, has no particular signal to noise advantage, as -is exhibited in more complex transmission systems, and is subject to phase errors between the two side bands. Such modulation presents particular problems if the communication medium exhibits non-uniform phase characteristics at varying frequencies within the channel band width. Frequency modulation for carrier multiplexed systems requires more complex and expensive equipment, and increases the signal-to-noise ratio only at the cost of wider occupied band width. Amplitude modulation is often modified to provide single side band transmission to reduce the required band width for transmission, and involves the suppression of the carrier and one side band at the transmission. oint. The signal-to-noise ratio can be made correspondingly higher. However, the equipment " RE ^ is much more complex than with standard amplitude modulation, particularly because the carrier must be reinjected at the receiver. t Some of the complexity of single side band reception can be eliminated by transmitting both a single side band- and the carrier, while suppressing the remaining side band. Such transmission, however, suffers high distortion in an environment where the carrier frequency at the receiver is subject to drift. In addition, such transmission requires an extremely accurate and costly transmission filter. True vestigial side band transmission systems slightly attenuate the carrier frequency in comparison with a main side band, and substantially attenuate the remaining (vestigial) side band. Such systems allow some band width economy in transmission, and are used, for example, to transmit picture information for television broadcast. True vestigial side band systems, however, suffer from high amplitude distortion if the receiver tuner drifts so that the carrier frequency is not accurately placed within the receiver pass band. Furthermore, the detected signal suffers from high harmonic distortion , at high modulation indices. Single side band, single side band plus carrier, and true vestigial side band transmission have classically been utilized in transmission systems to reduce the amount of band width required for transmission, so that more communication channels may occupy the same transmission band width, while the disadvantages of these modulating techniques have been reduced or eliminated through the use of expensive, complex equipment. However, in systems where equipment cost and complexity are of significant concern, it has been typical, in the prior art, to utilize double side band amplitude modulation and to limit overall system performance to a level consistent with this modulation. These limitations are a primary design consideration, for example, in multiple channel, carrier multiplexed telephone systems operating over extended wire pairs. Such transmission systems exhibit attenuation which increases with increasing frequency, such that higher frequency transmission necessitates either higher amplitude transmission or more complex repeaters placed at more frequent intervals. The use of frequent repeater amplifiers is costly, while the use of higher signal levels requires the transmission of greater power levels which, in turn, increases the cost and losses of the system. Thus, such communication systems are essentially bandwidth limited. In addition, these systems exhibit ' non-linear transmission medium phase characteristics at certain carrier channels as a consequency of the use of repeater amplifiers and extremely long cable runs, so that the effective usable band width and range of the systems must be compromised with one another to form a viable transmission network at reasonable cost. Summary of the Invention The present invention utilizes a modified vestigial receiver in a carrier multiplexed transmission system with standard double, side band AM transmitters. This does not result in a band width reduction for individual channels, but does permit channels to exist at frequencies within the pass band of the system which might otherwise be unusable. Thus, the system permits greater number of channels to be transmitted, without increasing overall system band width, and without substantially increasing system cost or complexity. At the same time, the invention substantially reduces distortion inherent in double side band modulating systems which results from phase non-linearities in the transmission medium. In addition, the invention reduces the susceptibility of the system to distortion OMPI caused by frequency drift from component aging or environmental factors.- The system transmitters produce double side band AM signals. On selected channels, a receiver is utilized which attenuates the received carrier frequency by a smaller degree than is common with true vestigial side band transmission, but which maintains a gentle receiver filter attenuation slope at the carrier frequency. This moderate slope reduces the susceptibility of the receiver to distortion caused by frequency drift of the receiver relative to the transmitted carrier signal. Although the resulting frequency response of the receiver within its pass band is not flat, as is common in true vestigial transmissions, a post detector filter flattens the response. In fact, the use of the post detector filter in this manner actually permits greater attenuation of harmonic distortion than would otherwise be possible. This transmission technique provides double side band transmission of all modulating frequencies, and reception of low modulating frequencies in a double side band mode, while essentially single side band reception is utilized for high modulating frequencies. Because phase non-linearities in transmission media, including repeater filters, are more acute at higher carrier modulating frequencies, and a filter band edges, this system greatly reduces distortion caused by these factors. Thus, the present invention permits the utilization of modified vestigial reception to eliminate distortion caused by phase non-linearities, while at the same time reducing the susceptibility of true vestigial transmission systems to receiver frequency drift. Because, in certain multiple channel transmission systems, range limiting phase non-linearities exist only on some of the multiple channels, the present invention contemplates the use of the receiver filtering technique, referred to above as modified vestigial side band receptio , only on certain channels, with standard AM transmission on all channels and standard AM reception on the remaining channels. Description of the Drawings • These, and other advantages, of the present invention are best understood through the following detailed description, which references the drawings, in which: Figure 1 is a bloc diagram schematic of the multiple channel modulated carrier transmission system of the present invention; Figure 2A is a plot of transmission line attenuation in the system of Figure 1; Figure 2B is a plot of the attenuation characteristics of the repeater filters of Figures 1 and 3; Figure 2C is a frequency plot showing the location of the transmission channel of the system of Figure 1 within the system, pass band; Figure 3 is a schematic block diagram of the repeater of Figure 1; ■ Figure 4 provides a plot of attenuation characteristics of receiver filters within the system of Figure 1, as well as certain prior art receiver filter characteristics for comparison; Figure 5 is a schematic diagram of a filter which may be used in the system of Figure 1 to achieve the filter characteristics of Figure 4 at selected channels within the system of Figure 1; Figure -6 shows a schematic block diagram of a post detector filter which may be utilized to reduce harmonic distortion and flatten system pass band response; and Figure 7 is a plot of the attenuation characteristics of the post detector filter of Figure 6 along with comparison attenuation plots. Detailed Description- of "the- Preferred Embodiment The preferred embodiment of this invention is incorporated in a frequency division multiplexed, analog carrier system for use, for example, in " multiplexing multiple subscriber telephone lines on one telephone cable pair, where the subscriber stations are located at a substantial distance from the central office. Systems of this type are used to expand service where existing cables have previously limited the number of available subscriber stations or where, because of the length of the transmission cables, .economic considerations justify the use of modulating electronics in preference to the installation of additional cable pairs over long cable runs. As is well known, the attenuation of such a cable pair increases with increasing frequency, and it is therefore common to modulate relatively low . frequencies with the voice signal in order to limit cost by limiting the number of repeaters and the transmissi power level required in the system. Figure 1 shows a station carrier system which, by AM carrier modulation, permits the multiplexing of eight subscriber channels, each permitting two-way communication, on one cable pair. In this instance, the cable pair carrie 16 carrier frequencies, 8 of which are used for transmittin voice signals from the central office to subscribers, and 8 used for transmitting voice signals from the subscribers to the central office. In the figure, the eight communication ' channels at the central office are designated C01 through COS, each including a voice output channel on lines- 11a through llh and 13a through 13h and a voice input channel on lines 15a through 15h and 17a through 17h. ___s i well known, the output channel, and input channel at each station are combined in a four, wire to two wire converter - (not shown) . Each of the voice input.channels on lines 15a-15h and. carrier signal provided by a carrier oscillator 21a-21h to provide a standard, double side band AM modulated signal at the output of the modulators 19a-19h on lines 23a-23h and 25a-25h. These output lines 23a-23h,25a-25h are summed in a summing amplifier and connected to a tranmission cable pair 27,29. The summing amplifier is not shown, but is well known in the art. In addition, the cable pair 27,29 is connected to a pair of regulated DC voltage sources 31 and 33, respectively, supplied at the central office. The DC power from the sources -31,33 is ' coupled through plural repeaters 35a and 35b (to be described below) , so that this power can be utilized at remote subscriber stations to power the filters, modulators, demodulators, and oscillators, described below. This DC coupling along the cable pair 27,29 permits the operation of remote subscriber stations without supplemental power at these stations. It will be understood by those skilled in the art that, if the length of the cables 27,29 is extreme, such DC power is substantially attenuated by line losses, supplementary DC power may be supplied. Each of the central office stations COl through C08 additionally includes a filter 37a through 37h connected by " a pair of lines 39a through 39h and 41a through 41h to the main cable pair 27,29 through a transformer. The use of such a transformer is well known, and is not shown. These filters 37a-h selectively filter incoming communication channels to provide a signal to plural demodulators 43a through 43h which provide the incoming voice signals on lines lla-h and 13a-h. The modulated carrier signals used for communication between the central office and subscriber channels are attenuated by the cable pair -27,29 and thus, for O-.-PI long cable runs , plural repeaters , such as the r°pea ers 35a and 35b , may be utilized in the system to amplify signals on lines 27, 29 utilizing power supplied by the DC voltage sources 31 and . 33 at various locations along the cable pair 27, 29. The subscriber stations may be located wherever required along the cable pair 27 ,29 , typically at substantially remote distances from the central office. As an example, it is typical to utilize a repeater 35 , approximately every five miles along the cable pair 27 ,29 to amplify the signals and thus avoid loss of the signals altogether due to cable attenuation. This amplification is accomplished utilizing the plus and. minus 135-volt DC power on the cable pair 27 ,29 from the sources 31 , 33. In the example shown in Figure 1 , plural subscriber stations. SUB1 through SUB 3 are connected in parallel to the line pair 27 ,29 at a location between the repeaters 35a and 35b. Additional subscriber stations SUB4 through SUB 8 are located even more remote from the central office, beyond the repeater 35b. In a configuration similar to that described for the central office communication channels C01-C08 , each of the subscriber stations includes an incoming voice line pair 51a-h and 53a-h and outgoing voice line pair 55a-h and 57a-h. The incoming voice signals are provided by filtering the incoming modulated carrier signals on the cable pair 27 , 29 utili zing a filter 59 a-h and, once a carrier and its side bands are thus selected, demodulating the voice signal , using a demodulator 61a-h. The outgoing voice channels on line pairs 55a-h , 57a-h are combined in modulators 63a-h with carrier frequency signals provided by plural oscillators :65a-h, in a manner identical to that utilized at the central office stations . Figure 2C provides a chart of the frequency spectrum ; utilized by the carrier system of Figure 1. It will be noted that 16 different carrier frequencies are utilized for two-way communication between the 8 central office channels C01-C08 and their corresponding 8 subscriber station channels SUB1-SUB8. Thus, voice signals transmitted from central office communication channel C01 modulate a 76 kilohertz carrier wave to ' provide the carrier signal 67a of Figure 2C, upper side band 69a, and lower side band 71a. Incoming voice signals for this channel COl produced by subscriber station SUBl are provided on carrier signal 73a, upper side band 75a, and lower side band 77a. It will be recognized, of course, that the diagram of Figure 2C . does not show the actual characteristics of the signal, but is only schematically representative of the carrier signals and their side bands to show the frequency locations thereof. The actual frequency content of the modulated signals will be described in detail below. Utilizing the same alphabetic designation for each channel (a for COl, SUBl; b for C02, SUB2,.etc), the location of the remaining carrier frequencies and side bands are shown in Figure 2C. In each case, the outgoing carrier signal from the central office channel C01-C08 is designated 67a-h and the incoming carrier signal to the central office channel C01-C08 is designated 73a-h, respectively. It can be seen that each of the voice channels is provided with 3 KHz modulation band width, providing a 6 KHz bandwidth, forced by a side band pair 69a-h, 71a-h; 75a-h, 77a-h surrounding the carriers 67a-h, 73a-h, respectively. The upper side band 69a-h, 75a-h of each carrier (69c, for example) is separated from the lower side band 71a-h, 77a-h (71d, for example) of the next adjacent carrier by a 2 KHz guard band. All of the subscriber station transmit channels are provided at frequencies belo " w 70 KHz, while all of the central office transmit channels are located at frequencies above 70 KHz, in order to permit satisfactory operation of the repeater amplifiers 53a and 53b in a manner described below. It will be recognized that the modulators 19a-h , 63a-h may include appropriate output filters in order to frequency limit the side bands 69 , 71, 75 , 77 to 3 KHz and thus permit the guard bands , ' shown in Figure 2C, to prohibit cross-talk between the communication channels. The filters 59a-h, 37a-h select "the appropriate carriers 67a-h, 73a-h and their related side bands for selective demodulation by the demodulators 61a-h , 43a-h, respectively. Thus , each of the communication channels is separately carried on a distinct carrier frequency to provide channel separation in the system. It will be recognized from the preceding description that the repeaters 35a and 35b , and other repeaters which may be added to the system to extend the cable pair 27,29 , must be capable not only of communicating voice modulated carrier signals in both directions , but also of coupling the DC voltage from the sources 31 ,33 which is used to power all of the electronics outside of the central office location. An exemplary repeater 35 is shown in Figure 3. This repeater 35 includes a high pass filter -81 connected to ' the line pair ' 27 ,29 " a ' t the central office end of the repeater 35. This high pass filter 81 will pass only the "carriers 67a-h and side bands 69a-h, 71a-h in the upper half of the frequency spectrum of Figure 2C, that is , the " central office transmit channels . These signals are amplified in an amplifier 83 and connected to the cable pair 27 ,29 at the subscriber station side of the repeater 35. Similarly , a low pass filter 87 is connected at the cable pair 27 ,29 on the subscriber station end of the . repeater 35 to pass the carriers 73a-h and associated side bands 75a-h and 77a-h to an amplifier 89 , the output of which is connected to the cable pair 27 ,29 at the central office side of the repeater 35. Figure 2B shows the pass band characteristics 81a of the high pass filter 81 of Figure 3 and the pass band characteristics 87a of the low-pass filter 87 of Figure 3. It will be seen from Figure 2B that the filters 81 and 87 are mutually exclusive in their pass band frequency ranges so that, for example , the output signals from the. amplifier 83 of Figure 3 cannot be amplified in the amplifier 89. The frequency arrangement of Figure 2 thus permits this bi-directional amplification in the repeaters 35 without interference between the channels in the two directions through the simple use of low and *»igh pass filters 87 , 81 , respectively. The repeater 35 must be constructed to pass DC power voltage from the sources 31 , 33. This may be accomplished by making the amplifier 89 DC coupled, and assuming that the low pass filter 87 will pass DC signals. Alternatively , the repeaters 35 may include a parallel circuit filtered to carry DC signal levels but to reject AC signals at frequencies even well below the lowest frequencies within the lower side band 77g of Figure 2C. ' Figure 2A shows the attenuation characteristics 91 of the cable pair 27 , 29 with frequency. Although the attenuation increases gradually with increasing frequency . the curve 91 is fairly uniform above approximately 13 KHz. Below 13 KHz , in the range designated generally 93 in Figure 2A, relatively drastic slope changes ocf.ur in the characteristic curve 91. These slope changes in the region 93 , along with the effects of the pas s band characteristics 81a and 87a of +-he repeaters 35 , shown in Figure 2B , particularly in the regions 95 and 97 (the roll-off portion of the filters 81 , 87) generate certain problems in the transmission -of modulated carrier signals in the system of Figure 1. While the use of more overall band width could alleviate these problems, permitting the lowest frequency carrier 73g to be placed at a higher frequency and increasing the guard band around 70 KHz to move the side bands 75h and 71a further away from the roll-off characteristics of . the filters 81 and 87, this solution is not entirely practical. Initially, this solution would require higher frequencies to be utilized. That is, the carrier 67h ■ and its side bands would have to be increased in frequency, increasing the attenuation over long cable lengths and requiring additional repeaters 35, or more sensitive amplifiers with more complex filtering, within the system. In addition, the standards which govern carrier multiplexed telephone communications require that the outgoing and incoming signals to be separated at approximately 70 KHz. In addition, the problem described above can be alleviated by reducing the number of channels, but only at the expense of higher cost per channel transmitted. Thus, the present invention, utilizing modified vestigial band pass reception, is designed to permit high quality voice transmission in two directions on 8 channels within these constraints and without unnecessarily increasing the complexity of repeaters 35 utilized in the system. As a result, the cost, of the overall " system is reduced to the greatest extent possible. In this regard, it should be recognized that, although single side band transmission permits a closer spacing of the carrier frequencies 67 and 73, such transmission substantially increases both the complexity and cost of the transmission and receiving electronics. In contrast, the present invention utilizes relatively inexpensive double side band AM modulated transmission equipment and provides a novel solution for alleviating distortion in channels a, g, and h caused by non-linear cable attenuation in the region 93 of Figure 2A and non-linear characteristics in the regions 95 and 97 of Figure 2B. The_ integrity of all system channels is therefore maintained while the cost of the system is kept at a moderate level. The drastic slope changes in the region 93 of .Figure 2A introduce, as is well known in the art, phase non-linearities in this same region. These phase ■ non-linearities over extended lengths of the cable pair- 27,29 can result in a phase reversal between the lower side band 77g and upper side band 73g. At higher . 0 modulation frequencies, this phase reversal can cause cancellation of the upper and lower side band signal portions, and resulting distortion, attenuation, and possible loss of higher frequency modulation signals. In order to make the guard band around 70 KHz 5 as narrow as possible in order to permit more modulated carrier channels to be utilized, the roll-off of the filters 81 and 87 of Figure 3 must be made extremely sharp. Thus, high order filters are used which produce, as an undesirable by-product, substantial phase 20 non-linearities in the filtered signal at the band edges. Because the roll-off, as can be seen in Figure 2B, occurs primarily in the area of the side bands 71a and 75h, there can be a phase reversal, particularly at the higher modulation frequencies, leading to distortion and 25 . cancellation in channels, a and h. Thus, the lower side, band modulation at high modulation frequencies can be shifted so that it is 180 degrees out of phase with the upper side band at the same frequency, leading to cancellation of the modulation signal. 30 As indicated previously, the equipment required to transmit single side band or true vestigial side band signals is complex and expensive. Thus, it is advantageous in this system to transmit double side band All signals. In order to alleviate the phase 35 cancellation and distortion problems, which exist at the low frequency end of the transmission spectrum, and in the area close to the guard band between subscriber sation transmit channels and central office transmit channels due to the repeater band splitting filter one solution would be to use single side band reception equipment to receive the double side band AM modulated signal. While this solution eliminates a possibility of side band cancellation with phase shift, and thus provides a satisfactory solution, it introduces problems of its own. Referring to Figure 4, there is shown a plot 101 of a theoretical single side band receiver filter response curve. Particular note should be taken of the fact that, in Figure 4, the frequencies offset from the carrier frequency are shown as +/- on the right side of the chart and -/+ on the left side of the chart. Thus, the chart, as shown, provides, in the case of the single side band filter 101, reception of the upper side band alone and corresponds directly to the higher frequencies being on the right side of the chart. If the higher frequencies are on the left side of the chart, then the plot 101 is a representation of a single side band reception filter receiving only the lower side band. As mentioned previously, the theoretical single side band filter, having an attenuation curve as plotted at 101, will pass substantially ali of one ,. side band up to approximately carrier frequency plus 3 KHz. Such a filter is a compromise, since it is desired to attenuate the carrier frequency to the greatest extent possible, while at the same time, permitting the-low frequency modulation signals to pass through the filter to the greatest extent possible. Thus, the plot 101 has a very steep roll-off through the carrier frequency. It will be appreciated- that, if this filter is utilized at central office station C07 (Figure 1) , to receive only the upper side band 75g, for example, of the 8-kilohertz carrier 73g, and is used, in addition, at the central office station C08 to pass only the lower side band 77h of the 64-kilohertz carrier frequency signal 73h; and, finally, at subscriber station SUBl, to pass only the upper side band 69a of the 76-kilohertz carrier frequency signal.67a, phase cancellation problems within the system, described in reference to Figures 1 and 2, will be eliminated. Thus, at certain receiver stations, one side band of the signal is substantially entirely removed at the receiver filter. Phase cancellation, due .to side band phase reversal, caused by the repeater« 35 and attenuation of the cable pair 27,29, cannot occur. However, due to the extremely fast roll-off of th«=» filter curve 101 at the carrier frequencies, frequency drift of the filters 59a, 37h, or 37g, in such an implementation, will introduce substantial signal distortion. Thus, referring to Figure.4, a shift in the center frequency of the filter of only 100 cycles will result in approximately 4 db signal level change at the carrier frequency and at low modulation frequencies. Center frequency drift, due to aging and environmental response of components within the filters, thus provides undesirable distortion due to drift in the filter frequency. Phase cancellation problems may also be cured with a true vestigial filter having a response curve shown at 103 in Figure 4, utilized in the same locations as suggested above for the single side band filter, thai- is, filters 59a, 37h, and 37g. A true vestigial filter, as shown by the plot 103, will attenuate the carrier frequency by 6 db, while passing virtually all of one side band and only a vestige of the remaining side band. Those signals provided by the vestige side band, at the low modulation frequencies, supplement the attenuated signals of the passed . -side band to provide virtually flat response. This system, however, suffers from problems similar to those described in reference to a single side band receiver, in that the slope of the filter curve 103 at the carrier frequency is still extremely steep. Thus, if the carrier frequency of the filter drifts slightly, due to aging or environmental causes, significant distortion, particularly at low modulation frequencies, " will be introduced. It should -be understood that, in accordance with the present invention, either the single side band filter, shown as plot 101 of Figure 4, or true vestigial side band filter, as shown b the plot 103, may be used for filters 59a, 37h, and 37g in the system of Figure 1, the remaining filters of Figure 1 being double side band filters. This solution to the phase cancellation problem will, however, introduce the distortion problem described above, and is thus usable only in systems where temperature and aging drift of filter components can be kept to a minimum. It will be understood that the single side band filter plot 101 and vestigial side band filter plot 103 are provided by filters which are well known in the art and which may be used at the locations within the circuit of Figure 1, described above, to eliminate phase cancellation problems, without other substantial modification to the AM carrier modulated system. Figure 4 also ••includes a plot 105 of a novel modified vestigial side band filter which not only solves the phase cancellation problems, discussed above, but at the same time reduces the susceptibility of the system to environmental and aging drift. Thus, carrier multiplexing systems employing a filter which provides plot 105 have significant advantages. This filter will be described in detail below. The plot 105 does not produce a flat pass " -band response, a matter which will be dealt with in the description below, but does provide a relatively gentle slope .at the point 107 where the curve passes through the carrier frequency. Because of this \ O ?I gentle slope characteristic at the carrier frequencv , distortion caused by frequency drift of the filter, or of the carrier frequency - itself , are minimized. A larger portion of the signal information is carried by the reduced side band than with either true vestigial side band 103 " or single side band reception 101. As in the case of the description above , with reference to single side band and true vestigial filtering, it wi ll be understood that the system of Figure 1 utilizing the modified vestigial side band characteristic 105 , provides double side band modulation from all modulators , but attenuates the incoming signal at the subscriber station SUBl and central office stations C07 and C08 , uti lizing a filter having an attenuation characteristic .105. In the case of filters 37g and 59a, the lower side band 77g, 71a is attenuated, so that the filter plot 105 is as shown in Figure 4. In the case of filter 37h, however, the plot 105 is reversed, the lower side, ban d 77 providing the main received side band , and the upper side band 75h providing the attenuated side band for reception. Plot 111 of Figure 4 shows the -effective modulation signal pass band for a receiver filter having the attenuation characteristics of plot 105. It will be noted that the pass band does not provide a flat response, but rather is attenuated at the higher modulation frequencies , particularly between 2 kilohertz and 3 kilohertz. While thi s might appear to provide a disadvantage of the modified vestigial band pass filter utilized in the:_present invention, it actually can be utilized in an overall circuit to provide an advantage , as described in more detail -below. A review of the plots 105 and 111 shows that this modified vestigial band pass receiver filter provides essentially AM doublfe side band reception between approximately 0 and 1.5 kilohertz , and essentially single side band reception betwee approximately 1. 5 kilohertz and 3 kilohertz modulation frequencies . Since phase cancellation problems exist primarily at higher modulation frequencies , this filter thus eliminates such phase cancellation , while at the same time providing the gentle slope characteristic " described above at the carrier frequency. Because single side band reception occurs only above approximately 1.5 KHz, and because distortion resulting from such single side band reception is likely to result in harmonic distortion, the lowest frequency distortion to be expected will be the first harmonic of approximately 2 KHz modulation signals , or at 4 KHz. Referring to . Figure 6 , the present invention utilizes a post detector filter 113 at the output of the demodulator electronics 114 in each demodulator 43 , 61 , utilizing modified vestigial side band filtering at the filter 37 ,59. As described above , the circuit of Figure 6 is utilized in the system of Figure 1 for the demodulators 61a, 43h, and 43g. Figure 7 provides a plot 115 of the theoretical pass band characteristic for the post detector filter 113. This filter 113 has an essentially flat pass band " from ' 0 to 3 kilohertz , and a very sharp roll-off 117 at 3 kilohertz. Unfortunately , such a theoretical filter is virtually impossible to build, and most such filters have an attenuation characteristic shown by the plot 119 , where the roll-off at 3 kilohertz is more gentle. As is well recognized in the filter design art, in order to make the post detector filter 113 more closely approximate the theoretical curve 117 , it is necessary to introduce non-linearities in the pass band,, resulting in a filter which provides a pass band attenuation pattern , as shown in plot 121 , this pass band attenuating low modulation frequencies to a greater extent than those modulation frequencies between 1.5 and 3 kilohertz. This pass band characteristic 121, for the post detector -filter 113, offsets the non-linearity in the pass band characteristic of the plot 111 of Figure 5 for the vestigial filter 37,59, resulting in a- " relatively flat output pass band, characteristic. Thus, what appears to ^e a disadvantage in the vestigial filter characteristic plotted in Figure 4 at 111, actually results in the ability to provide a more discriminating post detector filter 113. The use of the sharper post detector filter 113 plotted at 121 permits virtual elimination of harmonic distortion from the received voice channels. Figure 5 shows a circuit diagram of the filter used in Figure 1 for filters 37a through 37h and 59a through 59h. For channels 59b through 59h and • 37a through 37f, the components of this filter are selected to provide band pass filtering of +/- 3 kilohertz around the received carrier, to generate double side band AM reception. For filters 59a and 37g and h, however, as described above, the filter elements are selected to provide modified vestigial band pass characteristics in accordance with plot 105 of Figure 5. It has also been found convenient to provide modified vestigial band pass characteristics, utilizing the lower side band 77f as the main side band and the upper side band 75f as the modified vestigial side band, for the filter 37f at central office channel C06 for compatibility ™ith . prior systems. In Figure 6, the carrier signal is input a- lines " 121 -and 123 and amplified in transistor 125. The filter includes shunt resistance elements 127 and 129, shunt variable inductance elements 131, 133. and 135, and shunt capacitors 137, 139, 141, 143, 145, O PI 147, and 149. In addition, series capacitors 151, 153, 155, and 157, along with series resistors 161 and 163, as well as series variable inductors 165 and 167 are provided. It will be understood that Figure 5 shows an exemplary design for the filters in the various channels of the circuit of Figure 1, bu+" that those skilled in the art can provide the..modified vestigial filtering of the present invention for any selected carrier frequency if they are provided with the information from the plot 105 of Figure 4. The elements described above provide a band pass filter 181 which is appropriately tuned for filters 37f, 37h, and 59a to provide the band pass response of plot 105 of Figure 4. In addition, for the filter 37g, a notch filter 183 may be combined with the band pass filter 181 to provide the modified vestigial response of plot 105. This filter 183 includes capacitors 185 and .187 and variable inductance 189. Additional amplification is provided in the circuit by transistors 189 and 191. The following chart shows the values of the elements in the filters 181 and 183 for each of the ' filters 37a through 37h of Figure 1 in this exemplary embodiment. From this information, the values for the remaining filters 59a through 59h are current. PART # Filter 37a Filter 37b Filter 37c Filter 37d RESISTOR 127 4.53K 9.76K 5.23K 9.09K II 129 NOT USED 1 Meg 1 Meg 1 Meg It 161 10 JUMPR JUMPR JUMPR II 163 JUMPR JUMPR JUMPR JUMPR INDUCTOR 131 15 15 4 4 II 133 15 15 4 - 4" II 135 15 15. 4 . 4 11 165 15 15. .- 4 4 II 167 15 . . 15 4 4 fl 189 NOT USED NOT USED NOT USED NOT USED CAPACITOR 137 4101 2060 4657 3152 II 139 10,000 4840 10,637 7,013 It 141 10,770 5061 11,000 7310 II 143 2164 1501 3759 2698 II 145 .10,770 5061 11,000 7310 It 147 10,000 4840 ' 10,637 7013 II 149 4126 2060 4657 3187 II 151 4010 1177 1746 880 II 153 3137 860 1350 . 680 . II 155 3137 880 1350 680 II 157 • 3981 1177 1746 880 II 185 NOT USED NOT USED NOT USED NOT USED II 187 NOT USED NOT USED NOT USED NOT USED Resistors in ohms Capacitors in pico farads PART i Filter 37e Filter 37f Filter 37g Filter 3 RESISTOR 127 10.7K 60.4K 11.3K 7.5K II 129 1 Meg 1 Meg NOT USED NOT USED II 161 JUMPR JUMPR 115 JUMPR II 163 " JUMPR JUMPR JUMPR 56.2 INDUCTOR 131 4 1.277 122 1.277 II 133 4 1. 27 122 1.27 tl 135 4 . 1.27 122 1.27 II 165 4 1.27 122 1.27 11 167 4 1.27 122 1.27 189 NOT USED NOT USED 122 NOT USED CAPACITOR 137 2293 5350 1560 4320 II 139 4, 972 11, 300 3 ,421 9 ,100 II 141 5147 11,300 4101 9100 II 143 2024 4972 288 4101 I I 145 5147 11 ,300 4101 9100 tl 147 4972 16,900 3421 8760 11. 149 2293 NOT USED 1560 4191 II 151 541 804 2799 541 II 153 400 600 2149 427 tl 155 400 600 2164 389 I I 157 541 JMPR 2756 680 II 185 NOT USED NOT USED 1942 NOT USED II - 187 NOT USED NOT USED 3812 ' NOT USED Resistors in ohms Capacitors in pico farads Inductors in milli henries, unless otherwise noted In this implementation, it will be seen that the filter characteristic 105 (Figure 4) is provided at the low-frequency carrier 73g (Figure 2C) by combining, ' in series, a band .pass filter 181 and notch filter 183 the notch of the notch filter being placed .at 5 KHz, 3 KHz below the carrier frequency 73g. In- the remaining vestigial band pass filters 37f, 37h, and 59a (Figure 1) , the carrier frequency is high enough that it is possible to implement the band pass characteristic 105 (Figure 4) by detuning the poles of the band pass filter 181 on the modified vestigial side..band.side of the band pass filter characteristic. This modification is provided by the circuit values in the table above, with the variable inductances being tuned for each of - the modified vestigial channels to approximate, as closely as possible, the plot 105 of Figure.4. While an exemplary embodiment has been shown for the filters, it should be recognized that the invention contemplated provides a multiple- frequency modulated carrier system in which modified vestigial side band filters are utilized at the receiving stations for those channels which exhibit phase cancellation t>roblems due to the filter characteristic of the repeater 35 or due to line attenuation of the cable pair 27,29. In this system, all of the modulators 19 and 63 provide double side band AM modulated output signals so that the vestigial characteristics of the receiver ' filters 37,59 are utilized, not to closely space the carriers 67,73, but rather to avoid phase cancellation problems on certain channels. The implementation of the modified vestigial reception characteristics 105 of Figure 4 is well within the skill of filter designers at anv carrier frequency, the detuning of band pass filter poles and use of a notch filter in combination with a band pass filter provided as one possible implementation only.

Claims: 1. A transmission system providing multiple modulated carrier communication channels on a single transmission medium, comprising: 5 plural transmitters, each generating double side band AM modulated communication signals at different carrier frequencies; and plural receivers, each tuned to one of said . different carrier frequencies, at least one of 10 said plural receivers attenuating one of said double side bands more than the other of said side bands to eliminate side band phase cancellation. 2. A transmission system, as defined in Claim 1, 15 wherein said transmission medium exhibits phase nonlinearities at certain frequencies, said one of said plural receivers tuned to a carrier frequency at which said medium exhibits said. onlinear phase characteristics. 20 3. A transmission system, as defined in Claim 2, wherein said transmission medium exhibits nonlinear phase characteristics at plural separated frequencies, and wherein plural of said receivers, which attenuate one of said double side bands more than the other of said double 25 . side bands, are utilized to receive different carrier frequencies at said plural separated frequencies. 4. A transmission system, as defined in Claim 1, wherein at least one of said plural receivers receives full double side band AM modulated communication signals.. 30 5. A transmission system, as defined in Claim 1, wherein said one of said plural receivers provides substantially double side band reception-at modulation frequencies below a first predetermined frequency, and substantially single side band- reception at modulation 5 frequencies above a second predetermined frequency. 6. A transmission system, as defined in Claim 1, wherein said one of said plural receivers attenuates the received carrier frequency by approximately 3.5 db. 7. A transmission system, as defined in Claim 1, wherein said one of said plural receivers provides an attenuation slope at the received carrier frequency which is selected to avoid distortion caused by frequency drift- 8. A transmission system, as defined in Claim 1, wherein said one of said plural receivers includes a band pass filter, the pass band of which extends on both sides of the received carrier frequency, the poles on one side of the pass band having a relatively lower Q than the poles on the other side .of the pass band. 9. A transmission system, as defined in Claim 1, wherein said one of said plural receivers includes a band pass filter providing a pass band which extends above and below the received carrier frequency by ' a predetermined frequency amount, and a notch filter-, the notch of which is frequency positioned adjacent °ne edge of said band pass filter. 10. A method of carrier multiplexing multiple communication channels on a single communication medium, comprising: transmitting said multiple channels as double side band AM modulated carrier signals having carriers at different frequencies; and avoiding communication medium induced distortion by receiving at least some modulation, frequencies of at least one of said multiple channels as a single side band AM modulation signal. 11. A method of carrier multiplexing, as defined in Claim 10, additionally comprising: receiving another one of said multiple channels -as a double side band AM modulation signal. 12. A method of carrier multiplexing, as defined in Claim 11, wherein said one of ' said multiple channels is. selected to receive a carrier frequency at which said single communication medium exhibits nonlinear phase, characteristics. 13. A method of carrier multiplexing, as defined in Claim 10, wherein said avoiding step comprises receiving said modulation frequencies in a manner which provides a non-flat pass band response to modulation frequencies. 14. A method of carrier multiplexing, as defined in Claim 10, wherein said avoiding step avoids medium induced distortion, including that produced by repeaters, in said communication medium. 15. A method of carrier multiplexing, as defined in Claim 10, wherein said transmitting step comprises transmitting said multiple channels as double side band AM modulated carrier signals which do not overlap within the frequency spectrum. 16. A receiver filtering method for use with a carrier multiplexed, double side band transmission, comprising: attenuating a first side band to a greater extent than a second side band at frequencies displaced from the carrier by less than a first amount; essentially eliminating said first side band, in comparison with said second side band, at frequencies displaced from the carrier by more than a second amount; and maintaining the slope of said filter at said carrier sufficiently gentle to avoid frequency drift induced distortion. 17. A receiver filtering method, as defined in Claim 16, additionally comprising: attenuating said carrier by approximately 3.5 db. 18. A receiver filtering method, as defined in Claim 16, additionally comprising: -generating a non-flat modulation frequency pass band. iRi O PI » . IPO 19. A receiver filtering method,, as defined in Claim 18, additionally comprising: demodulating said carrier multiplexed transmission; and post demodulation filtering said carrier multiplexed transmission to generate a flat modulation frequency pass band response. 20. A receiver filtering method, as defined in Claim 16, wherein said first amount is approximately 1.5 kilohertz and wherein said second amount is approximately 2 kilohertz. 21. A receiver filtering method, as defined in Claim 16, wherein said carrier multiplexed double side band transmission is carried by a nonlinear transmission mediumand wherein said attenuating, eliminating, and maintaining steps are applied to frequency transmissions wherein said medium has nonlinear phase characteristics. 22. A method of designing a carrier multiplexed, multiple channel transmission system, for use with a transmission medium having nonlinear phase characteristics comprising: selecting carrier frequencies for double side band AM transmission of each of said multiple . channels; determining the frequencies at which said transmission medium exhibits acute nonlinear phase characteristics; designing first double side band AM receivers for frequencies at which said transmission medium does not exhibit acute nonlinear phase characteristics; and designing second receivers which attenuate one side band more than the other side band for said frequencies at which said transmission medium exhibits acute nonlinear -phase ' characteristics. • 23. A method of designing a carrier multiplexed, multiple channel transmission system, as defined in Claim 22, additionally comprising: 5 maintaining the attenuation slope of said receivers sufficiently gradual at the received carrier frequency to avoid frequency drift produced distortion. 24. A method of designing a carrier multiplexed,- 10 multiple channel transmission system, as defined in Claim 22, wherein said second designing step provides receiver attenuation which permits essentially double side band reception below a first modulation frequency and essentially single side band reception above a second 15 modulation frequency. 25. A method of designing a carrier multiplexed, multiple channel transmission system, as defined in Claim 22, wherein said transmission medium includes a repeater having a repeater filter, said second receiver 20 designing step being utilized for receivers operating at frequencies adjacent the cut-off point of said repeater filter. 26. A method of designing a carrier multiplexed, multiple channel transmission system, as defined in 25 Claim 22, wherein said transmission medium includes cables ' having nonlinear phase characteristics at low transmission frequencies, said second receiver designing step being utilized at said low transmissio frequencies. 27. A receiver filter for use with a double side band, 30 AM modulated carrier signal in a carrier multiplexed transmission system, comprising: means for providing a pass band frequency displaced on both sides of said carrier signal; and - 35 means for attenuating said double side band, -AM modulated carrier to a greater extent at frequencies within said pass band displaced in one direction from said carrier signal than at frequencies within said pass band displaced by an equal amount in the other direction from said carrier signal. 28. A receiver filter, as defined in Claim 27, wherein said means for providing a pass band frequency comprises a band pass filter and wherein said means for attenuating comprises a notch filter. - 29. A receiver filter, as defined in Claim 28, wherein said receiver filter is used at low carrier frequencies within said carrier multiplexed transmission system. 30. - A receiver filter, as defined in Claim 27, wherein said means for providing a pass band frequency comprises a band pass filter and wherein- said means for attenuating comprises poles of said band pass filter designed to have reduced Q. 31. A receiver filter, as defined in Claim 30, wherein said carrier multiplexed transmission system includes repeaters having filter attenuation cut-off curves, said filters being used at frequencies adjacent said cut-off curves. 32. A receiver filter, as defined in Claim 27, wherein said means for attenuating attenuates the received carrier frequency by approximately 3.5 db. 33. A receiver filter, as defined in Claim 32, wherein said attenuating means maintains the attenuation slope at said received carrier frequency gradual enough to avoid frequency drift generated distortion. 34. A receiver filter, as defined in Claim 27, wherein said means for attenuating provides a non-flat modulation frequency pass band. 35. A receiver filter, as defined in Claim 34,- additionally comprising: a post detection filter providing a non-flat modulation frequency pass band complementing the non-flat pass band of said attenuating means. 36. A carrier multiplexed telephone transmission system, comprising: a pair of transmission lines; plural central office channels, each comprising: a transmitter for generating on said pair, of transmission lines a double side band AM modulated carrier output signal carrying voice information; and a " receiver for receiving AM modulated carrier signals from said pair of transmission lines to' demodulate voice information; plural subscriber channels, each comprising: a transmitter for generating through said pair of transmission lines a double side band AM modulated carrier output signal carrying voice information; and a receiver for receiving AM modulated carrier signals from said pair of transmission lines to demodulate voice information; at least one of said central office and subscriber channel receivers demodulating both the upper and lower side bands of said AM modulated carrier; and at least one other of said central office and subscriber channel receivers demodulating one side band to the substantial exclusion of the other side band of said AM modulated carrier at selected modulation frequencies. 37. A carrier multiplexed telephone transmission system, as defined in Claim 36, wherein said selected modulation frequencies are frequencies above a predetermined modulation frequency. 38. A carrier multiplexed telephone transmission system,- as-defined in Claim 36, additionally comprising: a repeater amplifier connected in series with said pair of transmission lines, said repeater O Λ, W amplifier including a filter having a filter attenuation"curve at a selected transmission frequency. 39. A carrier multiplexed telephone transmission system, as defined in Claim 38, wherein said at least one other of said central office and subscriber channel receivers receives an AM modulated carrier signal adjacent said repeater filter curve. 40. A carrier multiplexed telephone transmission system, as defined in Claim 36, wherein said at least one other of said central office and subscriber channel receivers receives a modulated carrier signal at a frequency at which -said pair of transmission lines exhibits nonlinear phase characteristics. 41. A. carrier multiplexed telephone transmission . . system, as defined in Claim 36, wherein said at least one other of said central office and subscriber channel receivers includes a band pass and notch filter connected in series. 42. A carrier multiplexed telephone transmission system, as defined in Claim 36, wherein said at least one other of said central office and subscriber channel receivers includes a pass band filter with selected filter poles offset to provide non-flat pass band response.

Inventors: BENCH S, CAMPBELL G, STUMP T, THIBODEAU P
Applicants: ANACONDA CO
Application year: 1980
Main classification: H04J1
IPC codes: H04B1, H04B3, H04J1