Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J1/00—Frequency-division multiplex systems
  • H04J1/02—Details
  • H04J1/08—Arrangements for combining channels

Definitions

• 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.
• 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 a greater number of channels to be transmitted, without increasing overall system band width, and without substantially increasing system cost or complexity.
• the invention substantially reduces distortion inherent in double side band modulating systems which results from phase non-linearities in the transmission medium.
• the invention reduces the susceptibility of the system to distortion caused by frequency drift from component aging or environmental factors.
• 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.
• 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;
• Each of the voice input channels on lines 15a-15h and. 17a-17h are combined in a modulator 19a-19h with a 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.
• 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 long cable runs , plural repeaters , such as the repeaters 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.
• 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 CO1-CO8 and their corresponding 8 subscriber station channels SUB1-SUB8. Thus, voice signals transmitted from central office communication channel CO1 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 SUB1 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.
• each channel Utilizing the same alphabetic designation for each channel (a for CO1, SUB1; b for CO2, SUB2, etc), the location of the remaining carrier frequencies and side bands are shown in Figure 2C.
• the outgoing carrier signal from the central office channel CO1-CO8 is designated 67a-h and the incoming carrier signal to the central office channel CO1-C08 is designated 73a-h, respectively.
• each of the voice channels is provided with 3 KHz modulation band width, providing a 6 KHz bandwidth, forced by a side band pair
• 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 high 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.
• 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.
• the standards which govern carrier multiplexed telephone communications require that the outgoing and incoming signals to be separated at approximately 70 KHz.
• the problem described above can be alleviated by reducing the number of channels, but only at the expense of higher cost per channel transmitted.
• this filter is utilized at central office station CO7 ( 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 CO8 to pass only the lower side band 77h of the 64-kilohertz carrier frequency signal 73h; and, finally, at subscriber station SUB1, 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.
• 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 repeaters 35 and attenuation of the cable pair 27,29, cannot occur.
• 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 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.
• the carrier frequency of the filter drifts slightly, due to aging or environmental causes, significant distortion, particularly at low modulation frequencies, will be introduced.
• 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.
• 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 gentle slope characteristic at the carrier frequency , 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.
• 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 SUB1 and central office stations CO7 and CO8 , uti lizing a filter having an attenuation characteristic 105.
• the lower side band 77g, 71a is attenuated, so that the filter plot 105 is as shown in Figure 4.
• the plot 105 is reversed, the lower side, ban d 77h 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 double side band reception between approximately 0 and 1.5 kilohertz , and essentially single side band reception between approximately 1.
• 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.
• 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.
• the carrier signal is input at 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, 147, and 149.
• 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.
• Figure 5 shows an exemplary design for the filters in the various channels of the circuit of Figure 1, but 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.
• 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.
• 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.
• 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.
• 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 problems due to the filter characteristic of the repeater 35 or due to line attenuation of the cable pair 27,29.
• 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 any 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.

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Cable Transmission Systems, Equalization Of Radio And Reduction Of Echo (AREA)
• Reduction Or Emphasis Of Bandwidth Of Signals (AREA)

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Cited By (40)

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Citations (13)

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• 1980
  • 1980-01-23 JP JP80500497A patent/JPS56500198A/ja active Pending
  • 1980-01-23 BR BR8006228A patent/BR8006228A/pt unknown
  • 1980-01-23 WO PCT/US1980/000067 patent/WO1980001633A1/en not_active Application Discontinuation
  • 1980-08-15 EP EP19800900372 patent/EP0023223A4/en not_active Withdrawn

Patent Citations (13)

Non-Patent Citations (1)

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