IE46625B1 - Improvements in or relating to equivalent four-wire carrier frequency systems - Google Patents

Abstract

System for transmission in up to four channels by symmetrical lines, a pilot frequency being transmitted only in one direction and operated in a terminal station. System characterized in that one of the terminal stations (station A) transmits to the other terminal station (station B) in a frequency range of 32 to 48 kHz, and, in the opposite direction, in a frequency domain of 92 at 108 kHz. Provision applicable in the technique of multiple cable telecommunications.

Description

The invention relates to carrier frequency systems. Such a system may he provided, with pilot control for up to four low-frequency (IF) channels for operation via balanced twowire lines by the equivalent four-wire method, i.e. the outgoing and incoming transmission bands have different frequencies.

Simple carrier frequency systems with a low number of channels, for example, four channels, are used in civil as well as in military service. There is often a demand for using such, carrier frequency systems on the unoccupied trunks of cables which are already occupied with 12-channel carrier frequency systems (Z12) which operate by the equivalent four-wire method. In Z12 systems, transmission from one terminal station, referred to as A station, to the other terminal station (B station) is . performed in the frequency range 6-54 kHz and in the opposite direction it is performed in the range 60 - 108 kHz.

Owing to the narrower band width required, the entire effective transmission band of known 4-channel carrier frequency systems is necessarily in a frequency range which wholly or partially overlaps the range used by Z12 systems for transmission from the A station to the B station. For example, a printed brochure by Messrs. Telefuhken discloses a 4-channel carrier frequency system in which the range 4-20 kHz is uded for transmission tom the A station to the B station and the frequency range 24 - 40 kHz is used for transmission in the opposite direction.

If part of the trunks of a cable is occupied, with Z12 systems and the remaining trunks are wholly or partially occupied with 4-channel systems, it will not be possible to provide unrestricted operation for the latter. In this case, interfering cross-talk of the signal- transmitted from the A station of a Z12 carrier frequency system will occur on a reception signal of a 4-channel carrier frequency system in the same frequency range.

Such cross-talk is particularly detrimental if repeater field attenuation is high and the reception signal levels are therefore correspondingly low. Mixed operation of 4-channel and Z12 carrier frequency systems over the trunks of a cable is therefore possible only over short distances with low attenuation and then only in the case of cables which have good cross-talk equalization between the individual trunks.

German Offenlegungsschrift 25 43 492 discloses a particularly simple and efficient carrier frequency system which operates by the equivalent four-wire method with pilot control for transmitting up to four low-frequency channels over balanced conductors in which the effective transmission band is in the frequency range 12 - 43 kHz and a pilot frequency, outside the effective transmission band, is transmitted in only one direction and is gated in a terminal station.

According to the invention, there is provided a carrier frequency system operating by the equivalent four-wire method 46626 for transmitting up to four low-frequency channels over balanced conductors in which a pilot frequency is transmitted in only one direction and is evaluated in one terminal station, the transmission from one terminal station to another terminal station being performed, in the frequency range 32 - 48 kHz and transmission in the opposite direction being performed in the frequency range 92- 108 KHz.

A preferred low-cost 4-channel carrier frequency system permits unrestricted operation over the trunks of a cable of which other trunks are occupied by Z12 systems and can make substantial use of subassemblies of the carrier frequency system disclosed in the German Offenlegungsschrift 25 43 492.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Pig. 1 illustrates the frequency distribution of a preferred 4-channel carrier frequency s'ystem, using the pre-modulation method; and Pigs. 2 and 3 are block circuit diagrams of a circuit system for forming the transmission bands 32 - 48 kHz and 92 - 108 kHz, also referred to as effective transmission bands, using the premodulation method.

Each frequency band of a low-frequency signal which is to be transmitted covers the range of 0.3 - 3.4 kHz. To illustrate the transmission band, a frequency band of 0 - 4 kHz will be assumed hereinbelow in the interests of simplicity. The low-frequency signal (AF signal) can he a speech signal or data signal.

Ihe formation and lay-out of the transmission hands will be explained hereinbelow by reference to an example with premodulation. Fig. 1 shows the frequency distribution for this kind of generation of the two effective transmission hands.

The AF channels are shifted or translated by means of a carrier frequency of 48 kHz into the pre-modulation band of 48 - 52 kHz.

From this band each of the channels is translated with one of the carrier frequencies 84 kHz,88 kHz,92 kHz and 96 kHz and the resultant lower side hands are combined as signal bands into a band covering the frequencies 32 - 43 kHz. This band covers the frequencies for transmission from A station to 3 station. By means of a carrier frequency of 60 kHz, also referred to as directional carrier frequency, the aforementioned band is translated into the transmission band 92 - 108 kHz for transmission from the 3 station to the A station. The frequency of 60 kHz is also used as pilot frequency.

An output designated a in Fig. 2 is connected to an input designated a in Fig. 3 and. an input designated b in Fig. 2 is connected to. an output designated b in Fig. 3· The solid lines in Fig. 3 apply if the preferred carrier frequency system is connected as the A station. The broken lines of Fig. 3 apply - 6 if it is connected as thb B station.

The translation of the AF (audio frequency) signals into the transmission hand of an A station will first be explained. A channel translator (ΚΠ1 - HUA) is provided in the A station as well as in the B station for each AF signal which is to be transmitted. In the interests of clarity Fig. 2 shows only the channel translator KU1 ia detail, the remaining channel translators E(J2 - EDA being merely indicated..

The AF’signal which is supplied to'a terminal pair F2an of the — I channel translator KU1 is transferred via a transformer U1 to a low-pass .filter TP*i which limits the upper spectrum of the AF signal. The AF sign’al is translated in a modulator H1 hy means of a carrier frequency fΊ = AS kHz into the pre-modulation hand AS - 52 kHz and interfering modulation products are removed •15 hy a band pass filter BP1. In a further stage of modulation the signal is translated with a carrier frequency f2 = 8A kHz in a modulator K2 from the pre-modulation band into the frequency band 32 - 36 kHz. The carrier frequencies 83 kHz, 92 kHz and 96 kHz are used ’for the modulator M2 in the additional channel translators KH2 - KHA respectively. Each of the bands 32 - 36 kHz, 56 - AO kHz, AO - AA kHz -and AA - AS kHz produced in this stage of modulation is supplied to a low-pass filter TP2 as the frequency band 32 - A8 kHz. The upper side band 132 - 1A8 kHz produced in the modulators M2 of the channel translators KH1 - KU4 is blocked and only the lower side band of 32 - AS kHz is passed - 7 through as the signal hand. The said signal band is transferred to a pilot controlled amplifier V1 of a pilot control system PE. The pilot control system PE also contains a pilot controller amplifier VA which is connected in the reception direction. The amplifiers V1 and VA are constructed as frequency-independent amplifiers whose gain can be varied by means of the dc voltages which are applied to control inputs V1.1 or VA.1. The pilot frequency fn is coupled out by means of a selective pilot amplifier V3 from the output of the amplifier VA. Each of the two outputs of the selective pilot amplifier V3 delivers a dc voltage, proportional to the pilot voltage, for controlling the two amplifiers V1 and VA. The advantage of transmission side equalization of changes in the cable attenuation of the A station is that'it is possible to completely dispense vzith pilot‘control means in the B station. The cable attenuation itself is equalized by means of an amplifier V2 and in the opposite direction by means of an amplifier V5 while frequency-independent changes in cable attenuation, for example of the kind resulting from temperature effects, are equalized on the reception side by means of the pilot-controlled amplifier VA and on the transmission side by means of the amplifier V1 which is controlled in synchronism. The signal, amplified by the amplifiers V1 and V2, is supplied via the low-pass part to the direction filter EW and to the high-pass part of the line separating filter LW of the transmission Line L.

The transmission signal in the frequency range between 92 and 108 kHz which arrives from the opposite station is transferred via the high-pass parts of the line separating filter LW and directional filter SW of a receiver amplifier unit V5 comprising an amplifier and an equalizer. A frequency-indep5 endent gain and slant distortion, opposite to the cable attenuation characteristic, can be adjusted by means of this unit. Optimum equalization is thus possible for the attenuation characteristics of the cables in use. After passing through the reception amplifier unit V5j the transmitted signal is supplied to the amplifier V4 of the pilot control, system. The output of the amplifier 74 is connected to the hand pass filter BP3 which allows the equalized transmitted signal to pass without obstruction. This signal is subsequently translated ih a modulator M3 by means of the carrier frequency £3 = 60 kHz.

The adjoining low-pass filter TP4 allows the lower side band 32 - 48 kHz of this translation to pass through and blocks the upper side band which commences at 152 kHz. The maximum of 4 AF channels appear at the output of the low-pass filter TP4 in the frequency bands 32 - 36 kHz, 36 - 40 kHz, 40 - 44 kHz and 44 - 48 kHz, in each case in the reverse position. The input of a demodulator D1 of each of the channel translators KU1 - KH4 is connected in this case. The demodulator D1 of each channel translator is operated at the same carrier frequency as the associated modulator M2 and translates each of the four AF signals of the frequency band 32 - 48 kHz into the pre-modulation hand of 48 - 52 kHz. Interfering modulation products are suppressed by means of a band pass filter BP2 which is constructed in the same way as the band pass filter BP1. Subsequently, each signal - 9 46625 in the pre-modulation hand is translated into the original AF hand hy means of a demodulator D2 which operates at a carrier frequency of 48 kHz and hy a low-pass filter TP5· After amplification, each of the maximum of four AF signals can he picked off from a terminal pair F2ah of the relevant channel translator through a separate transformer U2.

The channel translators KU1 - EC4 operate irrespective’ of whether the station is connected as an A station or a B station. In the case of the B station, the output of the low-pass filter TP2 is connected to the modulator M3 which operates at the directional carrier frequency f3 - 60 kHz. This modulator translates the transmission hand 32-48 kHz at the output of the low-pass filter TP2 into the two side hands of 12 - 28 kHz and 92 - 108 kHz.

This spectrum is supplied to the hand pass filter BP3 which allows the upper side hand 92 - 108 kHz to pass and blocks the lower side hand .12 - 23 kHz. The pilot control system FE is omitted in one B station. A pilot coupling stage, whose input is connected to the band pass filter BP5 and whose output is connected to the amplifier V2, is connected in circuit in place of the amplifier V1 of the pilot control system PE. The signal transmission hand is supplied with a pilot frequency fo of 60 kHz in the above-mentioned pilot coupling stage. The transmitted band which also contains the pilot frequency fp, is amplified hy the amplifier V2 and supplied via the high-pass parts of the direction ^5 filter EW and line separating filter LW to the transmission line L. The transmitted signal from the opposite station and having the frequency range of 32 - 48 kHz is supplied via the low -pass part of the line separating filter LW and the low-pass part of >1.0 6 25 - 10 the directional filter RW to the reception amplifier unit V5· The output of the said reception amplifier unit V5 will then be connected to the low-pass filter TP4 which allows the equalized transmission signal to pass without obstruction. The maximum of four AF signals is subsequently obtained in the manner already described for the A station.

A specific mean transmission level prevails at a mean cable temperature at the output of the amplifier V2 of the A station. This transmission level can increase by a specific amount, for . example 4 dB, to equalize positive changes of attenuation which occur at higher cable temperatures. Owing to the lower cable attenuation of the transmission cable in the frequency range of the signal transmitted by the A station, it is however possible for the mean transmission level- of the-A station to be smaller than the constant transmission level of the B station approximately by the above-mentioned amount. The transmission level of the A station itself cannot therefore exceed that of the B station in the least favourable case so that the amplifier V2 of both stations can have the same modulation limit.

Channel translators of the carrier frequency system of 7R construction can be used as the channel translators KU1 - KU4 (TE KA DE Fachbericht A3-73-2,O/O2; TrSgerfrequenztechnik I (1975) page 17). These channel translators are commercially available, are produced in large numbers and are therefore not costly.

It is therefore not necessary to either newly develop or specially manufacture the channel translators for the preferred 43625 4-channel carrier frequency system.

The two signal transmission bands 32 - 48 kHz and 92 - 108 kilt are in a frequency range in which temperature dependence of cable attenuation is independent of frequency. A controllable, frequency-independent amplifier can therefore be used in place of apparatus with equalizing characteristics for regulating attenuation fluctuations which are dependent on temperature.

The pilot frequency fp is transmitted in only one direction and is gated at the reception point. This procedure offers ths advantage that the pilot control device, which is controlled on the transmission side as well as on the reception side by the pilot frequency fp is required only at the reception point.

In the exemplified embodiment of the invention described above, the directional carrier frequency fp = 60 kHz is also used as the pilot frequency. The choice of this frequency offers the advantage of dispensing with the need for generating a separate pilot frequency. It is another advantage of the pilot frequency of 60 kHz that it provides a sufficiently large frequency gap from the nearest signal frequencies of 48 kHz and. 92 kHz. Ho separate filter expenditure for a crystal filter or an electromechanical filter is therefore required for blocking and selecting the pilot frequency.

It is possible in practice to make use of the same modules which are employed for the carrier frequency system disclosed in the German Offenlegungsschrift 25 45 492. Only the filter BP3S the directional filter RW and the equalizer in the reception - 12 amplifier unit V5 need to be adapted to the signal transmission hand employed in the preferred method. There is the additional advantage that the corresponding modules of the Z12 system can he used as the directional filter RW and the equalizer of the reception amplifier unit_V5.

Claims (4)

1. CLAIMS:1. A carrier frequency system operating by the equivalent fourwire method for transmitting up to four low-frequency channels over balanced conductors in which a pilot frequency is transmitted in only one direction and is evaluated in one terminal station, the transmission from one terminal station to another terminal station being performed in the frequency range 32 - 48 kHz and transmission in the opposite direction being performed in the frequency range 92 - 108 kHz.

2. A carrier frequency system as claimed in Claim 1, in which up to four low-frequency channels with a carrier frequency of 48 kHz are translated into the frequency band 48 - 52 kHz and are translated by means of the carrier frequencies 84 kHz, 83 kHz, 92 kHz and 96 kHz into a transmission band covering the frequencies 32 - 48 kHz, from whence they are all translated, hy means of the carrier frequency 60 kHz from the aforementioned frequency band into the frequency band 92 - 108 kHz for the purpose of transmission in the opposite direction.

3. A carrier frequency system as claimed in Claim 1 or 2, in which the frequency of 60 kHz, which.is also used as the carrier frequency for translation into a transmission band, is used as pilot frequency.

4. A carrier frequency system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.