GB1580279A - Equivalent four-wire carrier frequency systems - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO EQUIVALENT FOUR-WIRE CARRIER FREQUENCY SYSTEMS (71) We, TE KA DE FELTEN & BR< GUILLEAUME FERNMELDEANLAGEN GM B H, of Thurn-und-Taxis-Strasse 10, 8500 Nurnberg 1, Germany, a German Body Corporate, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described, in and by the following statement: The invention relates to carrier frequency systems. Such a system may be provided with pilot control for up to four low-frequency (LF) channels for operation via balanced two-wire 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 1 2-channel carrier frequency systems (Z12) which operate by the equivalent fourwire 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. Telefunken discloses a 4-channel carrier frequency system in which the range 4-20 kHz is used for transmission from 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 4channel 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 12-48 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 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: Figure 1 illustrates the frequency distribution of a preferred 4-channel carrier frequency system, using the pre-modulation method; and Figures 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 pre-modulation 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 be a speech signal or a data signal.

The formation and lay-out of the transmission bands will be explained hereinbelow by reference to an example with pre-modulation.

Figure 1 shows the frequency distribution for this kind of generation of the two effective transmission bands. 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 of 84 kHz, 88 kHz, 92 kHz and 96 kHz and the resultant lower side bands are combined as signal bands into a band covering the frequencies 32-48 kHz. This band covers the frequencies for transmission from A station to B 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 B station to the A station.

The frequency of 60 kHz is also used as pilot frequency.

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

The translation of the AF (audio frequency) signals into the transmission band of an A station will first be explained. A channel translator (KU1-KU4) 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 Figure 2 shows only the channel translator KU1 in detail, the remaining channel translators KU2-KU4 being merely indicated.

The AF signal which is supplied to a terminal pair F2ao of the channel translator KUl is transferred via a transformer Ul to a low-pass filter TPl which limits the upper spectrum of the AF signal. The AF signal is translated in a modulator M1 by means of a carrier frequency fl = 48 kHz into the premodulation band 48-52 kHz and interfering modulation products are removed by a band pass filter BP1. In a further stage of modulation the signal is translated with a carrier frequency (2 = 84 kHz in a modulator M2 from the premodulation band into the frequency band 32-36 kHz. The carrier frequencies 88 kHz, 92 kHz and 96 kHz are used for the modulator M2 in the additional channel translators KU2-KU4 respectively.Each of the bands 32-36 kHz, 36-40 kHz, 40-44 kHz and 44-48 kHz produced in this stage of modulation is supplied to a low-pass filter TP2 as the frequency band 32-48 kHz. The upper side band 132-148 kHz produced in the modulators M2 of the channel translators KUl-KU4 is blocked and only the lower side band of 32-48 kHz is passed through as the signal band. 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 V4 which is connected in the reception direction. The amplifiers Vl and V4 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 V4.1.The pilot frequencyjp is coupled out by means of a selective pilot amplifier V3 from the output of the amplifier V4. 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 Vl and V4. The advantage of transmission side equalization of changes in the cable attenuation of the A station is that it is possible to completely dispense with pilot control means in the B station.The cable attenuation itself is equalized by means of an amplifier V2 and in the oposite 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 V4 and on the transmission side by means of the amplifier Vl which is controlled in synchronism. The signal, amplified by the amplifiers Vl and V2, is supplied via the low-pass part to the direction filter RW 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 RW of a receiver amplifier unit V5 comprising an amplifier and an equalizer. A frequency-independent gain and slant distortion, opposite to the cable attenuation charateristic, can be adjusted by means of this unit. Optimum equalization is thus possible for the attenuation charateristics of the cables in use. After passing through the reception amplifier unit V5, the transmitted signal is supplied to the amplifier V4 of the pilot control system. The output of the amplifier V4 is connected to the band pass filter BP3 which allows the equalized transmitted signal to pass without obstruction.This signal is subsequently translated in a modulator M3 by means of the carrier frequency f3 = 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 of4 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 Dl of each of the channel translators KU1-KU4 is connected in this case. The demodulator Dl 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 premodulation band 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 in the pre-modulation band is translated into the original AF band by means of a demodulator D2 which operates at a carrier frequency of 48 kHz and by a low-pass filter TP5. After amplification, each of the maximum of four AF signals can be picked off from a terminal pair F2ab of the relevant channel translator through a separate transformer U2.

The channel translators KU 1 -KU4 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 band 32-48 kHz at the output of the low-pass filter TP2 into the two side bands of 12-28 kHz and 92-108 kHz. This spectrum is supplied to the band pass filter BP3 which allows the upper side band 92--108 kHz to pass and blocks the lower side band 12-28 kHz.

The pilot control system PE is omitted in one B station. A pilot coupling stage, whose input is connected to the band pass filter BP3 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 band is supplied with a pilot fre quencyjp of 60 kHz in the above-mentioned pilot coupling stage. The transmitted band which also contains the pilot frequency fp, is amplified by the amplifier V2 and supplied via the high-pass parts of the direction filter RW 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 high-pass part of the line separating filter LW and the low-pass part of 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 abovementioned 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 KUl-KU4 (TE KA DE Fachbericht A3-73.2,0/02; Trigerfrequenz- technik 1(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 4-channel carrier frequency system.

The two signal transmission bands 32 48 kHz and 92---108 kHz are in a frequency range in which temperature dependence of cable attenuation is independent of frequency.

A controllable, frequency-independent amplifier can therefore by used in place of apparatus with equalizing characteristics for regulating attenuation fluctuations which are dependent on temperature. The pilot frequency jp is transmitted in only one direction and is gated at the reception point. This procedure offers the advantage that the pilot control device, which is controlled on the transmission side as well as on the reception side by the pilot frequency W) is required only at the reception point.

In the exemplified embodiment of the invention described above, the directional carrier frequencyj3 = 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. No 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 43 492. Only the filter BP3, the directional filter RW and the equalizer in the reception amplifier unit VS need to be adapted to the signal transmission band employed in the preferred method. There is the additional advantage that the corresponding modules of the Z12 system can be used as the directional filter RW and the equalizer of the reception amplifier unit V5.

WHAT WE CLAIM IS: 1. A carrier frequency system operating by the equivalent four-wire 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, 88 kHz, 92 kHz and 96 kHz into a transmission band covering the frequencies 32-48 kHz, from whence they are all translated by means of the carrier frequency 60 kHz from the aforementioned frequency band into the frequency band 92-108kHz 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.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. 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 43 492. Only the filter BP3, the directional filter RW and the equalizer in the reception amplifier unit VS need to be adapted to the signal transmission band employed in the preferred method. There is the additional advantage that the corresponding modules of the Z12 system can be used as the directional filter RW and the equalizer of the reception amplifier unit V5. WHAT WE CLAIM IS:

1. A carrier frequency system operating by the equivalent four-wire 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, 88 kHz, 92 kHz and 96 kHz into a transmission band covering the frequencies 32-48 kHz, from whence they are all translated by means of the carrier frequency 60 kHz from the aforementioned frequency band into the frequency band 92-108kHz 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.