AU677330B2

AU677330B2 - AGC for broadband transmitter

Info

Publication number: AU677330B2
Application number: AU67400/94A
Authority: AU
Inventors: Klaus Braun; Ulrich Steigenberger
Original assignee: Alcatel NV
Current assignee: Alcatel Lucent NV
Priority date: 1993-07-26
Filing date: 1994-07-13
Publication date: 1997-04-17
Anticipated expiration: 2014-07-13
Also published as: EP0637129A2; EP0637129A3; DE59406962D1; NZ260963A; EP0637129B1; ES2124818T3; DE4325020A1; AU6740094A

Description

P/00/011 28/5/91 Regulation 3.2

AUSTRALIA

Patents Act 1990 oooo 0 ooze oeee o o o o ooo *ooe *oo

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title: "AGC FOR BROADBAND TRANSMITTER" The following statement is a full description of this invention, including the best method of performing it known to us:- This invention relates to an automatic gain control for a broadband transmitter of a television distribution system.

These type of broadband transmitters are already known from existing optical television distribution systems. Such broadband transmitters are implemented for example, in higher-level amplifying stations where various radio and television channels coming from different carriers are combined and processed. Carriers can be, e.g. supra-regional long-distance traffic, co-axial or fibre-optic systems, as well as regional radio or television stations, or terrestrial Il.Q transmitters. The combined channels are injected e.g. into an optical or co-axial cable television distribution network as a combined broadband signal. Because of the plurality of the television and radio channels coming from the various signal sources, level fluctuations are unavoidable in the combined sum signal which is transmitted from the amplifying station. It is also possible that the number of transmitted channels can change from amplifying station to S amplifying station, or over a period of time in a particular amplifying station.

However, with a television distribution system a constant channel level is expected at its electrical input, and the maximum modulation of a channel is approximately limited by the product of the root of the number of channels with the corresponding channel level. I.e. the television channels can be better modulated with a lesser number of available channels and can therefore be transmitted with a better transmission quality than can be achieved when a large number of channels is to be transmitted. To guarantee a transmission of constant high quality a control circuit which allows for the number of television and radio channels is provided in the form of a sum load control. This sum load control maintains the modulation factor constant, i.e. it is always capable of triggering the broadband transmitter at optimum level.

With a plurality of cable television distribution systems, the receivers at the output of the cable television distribution system are amplitude controlled with a pilot signal contained in the receiver signal. This pilot signal is generally injected in the transmitter of the system at a carrier wave length of A 1300 nm, as for example in the existing optical television distribution system of the Deutsche Bundespost Telekom. With the known system, the pilot signal is injected after combining the various radio and television channels in the transmitter, namely before its sum load control.

This always results in the fact that the one pilot signal in the case of an optical transmitter, or the pilot signals in the case of an electrical transmitter does/do not have a fixed relationship to the number of television channels and their corresponding channel levels. The pilot signals can therefore only be used reliably to compensate transmission-path-dependent attenuations but not for an optimum modulation of the channels in the receiver. For example, when the level of the transmission signal is too high at the input of the television distribution system, the transmitter is protected from overmodulation by the sum load control, but the receiver can be over-modulated. On the other hand, too low channel levels in the transmitter lead to a too small output level in the receiver, which results in a poor image quality.

It is therefore an object of the present invention to create an automatic gain control of a broadband transmitter for a television distribution system for the transmission of a plurality of transmission channels, where the number of transmission channels is variable, the channels may have different levels, and where, in the television distribution system, the receivers are amplitude- S" controlled at a constant modulation factor, via a pilot signal.

According to the invention, there is provided an automatic gain control for a broadband transmitter of a television distribution system wherein a *o***plurality of transmission channels are combined into a broadband transmission signal, comprising an amplitude control, having a first coupling device for inserting a pilot signal into the transmission signal and a sum load control for achieving a constant modulation factor of the transmission signal, wherein a facility is provided for selecting a reference signal having a representative amplitude from the transmission signal, and wherein a device is provided which brings said reference signal and the pilot signal into a predetermined relationship.

In order that the invention may be readily carried into effect, i-embodiments thereof will now be described in relation to the accompanying drawings, in which: Figure 1 shows schematic display of a cable television distribution system, Figure 2 shows schematic display of a broadband transmitter of an optical cable television distribution system, Figure 3 shows a first embodiment of an automatic gain control for the transmitter of an optical cable television distribution system, and Figure 4 shows a second embodiment of an automatic gain control ~for the transmitter of a cable television distribution system.

Figure 1 shows an optical cable television system KFVS schematically and representative for other television distribution systems with other transmission media. The cable television distribution system KFVS shows a head station KS with an input E and outputs A 1 An. The various radio and television channels are fed individually or in groups via the input E, are combined to a transmission signal which is to be distributed and then transmitted via the outputs A, An of head station KS. In Figure 1, only one line L 1 is shown as an example, through which the transmission signal is fed to a distribution station VS, where for example, it is distributed to 32 different .2 forefront devices VFE or directly to individual buildings. In Figure 1, forefront S: device VFE 1 is shown representative for other forefront devices. Its final connection is to the subscribers T 1 Ti.

The concern of the invention is the automatic gain control AGC of a transmitter S which is arranged in the head station KS. A signal containing the various radio and television channels is fed to transmitter S. In the automatic gain control AGC this signal is injected with a pilot signal and then, via a further signal preparation SAU, it is passed on to a laser L as a modulation signal.

Other general measures known to the expert, such as distortion compensation, pre-emphasis, etc. are also conducted in the signal preparation SAU.

A first practical example of an automatic gain control AGC 1 according to the invention is shown in Figure 3. The gain control AGC 1 comprises a channel amplitude control KAR 1 and a sum load control SLR 1 arranged in series. The channel amplitude control KAR, has a first variable attenuator VD, which is controlled via a feedback loop RKS, as the input element for the combined transmission signal. The variable attenuator VD, is followed by a coupling device PE, for inserting the pilot signal, then by a broadband amplifier BK,, a coupling device RA, for extracting a signal for the feedback loop RKS, and an equaliser EZ 1 The feedback loop RKS, first comprises a high-frequency amplifier HFV, followed by a bandpass filter BF,, which leads to a rectifier GLR,. The rectifier GLR, leads to a controller REG,, which then supplies the control signal for the variable attenuator VD,. The output of equaliser EZ, 0 simultaneously represents the output of channel amplitude control KAR, and is connected to the input member of the sum load control SLR 1 i.e. a broadband amplifier BK 2 via a quadruple-splitter VFS 1 The broadband amplifier BK 2 is connected to the output of the sum load control SLR,, which also represents the output of the gain control AGC,, via an equaliser EZ 2 a second variable attenuator VD 2 a further broadband amplifier BK 3 and a coupling device RA 2 for extracting a signal for a broadband feedback loop RKS 2 The feedback loop

RKS

2 comprises a high-frequency amplifier HFV 2 followed by a rectifier GLR 2 and a controller REG 2 designed as a differential amplifier. Controller REG 2 supplies the control signal for the variable attenuator VD 2 .2Q For a better understanding of the invention, its function method is S explained with the help of some numerical value examples. In Figure 2 the points a-k have been entered to denote the measuring points for the corresponding amplitude values. It is now to be presumed that the combined transmission signal shows a level of 79 dB/pV at point a. In the variable attenuator VD 1 the value is controlled to 74 dBpV (point b) independent of the value in point a. The pilot signal shows a value of 100 dBpV at point c, with a pilot frequency of v 80.15 MHz. It is inserted in the transmission signal in the coupling device PE, for inserting the pilot signal. Due to insertion attenuation, the transmission signal shows a level of 73 dBpV and the pilot signal a level of 69 dBpV at point d. In the broadband amplifier BK,, the transmission signal and the pilot signal are amplified to 90 dBpV and 86 dBpV respectively, before it is fed to the coupling device RA 1 for extracting a signal and the equaliser EZ 1 At the input of the quadruple-splitter, the point f, the levels are still 87, resp. 83 dBiJV, at the output, the point g, only 79, rsp. dBiV.

The advantage of this channel amplitude control is that the maximum level of the transmission signal has a fixed predetermined relationship to the pilot signal. In this practical example, a constant difference of 4 dB is implemented. The relationship is initially set in controller REG "hich then determines the level of the transmission signal via the variable ,'ienuator VD 1 As a measure for the level of the transmission signal, a signal part is used as a reference signal whose amplitude is constant and known and is constantly transmitted. The reference signal could be a pilot signal injected into one of the channels to be transmitted and purposely designated to control the channel amplitude. If such a pilot signal is not available, it is advantageous to use another suitable part of the transmission signal, e.g. the frequency modulated 1.5 part, which is then the VHF band with a constant amplitude. The reference signal is selected by the bandpass filter BF, as the reference signal for control purposes.

i o o To allow for the fact that the transmission signal may contain a different number of transmission channels, and that with optimum utilisation of the bandwidth, the modulation of the amplitude depends on the number of channels to be transmitted, the transmission signal is fed to the sum load control SLR 1 after passing through the channel amplitude control KAR 1 A broadband amplifier BK 2 is arranged at the input of the sum load control SLR 1 said amplifier increasing the level at point g from 79 dBIV for the transmission signal and 75 dB/jV for the pilot signal to the values of 96, resp. 92 dB/pV at point h.

In the sum load control SLR 1 the channel allocation, i.e. the number of channels available in the transmission signal, is taken into account. For the example we are now assuming three cases, each with a different number of channels to be transmitted. Case 1 10 channels, case 2 25 channels and case 3 30 channels.

After passing through the broadband amplifier BK 2 all three cases show the same value of 97 dBpV for the transmission signal and 92 dB/V for the pilot signal. As the second feedback loop RKS 2 has the same design as the first feedback loop RKS 1 and is also arranged in the same manner, it basically functions in the same way. The only difference is that the second feedback loop RKS 2 does not comprise a bandpass filter, which means that to control the variable attenuator VD2, the complete transmission signal, i.e. the sum of all transmitted channels, is used. Correspondingly, the variable attenuator VD 2 attenuates the transmission signal harder in case 3 than in case 2 or 1. The values for the transmission signal at point i are 86 dB/pV in case 1, 85 dB/pV in case 2 and 84 dBp/V in case 3. The values for the pilot signal are 4 dB lower in all three cases. At the output of sum load control SLR 1 there is a final level for the transmissions signals, after another amplification of the signals in point k, of 101 dB/JV in case 1, of 100 dB/jV in case 2 and 99 dBpV for case 3. The levels of the pilot signals are again 4dB lower than the corresponding transmission signal levels.

The automatic gain control AGC therefore fulfils both requirements, the balance of input signal control fluctuations as well as achievement of a constant modulation factor, where the distance of the pilot signal to the transmission channels remains constant.

A second practical example of an automatic gain control AGC 2 comprises a sum load control SLR 2 with a pilot control PS in parallel, as channel amplitude control and this is shown in Figure 4. Elements which are the same as those already used in the first practical example are designated with. the same reference symbols. A broadband amplifier BK 2 represents the input for the transmission signal. It is connected to a coupling device PE 2 for inserting the pilot signal via an equaliser EZ 2 The coupling device PE 2 for inserting the pilot signal is led to a variable attenuator VD 2 said attenuator being connected to the output of the circuit via a first broadband amplifier BK 1 and a coupling device RA 2 for extracting a signal. A narrow-band feedback loop RKS 3 is connected to the coupling device PE 2 for inserting the pilot signal via the coupling device RA 2 for extracting the signal. The narrow-band feedback loop

RKS

3 comprises the pilot control PS. The narrow-band feedback loop RKS 3 is partly designed, like the narrow-band feedback loop RKS 1 for channel amplitude control of the first practical example. It contains a high-frequency amplifier 8

HFV

1 which is followed by a bandpass filter BF 1 which is led to the control input of a third variable attenuator VD 3 via a rectifier GLR 1 followed by a direct current amplifier DCV. At the signal input of the variable attenuator VD 3 a constant pilot signal of 100 dBpV is available which, controlled through the narrow-band feedback loop RKS 3 is now fed to the coupling device PE 2 for inserting the pilot signal.

A second feedback loop RKS 2 is fed from the coupling device RA 2 for extracting a signal to the control input of the variable attenuator VD 2 The second feedback loop RKS 2 comprises a high-frequency amplifier HFV 2 a rectifier GLR 2 and a control device REG 2 It is designed like the second oooo feedback loop of the first practical example and represents a component of the sum load control SLR, just like in the first practical example.

With regard to the function mode of the sum load control of the automatic gain control AGC 2 we refer to the first practical example. The channel amplitude control designed as the pilot control PS fulfils the same task as the channel amplitude control of the first practical example, i.e. the establishment of a fixed relationship between a reference signal contained in the incoming transmission signal and a pilot signal inserted in the automatic gain control. In the first practical example the pilot signal is maintained constant and 2 the reference signal is controlled to a predetermined relationship to the pilot signal. In the second practical example a level control of the pilot signal is conducted dependent on the level of the reference signal while maintaining the transmission signal level, e.g such that the modulated level of the reference signal shows 99 dB/V and that of the pilot signal 95 dB/V. This corresponds with the same distance of 4 dB which is already known from the first practical example. The actual modulation of amplitude deviations of the input signal is taken over by the sum load control via the variable attenuator VD 2 In a third practical example (not shown), the automatic gain control AGC shows a similar structure to the one in the second practical example. The only difference is that the pilot control PS does not have a narrow-band feedback loop RKS, but is connected to a microprocessor contained in transmitter S. The microprocessor is connected on the one side to a central processing unit of the

L

head station, which knows the number of channels to be transmitted and which forwards these to the microprocessor. The microprocessor now controls the level of the pilot signal, dependent on the number of channels. This can be done in such a way that the pilot level can be controlled e.g. in preset steps at a distance of 1/2 or 1/4 dB/V. As the pilot level is controlled dependent on the number of channels, the coupling device PE 2 for inserting the pilot signal is arranged after the variable attenuator.

The arrangement and number of amplifiers, equalisers and splitters shown in the practical examples is carried out in a manner and method known to the expert for each individual case. The components used can also be arranged in another sequence or combination. For example, the arrangement of the quadruple-splitter in the first practical example can be dispensed with, or a quadruple-splitter can be implemented between the equaliser EZ 2 and the coupling device PE 2 for inserting the pilot signal in the second practical example, which of course necessitates a different arrangement or dimensioning of the broadband amplifier. In the same manner, the bandpass filter BS1 and the high frequency amplifier HFV 1 can be arranged in the reverse sequence. The invention was described for an optical cable television distribution system. It is also suitable for use in cable television distribution systems with coaxial cables or for cable television distribution systems with radio-paths.

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Claims

1. An automatic gain control for a broadband transmitter of a television distribution system wherein a plurality of transmission channels are combined into a broadband transmission signal, comprising an amplitude control means, having a first coupling device for inserting a pilot signal into the transmission signal and a sum load control for achieving a constant modulation factor of the transmission signal, wherein a facility is provided for selecting a reference signal having a representative amplitude from the transmission signal, and wherein a device is provided which brings said reference signal and the pilot signal into a predetermined relationship. S

2. An automatic gain control as claimed in claim 1, wherein the first coupling device for inserting the pilot signal is contained in the channel Si° amplitude control, that the channel amplitude control comprises a variable attenuator and a narrow-band feedback loop via which the variable attenuator is controlled, and wherein the transmission signal is fed through the variable attenuator to the said first coupling device for inserting the pilot signal and to the coupling device for extracting a signal for the feedback loop.

3. An automatic gain control as claimed in claim 1, wherein the channel amplitude control is designed as a pilot level control, that the coupling device for inserting the pilot signal being followed by a coupling device for extracting a signal for a narrow-band feedback loop in which the reference signal is selected with which the amplitude of the pilot signal is controlled via a variable attenuator, and wherein the sum load control is connected in parallel with the pilot level control.

4. An automatic gain control as claimed in claim 2 or claim 3, wherein the narrow-band feedback loop includes a bandpass filter for audio signals. An automatic gain control as claimed in claim 1, wherein the sum load control comprises a variable attenuator followed by a broadband amplifier and a second coupling device for extracting a signal, the variable attenuator being controlled via a feedback loop connected to said second coupling device. i An automatic gain control for a broadband transmitter of a television 11 distribution system wherein a plurality of transmission channels are combined into a broadband transmission signal, comprising an amplitude control means having a coupling device for inserting a pilot signal into the transmission signal, and a sum load control for achieving a constant modulation factor of the transmission signal, comprising a pilot level control, the level of the pilot signal being controlled via a variable attenuator, with the controlled variable being a channel-loading-dependent quantity which can be fed to the pilot level control through a computer.

7. An automatic gain control as claimed in any one of the preceding claims, wherein the television distribution system is a cable television distribution S: system.

8. An automatic gain control for the broadband transmitter of a television distribution system wherein a plurality of transmission channels are combined 15 into a broadband transmission signal, substantially as herein described with reference to Figures 1 3 of the accompanying drawings. DATED THIS TWELFTH DAY OF FEBRUARY 1997 ALCATEL N.V I 'T 'NT O I, I SUMMARY To control the channel amplitude in tihe receiver of a television distribution system, the receivers are controlled by a pilot signal. A sum load control is used in the transmitter to achieve a constant modulation factor. The pilot signal is injected into the gain control (AGC) in known systems. Regardless of wether sum load control is effected first or the pilot signal injected, such a television distribution system has its disadvantages because the pilot signal does not have a fixed relationship to the signal amplitude of the incoming channels, or the modulation factor cannot be kept constant. It is the essence of the invention to bring a reference signal selected from 0 a transmission signal into a fixed relationship with the pilot signal. This is done in an advantageous way via a feedback loop (RKS 1 RKS 3 via which either the channel amplitude can be controlled to a preset value or a used pilot signal can be controlled to a preset value. (Figure 3) 060 a0