GB2303278A

GB2303278A - Adaptive angle demodulation systems

Info

Publication number: GB2303278A
Application number: GB9514126A
Authority: GB
Inventors: Remo Giovanni Andrea Marzolini
Original assignee: Individual
Current assignee: Individual
Priority date: 1995-07-11
Filing date: 1995-07-11
Publication date: 1997-02-12
Anticipated expiration: 2015-07-11
Also published as: GB2303278B; GB9514126D0

Abstract

A threshold extension system 4 for use in angle demodulation systems to minimise the effect of impulse noise, usually referred to as "clicks", that normally appear in the demodulated output signal 10 for weak input carrier signals 1, includes an error decoder 7 for determining the errors occurring in the digital signal structure, an error processing circuit 9 for deriving control signals 3 from the error signals, and a means 6 in the angle demodulator 2 for altering its characteristics in response to the control signal. The input signal is a TV signal, with part or all digital structure, and the control signals affect bandwidth e.g. via an adaptive bandpass filter.

Description

Field of the Invention The present invention relates generally to angle modulated demodulators, and more particularly to a threshold extension system for improving the operating performance of such demodulators for input signals having relatively low signal-to-noise ratios.

Backgound Ot The Invention Angle modulated receivers are sometimes employed to receive extremely weak incoming angle modulated radio signals and to recover therefrom the intelligence or information originally transmitted from a distant angle modulated transmitter.

It is therefore an objective of the present invention to provide an improved threshold extension device for improving the performance of angle modulated receivers in receiving relatively weak signals. A good example of this occurs in the case of direct broadcast by satellite systems (DBS) where satellites transmit, via a down-link communication system, directly to domestic television receivers in the home. The transmitter in orbit is at a great distance from the terrestrial domestic television receivers and the space and power limitations aboard the satellite dictate that the transmitter be of relatively low power.Since the low transmitter power and the great distance separating the satellite from the domestic receiver combine to cause the television signal received to be extremely weak, the domestic television receiver must be capable of processing the signal without superimposing thereon any appreciable amount of electrical noise if the transmitted intelligence is to be recovered with a minimum of degradation.

One of the factors having a substantial effect on the operational capability of the domestic television receiver is the operating performance factor of the angle demodulator used in the receiver. As the received television signal from the satellite becomes weaker, a threshold point is reached below which the demodulator usually begins to itself introduce electrical noise of noticeable and objectionable character.

This electrical noise is of several forms, termed clicks, false clicks and doublets, each with differing characteristics. In addition modulation excursions can occur that make the specific identification of these noise components difficult. Normally the most objectionable of these noise components are the click type noise impulses that are generated by the demodulation process itself, and that are the result of the interaction between randomly varying electrical noise and the signal input to the demodulator.

More particularly, when operating below a certain threshold level the signal amplitude will often be of the same order of magnitude as some of the random noise fluctuations within the demodulator and as a result, the noise components will combine with the signal components and produce a 2R radian phase excursion in the demodulator output and an undesirable impulse will be impressed upon the output of the demodulator.

Such click impulses are particularly undesirable in that they have substantial low frequency content in the information bandwidth that is not suppressed by the conventional demodulator output filters. A major difficulty exists in discriminating between clicks that cause problems (i.e. those that have a low frequency component in the wanted information passband) and those false clicks, doublets and modulation excursions that do not.

Prior art approaches towards solving the problem of click impulses have been many, and not totally successfiil in television applications with the rapid change of picture scene from say a static scene to a rapidly changing scene, giving what has been termed the static demodulator threshold and dynamic demodulator threshold. One approach has been to provide means responsive to the demodulator output for detecting the presence of the click impulses and then removing the click impulses from the output signal. Such an approach does not completely solve the problem since any click impulse that does not reach the a predetermined detection level is ignored, as well as the failure to differentiate between those noise impulses that cause problems and those that do not.Other means such as the Phase Lock Loop demodulator and the Frequency Modulation with Feedback (FMFB) demodulator have attempted to solve the click problem by either narrowing the bandwidth or by effectively reducing the deviation of the signal.

These prior art approaches show that the basic mechanisms for achieving threshold extension exist in a number of circuit approaches. These for a constant signal structure do exhibit a modicum ofthreshold extension. The fundamental problem is that dynamic or varying signal structures such as those that occur with rapid changing television pictures, do not maintain this threshold improvement. Because of the inherent nonlinear nature of angle modulation, it is currently considered very difficult, if not impractical to design a threshold extension demodulator that will fimstion for all values of input signal-to-noise ratio and for all values of modulation index by just using the characteristics of input signal envelope amplitude and the demodulated signal.

My invention described below makes use of an additional parameter, namely the information content ofthe signal resulting in various solutions that are described below and that will operate over a wide range of input signal-to-noise ratio and modulation indexes. The information characteristic used in my invention is the increase in error rate that occurs at threshold with those digital signal structures contained in part or all, of the signal structure. This error rate information characteristic is used to alter the configuration and/or the characteristics of the demodulator at the onset of threshold to delay said onset.

Summary of the oresent Invention According to the present invention there is provided a threshold extension demodulation apparatus for use in a receiver, comprising means for extracting the instantaneous frequency information from input carrier; means for extracting information in error from said instantaneous frequency information; means for processing said information in error for providing control signals; means responsive to said control signals for controlling said means for extracting the instantaneous frequency information from input carrier.

It is therefore an object of the present invention to provide an improved threshold extension system for improving the performance of angle modulated receivers in receiving relatively weak signals.

Another object of the present invention is to provide a new and improved system for extending the normal operating range of angle demodulators to include very low values of input signal-to-noise ratios.

An important advantage of the present invention is to provide a new and improved threshold extension system that will function for a dynamic signal structure such as those occurring rapid changing television pictures.

Another object of my invention is to minimise those clicks that occur at threshold and that result in errors in those digital structures contained within the signal modulation.

Still another object of the present invention is to provide a new and improved threshold extension system that uses information within the signal structure to identity that the demodulator is operating in the threshold region.

Still another object of the present invention is to provide a new and improved threshold extension system that uses information within the signal structure to provide a feedback signal to alter the structure and/or characteristics of the demodulator such as to minimise the corruption ofthat information.

A further important advantage of the present invention is to improve the stability of the threshold demodulation apparatus by using a feedback function that is independent of the demodulated output amplitude and phase characteristic. The feedback function is the means for extracting information in error from said instantaneous frequency information and for processing said information in error for providing control signals.

Still another object of the present invention is to provide a new and improved threshold extension system that uses information within the signal structure to provide a feedback signal to alter the structure and/or characteristics of the demodulator such as to match the bandwidth of the demodulator to the bandwidth of the input carrier such as to minimise the corruption of that information.

The foregoing and other objects, features and advantages of the invention will be apparent from the following description of a preferred embodiment of the invention that is illustrated in the several figures of the accompanying drawing.

Brief Description of the Diiwins Figure 1 is a block diagram of a threshold extension demodulation apparatus in accordance with the present invention.

Figure 2 is a block diagram of a threshold extension demodulation apparatus in accordance with the present invention and implemented as a tracking filter demodulator.

Figure 3 is a block diagram of a threshold extension demodulation apparatus incorporating a switched demodulator configuration and which is useful in illustrating of my invention.

Figure 4 is a block diagram of a threshold extension demodulation apparatus incorporating an alternative switched demodulator configuration and which is usefUl in illustrating of my invention..

Figure 5 is a block diagram of a threshold extension demodulation apparatus incorporating an alternative switched demodulator output configuration and which is useftil in illustrating of my invention.

Figure 6 is a block diagram of a threshold extension demodulation apparatus incorporating Teletext derived error feedback and which is useful in illustrating of my invention.

Figure 7 is a block diagram of a threshold extension demodulation apparatus incorporating MAC format sync pattern error derived feedback and which is usefil in illustrating of my invention.

Figure 8 is a block diagram of a threshold extension demodulation apparatus incorporating an all-digital television error feedback and which is useflil in illustrating of my invention Figure 9 is a block diagram of a threshold extension demodulation apparatus incorporating a frequency modulation with feedback demodulator and which is useful in illustrating of my invention.

Figure 10 is a block diagram of a threshold extension demodulation apparatus incorporating a frequency lock loop demodulator and which is useflil in illustrating of my invention.

Figure 11 is a block diagram of a threshold extension demodulation apparatus incorporating a phase lock loop demodulator and which is useflil in illustrating of my invention.

Figure 12 is a block diagram of a threshold extension demodulation apparatus incorporating a phase lock loop demodulator with AGC and which is useful in illustrating of my invention.

Detailed descotion of the Preferred embodiments Referring now to figure 1 of the drawing, a block diagram of a portion of an angle modulated receiver is shown for receiving the RF signal at input terminal 1, the input to the threshold extension demodulation apparatus 4 in accordance with the present invention. The principle components of my invention shown in the threshold extension demodulation apparatus 4 of figure 1, include an angle modulation discriminator arrangement 2 that may or may not include a liming device, and whose input is I. The demodulated output 5 is connected to an error decoder circuit 7 whose function is to detect the errors that occur at low input signal-to-noise ratio in the digital structure that is contained in part or in whole in the signal.The output measure of error 8 in the error decoder 7 is connected 8 to the error processor 9. The error processor 9 measures the errors that are occurring and produces a plurality of control signals 3 that is connected to the angle modulation discriminator arrangement 2 and, as is explained below alters the structure, including the characteristics, of the discriminator arrangement 2 in such a manner as to improve the output signal-to-noise ratio at output 10 and hence the error measured by the decoder 7. It is a characteristic of my invention that those clicks that occur at threshold result in errors in those digital structures contained within the signal modulation. As will be explained below the error processor 9 object is to minimise the error rate that occurs in the signal.The discriminator arrangement 2 has a control input 3 whose fiction is to modify the structure or the characteristics ofthe basic discriminator 6 that may or may not contain a limiting circuit. As will be illustrated below the plurality of control signals at control input 3 can be used to control the input level to input 1, the bandwidth of the discriminator arrangement 2 and even change the type of discinator 6 as well as other modifications to the discriminator arrangement 2 structure or characteristics. An advantage of my invention is that by using a feedback function 3 that is independent of the demodulated output 5 amplitude and phase characteristic improves the stability of the threshold extension demodulation apparatus 4.Although figure 1 shows the output of the threshold extension demodulation apparatus 4 is taken frorn terminal 10, the output of the threshold extension demodulation apparatus 4 may also be obtained from terminal 5.

Figure 2 shows a simplified block diagram of my invention implemented as a tracking filter demodulator for demodulating a television signal that for illustrative purposes is assumed to be a PAL format signal. Figure 2 shows the block diagram of a portion of a frequency modulated receiver for receiving a signal at input terminal 21, the input to the threshold extension demodulator apparatus 20 in accordance with the preserrt invention. The principle components of my invention shown in 20 include an adaptive bandpass filter 22 whose bandwidth is determined by the control signal 31. The output of the adaptive bandpass filter 22 is connected 23 to a limiter 24 that may or not be included in the system. The output 25 of the limiter 24 is connected to the frequency modulation discriminator 6. The frequency demodulated output 26 is connected to a video processing circuit 27 that conditions the signal to produce a video output 28.

This output 28 is connected to a digital content extraction circuit 29 that extracts that portion of the video signal that is digital in content. The output 30 of this digital content extraction circuit 29 is applied to the error decoder 7 that determines the number of bits in error in the error coding structure portion of the digital content of the video signal. This continuous measure of error 8 is fed to an error processor 9 that processes the error rate and depending on the error magnitude produces control signals 31 that modify the characteristics of the adaptive bandpass filter 22. The error processor 9 can be arranged to follow a variety of strategies in controlling the characteristics of the adaptive bandpass filter 22. Assume for the purposes of illustration one such strategy. Assume a condition where there is a high signal-to-noise ratio at the input 21.This will produce a negligible error rate in the demodulated signal digital structure. In this condition the error processor 9 will cause the adaptive bandpass filter 22 bandwidth to be set to a very wide value such that the bandwidth of the system is determined by the preceding part of the receiver to the threshold extension demodulation apparatus 20. As the input 21 signal-to-noise ratio reduces to a low value and the error rate in the digital portion of the signal increases, the error processor 9 control signal 31 will commence to reduce the bandwidth of the adaptive bandpass filter 22 resulting in a reduction in error rate as the sidebands ofthe signal are attenuated. This reduction in error rate will continue until the truncation of the sidebands results in distortion of the signal and the error rate increases.In parallel a fiuther control signal 31 could be used to adjust the centre frequency of the adaptive bandpass filter 22. This would follow the centre frequency component of the signal and also seek to minimise the error. Note, yet an alternative strategy is, in conditions of high input signal-to-noise ratios, for the adaptive bandpass filter 22 to be shorted out of circuit instead of its bandwidth being set to a wide value.

It is well known to those skilled in the art that various types of discriminators differ in their performance in the signal threshold region; some offering a superior performance to other types. Figure 3 shows a simplified diagram of my invention where the error control signal is used to change the configuration of the threshold extension demodulation apparatus, namely by replacing one type of discriminator by another depending upon the level of the input signal-to-noise and hence the demodulated output error rate. In the following example shown in figure 3 a conventional limiter discriminator circuit is replaced by a envelope multiplication discriminator (see J.H.

Roberts "Angle Mohrlation" Peter Peregrinus Ltd. 1977). The envelope multiplication discriminator has a superior characteristic at an input signal-to-noise ratio below the threshold of a conventional limiter discriminator and thus provides a superior performance and hence reduces the bits in error in the demodulated output. Although in the following example, shown in figure 3, a conventional limiter discriminator 44 and an envelope multiplication discriminator 45 are used, a wide range of discriminators types could be used in my invention.Figure 3 shows the block diagram of a portion of a frequency modulated receiver for receiving a modulated RF signal and applying it to the input terminal 41 of the threshold extension demodulation apparatus 40 in accordance with the present invention The principle components of my invention shown in 40 include a changeover switch 42, that in conditions of high signal-to-noise ratio and hence low data error rate in the demodulated signal SO, normally connects the input signal 41 to the input 43 of the conventional limiter discriminator 44. The output 47 of the conventional limiter discriminator 44 is connected to one input terminal 47 of the output changeover switch 46.In the signal condition of high signal-to-noise ratio and hence low data error rate in the demodulated signal So, the changeover switch 46 connects the input line 47 to the output 50. The output 50 of the changeover switch 46 is connected to the error decoder 7. This error decoder 7 determines the number of bits in error in the error coding structure portion of the data digital content output 50. The output 52 of the of the error decoder 7 is the demodulated output of the threshold extension demodulator 40. The output 8 of the error decoder 7 is applied to the error processor circuit 9.The error processor 9 is arranged to have a strategy such that when the number of bits in error exceed a preset value it produces an output St that causes the input changeover switch 42 and the output changeover switch 46 to operate and change input 41 from output 43 to output 48 and change output 50 from input 47 to input 49 respectively. This action introduces the envelope multiplication demodulator 45 into circuit in place of the conventional limiter discriminator 44. The envelope multiplication demodulator 45 has a superior characteristic at signal-to-noise ratio below the threshold of a conventional limiter discriminator 44 and thus provides a superior performance and hence reduces the bits in error in the output 52. Various strategies can be implemented by those skilled in the art to operate the system to select the lowest error rate in the output 52.Although the illustration shown in figure 3 uses two types of discriminators, those skilled in the art can clearly see that my invention could use many discriminators each with a different performance characteristic in various parts of the signal threshold region and that the switching scheme could be extended to select any of these demodulators, that has the lowest error rate, in response to a control signal.

Figure 4 shows a simplified diagram of my invention and is an alternative to that shown in figure 3 where the error control signal is used to change the configuration of the threshold extension demodulation apparatus. Figure 4 shows the block diagram of a portion of a frequency modulated receiver for receiving a modulated RF signal and applying it to the input terminal 61 of the threshold extension demodulation apparatus 60 in accordance with the present invention. The principle components of my invention shown in 60 include an input changeover switch 62, that in conditions of high signalto-noise ratio and hence low data error rate in the demodulated signal 65, normally connects the RF signal from input 61 to the input 63 of the conventional limiter discriminator 64.The output of the conventional limiter discriminator 64 is connected to the input terminal 65 ofthe error decoder 66. The output 67 of the error decoder is connected to one input terminal of the changeover switch 68. In conditions of high signal-to-noise ratio and hence low data error rate in the demodulated signal 65 the output 67 of the error decoder 66 is normally connected to the output 69 of the output changeover switch 68. The output 69 of the of the changeover switch 68 is the demodulated output of the threshold extension demodulation apparatus 60. The error decoder 66 determines the number of bits in error in the error coding structure portion of the data digital content of the demodulated signal on output 65. The output 75 of the error decoder 66 is applied to one input of the error processor 72. The error processor 72 can be arranged to have a variety of control strategies. For this illustration the error processor 72 is arranged to have a strategy such that when the number of bits exceed a predetermined value it produces an output 71 that causes the input changeover switch 62 and the output changeover switch 68 to operate and connect input 61 to output 79 and connect input 70 to output 69 respectively. This action introduces the envelope multiplication discriminator 77 into circuit in place of the conventional limiter di discriminator ator 64. The envelope multiplication discriminator 77 has a superior characteristic at signal-to-noise ratio below the threshold of a conventional limiter discriminator 64 (see J.H.Roberts "Angle MatirlbtioPr" Peter Peregrinus Ltd. 1977) and thus provides a superior performance and hence reduces the bits in error in the output 69. In this illustration the output 76 of the envelope multiplication discriminator 77 is connected to the input of a second error decoder 73 whose output 70 is applied to an input of the changeover switch 68. This input 70 is connected to the output 69 ofthe threshold extension demodulation apparatus 60.The error processor 72 is arranged to have a strategy such that when the number of bits in error exceed a predetermined value it produces an output 71 that causes the input changeover switch 62 and the output changeover switch 68 to operate and connect input 61 to output 63 and connect input 62 to output 69 respectively to determine if the number of bits in error in the demodulated signal 65 is lower than those on 76. If the bits in error are not lower then the changeover switches 62 and 68 revert back to their previous setting for the envelope multiplication discriminator 77. Various strategies can be implemented in error processor 72 by those skilled in the art to operate the system to select the lowest error rate in the output, including those conditions where the error rate from each discriminator is equal.

Figure 5 shows a simplified dam of my invention and is yet another alternative to that shown in figures 3 and 4 where the error control signal is used to change the configuration of the threshold extension demodulation apparatus. Figure 5 shows the block diagram of a portion of a frequency modulated receiver for receiving a modulated RF signal and applying it to the input terminal 81 of the threshold extension demodulation apparatus 80 in accordance with the present invention This illustration makes use of only one changeover switch 94 at the error decoders 86 and 89 outputs 92 and 93 respectively. The inputs 81 to discriminators 82 and 83 are in parallel.

Although a signal loss is introduced by connecting the inputs 81 to the discriminators 82 and 83 in parallel, this approach offers advantages in allowing the error rate to be continuously monitored. The principle components of my invention shown in SO include a conventional limiter discriminator 83 and an envelope multiplication discriminator 82, the inputs 81 of which are connected in parallel. Hence when a signal is obtained from a stage of the receiver previous to the threshold extension demodulation apparatus 80 and applied to the input 81 of the latter, both discriminators 82 and 83 demodulate the signal and apply it to their respective error decoders 86 and 89.The output 84 of the envelope multiplication discriininator 82 is connected to the input af the error decoder 86 and the output 85 of the conventional limiter discriminator 83 is connected to the input of the error decoder 89. The error decoder 86 determines the number of bits in error in the error coding structure portion of the data digital content of the signal received via the connection 84 from the envelope multiplication disctiminator 82.The error decoder 89 determines the number of bits in error in the error coding structure portion of the data digital content of the signal received via the connection 85 from the conventional limiter discriminator 83, The output 84 of the envelope multiplication discriminator 82 is, after processing in the error decoder 86 and detection of the number of bits in error, connected to the output 92. The output 85 of the conventional limiter discriminator 83 is, aeter processing in the error decoder 89 and detection of the number of bits in error, connected to the output 93. Connection 87 from the error decoder 86 provides a measure of the number of bits in error in the demodulated signal from the envelope multiplication discriminator 82, to an input 87 of the error processor 90.Connection 88 from the error decoder 89 provides a measure of the number of bits in error in the demodulated signal from the conventional limiter discriminator 83, to an input 88 of the error processor 90. The function of the error processor 90 is to compare the number of bit in error on inputs 87 and 88 and determine which discriminator, 82 or 83, is producing the lowest error rate, and produce an output 91 that controls the changeover switch 94. The function of the changeover switch 94 is to select the error decoder output 92 or 93 that has the lowest number of bits in error, to the threshold extension demodulation apparatus 80 output 95.Assume as an illustration of the operation of figure 5 that a high signal-to-noise ratio signal, and hence low error rate, is received from the preceding part of the receiver at input 81. In this condition the conventional limiter discriminator 83 would have a lower number of bits in error in its output 85 than the envelope multiplication discriminator 82 in its output 84. Hence the measure of bits in error in error decoder 89 output 88 would be lower than the number of bits in error in output 87 from error decoder 86. The error processor 90 would therefore recognise that input 88 had a lower error rate than input 87 and produce an output 91 that would cause the changeover switch 94 output 95 to be connected to changeover switch input 93, the processed output of the conventional limiter discrirninator 83 output 85.Assume now as an illustration of the operation of figure 5 that a low signalto-noise ratio signal, and hence high error rate, is received from the preceding part of the receiver at input 81. In this condition the conventional limiter discriminator 83 would have a higher number of bits in error in its output 85 than the envelope multiplication discriminator 82 in its output 84. Hence the measure of bits in error in error decoder 86 output 87 would be lower than the number of bits in error in output 88 from error decoder 89. The error processor would therefore recognise that input 87 had a lower error rate than input 88 and produce an output 91 that would cause the changeover switch 94 output 95 to be connected to changeover switch input 92, the processed output of the envelope multiplication discriminator 82.Various strategies can be implemented in error processor 90 by those skilled in the art to operate the system to select the lowest error rate in the output, including those conditions where the error rate from each discriminator is equal.

Figure 6 shows a simplified block diagram of my invention implemented as a tracking filter demodulator for demodulating a television signal that for illustrative purposes is assumed to be a PAL format television signal containing Teletext. Note that the same approach with minor modification could be used by those skilled in the art with SEC AM format or NTSC format television signals. Figure 6 shows the block diagram of a portion of a frequency modulated receiver for receiving RF signal at input terminal 101, the input to the threshold extension demodulation apparatus 100 in accordance with the present invention. The principle components of my invention shown in the threshold extension demodulation apparatus 100 include an adaptive bandpass filter 102 whose bandwidth is determined by the control signal 111. The output of the adaptive bandpass filter 111 is connected 103 to a limiter 104 that may or not be included in the apparatus. The output 105 of the limiter 104 is connected to the frequency modulation discminator 6. The frequency demodulated output 106 is connected to video filter processing circuit 107 that conditions the signal to produce a video output 108. This output is connected 108 to a Teletext decoder 199 that is arranged to extract the number of bits in error from that portion of the Teletext signal that contains error coding. In the Teletext system Hamming error coding is used in the address words for detecting double and correcting single errors. The Teletext decoder 109 makes use of this facility in the invention described herein.The output 110 of the Teletext decoder 109 is applied to the error decoder 7 that produces an output 8 that is a continuous measure of error. This continuous measure of error 8 is fed to an error processor 9 that processes the Teletext error rate and, depending on the error magnitude, produces control signals 111 that modify the characteristics of the adaptive bandpass filter 102. The error processor 9 can be arranged to follow a variety of strategies in controlling the characteristics of the adaptive bandpass filter 102. Assume for the purposes of illustration one such strategy. Assume a condition where there is a high signal-to-noise ratio at the input 101. This will produce a negligible error rate in the output signal structure 108.In this condition the error processor 9 will cause the adaptive bandpass filter 102 bandwidth to be set to a very wide value such that the bandwidth of the system is determined by the previous portion of the receiver to the threshold extension demodulation apparatus 100. As the input 101 signal-to-noise ratio reduces to a low value and the error rate in the Teletext signal address words increases, the output 111 of the error processor 9 will commence to reduce the bandwidth of the adaptive bandpass filter 102 resulting in a reduction in error rate as the sidebands of the signal are attenuated. This reduction in error rate will continue until the truncation ofthe sidebands results in distortion ofthe signal and the error rate increases.In parallel a further control signal 111 could be used to adjust the centre frequency of the adaptive bandpass filter 102. This would follow the centre frequency component of the signal and also seek to minimise the error. Note an alternative strategy is, in conditions of high signal-to-noise ratios, for the adaptive bandpass filter 102 to be shorted out.

Figure 6 can also be used to illustrate a further application of my invention - that of matching the bandwidth of the demodulator, and hence the receiver, to a signal of unknown bandwidth. Assume for the purpose of this illustration that the receiver was tuned to a signal whose structure, including its digital content, was known but that its occupied bandwidth was not. The strategy that could be implemented in the adaptive bandpass filter 9 for this signal would be to start with a wide bandwidth setting for the adaptive bandpass filter 9, and to reduce this bandwidth until the error rate was minimized, thus achieving an optimum bandwidth setting. As part of this strategy it may be necessary for the adaptive bandpass filter 9 to consist of several filters each of a dishing bandwidth to provide the range of bandwidth required.The strategy followed would in this case be to select and adjust each filter in turn, in decreasing bandwidth steps, say, until the bandwidth that gave the minimum error was attained.

Figure 7 shows a simplified block diagram of my invention implemented as a tracking filter demodulator for demodulating a television signal that for illustrative purposes is assumed to be a MAC format television signal, and in particular the D2/MAC format.

Note that the same approach with minor modification could be used by those skilled in the art, with any of the MAC format television signals. Figure 7 shows the block diagram of a portion of a frequency modulated receiver for receiving a RF signal at input terminal 121, the input to the threshold extension demodulation apparatus 120 in accordance with the present invention. The principle components of my invention shown in 120 include an adaptive bandpass filter 122 whose bandwidth is determined by the control signal 131. The output of the adaptive bandpass filter 122 is connected 123 to a limiter 124 that may or not be included in the system. The output 125 of the limiter 124 is connected to the frequency modulation discriminator 6. The frequency demodulated output 126 is connected to video filter processing circuit 127 that conditions the signal to produce a video output 128.As well as being the output of the threshold extension demodulation apparatus 120 this output 128 is connected to the input of a framelline extraction circuit 129 that is arranged to extract the line and frame synchronisation. The output 130 of the framelline extraction circuit 129 is connected to the error decoder 7. Digital synchronisation is used in the MAC system, where for the frame and line synchronisation a 6 bit word is used at the start of each line. In addition there is a further sequence of 64 bits transmitted within the data of line 625 for additional frame synchronisation. The pattern of these synchronisation structures are pre-defined and thus any bits in error can be readily established in the error decoder circuit 7. The output 8 of the error decoder 7 is applied to the error processing circuit 9.This continuous measure of bit error 8 in the synchronisation patterns is fed to an error processor 9 that processes the bits in error and depending on the error magnitude produces control signals 131 that modify the characteristics of the adaptive bandpass filter 122. The error processor 9 can be arranged to follow a variety of strategies in controlling the characteristics of the adaptive bandpass filter 122. Assume for the purposes of illustration one such strategy. Assume a condition where there is a high signal-to-noise ratio at the input 121.This will produce a negligible error rate in the output 128 signal structure. In this condition the error processor 9 will cause the adaptive bandpass filter 122 bandwidth to be set to a very wide value such that the bandwidth ofthe system is determined by the preceding receiver stage to the threshold extension demodulation apparatus 120. As the input 121 signal-to-noise ratio reduces to a low value and the error rate in the demodulated 128 MAC synchronisation words increases, the output 131 of the error processor 9 will commence to reduce the bandwidth of the adaptive bandpass filter 122 resulting in a reduction in error rate at output 128 as the sidebands of the signal are attenuated.This reduction in error rate will continue until the truncation of the sidebands results in distortion of the demodulated signal 128 and the error rate increases. In parallel a further control signal 131 could be used to adjust the centre frequency of the adaptive bandpass filter 122.

This would follow the centre frequency component of the signal at input 121 and also seek to minimise the error at output 128. Note an alternative strategy is, in conditions of high signal-to-noise ratios, for the adaptive bandpass filter 122 to be shorted out.

Figure 8 shows a simplified block diagram of my invention implemented as a tracking filter demodulator for demodulating a television signal that for illustrative purposes is assumed to be an all-digital television format signal. Figure 8 shows the block diagram of a portion of a frequency modulated receiver for receiving a RF signal at input terminal 141, the input to the threshold extension demodulation apparatus 140 in accordance with the present invention. The principle components of my invention shown in 140 include an adaptive bandpass filter 142 whose bandwidth is determined by the control signal 151. The output of the adaptive bandpass filter 142 is connected 143 to a limiter 144 that may or not be included in the system. The output 145 of the limiter 144 is connected to the frequency modulation discriminator 146. The frequency demodulated output 147 from the discrininator 146 is connected to an error decoder circuit 148. The output 155 ofthe error decoder 148 provides the output to the part of the receiving system following the threshold extension demodulation apparatus 140. A fUrther output 149 from the error decoder is connected to the error processing circuit 150. This error decoder output 149 provides a measure of the number of bits in error in the demodulated signal.Typically an all-digital television signal would contain a number of signal structures, including field synchronisation and segment synchronisation patterns, that may be used to determine the number of bits in error For example the received demodulated signal structure at output 147 would for illustrative purposes use a Reed Solomon error coding with error control parity bits added to each data segment with additional protection being provided by a data interleaver. In practice, nether field synchrotlisation or segment synchronisation patterns are error encoded or interleaved. The pattern of these synchronisation structures are pre-defined and thus any bits in error can be readily established in the error decoder circuit 148.This continuous measure of bit error in the Reed Solomon error coding and/or synchronisation patterns t49 is fed to an error processor 150 that processes the bits in error and depending on the error magnitude produces control signals 151 that modify the characteristics of the adaptive bandpass filter 142. The error processor 150 can be arranged to follow a variety of strategies in controlling the characteristics of the adaptive bandpass filter 142. Assume for the purposes of illustration one such strategy. Assume a condition where there is a high signal-to-noise ratio at the input 141. This will produce a negligible error rate in the signal structures at output 147.In this condition the error processor 150 will cause the adaptive bandpass filter 142 bandwidth to be set to a very wide value such that the bandwidth of the system is determined by the preceding part of the receiver to the threshold extension demodulation apparatus 140. As the input 141 signal-to-noise ratio reduces to a low value and the error rate in the demodulated signal structure 147 increases, the output 151 of the error processor 150 will commence to reduce the bandwidth of the adaptive bandpass filter 142 resulting in a reduction in error rate as the sidebands of the signal are attenuated. This reduction in error rate will continue until the truncation of the sidebands results in distortion of the signal and the error rate increases.In parallel a further control signal 151 could be used to adjust the centre frequency of the adaptive bandpass filter 142. This would follow the centre frequency component ofthe signal and also seek to minimise the error. Note an alternative strategy is, in conditions of high signal-to-noise ratios, for the adaptive bandpass filter 142 to be shorted out. A fUrther function is shown in figure 8, that ofthe clock recovery circuit 152 whose input is obtained from discriminator output 147. The function of this circuit is to provide a timing or clock signal to the error decoder by means of output 153.The output 154 from the clock recovery circuit 152 provides a tnrn.'g or clock signal to the remainder ofthe receiver following the threshold extension demodulation apparatus 140.

Figure 9 shows a simplified block diagram of my invention of a threshold extension demodulation apparatus implemented as a frequency modulation with feedback error controlled (FMFB) demodulator; a demodulator that utilises FM deviation compression techniques, and that for illustrative purposes is shown demodulating a frequency modulated signal. Figure 9 shows the block diagram of a portion of a frequency modulated receiver for receiving an input FM signal of prescribed FM deviation signal at input terminal 161, the input to the threshold extension demodulation apparatus 160 in accordance with the present invention The principle components of my invention shown in 160 includes the basic FMFB demodulator arrangement r70 whose I.F. adaptive filter 164 characteristics are modified by the error control signal 172.In the embodiment of figure 9 the basic FMFB demodulator 170 receives from the previous part of the receiver to the threshold extension demodulation apparatus 160, an input FM signal of prescribed FM deviation at input 161 to the mixer 162 and mixed with the output 169 of the voltage controlled oscillator 173 converting said input FM signal of prescribed reduced FM deviation to an I.E. signal having a prescribed reduced F.M. deviation at mixer 162 output 163. The I.F. signal 163 of reduced deviation passes through the I.F. adaptive filter 164 typically having a bandpass characteristic that removes as much broad band noise as possible yet not adversely affecting the said signal. In addition the broad band characteristic must frequently also be wide enough to tolerate the effect of circuit instabilities and drifts.

The I.E. output signal 165 is applied to the FM discriminator 6 that produces a demodulated output 166. This demodulated output 166 is then applied to a baseband filter 167, that typically has a low pass characteristic. The filtered output 168 of the baseband filter 167 is connected to the input of the voltage controlled oscillator 173 and the input to the error decoder 7. The demodulated output at 168 controls the frequency of the voltage controlled oscillator so as to closely track the FM input signal at mixer input 161 in a fashion as to greatly reduce the FM deviation of the converted I.F. signal at mixer output 163. The output 171 of the error decoder 7 provides the demodulated output to the part of the receiving system following the threshold extension demodulation apparatus 160.A further output 8 from the error decoder 7 is connected to the error processing circuit 9. This error decoder output 8 provides a measure of the number of bits in error in the demodulated signal. This continuous measure 8 of bit error in the demodulated output is fed to an error processor 9 that processes the bits in error and depending on the error magnitude produces control signals 172 that moddy the characteristics of the adaptive I.F. adaptive filter 164. The error processor 9 can be arranged to follow a variety of strategies in controlling the characteristics of the adaptive I.F. adaptive filter 164. Assume for the purposes of illustration one such strategy. Assume a condition where there is a high signal-to-noise ratio at the input 161. This win produce a negligible error rate in the signal structures.

In this condition the error processor 9 will cause the adaptive I.F. adaptive filter 164 bandwidth to be set to a predetermined bandwidth. As the input 161 signal-to-noise ratio reduces to a low value and the error rate in the demodulated signal structure 168 increases, the output 172 of the error processor 9 will commence to reduce the bandwidth of the I.E. adaptive filter 164 resulting in a reduction in error rate as the sidebands of the input signal are attenuated. This reduction in error rate will continue until the truncation ofthe sidebands results in distortion ofthe signal and the error rate increases. In parallel a further control signal 172 could be used to adjust the centre frequency of the I.E. adaptive filter 164. This would follow the centre frequency component of the signal and also seek to minimise the error. Yet a further strategy could be to utilise a further output 172 of the error processor 9 to control the centre frequency of the voltage controlled oscillator 173; one way of achieving this would be to sum the baseband filter output 168 and the error processor output 172, at the input to the voltage controlled oscillator 173. Yet another strategy could be to use a further control output 172 of the error processor 9 to alter the characteristics of the baseband filter 167.

Figure 10 shows a simplified block diagram of my invention implemented as a frequency lock loop (elm) demodulator and that for illustrative purposes is shown demodulating a frequency modulated signal. Figure 10 shows the block diagram of a portion of a frequency modulated receiver for receiving an input FM signal of prescribed FM deviation signal at input terminal 181, the input to the threshold extension demodulation apparatus 180 in accordance with the present invention. The circuit contained within 190 is a frequency lock loop demodulator whose loop filter 188 characteristics can be modified by a control signal 193.The input FM signal of prescribed FM deviation signal at input terminal 181 is applied to a limiter and discriminator 182, that produces an output 183 proportional to the instantaneous frequency, and an envelope detector 19S. The output of the limiter and discrnnator 182 whose output 183 is applied to the feedback loop formed by the subtraction device or error junction, 184, connection 185, the multiplier 186 that multiplies the output 185 of the subtraction device 184 by a measure of the instantaneous envelope 194, connection 187, the loop filter 188, whose characteristics may be controlled by input 193, and connection 189 and 191. The output 189 of the loop fìlter 188 can be used as the frequency lock loop 190 output.The gain of the said feedback loop may be controlled directly by the output 194 of the envelope demodulator 195 that envelope detects the said FM signal at input 181. The frequency demodulated output 189 is connected to an error decoder circuit 7. The output 192 of the error decoder 7 provides the output to the part of the receiving system following the threshold extension demodulation apparatus 180. A fiirr output 8 from the error decoder is connected to the error processing circuit 9. This error decoder output 8 provides a measure of the number of bits in error in the demodulated signal.This continuous measure 8 of bit error in the demodulated signal is fed to an error processor 9 that processes the bits in error and depending on the error magnitude produces control signals 193 that moddy the characteristics of the adaptive loop filter 188. The error processor 9 can be arranged to follow a variety of strategies in controlling the characteristics of the adaptive loop filter 188. Assume for the purposes of illustration one such strategy. Assume a condition where there is a high signal-to-noise ratio at the input 181. This will produce a negligible error rate in the signal structures. In this condition the error processor 9 will cause the adaptive loop filter 188 bandwidth to be set to a predetermined bandwidth.As the input 181 signal-to-noise ratio reduces to a low value and the error rate in the demodulated signal structure 189 increases, the output 193 of the error processor 9 will commence to reduce the bandwidth of the adaptive loop filter 188 resulting in a reduction in error rate as the bandwidth of the demodulated signal is reduced. This reduction in error rate will continue until the reduction in bandwidth resits in distortion of the signal and the error rate increases.

An alternative strategy is that the control signal 193 could be used to adjust the characteristics, e.g. the amplitude function, of the envelope detector 195. This would also seek to minimise the error. Yet a further strategy is to use the control signal 193 to adjust the characteristics, including loop gain, of the feedback loop formed by the subtraction device or error junction, 184, connection 185, the multiplier 186 that multiplies the output 185 of the subtraction device 184 by a measure of the instantaneous envelope 194, connection 187, the loop filter 188, and connection 189 and 191.

Figure 11 shows a simplified block diagram of my invention implemented as a phase lock loop (PLL) demodulator and that for illustrative purposes is shown demodulating a frequency modulated signal. Figure 11 shows the block diagram of a portion of a frequency modulated receiver for receiving an input FM signal of prescribed FM deviation signal at input terminal 201, the input to the threshold extension demodulation apparatus 200 in accordance with the present invention. The circuit contained within 210 constitutes a phase lock loop demodulator whose adaptive loop filter 206 characteristics can be modified by a control signal 209. The input FM signal of prescribed FM deviation signal at input terminal 201 is applied to one input of a phase detector.The phase detector 202 compares the phase of the input signal 201 with the phase of the signal output 208 of the voltage controlled oscillator 207. The output 203 of the phase detector 202 is applied to the adaptive loop filter 206. The output 204 of the adaptive filter is a filtered demodulated signal that is applied to the input of the error decoder 7 as well as the input 205 to the voltage controlled oscillator 207. The output 211 of the error decoder 7 provides the output to the part of the receiving system following the threshold extension demodulation apparatus 200. A further output 8 from the error decoder is connected to the error processing circuit 9.

This error decoder output 8 provides a measure of the number of bits in error in the demodulated signal. This continuous measure 8 of bit error in the demodulated signal is fed to an error processor 9 that processes the bits in error and depending on the error magnitude produces control signals 209 that modify the characteristics of the adaptive loop filter 206. The error processor 9 can be arranged to follow a variety of strategies in controlling the characteristics of the adaptive loop filter 206. Assume for the purposes of illustration one such strategy. Assume a condition where there is a high signal-to-noise ratio at the input 201. This will produce a negligible error rate in the demodulated signal structures.In this condition the error processor 9 will cause the adaptive loop filter 206 bandwidth to be set to a predetermined bandwidth. As the input 201 signal-to-noise ratio reduces to a low value and the error rate in the demodulated signal structure 211 increases, the output 209 of the error processor 9 will commence to reduce the bandwidth of the adaptive loop filter 206 resulting in a reduction in error rate as the bandwidth of the demodulated signal is reduced. This reduction in error rate will continue until the reduction in bandwidth of the demodulated signal results in distortion of the signal and the error rate increases. In parallel or as a separate strategy a further control signal 209 could be used to adjust the centre frequency of the voltage controlled oscillator 207.This would follow the centre frequency component ofthe signal and also seek to minimise the error; one way of achieving this would be to sum the adaptive loop filter output 204 and the error processor output 209, at the input to the voltage controlled oscillator 207. Yet a further variant of my invention is where the error control signal 209 could be used to alter the order of the loop filter 206. Yet another strategy is where the error control signal 209 could be used to alter the loop gain of the phase lock loop demodulator, namely phase detector 202, connection 203, adaptive loop filter 206, connections 204 and 205, voltage controlled oscillator 207 and connection 208.

Figure 12 shows a simplified block diagram of my invention implemented as a further version of the phase lock loop (PLL) demodulator and that for illustrative purposes is shown demodulating a frequency modulated signal. Figure 12 shows the block diagram of a portion of a frequency modulated receiver for receiving an input FM signal of prescribed FM deviation signal at input terminal 221, the input to the threshold extension demodulation apparatus 220 in accordance with the present invention. The circuit contained within 230 constitutes a phase lock loop demodulator. The input FM signal of prescribed FM deviation signal is applied at input terminal 221 of the Automatic Gain Control (AGC) controller 222 whose gain is determined by the error control function at input 234.The output 223 of the AGC controller 222 is applied to one input of a phase detector 224. The phase detector 224 compares the phase of the input signal 223 with the phase of the signal output 233 of the voltage controlled oscillator 232. The output 225 of the phase detector 224 is applied to the loop filter 226. The output 227 of the loop filter 226 is a filtered demodulated signal that is applied to the input of the error decoder 7 as well as the input 231 to the voltage controned oscillator 232. The output 228 of the error decoder 7 provides the output to the part of the receiving system following threshold extension demodulation apparatus 220. A further output 8 from the error decoder is connected to the error processing circuit 9. This error decoder output 8 provides a measure of the number of bits in error in the demodulated signal.This continuous measure 8 of bit error in the demodulated signal is fed to an error processor 9 that processes the bits in error and depending on the error magnitude produces control signals 234 that modify the characteristics of the AGC controller 222. The error processor 9 can be arranged to follow a variety of strategies in controlling the characteristics of the AGC controller 222. Assume for the purposes of illustration one such strategy. Assume a condition where there is a high signat-to-noise ratio at the input 221. This will produce a negligible error rate in the demodulated signal structure. In this condition the error processor 9 will cause the AGC output 223 to the phase lock loop demodulator 230 to be high resulting in a high loop gain and hence wider bandwidth.As the input 221 signal-to-noise ratio reduces to a low value and the error rate in the demodulated signal structure 227 increases, the output 234 of the error processor 9 will commence to reduce the output signal level 223 of the AGC stage 222 resulting in a reduction in error rate, as the loop gain, and hence bandwidth of the phase lock loop demodulator 230 is reduced, thus providing threshold extension. This reduction in error rate will continue until the reduction in signal results in distortion of the signal and the error rate increases. In parallel or as a separate strategy a firrther control signal 234 could be used to adjust the centre frequency ofthe voltage controlled oscillator 232. This would follow the centre frequency component of the signal and also seek to minimise the error; one way of achieving this would be to sum the loop filter output 227 and the error processor output 234, at the input to the voltage controlled oscillator 231. Yet a fiuther variant of my invention is where the error control signal 234 could be used to alter the order of the loop filter 226.

Although an exemplary embodiment is described above, it will be obvious to those skilled in the art that many alterations and modifications may be made without departing from the invention. Accordingly, it is intended that all such alterations and modifications be included within the spirit and scope of the invention defined in the appended claims.

Claims

I claim:

1. A threshold extension demodulation apparatus for use in a receiver, comprising - an input terminal means for receiving a carrier signal; - means for extracting the instantaneous frequency information from said input carrier signal; - means for extracting information in error from said instantaneous frequency information, - means for processing said information in error for providing control signals; - means responsive to said control signals for controlling said means for extracting the instantaneous frequency information from input carrier; - an output terminal means coupled with output of said instantaneous frequency information.

2. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 1 in which said means for extracting instantaneous frequency information from input carrier signal comprises a discriminator.

3. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 1 in which said means for extracting information in error from said instantaneous frequency information comprises an error detector that detects bits in error in the discriminator output signal.

4. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 1 in which said means for processing said information in error for providing control signals comprises an error processor that produces control signal outputs.

5. A threshold extension demodulation apparatus for use in a receiver as set forth in claim I in which said means responsive to said control signals for controlling said means for extracting the instantaneous frequency information from input carrier signal comprises a discriminator whose structure and characteristics are determined by said control signal.

6. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 1 in which an input terminal is coupled to the means for extracting the instantaneous frequency information from input carrier signal; an output terminal is coupled to the output of said means for extracting the instantaneous frequency information from input carrier.

7. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 1 in which an input terminal is coupled to the means for extracting the instantaneous frequency information from input carrier signal; an output terminal is coupled to the output of said means for extracting information in error from said instantaneous frequency information.

8. A threshold extension demodulation apparatus for use in a receiver as set forth in caims 1, 2, 3, 4, S and 6 or 7.

9. A threshold extension demodulation apparatus for use in a receiver, comprising - an input terminal means for receiving a carrier signal; - filter means for conditioning said input carrier signal and whose said filter characteristics are responsive to control signals; - limiter means coupled to output of said filter means; means for extracting the instantaneous frequency information from output of said limiter means; - means for extracting information in error from said instantaneous frequency information; - means for processing said information in error for providing said control signals; - means responsive to said control signals for controlling said filter means; - an output terminal means coupled with output of said instantaneous frequency information.

10. A threshold extension demodulation apparatus for use in a receiver, comprising - an input terminal means for receiving a carrier signal; - filter means for conditioning said input carrier signal and whose said filter characteristics are responsive to control signals; - means for extracting the instantaneous frequency information from output of said filter means; - means for extracting information in error from said instantaneous frequency information, - means for processing said information in error for providing said control signals; - means responsive to said control signals for controlling said filter means; - an output terminal means coupled with output of said instantaneous frequency information.

11. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 9 and 10 in which said filter means comprises divers filters; each filter being selectable and comprising a bandpass filter whose centre frequency and bandwidth can be controlled.

12. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 9 in which said limiting means comprises a lintiting circuit.

13. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 9 and 10 in which said means for extracting instantaneous frequency information from input carrier signal comprises a discriminator.

14. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 9 and 10 in which said means for extracting information in error from said instantaneous frequency information comprises an error detector that detects bits in error in the discriminator output signal.

15. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 9 and 10 in which said means for processing said information in error for providing control signals comprises an error processor that produces control signal outputs.

16. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 9 and 10 in which said means responsive to said control signals for conditioning the input carrier signal comprises a bandpass filter whose centre frequency and bandwidth are determined by said control signal.

17. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 9 or 10 in which an input terminal is coupled to the means for extracting the instantaneous frequency information from input carrier signal; an output terminal is coupled to the output of said means for extracting the instantaneous frequency information from input carrier signal.

18. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 9 or 10 in which an input terminal is coupled to the means for extracting the instantaneous frequency information from input carrier signal; an output terminal is coupled to the output of said means for extracting information in error from said instntane frequency information.

19. A threshold extension demodulation apparatus for use in a receiver as set forth in claims 9 or 10 and 11,12,13,14,15,16, 17 or 18.

20. A threshold extension demodulation apparatus for use in a receiver, comprising - an input terminal means for receiving a carrier signal; - a first switching means responsive to a control signal to connect said input terminal to a selected input of divers means ofextracting instantaneous frequency information from input carrier signal; - divers means ofextracting instantaneous frequency information from input carrier signal, each means input selectable by said first switching means and each means output selectable by second switching means.

- said second switching means responsive to said control signal to connect a selected output of said divers means of extracing said instantaneous frequency information from said input carrier signal; - means for extracting information in error from said instantaneous frequency information; - means of processing said information in error for providing said control sisnai; - an output terminal means coupled with output of said instantaneous frequency information.

21. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 20 in which said divers means of extracting frequency information from carrier signal consists oftwo discriminators each selectable by said first and second switching means.

22. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 20 in which said divers means of extracting frequency information from input carrier signal consists a conventional limiter discriminator and an envelope multiplication discriminator, each selectable by said first and second switching means.

23. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 20 in which said means for extracting information from said instantaneous frequency information comprises an error detector that detects bits in error in the discriminator output signal.

24, A threshold extension demodulation apparatus for use in a receiver as set forth in claim 20 in which said means for processing said information in error for providing control signals comprises an error processor that produces control signal outputs.

25. A threshold extension demodulation apparatus for use in a receiver as set forth in claims 20, 21, 22, 23 and 24.

26. A threshold extension demodulation apparatus for use in a receiver, comprising - an input terminal means for receiving a carrier signal; - a first switching means responsive to a control signal to connect said input terrninal to a selected input of divers means of extracting instantaneous frequency information from input carrier signal; - divers means of extracting instantaneous frequency information from said input carrier signal, whose input means are selectable by said first switching means; - each output of said divers means of extracting said instantaneous frequency information connected to a means of extracting information in error from said instantaneous frequency information;; - said second switching means responsive to said control signal to connect a selected first output of said divers means of extracting information in error to provide said instantaneous frequency information; - each second output of said divers means of extracting information in error connected to a means of selecting and processing said information in error for providing said control signal; - an output terminal means coupled with output of said instantaneous frequency information.

27. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 26 in which said divers means of extracting frequency information from input carrier signal consists oftwo discriminators whose inputs are selectable by a first switching means; each discriminator output connected to an error detector and whose said error detector outputs providing instantaneous frequency information are selectable by said switching means.

28. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 26 in which said divers means of extracting frequency information from input carrier signal consists a conventional limiter discriminator and an envelope multiplication discriminator whose inputs are selectable by a first switching means; each discriminator output connected to an error detector and whose said error detector outputs providing instantaneous frequency information are selectable by said switching means.

29. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 26 in which said means for extracting information in error from said instantaneous frequency information comprises divers error detectors that detects bits in error in divers discriminator output signal.

30. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 26 in which said means for processing said information in error for providing control signals comprises an error processor that produces control signal output to select the discriminator output signal with the lowest error rate.

31. A threshold extension demodulation apparatus for use in a receiver as set forth in claims 26, 27, 28, 29 and 30.

32. A threshold extension demodulation apparatus for use in a receiver, comprising - an input terminal means for receiving a carrier signal; - said input terminal coupled to inputs in parallel of divers means of extracting instantaneous frequency information; - each output of said divers means of extracting said instantaneous frequency information connected to a means of extracting infbrmation in error from said instantaneous information; - each first output of said divers means of extracting information in error connected to a means of processing said information inerror; - means of selecting and processing said information in error for providing said control signal;; - switching means responsive to said control signals to connect selected second output of said divers means of extracting information in error to provide said instantaneous frequency information.

- an output terminal means coupled with output of said instantaneous frequency information.

33. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 32 in which said divers means of extracting frequency information from carrier consists oftwo discminators whose inputs are in parallel; each disciminator output is connected to an error detector and whose said error detector outputs that provide the wanted instantaneous frequency information are delectable by a switching means.

34. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 32 in which said divers means of extracting frequency information from carrier consists a conventional limiter discriminator and an envelope multiplication discriminator, whose inputs are in parallel; each discriminator output connected to an error detector and whose said error detector outputs that provide the wanted instantaneous frequency information are delectable by said switching means.

35. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 32 in which said means for extracting information in error from said instantaneous frequency information comprises divers error detectors that detects bits in error in divers discriminator output signal.

36. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 32 in which said means for processing said information in error for providing control signals comprises an error processor that produces control signal output to select the discriminator output signal with the lowest error rate.

37, A threshold extension demodulation apparatus for use in a receiver as set forth in claims 32, 33,34, 35 and 36.

38. A threshold extension demodulation system for use in a receiver utilising FM compression deviation, comprising - an input terminal for receiving a carrier signal - a voltage controlled oscillator supplying local oscillations; - a mixer means having an input receiving an FM signal of prescribed FM deviation and an input receiving said local oscillations and converting said FM signal to a converted I.F. signal having a prescribed reduced FM deviation; - an I.F. adaptive amplifier means coupled to said niixer means for amplifying and selective filtering of said converted I.F. signal within a bandwidth that is determined by a control fimction means.

- a FM discriminator means coupled to said I.F. adaptive amplifier, - a baseband filter means coupled to said FM discriminator means for selective filtering said demodulated signal within a prescribed bandwidth; - a feedback loop coupling output of said baseband filter means to input of said voltage controlled oscillator means for controlling said local oscillations; - an error detection means coupled to said baseband filter that detects the information in error in the signal; - an error processor means coupled to said error detection means that produces said control function means for controlling said I.F. adaptive amplifier bandwidth; - an output terminal means coupled with output of said demodulated signal.

39. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 38 in which said error processor means coupled to said error detection means produces said control function means for controlling said I.F.

adaptive amplifier centre frequency.

40. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 38 in which said error processor means coupled to said error detection means produces said control function means for controlling said voltage controlled oscillator centre frequency.

41. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 38 in which said FM discriminator means comprises a limiter-FM discriminator means.

42. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 38 in which said error processor means coupled to said error detection means produces said control function means applied to one input of a summing network; a feedback loop coupling output of said baseband filter means to a second input of said summing network; the output of said summing network is coupled to the input of a voltage controlled oscillator means for controlling said local oscillations.

43. A threshold extension demodulation apparatus for use in a receiver as set forth in claims 38,39, 40, 41 and 42.

44. A threshold extension demodulation system for use in a receiver comprising - an input terminal for receiving a carrier signal; - means of extracting the instantaneous frequency information from said carrier signal; - means for extracting instantaneous envelope information from said carrier signal for providing a first control signal; - a transmission channel for said instantaneous frequency information comprising the series combination of a feedback loop and a baseband filter;; - said feedback loop comprising a subtraction device, a multiplication device and a loop filter, the output of said subtraction device being supplied to the first terminal of said multiplication device, the output of said multiplication device being supplied to input of said loop filter, the output of said loop filter being supplied to first input terminal of said subtraction device; - said instantaneous frequency information signals being supplied to said feedback loop through the second input terminal on said subtraction device; - said feedback loop output being obtained from said loop filter output terminal;; - said first control signal means being introduced into said feedback loop at the second input terminal of said multiplication device to thereby control the parameters of said feedback loop; - said loop filter characteristics being responsive to a second control signal; - an error detection means coupled to said loop filter output that detects the information in error bits in the instantaneous frequency information; - an error processing means coupled to said error detection means that produces a second control means for controlling said loop fitter characteristics; - an output terminal means coupled with output of said instantaneous frequency information.

45. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 44 in which said instantaneous envelope information characteristics is responsive to the said second control means.

46. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 44 in which an input terminal is coupled to the parallel inputs of the said means of extracting the instantaneous frequency information from said carrier signal and said means for extracting instantaneous envelope information; an output terminal is coupled to the output of said error decoder.

47. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 44 in which an input terminal is coupled to the parallel inputs of the said means of extracting the instantaneous frequency information from said carrier signal and said means for extracting instantaneous envelope information; an output terminal is coupled to the output of said loop filter.

48 A threshold extension demodulation apparatus for use in a receiver as set forth in claim 44 in which an output terminal is coupled to the output of said means for extracting information in error from said instantaneous frequency information.

49. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 44 in which an output terminal is coupled to the output of said loop filter means to provide said instantaneous frequency information.

50. A threshold extension demodulation apparatus for use in a receiver as set forth in clauns 44, 45, 46 or 47, and 48 or 49.

51. A threshold extension demodulation system for use in a receiver comprising - an input terminal for receiving a carrier signal; - a phase detector means having a first and second input terminal that compares the phases oftwo signals applied to the first and second input terminals and an output terminal; - the first terminal of said phase detector being coupled to said input carrier signal; - a loop filter that is connected to said output terminal of said phase detector and whose characteristics are responsive to a control means and whose output is the instantaneous frequency infbrmation;; - a voltage controlled oscillator means controlled by the output of said loop filter and connected to supply a signal to said second input terminal of said phase detector, - an error detection means coupled to said loop filter output that detects the information in error in said instantaneous frequency information; - an error processing means coupled to said error detection means that produces said control function means for controlling said loop filter characteristics; - an output terminal means coupled with output of said instantaneous frequency information

52.A threshold extension demodulation apparatus for use in a receiver as set forth in claim 51 in which said error processor means coupled to said error detection means produces said control function means applied to one input of a sunning network; coupling output of said loop filter means to a second input of said summing network; the output of said summing network is coupled to the input of a voltage controlled oscillator means for controlling said local oscillations.

53. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 51 in which order of said loop filter characteristics is responsive to the said control means.

54. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 51 in which an input terminal is coupled to the first input ofthe said phase detector means; an output terminal is coupled to the output of said error decoder.

55. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 51 in which an input terminal is coupled to the first input of the said phase detector means; an output terminal is coupled to the output of said loop filter.

56. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 51 in which an output terminal is coupled to the output of said means for extracting information in error from said instantaneous frequency information.

57. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 51 in which an output terminal is coupled to the output of said loop filter means to provide said instantaneous frequency information.

55. A threshold extension demodulation apparatus for use in a receiver as set forth in claims 51, 52, 53, 54 or 55 or 56 or 57.

59. A threshold extension demodulation system for use in a receiver comprising - an input terminal for receiving a carrier signal; - an Automatic Gain Control controller means responsive to a control means and connected to the first input terminal of a phase detector means; - said phase detector means having a first and second input terminal which compares the phases of two signals applied to the two input terminals and an output terminal; - the first terminal of said phase detector being coupled to said input carrier signal; - a loop filter means that is connected to the output terminal of said phase detector; - a voltage controlled oscillator means controlled by the output of said loop filter means and connected to supply a signal to the second input terminal of said phase detector;; - an error detection means coupled to said loop filter output that detects the information in error in the signal; - an error processing means coupled to said error detection means that produces said control function means for controlling said Automatic Gain Control controller; - an output terminal means coupled with output of said instantaneous frequency information.

60. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 59 in which order of said loop filter characteristics is responsive to the said control means.

61. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 59 in which an input terminal is coupled to the input of the Automatic Gain Control controller means; an output terminal is coupled to the output of said error decoder.

62. A threshold extension demodulation apparatus for use in a receiver as set forth in claim S9 in which an input terminal is coupled to the input ofthe Automatic Gain Control controller means; an output terminal is coupled to the output of said loop filter.

63. A threshold extension demodulation apparatus for use in a receiver as set forth in claim 59 in which said error processor means coupled to said error detection means produces said control function means applied to one input of a summing network; coupling output of said loop filter means to a second input of said summing network; the output of said suming network is coupled to the input of a voltage controlled oscillator means for controlling said local oscillations.

64. A threshold extension demodulation apparatus for use in a receiver as set forth in claims 59, 60, 61 or 62 and 63.

65. A threshold extension demodulation apparatus substantially as described herein with reference to figures I to 12 of the accompanying drawing.