MXPA99011791A - Method and apparatus for filtering interference and nonlinear distortions - Google Patents

Abstract

Interference and/or nonlinear distortions are reduced in a signal communicated from a transmitter to a receiver. To reduce interference, the transmitter is momentarily disrupted, during which time the receiver analyzes interference on the communication path to determine the frequency of at least one peak thereof. Information is communicated from the receiver to the transmitter identifying the frequencies of the interference peaks. Based on this information, the transmitter pre-distorts the signal to accentuate the signal magnitude at the identified frequencies. The pre-distorted signal is then communicated to the receiver, where it is filtered to attenuate the signal magnitude at the identified frequencies. An adaptive scheme periodically determines the interference peak frequencies. For nonlinear distortion cancellation (e.g., CSO/CTB), there is no need to disrupt the transmitter since the frequencies of the distortion are already known. Pre-distortion at the transmitter and filtering at the receiver are performed at the known, fixed frequencies.

Description

METHOD AND APPARATUS FOR FILTERING INTERFERENCE AND NON-LINEAR DISTORTIONS BACKGROUND OF THE INVENTION The present invention relates to electronic communication systems, and more particularly to a method and apparatus for filtering non-linear interference and distortions in a signal communicated from a transmitter to a receiver via a communication path. In particular, the invention is suitable for use in conjunction with a television distribution system, such as a hybrid fiber / coaxial (HFC) network, in which a subscriber terminal, such as an adapter unit or cable modem, it receives television and / or data signals from the "head" of the distribution system via a "downstream" communication route and sends the information back to the header on the "upstream" return route. In this environment, the interference filtered by the present invention is often referred to as the "ingress" or "ingress noise" and the non-linear distortions of interest comprise composite triple-beat (CTB) and second-order composite distortions (CSO). ). Hybrid fiber / coax networks, which are based on a branching and tree architecture, provide a cost effective means of P1741 / 99MX distribution of downstream broadcast services, such as analog / digital and high-speed video data. In addition, they provide subscribers with an upstream transmission of high-speed data, for example, in the 5-42 MHz portion of the RF spectrum. Cumulative revenue noise is the main deterioration in the portion of the return path of HFC networks. The types of input noise that appear in the return path can be classified as follows: A. Narrow-band, shortwave signals, which originate from radio stations and other sources, coupled to the cable plant. the return route at the subscriber's location or at the distribution plant. B. Common mode distortion that originates from the nonlinearities in the cable plant. C. Specific interference to the location generated by an electrical device at the subscriber's location. See, for example, C. A. Eldering, N. Himayat, and F. M.

Gardner, "CATV Return Path Characterization for Reliable Communications ", IEEE Communications 8, 62-68 (nineteen ninety five) . The amount of cumulative revenue noise in the return route network is essentially a limiting factor in determining the maximum number of simultaneous users and the maximum data transmission speed that can be achieved.

P1741 / 99MX The video signals sent to the adaptation unit are frequently subjected to "explosion / impulse noise" that originates from composite second order (CSO) and / or triple-pulse compound (CTB) distortions. These distortions occur in general at known frequencies, which depend on the plan of television frequencies used by the analog video signals. In cable television systems, frequency plans include the fully related carrier plan (IRC) and the carrier plan harmonically related (HRC) The distortions of CSO and CTB can lead to blocking of video and visually degraded areas in a television image. A method for dealing with the problem of CSO / CTB distortion in AM-VSB video transmission systems (rudimentary amplitude-modulated lateral band) / QAM (quadrature amplitude modulation) of several channels and the like is described in US Patent Application, commonly assigned, co-pending, Serial No. 09 / 170,852 filed October 13, 1998 and entitled "Method and System for Enhancing Digital Video Transmission to a Set-Top Box". In the system described in that patent application, the performance of digital and analogue video transmission systems, hybrids are improved by determining the relative magnitude and frequency locations of the non-linear distortions, identifying the frequency plan of the channel Analog P1741 / 99MX and then selecting a digital channel map based on this. It would be advantageous to have a strong and cost effective method and apparatus for filtering out interference (such as input noise or other interference noise) and non-linear distortions in a signal communicated from a transmitter to a receiver via a communication path , such as a signal of the return path from an adaptation unit or the like. It may be additionally advantageous to provide a method and apparatus that operate in an adaptive manner, so that interference (eg, ingress) is efficiently filtered even when the frequency of the interference peaks changes over time. This method and apparatus must allow a plurality of interference peaks to be filtered, and must adapt automatically to changing conditions in the interference. It would be still further advantageous to provide a method and apparatus for filtering the non-linear distortion in a signal communicated from a transmitter to a receiver via a communication path. This method and apparatus would be particularly useful in the downstream channel of an HFC television cable distribution system, where either an IRC or HRC frequency plan is used. The present invention provides methods and apparatus that enjoy the advantages mentioned with P1741 / 99MX before and others.

SUMMARY OF THE INVENTION In accordance with the present invention, there is provided a method and apparatus for filtering the interference in a signal communicated from a transmitter to a receiver in a communication path. The transmitter is momentarily interrupted from transmission over the communication path, for example, by placing it in an inactive state. During the momentary interruption, the receiver analyzes the interference in the communication path to determine the frequency of at least one peak of interference noise. The information is communicated from the receiver to the transmitter which identifies the frequency of at least one peak of interference noise. Based on this information, the transmitter pre-distorts the signal to accentuate the signal magnitude at the identified frequency or frequencies of the frequency peak (s). The pre-distorted signal is then transmitted by the transmitter to the receiver, which filters the pre-distorted signal to attenuate the signal magnitude at the frequency or frequencies identified. In an illustrated embodiment, the receiver performs a real or complex analysis of the signal frequency in the interference to determine the frequency peak (s) therein. The filtration in the P1741 / 99MX receiver can use, for example, a transfer portion that is the inverse of the transfer function to pre-distort the signal in the transmitter. In a possible implementation, filtering in the receiver uses the Z transform transfer function: where • = ex (2j »f), f is the normalized center frequency of the filter and R is a constant. Pre-distortion in the transmitter can implement the inverted transfer function H (z) "1. A power threshold detection can be used during the analysis to identify the location (s) of frequency of the peak (s). For example, only peaks that exceed a predefined power threshold level could be identified for pre-distortion in the transmitter and subsequent filtering in the receiver In an adaptive method, the transmitter is periodically interrupted Transmission over the communication path Interference in the communication path is analyzed in the receiver during periodic interruptions and information is communicated from the receiver to the transmitter identifying the changes in the determined interference peak (s) (s) during periodic interruptions.

P1741 / 99MX The transmitter then pre-distorts the signal to accentuate the signal magnitude according to the changes of the interference peaks. Also disclosed is a method and apparatus for filtering the non-linear distortion in a signal communicated from a transmitter to a receiver via a communication path. The signal is pre-distorted at the transmitter to accentuate the signal magnitude at a fixed frequency where non-linear distortions take place. The pre-distorted signal is transmitted to the receiver, which provides filtering to attenuate the signal magnitude at the fixed frequency. For example, if the signal is an integrally related bearer television channel (IRC) signal having composite second order (CSO) and triple-beat composite (CTB) distortions at different fixed frequencies, the distortions of CSO and CTB are they filter by pre-distorting the signal in the transmitter to accentuate the signal magnitude to a first fixed frequency where the CSO distortion resides, and to a second fixed frequency where the CTB distortion resides. The pre-distorted signal is then filtered in the receiver at the first and second fixed frequencies. For example, if the signal is a harmonically related bearer television channel (HRC) signal having composite second order (CSO) and triple-beat composite distortions P1741 / 99MX (CTB) at a common fixed frequency, the distortions of CSO and CTB are filtered by pre-distorting the transmitter signals to accentuate the signal magnitude at the common fixed frequency. The pre-distorted signal is then filtered in the receiver at the fixed, common frequency.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram for illustrating a cable television headend or cable modem termination (CMTS) system with a return path receiver; Figure 2 is a more detailed block diagram of the component of the return path receiver of Figure 1; Figure 3 is a detailed block diagram of an embodiment of an explosion receiver that can be used in the receiver of Figure 2 according to the invention; Figure 4 is a detailed block diagram of an alternative embodiment of an explosion receiver that can be used in the receiver of Figure 2 according to the invention; Figure 5 is a block diagram of a second order notch filter structure, which can be used in a receiver according to the invention; Figure 6 is a graph illustrating a P1741 / 99MX baseband 256-QAM spectrum, simulated with a narrow band interference peak located at 0.5 MHz from the center of the channel with C / (N + I) = 0dB; Figure 7 is a graph illustrating a simulated baseband 256-QAM spectrum of a pre-distorted signal to be transmitted according to the invention; Figure 8 is a graph illustrating the simulated 256-QAM baseband spectrum of the signal of Figure 7 after the interference has been removed by filtering into the receiver according to the invention; Figure 9 is a graph illustrating the amplitude response of a notch filter that can be used in the receiver to filter the pre-distorted signal; Figure 10 is a graph illustrating the phase response of a notch filter that can be used in the receiver to filter the pre-distorted signal; Figure 11 is an illustration of a simulated quadrant of a 256-QAM I / Q constellation with narrow band interference that has not been filtered according to the invention; and Figure 12 is an illustration of a simulated quadrant of a 256-QAM I / Q constellation with narrow band interference that has been filtered according to the invention.

P1741 / 99MX DETAILED DESCRIPTION OF THE INVENTION The present invention provides techniques for filtering both interference and non-linear distortions in a communication system. For example, the invention is useful for filtering the input noise which may include, for example, narrow band interference, common mode distortion and location-specific interference in an upstream channel (eg, return path) of a cable television system. The invention can also be used to filter non-linear distortions such as distortions of CSO and CTB, which for example, may be present in the downstream path of a cable television system. Additional uses of the invention for other types of interference and distortion will be apparent to one skilled in the art. Figure 1 illustrates, in a simplified block diagram form, a cable modem termination system (CMTS) in a cable television headend. The CMTS is controlled by a computer processor (CPU) 10 which communicates with the other CMTS components on a common bi-directional bar coupled to a media access controller (MAC) 12. The MAC 12 controls the physical layer of the CMTS. the communication signals and coordinates the various aspects of the data transported by such signals P1741 / 99MX as the time data that is sent, the time data received, etc. The MAC 12 also receives signals from a return path receiver 20 which, in turn, receives signals from a remote subscriber signal (e.g., a cable modem or adaptation unit). The data signals to be transmitted are provided by the MAC to a QAM modulator 14 for modulation in a conventional manner. An upconverter 16 converts the output of the QAM modulator to a suitable radio frequency (RF) for transmission over, for example, a bidirectional HFC network 22. The CMTS is coupled to the HFC network via a diplexer 18 in a manner conventional. The input section of the return path receiver 20 converts an analogue explosion signal received from the subscriber terminal to a sampled digital signal, which can be fed to a digital signal processing (DSP) circuit 38, off-line ( that is, in non-real time) in the receiver, as illustrated in Figure 2. In order to filter the interference (for example, narrow band interference, such as input), the interference must first be detected. To do this, an initialization process is started where the digital transmitter (for example, QPSK or QAM) in the cable modem (CM) or in the adaptation unit is placed in an inactive mode, so that it will only be received cumulative interference by the receiver P1741 / 99MX 20 return path in the CMTS. The off-line DSP 38 provided within the return path receiver 20 analyzes the received noise. This analysis can be performed using a signal frequency analyzer, which complex uses, for example, the Fourier transform, discrete (DFT) algorithm. The spectral power density of the interference is significantly higher than that of the white Gaussian noise (WGN), and the noise peaks can be easily identified using a threshold power detector. The threshold level can be adjusted, for example, to be 10-dB greater than the floor of WGN to ensure that only large interference peaks are identified. To achieve, for example, a resolution of the 10 kHz frequency peaks in a 3.2 MHz data channel (the worst case), 640 sampling points are required for the DFT. Once the frequencies of the interference peaks have been determined, the information identifying these specific frequencies is transmitted to the subscriber terminal via the downstream QAM modulator 14, the upconverter 16 and the diplexer 18 as shown in FIG. Figure 1. It is noted that although a QAM modulator is shown for illustration purposes, any suitable form of digital modulation may be used to transmit the identification information of the frequency peaks to the subscriber terminal. As P1741 / 99MX an alternative, an alternative channel (which can be digital or analog) could be used to pass this information to the subscriber's terminal. Relevant portions of a possible implementation of the return path receiver 20 are illustrated in the form of block diagrams in Figure 2. It is noted that the illustrated portions are provided as an example only and that other implementations will be apparent to an expert in The technique. In a cable television implementation, the illustrated receiver portions may be provided in a CMTS or other header mode. The HFC network 22 is coupled to a tuner 30 provided in the receiver. A desired upstream signal received from the HFC network, such as a data signal, is tuned using the tuner 30 and passed to an analog-to-digital (A / D) converter 32. The digitized A / D signal 32 is passed to an appropriate receiver, such as an explosion receiver 36 and a DSP 38. The DSP performs a real or complex analysis of the signal frequencies of the return route signals to determine the frequency of each of the peaks. of interference. The DSP sends information indicative of the frequency of each peak (e.g., filter coefficient data) to the microprocessor 40 to adjust the notch filters in the explosion receiver 36. This information is also communicated to the adaptation unit of the P1741 / 99MX subscriber or cable modem for use in adjusting the pre-distortion, complementary filters. The purpose of the pre-distortion filters in the cable modem or subscriber adaptation unit is to accentuate the signal transmitted in the return path (upstream) to the frequencies where interference is expected to occur in the receiver. The corresponding notch filters in the CMTS will then attenuate the same sequences in the return path receiver, thereby effectively filtering the effect of the interference. The attenuation at the explosion receiver 36 not only filters the interference; the level of the signal also returns to its appropriate magnitude at the frequencies of the interference peaks. Figures 3 and 4 illustrate the additional detail for two different modes of a return path receiver using the notch filters according to the present invention. The receivers illustrated in these figures can be used to provide the functions of the return path receiver 20 shown in FIG. 1. In particular, the intermediate frequency return (IF) signal of the tuner 30 of FIG. 2 is entered. to an A / D converter 32 (Figure 2), which digitizes the signal and passes it to the phase I and Q quadrature mixers 52, 54, respectively (Figure 3 or 4). In the embodiment of Figure 3, Nyquist filters 56, 58 P1741 / 99MX square root filters the I and Q signals, and passes them to the respective notch filters (NF) combined with -r-4 decimation filters 60, 62, thereby providing I and Q signals, sampled down that have been attenuated back to normal levels at the interference frequency peaks. In the embodiment of Figure 4, the notch filters 51 are provided immediately before the quadrature mixers 52, 54 instead of being combined with the decimator filters 61, 63. The I and Q signals, filtered and sampled downwardly. they are passed to a forward feed compensator (FFE) 64 and decision feedback compensator (DFE) 66 for conventional compensation. A decoder 68 of early error correction (FEC) then processes the compensated I and Q signals in a conventional manner. The acquisition and tracking circuit 70 allows the receiver to acquire and follow the received signal appropriately, as is known in the art. The following difference equation describes a second-order digital notch filter design that can be used to implement the invention: • (") = hü •? () + Bx • x (n - 1) + b2 • x { N - 2) - a, • y { N - 1) -«, • y { n - 2) where: x (n) e and (n) are the input sequences and P1741 / 99MX discrete time output, b2 = b0 = 1, bi = 2Re (•), ax = -2R2.Re (.), A2 = R2, and • = exp (2jf) f = normalized central frequency of the notch, and R is a constant related to the notch magnitude. In this way, only two coefficients are needed for this filter. The second order notch filter structure, shown in Figure 5, is referred to as a direct structure II. The transfer function or the notch filter Z transform 80 is described by the following equation.

The pre-distortion filter in the transmitter of the subscriber terminal has exactly the inverted transfer function as the second-order notch filter shown above [H (z) ~ 1] . Pre-distortion filters can be inserted, for example, after the symbol mapper for a QPSK (quadrature phase shift modulation) or QAM implementation (modulation by quadrature amplitude), and before P1741 / 99MX a programmable transmitter pre-compensator used to cancel the effects of inter-symbol interference (ISI). It is noted that although a notch filter is disclosed herein for use with the return path receiver, other types of filtration may be used to provide similar or equal results. Figure 6 illustrates a baseband 256-QAM spectrum 82, simulated with a narrow band interference peak 83 located 0.5 MHz outside the center of the channel and with C / (N + I) = OdB. Figure 7 illustrates the spectrum 84 of the simulated baseband 256-QAM spectrum with the pre-distortion 85 added to the transmitter. Figure 8 shows the simulated baseband 256-QAM spectrum 86 in the upstream receiver after the narrow interference peak 83 and the pre-distortion 85 have been removed. Figures 9 and 10 illustrate the responses 88, 90 of amplitude and phase, respectively, of the second order notch filter of Figure 5, as a function of the normalized frequency. Figure 11 shows a single, simulated quadrant 92 of the 256-QAM constellation when the interference peak illustrated in Figure 6 is presented. It is clear from this figure that the received signal contains many errors. Figure 12 shows a quadrant 94 Individual P1741 / 99MX, simulated 256-QAM constellation in the presence of the interference peak, but after the application of the interference filtering technique of the present invention. The simulation results indicate an error-free reception for the 256-QAM signal, demodulated. In order to further clarify the present invention, the filtering of the interference such as the incoming noise can be made adaptable. In particular, between burst intervals upstream, (for example, once every second), the transmitter of the subscriber terminal can be inactivated to "quiet" the return path. This will allow the CMTS to monitor incoming peaks in the quieted return path. The DSP circuit 38 off-line at the receiver of the return path in the CMTS can determine whether the previously detected input peaks are still present or whether new peaks of entry exist. The updated information (parameter • in the transfer function for H (z)) in the frequency of the interference peak (s), stream (s) is then sent to the transmitter of the subscriber's terminal (eg, unit adapter or cable modem) via the downstream modulator to enable and / or disable the appropriate pre-distortion filters on the return path transmitter. At the same time, the appropriate notch filters in the receiver of the return path are P1741 / 99MX enable and the inappropriate notch filters are disabled. A variation of the techniques described above can be applied to the transmission of the downstream signal (e.g., digital video) to the subscriber terminals to overcome the effect of non-linear distortions (CSO / CTB). The location of CSO and CTB distortions in an HFC network depends on the cable TV frequency plan used for the analog video signals. The two most widely used cable TV frequency plans are the plans of the fully related carrier (IRC) and the harmonically related carrier (HRC). In the IRC plan, the first frequency of the image carrier is located at 55.2625 MHz with successive image carriers located with 6 MHz of separation up to 1 GHz. In the HRC frequency plan, the image carrier frequencies are shifted towards below 1.25 MHz compared to the corresponding image carriers in the IRC plan. The advantage of HRC is that the distortion products of CSO and CTB fall into the image carrier, and in this way its effect becomes almost invisible. In the IRC plan, the CSO distortions are located ± 1.25 MHz of the corresponding image carrier frequency, and thus can become visible. The following Table 1 shows the various P1741 / 99MX options for IRC and HRC frequency plans TABLE 1- LQCALIZATION OF THE DISTORTIONS OF CSO AND CTB WITH REGARD TO THE FREQUENCY OF CENTRAL OF THE QAM CHANNEL To overcome the impact of CSO / CTB distortions using the IRC plan of the QAM receiver, two pre-distortion filters are enabled (one for CSO and one for CTB) with the frequency response of H (z) "1 In the QAM modulator, which is located in the cable header, two notch filters are also enabled with the frequency response H (z) and with the same coefficients, in the QAM receiver in the adaptation unit. requires adaptation here since the frequencies of the nonlinear distortions are always known in hybrid, analog / digital HFC networks For the HRC plan, only one pre-distortion filter and one corresponding notch filter are required, since the CSO and CTB P1741 / 99MX present on the same frequency. Now, it should be appreciated that the present invention provides methods and apparatus for filtering non-linear interference and distortions in communication systems. Although the invention has been described in conjunction with cable television systems, where the downstream path may suffer from non-linear distortions (CSO / CTB) and the upstream path may suffer from input noise, the invention is not limited to use in these systems or these types of interference and distortion. The new techniques for detection and reduction of interference and for filtering the non-linear distortion are applicable from the communication system where it receives interference at already known or detectable frequencies. By pre-distorting the transmitted signal and filtering the pre-distorted receiver signal, an effective reduction or elimination of non-linear interference and distortion is obtained. Accordingly, various adaptations and modifications to the invention can be made without departing from the scope thereof as set forth in the claims.

P1741 / 99MX

Claims (20)

1. NOVELTY OF THE INVENTION Having described the present invention, it is considered as a novelty and, therefore, the content of the following CLAIMS is claimed as property 1. A method for filtering a signal communicated from a transmitter to a receiver via a route of communication to reduce interference, comprising the steps of: momentarily interrupting the transmitter in the transmission over the communication path; analyzing the interference in the communication path in the receiver during the momentary interruption to determine the frequency of at least one interference peak; communicating the information from the receiver to the transmitter identifying the frequency of at least one interference peak; pre-distort the signal at the transmitter to accentuate the signal magnitude at the identified frequency; communicate the pre-distorted signal to the receiver; and filtering the pre-distorted signal in the receiver to attenuate the signal magnitude at the identified frequency.
2. 2. A method according to claim 1, wherein the analysis step performs a real analysis or P1741 / 99MX complex of the signal frequency in the interference to determine the frequency peak (s) in it. A method according to claim 1 or 2, wherein the filtering of the receiver uses a transfer function which is the inverse of the transfer function used to pre-distort the transmitter signal. 4. A method according to one of the preceding claims, wherein the filtering in the receiver uses the transform transfer function z: where • = exp (2j «f), f is the normalized central frequency of the filter and R is a constant. A method according to claim 4, wherein the pre-distortion in the transmitter implements the reverse transfer function H (z) "1. A method according to one of the preceding claims, wherein: the transmitter is periodically interrupted of the transmission on the communication path, the interference of the communication path in the receiver is analyzed during the periodic interruptions; P1741 / 99MX communicates information from the transmitter receiver by identifying changes in the interference peak (s) determined during periodic interruptions; and the transmitter pre-distorts the signal to accentuate the signal magnitude according to the changes of the interference peaks. A method according to one of the preceding claims, wherein the analysis step identifies the frequency location (s) of the interference peak (s) according to a power threshold level. 8. Apparatus for filtering interference in a signal communicated from a transmitter to a receiver via a communication path, the receiver comprises: a real or complex signal frequency analyzer adapted to analyze the interference in the communication path during momentary interruptions of the signal for determining the frequency of at least one interference peak; and means for communicating information from the receiver to the transmitter that identifies the frequency of at least one interference peak; the transmitter comprises: a filter adapted to pre-distort the signal in the transmitter for central the signal magnitude at the identified frequency; Y P1741 / 99MX The receiver also includes: a filter adapted to attenuate the signal magnitude of the pre-distorted signal at the identified frequency. The apparatus according to claim 8, wherein: the communication path comprises a return path of the cable television system that couples a subscriber location to a headend of the cable television system; the transmitter is provided at the subscriber's location; the transmitter is provided at the head of the cable television system; and the interference comprises input noise. The apparatus according to claim 8 or 9, wherein: the transmitter communicates the pre-distorted signal to the receiver using digital modulation. The apparatus according to one of claims 8 to 9, wherein the filter in the receiver comprises a notch filter. The apparatus according to claim 11, wherein the notch filter of a transform transfer function Z described by: P1741 / 99MX where • = exp (2j »f), f is the normalized center frequency of the filter and R is a constant. The apparatus according to claim 12, wherein the pre-distortion filter in the transmitter implements the reverse transfer function H (z) "1. The apparatus according to claim 13, wherein: the complex frequency analyzer Signal periodically analyzes interference in the communication path during momentary signal interruptions to determine changes in the interference peak (s) over time, and pre-distortion notch filters can be programmed to provide the accentuation attenuation of the signal, respectively, in the interference peak (s) as the frequency of the peak (s) changes over time 15. The apparatus according to one of claims 8 to 14, wherein the complex signal frequency manager includes a threshold power level detector for use in determining the frequency of at least one interference peak. non-linear distortion in a signal communicated from a transmitter to a receiver via a communication path, comprising the steps of: P1741 / 99MX pre-distort the signal at the transmitter to accentuate the signal magnitude at a fixed frequency where it receives non-linear distortion; communicate the pre-distorted signal to the receiver; and filtering the pre-distorted signal in the receiver to attenuate the signal magnitude at the fixed frequency. A method according to claim 16, wherein: the signal is an integrally related carrier television (IRC) channel signal having composite second order (CSO) and triple composite pulse (CTB) distortions present at different frequencies fixed; and the distortions of CSO and CTB are reduced by pre-distorting the signal at the transmitter to accentuate the signal magnitude at a first fixed frequency where the CSO distortion receives a second fixed frequency where it receives the distortion of CTB, and filter the signal in the receiver at the first and second fixed frequencies. 18. A method according to claim 16 to 17, wherein: the signal is a harmonically related carrier television channel (HRC) signal having composite second order (CSO) and triple compound pulse (CTB) distortions present at a P1741 / 99 X common fixed frequency; and the distortions of CSO and CTB are reduced by pre-distorting the signal at the transmitter to center the signal magnitude at the common fixed frequency and filter the signal at the receiver at the common fixed frequency. 19. Apparatus for filtering non-linear distortion in a signal communicated from a transmitter or receiver via a communication path, comprising: a first filter in the transmitter to provide a pre-distorted signal having an accented magnitude at a fixed frequency where it receives non-linear distortion; and a second filter in the receiver adapted to filter the pre-distorted signal to attenuate the signal magnitude at the fixed frequency. The apparatus according to claim 19, wherein the second filter comprises a notch filter having a transform transfer function Z described by: where • = exp (2j »f), f is the normalized center frequency of the filter, and R is a constant; and the first filter increases the inverted transfer function E (z) '1. P1741 / 99MX