Adaptive method to remove ghosts in video signals

Abstract: An adaptive method for suppressing video signal x(k) echoes in equalizers of TV sets including digital filters whose coefficients are updated in an adaptive and iterative manner using an LMS (Least Mean Square) algorithm until the difference, or output error, between a target output signal d(k), called the reference signal, and an outgoing signal y(k) from the equalizer is gradually reduced, comprises the steps of: applying a "combing" technique to an original filter having K*N coefficients in order to select K comb filters having N coefficients each; applying said LMS algorithm, with a variable convergence factor (µ) to each individual comb filter for a predetermined number of iterations; gathering the resultant configurations of the comb filter coefficients and selecting a subfilter with N largest modulo coefficients therefrom; updating the values of said N coefficients again by reiterating the LMS algorithm to said subfilter for a limited number of iterations; zeroing all the coefficients with a lower modulo than a predetermined threshold; selecting a group of F coefficients by a slotting operation across those of said coefficients which have a cluster value: and updating the value of said group of F coefficients, by reiterating the LMS algorithm, until the output error e(k) becomes smaller than a predetermined value.

Description: Field of the Invention This invention relates to an adaptive method of suppressing video signal echoes in latest generation TV sets. More particularly, the invention relates to an adaptive method of suppressing video signal echoes x(k) in equalizers of TV sets incorporating digital filters whose coefficients are updated in an adaptive and iterative manner using an LMS (Least Mean Square) algorithm until the difference, or output error, between a desired output signal d(k) -- called the reference signal -- and an outgoing signal y(k) from the equalizer is progressively reduced. As is well known, one of the main causes for deterioration of the TV picture quality on the displays of today's analog TV receivers is the appearance of duplicates of the original signal which, by overlapping the primary image, create a so-called echo effect. TV echoes, best known as ghost effects, are usually due to reflections undergone by the primary TV signal against fixed rebound surfaces, such as mountains, buildings, etc.. Also, some moving objects, such as airplanes flying over the area between the TV transmitter and the TV receiver, may scatter the TV signal and generate such noise. The echoes thus generated are presented to the receiver in the form of delayed, attenuated, and distorted duplicates of the primary image. The video echo phenomenon actually is a major deteriorating factor of TV pictures, and its influence is most noticeable on the latest Improved Quality (IQTV) receivers and the High Definition (HDTV) wide screen ones. In fact, with these new generation TV sets, the doubled line scan frequency and the use of large size screens tend to enhance the ghost effect on the display. Accordingly, while a standard TV receiver equipped with a channel equalizer can provide a picture of significantly improved quality, the use of an equalizer is bound to become an absolute necessity in IQTV and HDTV sets. For a better understanding of the invention aspects, a review of the traditional ghost-suppressing techniques will be given first. The ghost cancelling operation is basically one of filtering the TV signal being input to the TV receiver. Background Art The TV ghost cancelling process is usually based on a so-called equalization system. Since the characteristics of an RF video channel vary over time because they are strictly dependent on environment and operating conditions, the ghost cancelling process is the most effective when an automatic equalization system of the adaptive type is used. Two different approaches are available for implementing a video channel equalization system, namely a direct method and an indirect method. Many video channel equalizers are based on the identification of the video channel pulsive response and its reverse. This enables the coefficients of the system filters to be generated by a direct method. Although performing well, this technique is highly complex from the computational standpoint. Alternatively, an adaptive system based on an indirect method utilizes a ghost-suppressing algorithm which updates the values of the internal filter coefficients in an adaptive manner, that is, so as to gradually reduce the difference between a desired reference signal and the outgoing signal from the ghost-suppressing system. The filtering section of an equalizer system, on the other hand, may be either implemented by a FIR method or an IIR method. The former technique uses F.I.R. filters to approximate the infinite pulsive response from the video channel equalizer. The latter suppresses the ghost effect by means of I.I.R. filters, that is, fed back FIR filters. The first-mentioned method may be defined a classic type and is widely used for suppressing TV echoes. However, while ensuring stability for its filters, it can provide no equalizer of the adaptive type unless recourse to a complicated hardware structure is made. In fact, each pole of the pulsive response from the ghost suppressor can only be approximated by a large number of zeroes, which significantly increases the number of the internal multipliers of the system. The second-mentioned method provides for the use of an equalizer based on IIR filters. The latter do streamline the system hardware and reduce the volume of the calculations required for the ghost suppressing operation, but their stability is not always ensured. Many adaptive algorithms intended for application to IIR filters have been defined and tested as to their stability, rate of convergence, and capability to remove TV echoes. Such algorithms stem all from a classic algorithm known as Least Mean Square (LMS). The updating laws for the coefficients of an LMS algorithm are the following: ai(k) = ai(k-1) + µ y(k-1-i) e(k-1) 1 < i < N bj(k) = bj(k-1) + σ x(k-1-j) e(k-1) 1 < j < M where, e(k)=d(k)-y(k) . The Greek letters µ and σ designate the adaptation pitch (also known as convergence factor) for the FIR and IIR filters, respectively; ai and bj are the coefficients of the IIR and FIR filters, respectively; x(k) and y(k) designate samples of the input and output signals to/from the equalizer; and e(k) is the output error, i.e. the difference between the target signal d(k) and the system output y(k). Simulations carried out by applying the classic LMS algorithm to an IIR filter with M coefficients or plugs have proved unsatisfactory especially in the presence of large amplitude echoes. Nonetheless, an analysis of the aggregate of the associated coefficients shows that, most of the coefficients have near-null values, the non-null values form clusters or local peaks during the adaptation process, and the locations of such clusters on the delay line of the filter are closely related to the delays of the echoes which overlap the primary image. Figure 2 shows a typical pattern for the coefficients generated by the application of an LMS algorithm until a lower output error than a predetermined threshold is obtained. The simulation conditions are instead entered into Table 1 in Figure 3, where the echo delays are set forth as sample numbers keeping in mind that the sampling period is of 74 ns. An example of the prior art referred to a ghost cancellation circuit using F.I.R. and I.I.R. filter combination is disclosed in the European Patent Application EP-A-0 421 526. A more up-to-date ghost cancelling technique provides a determination of the locations of the largest amplitude clusters corresponding to the main signal overlapping echoes such that only the most significant filter coefficients will be updated, thereby reducing the computation volume and improving the algorithm rate of convergence. Thus, the secondary coefficients will be cleared as the corresponding multipliers are removed, which relieves the structural complexity of the equalizer. Understandably, the reduced coefficient aggregate is bound to differ, at the end of the adaptation process, from the configuration of the original aggregate. However, by suitably selecting the coefficients of the new reduced configurations, it becomes possible to re-create the right pulsive response from the echo suppressing system, even with a smaller number of multipliers. To accomplish this, a so-called "combing" method has been used whereby several reduced coefficient configurations are selected by a "combing" procedure, starting with the original configuration. In other words, to create N combed filters, with M initial filter coefficients being given, one coefficient is selected every K, where K=M/N integer, starting with the first coefficient of the original filter. The second "comb" of coefficients is obtained by shifting the preceding one through one coefficient, as follows: In this way, the combination of the partial coefficient configurations will cover the original configuration, leaving their intersections empty. Accordingly, it can be ensured that the merging of the partial configurations which results from the adptation process will approximate in an optimum manner the original configuration of the coefficients, following the application of the algorithm. Once the K combing filters with N coefficients each are selected, one of the early methods under consideration implied that, the LMS algorithm be applied to each combing filter for a fixed number (e.g. four) of iterations; the subfilter coefficients be gathered into the original aggregate of M coefficients; and the LMS algorithm be applied to the most significant of the coefficients thus determined. This sequence of operations defines an algorithm called CDFF (Cluster Detection with Fixed-convergence Factor) which has led to considerable improvements over the mere application of the LMS algorithm. Further improvements in the performance of that algorithm have been attained by changing the adaptation pitch µ (Equation 1) according to the value of the output error. In fact, the parameter µ directly governs both stability and rate of convergence; however, this parmeter also controls the amplitude of the coefficient fluctuations about their final value (Grain Error). The higher the value of µ, the greater is the algorithm rate of convergence, but the greater becomes the grain error also. To obviate this drawback, it has been thought of varying the adaptation pitch with respect to the error e(k) according to a linear law whereby the angular coefficient changes between two values upon e(k) going below a given threshold. In this way, so far as the error e(k) between the cleaned signal from the adaptive equalizer and the desired signal stay high, µ will retain high values which decrease quickly with e(k); upon e(k) dropping below a predetermined threshold, µ will take low values which decrease very slowly as e(k) decreases. This situation is depicted schematically in Figure 5. Despite that improvement, the CDVF (Cluster Detection with Variable-convergence Factor) algorithm described above exhibits yet another drawback. During some simulations, a delay line was considered which could accommodate post-echoes carrying delays of up to 14.6 µs. Accordingly, on completion of the combing step which presupposes the creation of K=3 comb filters, a further subfilter is obtained which consists of the M/3=N highest modulo plugs. In the presence of three echoes, the N coefficients are more or less equally distributed over three clusters (approximately N/3 plugs per cluster). In the presence of a single echo, all the N coefficient will instead correspond to the single cluster provided. Since the algorithm time of convergence is heavily dependent not only on the cluster "framing" -- i.e. the determination of the peak largest coefficient -- but also on the determination of the actual width of the cluster, progressive deterioration of the algorithm performance was encountered as the number of the echoes decreased, due indeed to the aforesaid overabundance of coefficients. The underlying technical problem of this invention is to provide a novel adaptive method for suppressing video signal echoes in equalizer devices, which has such functional features as to overcome the drawbacks with which the various prior art methods mentioned above are beset. Summary of the Invention The idea on which this invention stands provides for the number of the coefficients of the final subfilter to be optimized in the adaptive algorithm to suit the number of the echoes, and hence the clusters, present. Based on this idea, the technical problem is solved by a method for suppressing ghost images as defined in the appended Claim 1. The features and advantages of the inventive method will become apparent from the following description of an embodiment thereof, given by way of illustration and not of limitation with reference to the accompanying drawings. Brief Description of the Drawings In the drawings: Figure 1 is a diagramatic view of the architecture of an equalizer operated in accordance with the method of this invention; Figure 2 is a diagram showing the typical pattern of coefficients generated by the application of an adaptive algorithm of the LMS type, according to the prior art; Figure 3 shows in chart form the delay and amplitude values of echoes to be suppressed by the LMS algorithm; Figures 4 and 5 are respective diagrams showing the pattern of a parameter µ which controls the rate of convergence of a second (CDVF) algorithm according to the prior art; Figure 6 shows a comparative diagram for "training" curves of the LMS algorithm, the CDVF algorithm, and the algorithm according to the invention; and Figures 7a, 7b and 7c are respective diagrams related to coefficient selection steps provided by the inventive method. Detailed Description With reference to the drawing figures, generally and diagramatically shown at 1 is an adaptive video channel equalizer or ACE (Adaptive Channel Equalizer) set up for operation according to the method of this invention. The equalizer 1 is, therefore, intended to implement an adaptive method for suppressing echoes of video signals with the aid of I.I.R. filters. This method will be described hereinafter, it being sufficient for the time being to know that it is effective to update the coefficients of the filters which make up the equalizer system 1 such that the difference between an outgoing signal y(k) from the equalizer and a target signal d(k) outgoing equalization can be minimized. The equalizer 1 comprises a first digital filter 2 of the FIR (Finite Impulse Response) type to which is input a sampled video signal x(k) and having an output connected to a summing node 3. The signal y(k) is output from the node 3 to form the equalizer output. That signal y(k) is input to a second digital filter 5 of the FIR type which has an output fed back to the node 3. This second filter 5 will operate, therefore, as a filter of the IIR type. Additionally, the output y(k) is addressed to a second summing node 6, whereto said output y(k) is transmitted with negative sign. The node 6 also receives a signal d(k) which represents the target equalizer output. A signal e(k) is output from the node 6 and represents the output error. In practice, the output y(k) from the equalizer is compared with a reference signal d(k) which constitutes the ideal output from the equalizer. This operation is performed using an appropriate off-line processing program. The difference between the two signals supplies the error e(k) whereby the method of this invention allows the process of adapting the filter coefficients to be carried out. On completion of the process of selecting the most significant coefficients and adaptively updating them, thereby minimizing e(k), the computed coefficients are conveyed to the filters 2 and 5 through a block 4 identified by its associated function C(z) in the complex transform z. At this point, an on-line process of suppression of the video signal echo may start. The method of this invention, also referred to by the acronym ACDVF (Advanced Cluster Detection with Variable-convergence Factor) hereinafter, is essentially coincident with that of the algorithm CDVF up to the determination of the subfilter which comprises the largest modulo coefficients N. Presently the method of this invention is run through a process of fine adaptation of the selected coefficients based on application of the LMS law with variable µ. This process is applied for L iterations to better define the main clusters; in fact, a few iterations are already sufficient to segregate the primary peaks from the rest by 2-3 orders of magnitude. Thereafter, the most significant coefficients are selected for the echo suppressing process by clearing all the coefficients with a modulo below a predetermined threshold. The threshold value is selected each time based upon the value of the largest modulo coefficient of the N selected ones. But the clearing of the coefficients below a certain threshold might depress the algorithm performance where, for example, a cluster coefficient was cleared which is instead essential to the suppression of the associated echo, such as the coefficient directly adjacent to the highest modulo plug. To obviate this shortcoming, a slotting process is applied across the largest modulo coefficient of each cluster. The slotting operation consists of bringing to a logic value of "1" the F elements of a null value mask. The number of mask elements corresponds to the number of coefficients of the original filter. Such unitary values correspond to the F coefficients lying symmetrically across the highest modulo element of the cluster, as shown in Figure 7b. At this point, the F coefficients which correspond to non-null elements of the mask are updated according to the LMS law with variable µ, whilst the rest are cleared. Through the slotting operation, the number of the coefficients to be adapted is made equal to the product of the detected echoes by the width F of the slot. As a result, the largest number of coefficients to be updated, and hence the largest number of multipliers in the filtering section, will be 3*F, assuming an equalizer which reduces up to three echoes. On simulation, with F=5, excellent results could be obtained both as relates to the degree of echo suppression and the algorithm rate of convergence. In addition, the filtering section of the equalizer only requires fifteen multipliers; thus, the equalizer structure can be made far less complicated than in prior art approaches. Reverting to the previous example, wherein the threshold operation erroneously eliminates a significant cluster coefficient, the slotting operation allows the cleared coefficient to be recovered and subjected to the adaptation process. Therefore, the clearing of that significant coefficient is not influential because the adaptation process enables the coefficient to promptly attain its original value. In the event that the cluster is wider than the slot defined, as shown in Figure 7a, coefficients with a modulo exceeding the threshold may also happen to be cleared. Simulations carried out by the Applicant have shown that with 1-2 iterations the final error first increases, to then decrease sharply by virtue of the higher rate of convergence of the present method, as provided by carrying out the adaptation process on a reduced number of coefficients. Compared to the algorithms originating from the classic LMS algorithm, the method of this invention can have 4 to 5 times less iterations than the LMS algorithm, to bring the error output from the equalizer 1 below a predetermined threshold. Shown in Figure 6 is a comparison of the training curves for the plain LMS algorithm, the CDVF algorithm, and the method of this invention. The latter quickly determines the delay of the echoes from the main signal, thereby allowing the video channel equalizer hardware to be significantly simplified both in terms of the multipliers required and the volume of computations to be carried out. This enhances the equalizer system performance. The equalizer 1 of this invention can handle at least three video echoes, adequate to equalize the European UHF/VHF channels. As against the Japanese case, no reference signal for the GCR (Ghost Cancelling Reference) signal is available, as would be useful to immediately suppress ghost images. It is on this account that a signal called ITS (Insertion Test Signal) as supplied by R.A.I. Radiotelevisione Italiana (Italian Radio & TV Broadcasting Corporation) on line 17 has been used for reference. This ITS signal exactly conforms to many of the characteristics which are typical of ideal GCR signals. In particular, one of the worst faults of this ITS signal is that it cannot find out the delays of video echoes in any direct and accurate manner, as can instead the GCR signals defined in Japan. However, the method of this invention enables the location of ghosts along the time axis to be found quickly and accurately irrespective of the reference signal. Accordingly, this equalizer system can be readily adapted to the GCR signals adopted in various countries and is, therefore, highly versatile. It can be deduced from the foregoing considerations that the method of this invention is highly innovative, in that it, allows the locations of the echoes relative to the main signal to be quickly determined even if no suitable reference signal is available for suppressing the echoes; eliminates the problems from the redundant coefficients in the CDVF algorithm; and reduces substantially the number of the coefficients of the filtering section of the ghost suppressor from the CDVF algorithm, greatly simplifying the equalizer architecture in terms of the number of multipliers required. In particular, the number of the multipliers required will solely depend on the width of the selected slot, once the largest number of echoes to be suppressed is set. In other words, the method of this invention allows the locations of the video echoes to be quickly found relative to the location of the main signal, irrespective of the reference signal being used. Based on this information, the significant coefficients for the removal of ghost images can be selected, thereby drastically reducing the complexity of the video channel equalizer architecture by significantly cutting down the number of the multipliers required in the filtering structure. Furthermore, the time required to find the final values of the coefficients of the equalizer internal filters is made much shorter than that usually required by such classic algorithms as the LMS, because of the gradual decrease of the filter coefficients to be adapted affording reduced computational complexity for the method, and hence increased rate of convergence over classic approaches.

Claims: Adaptive method for suppressing video signal (x(k)) echoes in equalizers of TV sets including digital filters whose coefficients are updated in an adaptive and iterative manner using an LMS (Least Mean Square) algorithm such that the difference, or output error signal, between a target output signal (d(k)), called the reference signal, and an outgoing signal (y(k)) from the equalizer is gradually reduced, comprising the following steps: receiving an input signal (x(k)); computing a difference between an output (y(k)) of an equalizer including digital filters (2,5) having K*N coefficients, and supplied with the input signal (x(k)), and the reference output signal (d(k)); applying an LMS algorithm having a variable convergence factor (µ) for a predetermined number (L) of iterations, to reduce the error between said output (y(k)) and said reference signal (d(k)); processing the digital filter coefficients in order to select K subfilters having N coefficients each; applying said LMS algorithm, with a variable convergence factor (µ) to each individual subfilter for a predetermined number of iterations; characterized by the further following steps: collecting the resultant configurations of the subfilter coefficients; carrying out a slotting operation, comprising selecting groups of F coefficients, the coefficients of each group being larger than a threshold value and being symmetrically clustered about a peak coefficient value, the number of said groups being equal to the maximum number of echoes it is desired to suppress; clearing all the coefficients with a lower modulo value than a predetermined threshold value; reiterating the LMS algorithm, until the output error signal (e(k)) becomes smaller than a predetermined value; processing the input signal (x(k)) using the F coefficients provided by the previous step. Method according to Claim 1, characterized in that said threshold value is selected based on the largest modulo coefficient of the N coefficients selected. Method according to Claim 2, characterized in that said slotting operation consists of retaining the values of the F coefficients selected, wherein, in each selected group of F coefficients, F-1 coefficients are substantially symmetrically disposed about the coefficient of said group exhibiting the largest modulo value. Method according to Claim 1, characterized in that the step of selecting a group includes the step of slotting at least three coefficients about said peak coefficient value.

Inventors: AIROLDI FABRIZIO, D ALTO VIVIANA, PODESTA MARIA GRAZIA, SCALISE FABIO, AIROLDI, FABRIZIO, D'ALTO, VIVIANA, PODESTA', MARIA GRAZIA, SCALISE, FABIO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1993
Main classification: H04N5
IPC codes: H04N5, H04B7