JP2004328758A - Ghost cancel system for signal reception - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a system which efficiently eliminate a ghost and has high robustness against a noise. <P>SOLUTION: The system which cancels an echo of a composite video signal in one receiver includes the following : (1) at least one antenna for receiving a television signal; (2) at least one receiver for the television signal; (3) at least one converter for converting the composite video signal into a digital signal; (4) one synchronizer which detects a minimum value of a line synchronous signal and gives a reference signal that is in-phase to the video signal as an output; (5) at least one active filter which receives each video signal at an input as well as the reference signal and receives the synchronous signal. One equalizer which gives an equalized video signal at an output and gives information about a lead or lag of the reference signal to the synchronizer. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

The present invention relates to a receiver for a signal or an analog television signal which allows the elimination of echoes called ghosts appearing on the television screen.

The appearance of ghosts and / or signal echoes is due to the reflection of radio waves by obstacles such as buildings and mountains. The signal collected by one antenna includes a necessary radio wave and an unnecessary radio wave. The unnecessary radio wave is caused by reflection of the radio wave by an obstacle, and generates a ghost or an echo. In a mobile station's communication network, eg, one car receiving a television signal, the signal between the transmitter and the receiver passes through radio waves propagating in the transmission channel.

It is known that this transmission over a propagation channel often causes a large portion of transmission interference. In fact, the various situations that can occur hinder prior determination and realization of a proper solution in all cases. Among the more harmful ones, the loss between the transmitter and the receiver is particularly noticeable. The loss is due to the distance or masking between the transceivers and to the presence of echoes in the signal, to the fact that the signal arrives from several directions by the omni-directional antenna and is reflected by the environment. These numerous problems can be reduced by system improvements (eg, use of filters or adaptive equalizers, increased power or number of receivers).

However, the cost of these improvements in commercial competition has made implementation difficult. Furthermore, technical solutions that seem to be effective are difficult to implement on existing networks without major changes in infrastructure. Many improvements have been devised for antennas, in particular, many types of transmission and reception have been proposed for reducing jamming effects (directional antennas), but for mobile stations the use of omni-directional antennas has been suggested. The fact is that it is used because of its simplicity.

As a document related to the technology described above, there is a document described in Non-Patent Document 1 shown below.
Author: CC Kuo & YT Chen, "EEE TRANSACTIONS ON CONSUMER ELECTRONICS: Vol. No. 2, May 2002". New Method for the implementation of an NTSC digital video decoder.

In order to solve the above problem, it is known to use a synchronization signal included in a television signal as a reference signal for adapting an equalizer. These signals have the same interference as the television signal remnants, but since the reference signal is known, the output of the adaptive signal is reduced by one adaptive filter to reduce the interference effect on the synchronization signal and the television signal remnants. It is possible to equalize the television signal and adjust the filter coefficient. Further, conventional systems of fixed reception use the principle of synchronization at the rising edge (or falling edge) of the line synchronization signal of the composite video signal. For example, “Document 1” describes detection of a synchronization pulse at a rising edge. As shown in FIG. 1, one threshold value that responds to 50% of the synchronization pulse height is set.

However, the noise-free video signal level is already close to that threshold. In the case of noise, the video signal immediately exceeds the threshold, causing false detection of synchronization. High noise levels in mobile systems make this prior art use unsuitable. Therefore, the present invention aims to alleviate the above disadvantages. More precisely, one of the objects of the present invention is to correct these disadvantages with a ghost cancellation system for an accurate, fast and inexpensive television receiver. Other problems are achieved by the ghost cancellation system for the composite video signal of the receiver, as well as this system including:
(1) A system for canceling ghosts of a received composite video signal including a receiving antenna for at least one radiated television signal (2) At least one receiver for receiving television signals coupled to individual antennas (3) At least one analog-to-digital converter for converting the composite video signal received by an individual receiver into a digital signal; detecting a minimum value of a line synchronization signal of the individual video signal and synchronizing with the composite video signal; A synchronizer that receives the individual digital signals at the input to output the reference signal as an output, and an equalizer that receives the composite video signal at the input as well as the synchronous reference signal. At least one adaptive filter for receiving a signal Wherein the providing the video signal which is equalized at the output. The equalizer provides a synchronizer with shift information to advance or delay the reference signal.

Therefore, in order to solve the above-mentioned problems of the prior art, the technical measures taken in the present invention are:
In a system for canceling echo of a video composite signal of a receiver,
At least one antenna for receiving a television signal, at least one receiver coupled to the individual antenna, and at least one analog for converting a video composite signal received by the individual receiver into a digital signal A digital converter and one synchronizer receiving the digital signal at the input to detect a minimum of the line synchronization signal of the individual video signals and to provide a reference signal synchronized with the video signal at the output; and at the input An equalizer for receiving an individual video signal as well as the synchronization reference signal, the equalizer having at least one adaptive filter for receiving the synchronization signal and providing one equalized video signal at an output. , And furthermore, the equalizer (8) is used to determine whether the reference signal The given to synchronous device, it is equipped with a digital-to-analog converter for receiving the output signal of the equalizer at the input to provide one analog video signal at the output.

According to the characteristics obtained by the configuration of the above-mentioned solving means, it is possible to obtain a system having high robustness against noise while efficiently removing ghosts. Furthermore, the same principle can be applied to mobile receivers and also to fixed receivers suffering from ghost problems.

Next, embodiments of the present invention will be described with reference to the accompanying drawings through specific examples to which the present invention is applied.

A preferred embodiment of the present invention will be described with reference to FIG. In this example, there are two antennas A1 and A2. Of course, the same principle can be applied when the number of antennas is one or more than two. The inputs to the synchronizer and equalizer are one composite video signal. In fact, there is often interference due to interference, reflections of the signal on the environmental element (multipath), Doppler effects, etc., and results in increased noise and one or more echoes of the signal. This signal contains noise as well as the transmitted video signal, echo. An implementation of one system using a mobile receiver will be described. Of course, the same signal processing can be applied to a system that receives signals in a fixed receiver. A ghost cancellation system for a composite video signal in a receiver according to the present invention includes at least one receiving antenna of a radiated television signal and two antennas A1 and A2 referred to herein. Each antenna A1, A2 is coupled to one TV tuner 3, 4. The signals received by the individual receivers are then provided at input to a single analog-to-digital converter ADC 5,6 for converting the composite video signal to a single digital signal. The digital signal is provided to one synchronizer 7, which is intended to provide the equalizer with a synchronization signal in phase with the video signal by detecting the minimum of the line synchronization signal of the individual video signal. ing.

In accordance with the present invention, as schematically illustrated in FIG. 4, the synchronizer determines the start of the line sync signal contained within the composite video signal by detecting the minimum of the line sync pulse. I do. In fact, as mentioned above, in one mobile receiver, the presence of noise does not allow reliable synchronization by detecting the rising and falling edges of the line sync pulse. This detection is less accurate than edge detection because the synchronization pulse duration is 4.7 μs, while the edge build-up time is 0.25 μs.

Thus, more accurate synchronization needs to be performed. The first synchronization that detects the minimum is referred to herein as coarse synchronization. Advantageously, a threshold can be used in conjunction with this minimum detection to improve the minimum detection of the synchronization signal. When the minimum value is detected, the algorithm searches for the rising and falling of the line synchronization pulse in one time window. Preferably, the window duration is slightly shorter than the line duration to avoid two minimum detections. The center of this window is adaptively positioned at the previously detected minimum plus one line. As shown in FIG. 4, the window can predict the minimum standby position, so that this window combined with one threshold can be used to more accurately detect the reliable line sync signal.

For example, the threshold is a percentage of the amplitude of the synchronization pulse. If the threshold cross point is inside the search window, that value is saved. Otherwise, the minimum position is retained. Therefore, a better estimate of the start of an individual line is possible. The video signals for the individual input signals are also provided to the equalizer 8. The equalizer receives as input a reference signal generated by the synchronizer, the reference signal responding to the synchronization signal in phase with the video signal. As described in more detail below, the equalizer includes an adaptive filter and performs a second synchronization known as fine synchronization by detecting the position of the filter coefficient minimum. The equalizer provides, as an output, one composite signal that is equalized to one digital-to-analog converter DAC 9 that provides the composite signal with a low noise level and with almost no echo of the signal.

This output signal is sent to one television and displayed. As shown in more detail in FIG. 3, the equalizer comprises one adaptive filter for each antenna, one adder 14 for receiving the video signal and adding the filtered video signal. It includes one individual adaptive filter 11, 12 which provides as output the filtered video signal. Preferably, the adaptive filter is a finite impulse response filter (FIR). The output of the adder 14 is the equalized composite signal.

Where S (n) is the video composite signal received at time n (for one antenna) and Ci is the filter coefficient at index i.

In some cases, there may be an offset to the reference signal provided by the synchronization. Furthermore, this offset can evolve according to the evolution of the propagation channel or the image content. It is desirable that the equalizer compensate for this offset.

The adaptive filter need not necessarily be a FIR. It can be one infinite length impulse response filter (IIR). The output of the adder 14 is subtracted from the reference signal in one comparator 16 to update the coefficients of the adaptive filters 11 and 12. The comparator 16 outputs one error signal ε which is supplied to one estimator 17. The estimator is one processor that minimizes the error signal ε, ie, modifies the coefficients of the adaptive filters 11, 12 to bring ε closer to zero.

A preferred implementation of this estimator is an algorithm known as least squares (LMS), where the individual coefficients of the finite impulse response filter are multiplied by the error ε and the video signal Si multiplied by the filter coefficients. Are updated between individual sampling intervals.

Here, μ is one coefficient that makes it possible to tune the adaptation rate of the coefficient. This adaptation is only possible during reception of the synchronization signal of the video signal. If one offset correction is integrated into the equalizer filter, the fit is a function of the error ε.

Here, μo is one specific coefficient, and the adaptation rate of this coefficient can be increased as compared with the other. Finally, the equalizer detects the maximum position of the filter module and induces the shift so that the maximum coefficient is applied to the reference signal such that it is located in the middle of the filter. This information of the shift is a fine synchronization given to the synchronizer so that this shift is applied to the reference signal transmitted to the equalizer. Under certain reception conditions, it is desirable to assume additional filtering of the received signal of the synchronizer.

In fact, as described above, the accuracy of the synchronization signal can be improved by adding one threshold detection associated with the detection of the minimum value of the synchronization signal. The value of the detection position according to the minimum value of the synchronization signal is called MaxPosM. This value is filtered by a Kalman filter to better determine the true value of the location. The filter value is called MaxPos. The distance between the two minimum positions is called MeanLineLen, and in many cases the two positions are divided into MaxPoSp and MaxPoSp _-1 to correspond to the juration. The following formula defines the relationship between the two values MaxPoSp and MaxPoSp _-1 .

Where ε p is the error signal to be minimized and Gi is the gain. By using the steady state of the Kalman filter, the gain is constant. These can be calculated in advance by two parameters, measurement noise variation and normal noise variation. One example of a gain G is

During tracking, a small Kalman filter is needed to improve the stability of the next minimum predicted position, but this increases the acquisition time on loss of synchronization or on initialization. High gain improves acquisition and allows for fast synchronization. The gain adaptation can be performed by the error signal fluctuation εp. If the error signal is large, that is, if it exceeds a certain threshold for a defined period of time, synchronization is lost and it is necessary to increase the gain for a period of time for synchronization acquisition. When the error signal exceeds a threshold, the responsive measurement is not considered to avoid that this erroneous measurement has a negative effect on the determination of the next minimum predicted position, ie, εp = 0. This state is called a filter freeze.

FIG. 5 shows an example of the above error signal fluctuation. Synchronization can be forced by holding the high gain for a period of time. Kalman filter control is performed by a state machine. The principles of state machines are well known to experts. Also, by adding a white filter to the input, it is possible to improve the equalizer performance. An example of the data flow is shown in FIG. The white filter 21 is arranged to receive the measurement video signal and the output reference signal from the synchronizer 7. These signals are whitened in the white filter 21 using a filter that amplifies the high frequency components of the composite video signal. The filter signal enters the equalizer 8 while allowing the equalizer to converge fast. At the output of the equalizer, one black filter needs to be considered in order to remove the effect of the white filter 21 on the composite video signal. Of course, the invention is not limited to the implementation described above, but is only given as an example. Many modifications and improvements can be made to the present invention without departing from the framework of the invention.

FIG. 9 is a diagram schematically illustrating conventional detection for one line synchronization pulse. It is a figure showing an embodiment by the present invention. FIG. 3 is a diagram showing the equalizer of FIG. 2 in more detail. FIG. 4 is a diagram schematically illustrating detection of a line synchronization pulse according to the present invention. 4 shows an example of a variation of a synchronization error signal on a time axis. This shows the position of the white and black filters in the data flow.

Explanation of reference numerals

A1, A2: antenna 3, 4: receiver 5, 6: analog digital converter 7: synchronizer 8: equalizer 9: digital-analog converter 11, 12: adaptive filter

Claims (12)

In a system for canceling echo of a video composite signal of a receiver,
At least one antenna for receiving a television signal;
At least one receiver coupled to each antenna;
At least one analog-to-digital converter for converting video composite signals received by individual receivers to digital signals;
One synchronizer for receiving a digital signal at an input to detect a minimum of a line synchronization signal of an individual video signal and to provide a reference signal synchronized with the video signal at an output;
An equalizer for receiving, at the input, the individual video signals as well as the synchronization reference signal, the equalizer having at least one adaptive filter for receiving the synchronization signal, and one equalized video signal at the output; And an equalizer that further provides advance / delay information of the reference signal to the synchronizer;
A digital-to-analog converter that receives the output signal of the equalizer at the input to provide one analog video signal at the output. 2. The system according to claim 1, wherein the system has two antennas (A1, A2). The system of claim 1, wherein the system has four antennas. 4. The system according to claim 1, wherein said adaptive filter of said equalizer is one finite impulse response filter. 4. The system of claim 1, wherein said adaptive filter of said equalizer is one infinite impulse response filter. 6. The system according to claim 1, wherein said synchronizer further comprises a comparator for comparing a level detector of said line synchronization signal of an individual video signal with a synchronization signal having a defined threshold. I do. 7. The system of claim 1, wherein said synchronizer further comprises a Kalman filter for filtering said line synchronization signal position. 8. The system of claim 1, further comprising one white filter at each input of said equalizer, and a black filter located between said equalizer and said digital-to-analog converter. 9. The system according to claim 1, wherein said receiver is one mobile station. 9. The system according to claim 1, wherein said receiver is one fixed station. 2. The system of claim 1, wherein the system has an odd number of (n + 1) (n is 0 and a natural number) antennas. 2. The system of claim 1, wherein the system has an even number of (2n + 1) (n is a natural number) antennas.

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